All Exams >
Electrical Engineering (EE) >
Power Electronics >
All Questions
All questions of Three Phase Line Commutated Converter for Electrical Engineering (EE) Exam
In case of a three phase full controlled converter with 6 SCRs, commutation occurs every- a)120°
- b)60°
- c)180°
- d)30°
Correct answer is option 'B'. Can you explain this answer?
In case of a three phase full controlled converter with 6 SCRs, commutation occurs every
a)
120°
b)
60°
c)
180°
d)
30°
|
Kunal Sharma answered |
Every SCR conducts for 120°. This means that the SCRs from the positive group are fired 120° among themselves, same is true for SCRs from negative group. For example, if T1 starts conducting at 90° it will conduct till 90+120 = 210°. But while T1 is conducting, half of the time i.e. from 90 to 150, T6 is conducting and another half of the time T2 is conducting. Hence, commutation (change in the SCR which is conducting) takes place every 60 degrees irrespective of the firing angle. Construct the firing sequence table for better understanding.
A M-6 controlled converter or 6-pulse half-wave controlled converter is obtained by using a transformer having- a)a double delta connected secondary winding
- b)a double star connected secondary winding
- c)a double delta connected primary winding
- d)6-windings on both primary and secondary sides
Correct answer is option 'B'. Can you explain this answer?
A M-6 controlled converter or 6-pulse half-wave controlled converter is obtained by using a transformer having
a)
a double delta connected secondary winding
b)
a double star connected secondary winding
c)
a double delta connected primary winding
d)
6-windings on both primary and secondary sides
|
Sparsh Saini answered |
M-6 requires a transformer having a delta connected primary and a double star connected secondary such that 6 SCRs are connected to it on the secondary side.
In a three-phase semi-converter, at a time one SCR and one diode conduct simultaneously. With SCR T1 conducting which diode(s) is most likely to conduct along with T1?
- a)D2 only
- b)D3 only
- c)D1 and D2
- d)D2 and D3
Correct answer is option 'D'. Can you explain this answer?
In a three-phase semi-converter, at a time one SCR and one diode conduct simultaneously. With SCR T1 conducting which diode(s) is most likely to conduct along with T1?
a)
D2 only
b)
D3 only
c)
D1 and D2
d)
D2 and D3
|
Anoushka Kumar answered |
Understanding 3 – φ Semi Converter Operation
In a 3-phase semi converter, the operation of SCRs (Silicon Controlled Rectifiers) and freewheeling diodes is crucial for determining their conduction periods based on firing and extinction angles.
Firing Angle
- The firing angle (α) is given as 125°.
- This is the angle at which the SCR is triggered to start conducting.
Extinction Angle
- The extinction angle (β) is provided as 105°.
- This angle indicates when the SCR stops conducting due to the current dropping to zero.
Conduction Time for SCRs
- The conduction period for the SCR can be calculated as:
180° - (α - β)
- Here, the effective conduction period becomes:
180° - (125° - 105°) = 180° - 20° = 160°.
- Since it’s a 3-phase system, each SCR conducts for:
160° / 3 = 53.33°, which can be approximated to 55°.
Conduction Time for Freewheeling Diodes
- The conduction period for freewheeling diodes starts after the extinction of the SCR and runs until the next SCR conducts.
- This period is calculated as:
360° - (β + (180° - α)).
- Therefore, it becomes:
360° - (105° + 55°) = 360° - 160° = 200°.
- Divided among 3 phases gives:
200° / 3 = 66.67°, which can be approximated to 50°.
Conclusion
- Therefore, the SCR conducts for approximately 55° and the freewheeling diode for approximately 50°.
- This aligns with option 'A': 55°, 50°.
In a 3-phase semi converter, the operation of SCRs (Silicon Controlled Rectifiers) and freewheeling diodes is crucial for determining their conduction periods based on firing and extinction angles.
Firing Angle
- The firing angle (α) is given as 125°.
- This is the angle at which the SCR is triggered to start conducting.
Extinction Angle
- The extinction angle (β) is provided as 105°.
- This angle indicates when the SCR stops conducting due to the current dropping to zero.
Conduction Time for SCRs
- The conduction period for the SCR can be calculated as:
180° - (α - β)
- Here, the effective conduction period becomes:
180° - (125° - 105°) = 180° - 20° = 160°.
- Since it’s a 3-phase system, each SCR conducts for:
160° / 3 = 53.33°, which can be approximated to 55°.
Conduction Time for Freewheeling Diodes
- The conduction period for freewheeling diodes starts after the extinction of the SCR and runs until the next SCR conducts.
- This period is calculated as:
360° - (β + (180° - α)).
- Therefore, it becomes:
360° - (105° + 55°) = 360° - 160° = 200°.
- Divided among 3 phases gives:
200° / 3 = 66.67°, which can be approximated to 50°.
Conclusion
- Therefore, the SCR conducts for approximately 55° and the freewheeling diode for approximately 50°.
- This aligns with option 'A': 55°, 50°.
Which of the below mentioned converter can operate in both 3-pulse and 6-pulse modes?- a)3-phase half wave controller
- b)3-phase full converter
- c)3-phase semi-converter
- d)None of the mentioned
Correct answer is option 'C'. Can you explain this answer?
Which of the below mentioned converter can operate in both 3-pulse and 6-pulse modes?
a)
3-phase half wave controller
b)
3-phase full converter
c)
3-phase semi-converter
d)
None of the mentioned
|
Mihir Chawla answered |
Answer:
A 3-phase semi-converter is the converter that can operate in both 3-pulse and 6-pulse modes. Let's understand why this is the correct answer.
Explanation:
1. 3-phase half wave controller:
A 3-phase half wave controller is a single-phase controlled rectifier used for converting AC voltage to DC voltage. It can only operate in half-wave mode and cannot be used in both 3-pulse and 6-pulse modes. Therefore, option 'a' is incorrect.
2. 3-phase full converter:
A 3-phase full converter is a six-pulse converter that converts AC voltage to DC voltage. It operates by using six thyristors in a bridge configuration. It cannot operate in both 3-pulse and 6-pulse modes simultaneously. Therefore, option 'b' is incorrect.
3. 3-phase semi-converter:
A 3-phase semi-converter is a converter that can operate in both 3-pulse and 6-pulse modes. It consists of three thyristors connected in a bridge configuration. In the 3-pulse mode, only one thyristor conducts during each half-cycle of the input AC waveform, resulting in a 3-pulse output. In the 6-pulse mode, two thyristors conduct during each half-cycle, resulting in a 6-pulse output. The mode of operation can be switched by controlling the firing angles of the thyristors. Therefore, option 'c' is correct.
4. None of the mentioned:
This option is incorrect as a 3-phase semi-converter can operate in both 3-pulse and 6-pulse modes.
Therefore, the correct answer is option 'C' - 3-phase semi-converter.
A 3-phase semi-converter is the converter that can operate in both 3-pulse and 6-pulse modes. Let's understand why this is the correct answer.
Explanation:
1. 3-phase half wave controller:
A 3-phase half wave controller is a single-phase controlled rectifier used for converting AC voltage to DC voltage. It can only operate in half-wave mode and cannot be used in both 3-pulse and 6-pulse modes. Therefore, option 'a' is incorrect.
2. 3-phase full converter:
A 3-phase full converter is a six-pulse converter that converts AC voltage to DC voltage. It operates by using six thyristors in a bridge configuration. It cannot operate in both 3-pulse and 6-pulse modes simultaneously. Therefore, option 'b' is incorrect.
3. 3-phase semi-converter:
A 3-phase semi-converter is a converter that can operate in both 3-pulse and 6-pulse modes. It consists of three thyristors connected in a bridge configuration. In the 3-pulse mode, only one thyristor conducts during each half-cycle of the input AC waveform, resulting in a 3-pulse output. In the 6-pulse mode, two thyristors conduct during each half-cycle, resulting in a 6-pulse output. The mode of operation can be switched by controlling the firing angles of the thyristors. Therefore, option 'c' is correct.
4. None of the mentioned:
This option is incorrect as a 3-phase semi-converter can operate in both 3-pulse and 6-pulse modes.
Therefore, the correct answer is option 'C' - 3-phase semi-converter.
Each SCR of a 3-phase 6-pulse converter conducts for- a)120 degrees
- b)60 degrees
- c)180 degrees
- d)360 degrees
Correct answer is option 'A'. Can you explain this answer?
Each SCR of a 3-phase 6-pulse converter conducts for
a)
120 degrees
b)
60 degrees
c)
180 degrees
d)
360 degrees
|
Rounak Rane answered |
A 3-phase 6-pulse converter is nothing but the 3-pulse full controlled converter using 6 devices each conducting for 120°.
A 3-phase bridge converter is given a three-phase supply in the phase sequence R-Y-B. Let the neutral to R phase voltage be Vm sinωt. The first SCR (connected to R phase) is fired at an angle of 15°. What is the maximum value at the output terminals at this instant?- a)Vm/√2
- b)Vm
- c)1.5 Vm
- d)3 Vm
Correct answer is option 'C'. Can you explain this answer?
A 3-phase bridge converter is given a three-phase supply in the phase sequence R-Y-B. Let the neutral to R phase voltage be Vm sinωt. The first SCR (connected to R phase) is fired at an angle of 15°. What is the maximum value at the output terminals at this instant?
a)
Vm/√2
b)
Vm
c)
1.5 Vm
d)
3 Vm
|
|
Alok Khanna answered |
In case of a 3 phase bridge converter, the maximum value of voltage at the output terminal is always 1.5 Vm.
A three-phase half-wave controlled converter is fed from a 3-phase, 400 V source and is connected to a load which takes a constant current of 36 A. Find,the value of average output voltage and average current rating of SCR for a firing angle of 30°.- a)234 V, 36 A
- b)234 V, 12 A
- c)135 V, 36 A
- d)135 V, 12 A
Correct answer is option 'B'. Can you explain this answer?
A three-phase half-wave controlled converter is fed from a 3-phase, 400 V source and is connected to a load which takes a constant current of 36 A. Find,the value of average output voltage and average current rating of SCR for a firing angle of 30°.
a)
234 V, 36 A
b)
234 V, 12 A
c)
135 V, 36 A
d)
135 V, 12 A
|
Sparsh Nambiar answered |
Vo = (3√3/2π) x Vmp x cosα = 233.874 V.
Ia = Io/3 = 12 A.
Ia = Io/3 = 12 A.
The range of firing angle for a 3-phase, 3-pulse converter feeding a resistive load is __________ (in degrees).- a)0 to 180
- b)0 to 150
- c)30 to 150
- d)30 to 180
Correct answer is option 'B'. Can you explain this answer?
The range of firing angle for a 3-phase, 3-pulse converter feeding a resistive load is __________ (in degrees).
a)
0 to 180
b)
0 to 150
c)
30 to 150
d)
30 to 180
|
Mahesh Datta answered |
Firing angle for a 3-phase, 3-pulse converter feeding a resistive load is 0 to 150 degrees.
A three-phase full converter is driving a DC motor. If a continues current of Im amperes is flowing through the motor load, then find the rms value of supply current drawn by the converter to drive the motor.- a)Im/√2
- b)Im2/3
- c)√2Im/√3
- d)√2Im/3
Correct answer is option 'C'. Can you explain this answer?
A three-phase full converter is driving a DC motor. If a continues current of Im amperes is flowing through the motor load, then find the rms value of supply current drawn by the converter to drive the motor.
a)
Im/√2
b)
Im2/3
c)
√2Im/√3
d)
√2Im/3
|
|
Shivam Das answered |
The RMS value of the supply current IS over π radians would be
(IS)2 = (1/π) x (Im)2 x (2π/3) = Im√2/√3.
(IS)2 = (1/π) x (Im)2 x (2π/3) = Im√2/√3.
A three-phase full converter charges a battery from a three-phase supply of 230 V. The battery emf is 200 V. Find the value of the firing angle if the battery terminal voltage is 210 V.- a)36.54°
- b)56.7°
- c)89.3°
- d)47.45°
Correct answer is option 'D'. Can you explain this answer?
A three-phase full converter charges a battery from a three-phase supply of 230 V. The battery emf is 200 V. Find the value of the firing angle if the battery terminal voltage is 210 V.
a)
36.54°
b)
56.7°
c)
89.3°
d)
47.45°
|
Hrishikesh Yadav answered |
Vo = (3Vml/π) cos α
α = cos-1(210π/3√2×230) = 47.453°.
α = cos-1(210π/3√2×230) = 47.453°.
A three-phase M-3 converter is operated from a 3-phase, 230 V, 50 Hz supply with load resistance R = 10 Ω. Find the value of firing angle if an average output voltage of 50% of the maximum possible output voltage is required.Hint: α > 30°.- a)92.7°
- b)67.7°
- c)45°
- d)75.7°
Correct answer is option 'B'. Can you explain this answer?
A three-phase M-3 converter is operated from a 3-phase, 230 V, 50 Hz supply with load resistance R = 10 Ω. Find the value of firing angle if an average output voltage of 50% of the maximum possible output voltage is required.Hint: α > 30°.
a)
92.7°
b)
67.7°
c)
45°
d)
75.7°
|
|
Aniket Shah answered |
We need , Vo = 0.5 Vom.
α>30° hence we use the equation Vo = (3√3/2π) x Vmp x [1+cos(30+α)] √3Vmp = Vml = √2×230
Therefore, Vo = (3/2π) x √3 Vml x [1+cos(30+α)] = 0.5Vom
(1/√3) x [1+cos(30+α)] = Vo x 2π/3Vml = Vo/Vom = 1/2
α = 67.7°.
α>30° hence we use the equation Vo = (3√3/2π) x Vmp x [1+cos(30+α)] √3Vmp = Vml = √2×230
Therefore, Vo = (3/2π) x √3 Vml x [1+cos(30+α)] = 0.5Vom
(1/√3) x [1+cos(30+α)] = Vo x 2π/3Vml = Vo/Vom = 1/2
α = 67.7°.
In a 3-phase semi-converter, firing angle is less than 60°, as such each SCR and diode conduct respectively for __________ (in degrees)- a)60, 60
- b)90, 30
- c)120, 120
- d)180, 180
Correct answer is option 'C'. Can you explain this answer?
In a 3-phase semi-converter, firing angle is less than 60°, as such each SCR and diode conduct respectively for __________ (in degrees)
a)
60, 60
b)
90, 30
c)
120, 120
d)
180, 180
|
|
Raghav Majumdar answered |
Each will conduct for 120° per cycle whatever the firing angle is.
Semi-converters are- a)single quadrant converters
- b)double quadrant converters
- c)three quadrant converters
- d)none of the mentioned
Correct answer is option 'A'. Can you explain this answer?
Semi-converters are
a)
single quadrant converters
b)
double quadrant converters
c)
three quadrant converters
d)
none of the mentioned
|
|
Sandeep Saha answered |
Single Quadrant Converters
Single quadrant converters are a type of semi-converters that operate in a single quadrant of the input-output plane. They are capable of converting AC voltage or current into DC voltage or current in one direction only. They are also known as half-wave rectifiers.
Working Principle
The working principle of single quadrant converters is based on the fact that a diode conducts current in one direction only. During the positive half-cycle of the input AC voltage, the diode conducts and allows the flow of current through the load. During the negative half-cycle, the diode blocks the flow of current, and no current flows through the load.
Applications
Single quadrant converters are widely used in applications where unidirectional power flow is required, such as battery charging, electroplating, and DC motor control.
Advantages
- Simple and low-cost circuit design
- Easy to implement
- Low power losses
Disadvantages
- Only suitable for unidirectional power flow
- Low efficiency due to the use of a single diode
- Produces harmonic distortion in the output waveform
Conclusion
Single quadrant converters are simple and cost-effective solutions for converting AC voltage or current into DC voltage or current in one direction only. They are widely used in various applications and offer several advantages, including easy implementation and low power losses. However, they also have some limitations, such as low efficiency and harmonic distortion in the output waveform.
Single quadrant converters are a type of semi-converters that operate in a single quadrant of the input-output plane. They are capable of converting AC voltage or current into DC voltage or current in one direction only. They are also known as half-wave rectifiers.
Working Principle
The working principle of single quadrant converters is based on the fact that a diode conducts current in one direction only. During the positive half-cycle of the input AC voltage, the diode conducts and allows the flow of current through the load. During the negative half-cycle, the diode blocks the flow of current, and no current flows through the load.
Applications
Single quadrant converters are widely used in applications where unidirectional power flow is required, such as battery charging, electroplating, and DC motor control.
Advantages
- Simple and low-cost circuit design
- Easy to implement
- Low power losses
Disadvantages
- Only suitable for unidirectional power flow
- Low efficiency due to the use of a single diode
- Produces harmonic distortion in the output waveform
Conclusion
Single quadrant converters are simple and cost-effective solutions for converting AC voltage or current into DC voltage or current in one direction only. They are widely used in various applications and offer several advantages, including easy implementation and low power losses. However, they also have some limitations, such as low efficiency and harmonic distortion in the output waveform.
A three-phase three pulse type controlled converter is constructed using 3 SCR devices. The circuit is supplying an R load with α < 30°. As such, each SCR device would conduct for- a)60° each cycle
- b)120° each cycle
- c)180° each cycle
- d)360° each cycle
Correct answer is option 'B'. Can you explain this answer?
A three-phase three pulse type controlled converter is constructed using 3 SCR devices. The circuit is supplying an R load with α < 30°. As such, each SCR device would conduct for
a)
60° each cycle
b)
120° each cycle
c)
180° each cycle
d)
360° each cycle
|
Mahesh Singh answered |
Each conduct for 120° per cycle is the firing angle is less than 30°. 120 x 3 = 360°.
A three-phase three-pulse converter would operate as a line commutated inverter when- a)30° < α < 60°
- b)90° < α <180°
- c)90° > α
- d)it can never operate as a line commutated inverter
Correct answer is option 'B'. Can you explain this answer?
A three-phase three-pulse converter would operate as a line commutated inverter when
a)
30° < α < 60°
b)
90° < α <180°
c)
90° > α
d)
it can never operate as a line commutated inverter
|
|
Harshitha Pillai answered |
A) The converter is supplied with three-phase AC voltage and the thyristors are triggered in a specific sequence to convert the input AC voltage into a variable frequency AC output voltage.
A three-phase full converter supplied from a 230 V source is working as a line commutated inverter. The load consists of RLE type with R = 5 Ω, E = 200 V and L = 1 mH. A continues current of 10 A is flowing through the load, find the value of the firing angle delay.- a)119°
- b)127°
- c)156°
- d)143°
Correct answer is option 'A'. Can you explain this answer?
A three-phase full converter supplied from a 230 V source is working as a line commutated inverter. The load consists of RLE type with R = 5 Ω, E = 200 V and L = 1 mH. A continues current of 10 A is flowing through the load, find the value of the firing angle delay.
a)
119°
b)
127°
c)
156°
d)
143°
|
Dhruv Datta answered |
To find the value of the firing angle delay, we need to calculate the firing angle delay angle (α) using the formula:
α = tan^(-1)((E/V - I*R) / (I*X))
where:
E = 200 V (load voltage)
V = 230 V (source voltage)
I = 10 A (load current)
R = 5 Ω (load resistance)
X = ωL = ω * 1 mH = 2πfL = 2π(50)(0.001) = 0.314 Ω (load reactance)
Plugging in the values, we get:
α = tan^(-1)((200/230 - 10*5) / (10*0.314))
= tan^(-1)(-0.304)
≈ -17.62°
Since the firing angle delay cannot be negative, we take the positive value of the angle:
α ≈ 17.62°
Therefore, the value of the firing angle delay is approximately 17.62°.
α = tan^(-1)((E/V - I*R) / (I*X))
where:
E = 200 V (load voltage)
V = 230 V (source voltage)
I = 10 A (load current)
R = 5 Ω (load resistance)
X = ωL = ω * 1 mH = 2πfL = 2π(50)(0.001) = 0.314 Ω (load reactance)
Plugging in the values, we get:
α = tan^(-1)((200/230 - 10*5) / (10*0.314))
= tan^(-1)(-0.304)
≈ -17.62°
Since the firing angle delay cannot be negative, we take the positive value of the angle:
α ≈ 17.62°
Therefore, the value of the firing angle delay is approximately 17.62°.
A three-phase full converter charges a battery from a three-phase supply of 230 V. Find the value of the power delivered to the load if a continues current of 20A is flowing through the battery of emf 200 V and internal resistance of 0.5 Ω.- a)0 W
- b)5600 W
- c)4200 W
- d)1040 W
Correct answer is option 'C'. Can you explain this answer?
A three-phase full converter charges a battery from a three-phase supply of 230 V. Find the value of the power delivered to the load if a continues current of 20A is flowing through the battery of emf 200 V and internal resistance of 0.5 Ω.
a)
0 W
b)
5600 W
c)
4200 W
d)
1040 W
|
|
Nandita Kulkarni answered |
Iavg = Irms = 20 A
P = E x Iavg + Irms2 x R = 4200 W.
P = E x Iavg + Irms2 x R = 4200 W.
The effect of source inductance on the performance of a 3-phase controlled converter is to- a)increase the average load voltage
- b)reduce the average load voltage
- c)make the load current continuous
- d)remove ripples from the load current
Correct answer is option 'B'. Can you explain this answer?
The effect of source inductance on the performance of a 3-phase controlled converter is to
a)
increase the average load voltage
b)
reduce the average load voltage
c)
make the load current continuous
d)
remove ripples from the load current
|
|
Shivam Das answered |
It reduces the average value of the output voltage by introducing a overlap delay μ.
A three-phase full converter charges a battery from a three-phase supply of 230 V. The battery emf is 200 V and the internal resistance of the battery is 0.5 Ω. Find the value of the continuous current which is flowing through the battery if its terminal voltage is 210 V- a)10 A
- b)20 A
- c)0.5 A
- d)25 A
Correct answer is option 'B'. Can you explain this answer?
A three-phase full converter charges a battery from a three-phase supply of 230 V. The battery emf is 200 V and the internal resistance of the battery is 0.5 Ω. Find the value of the continuous current which is flowing through the battery if its terminal voltage is 210 V
a)
10 A
b)
20 A
c)
0.5 A
d)
25 A
|
|
Rajesh Kumar answered |
Vo = 210 V
Vo = E + Io x R
210 = 200 + 0.5 x Io
Io (Current through the battery) = 20 A.
Vo = E + Io x R
210 = 200 + 0.5 x Io
Io (Current through the battery) = 20 A.
In a 3 phase, 12-pulse controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.- a)π/6 radians
- b)60 degrees
- c)3π/2 radians
- d)12 degrees
Correct answer is option 'A'. Can you explain this answer?
In a 3 phase, 12-pulse controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.
a)
π/6 radians
b)
60 degrees
c)
3π/2 radians
d)
12 degrees
|
|
Gargi Basak answered |
Each SCR conducts for 30° or π/6 radians. 12 x 30 = 360°.
The commutation period when both incoming and outgoing SCRs are conducting due to source inductance is called as the- a)conduction delay
- b)overlap period
- c)one on one period
- d)distorting angle
Correct answer is option 'B'. Can you explain this answer?
The commutation period when both incoming and outgoing SCRs are conducting due to source inductance is called as the
a)
conduction delay
b)
overlap period
c)
one on one period
d)
distorting angle
|
Nikhil Iyer answered |
Due to source inductance, SCRs cannot start and stop conducting immediately, hence a time occurs when both incoming and outgoing SCRs are conducting together. This delay is called as commutation angle or overlap period (μ).
A three phase full converter will require __________ number of SCRs.- a)3
- b)6
- c)9
- d)2
Correct answer is option 'B'. Can you explain this answer?
A three phase full converter will require __________ number of SCRs.
a)
3
b)
6
c)
9
d)
2
|
|
Shail Nambiar answered |
Three legs having two SCRs each, six in total.
A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. Find the DF (distortion factor).- a)1.414
- b)0
- c)0.707
- d)0.569
Correct answer is option 'C'. Can you explain this answer?
A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. Find the DF (distortion factor).
a)
1.414
b)
0
c)
0.707
d)
0.569
|
|
Sneha Bose answered |
DF = cosα = cos45 = 0.707
Note that ripple free current does not mean that DF = 0. It means the current is continues in magnitude and direction, current will always contain harmonics (distortions) whether it is rippled or ripple free.
Note that ripple free current does not mean that DF = 0. It means the current is continues in magnitude and direction, current will always contain harmonics (distortions) whether it is rippled or ripple free.
A three phase six pulse full converter works as a ac to dc converter for firing angles in the range- a)α > 90
- b)90 < α < 180
- c)0 < α < 90
- d)0 < α < 360
Correct answer is option 'C'. Can you explain this answer?
A three phase six pulse full converter works as a ac to dc converter for firing angles in the range
a)
α > 90
b)
90 < α < 180
c)
0 < α < 90
d)
0 < α < 360
|
Raj Desai answered |
When α is less than 90°, the SCRs conduct for 120° and the current and voltage are positive on an average hence, the power flows from AC source to DC load.
For a three-phase full controlled converter with R load, the average value of output voltage is zero for- a)α = 0°
- b)α = 90°
- c)α = 180°
- d)It can never be zero
Correct answer is option 'B'. Can you explain this answer?
For a three-phase full controlled converter with R load, the average value of output voltage is zero for
a)
α = 0°
b)
α = 90°
c)
α = 180°
d)
It can never be zero
|
Vaibhav Joshi answered |
For α = 90 degrees, the voltage waveform is equally symmetrical about the ωt axis, hence the average value is zero. This can also be found by using the formula for average output voltage,
Vo = (3Vml/π) cos α,
For α = 90°, cosα = 0, Vo = zero.
Vo = (3Vml/π) cos α,
For α = 90°, cosα = 0, Vo = zero.
In a p-pulse converter, each SCR conducts for (per cycle)- a)p radians
- b)p degrees
- c)p/2π radians
- d)2π/p radians
Correct answer is option 'D'. Can you explain this answer?
In a p-pulse converter, each SCR conducts for (per cycle)
a)
p radians
b)
p degrees
c)
p/2π radians
d)
2π/p radians
|
|
Nitya Chopra answered |
In a p-pulse controlled converter, each device conducts for p radians. p-pulse converter would have p devices. Hence, 2π/p x p = 2π = 360°.
A 3-phase full converter feeds power to an R load of 10 Ω. For a firing angle delay of 30° the load takes 5 kW. An inductor of large value is also connected to the load to make the current ripple free. Find the value of per phase input voltage.- a)133 V
- b)230/√3 V
- c)191/√3 V
- d)298/√3 V
Correct answer is option 'C'. Can you explain this answer?
A 3-phase full converter feeds power to an R load of 10 Ω. For a firing angle delay of 30° the load takes 5 kW. An inductor of large value is also connected to the load to make the current ripple free. Find the value of per phase input voltage.
a)
133 V
b)
230/√3 V
c)
191/√3 V
d)
298/√3 V
|
Arpita Banerjee answered |
Ior = Vo/R = (3Vml/Rπ) cos α
P = 5 kW = Ior2 x R = [(3Vml/π) cos α]2 x 1/R] Therefore, Vs (line) = √50000 x (π/√2 x 3 x cos30) = 191.22 V
Vs (phase) = 191/√3 V.
P = 5 kW = Ior2 x R = [(3Vml/π) cos α]2 x 1/R] Therefore, Vs (line) = √50000 x (π/√2 x 3 x cos30) = 191.22 V
Vs (phase) = 191/√3 V.
What is the relationship between DF, CDF and PF?- a)PF = CDF = DF
- b)PF = CDF/DF
- c)PF = DF/CDF
- d)PF = CDF x DF
Correct answer is option 'D'. Can you explain this answer?
What is the relationship between DF, CDF and PF?
a)
PF = CDF = DF
b)
PF = CDF/DF
c)
PF = DF/CDF
d)
PF = CDF x DF
|
|
Bijoy Mehta answered |
PF (power factor) = (distortion factor) x (current distortion factor).
A three-phase, three-pulse, M-3 type controlled converter uses ____________ number of SCRs.- a)1
- b)2
- c)3
- d)4
Correct answer is option 'C'. Can you explain this answer?
A three-phase, three-pulse, M-3 type controlled converter uses ____________ number of SCRs.
a)
1
b)
2
c)
3
d)
4
|
|
Sanaya Basu answered |
Introduction:
A three-phase, three-pulse, M-3 type controlled converter is a type of power electronic converter used for converting AC power to DC power. It is commonly used in various industrial applications such as motor drives, voltage regulators, and power supplies.
Explanation:
- The M-3 type controlled converter is a three-phase converter that uses three SCRs (Silicon Controlled Rectifiers) in each phase.
- The term "three-pulse" indicates that each phase of the converter uses three SCRs.
- In a three-phase system, there are three phases: phase A, phase B, and phase C. Each phase requires three SCRs to control the conversion process effectively.
- The SCRs are semiconductor devices that can control the flow of current in a circuit by acting as switches. They are turned on and off using gate signals to control the power flow in the converter.
Advantages of using three SCRs:
- The use of three SCRs in each phase of the converter allows for better control and regulation of the output voltage and current.
- The three SCRs are fired at specific angles during each half-cycle of the input AC waveform to control the output power.
- By controlling the firing angles of the SCRs, the converter can regulate the output voltage and current, resulting in better power quality and efficiency.
Other types of controlled converters:
- Apart from the M-3 type controlled converter, there are other types of controlled converters, such as the M-1, M-2, and M-6 types.
- The M-1 type controlled converter uses one SCR in each phase, the M-2 type uses two SCRs in each phase, and the M-6 type uses six SCRs in each phase.
- The choice of the type of controlled converter depends on the specific application and the desired level of control and regulation required.
Conclusion:
In summary, a three-phase, three-pulse, M-3 type controlled converter uses three SCRs in each phase. The use of three SCRs allows for better control and regulation of the output voltage and current, resulting in improved power quality and efficiency.
A three-phase, three-pulse, M-3 type controlled converter is a type of power electronic converter used for converting AC power to DC power. It is commonly used in various industrial applications such as motor drives, voltage regulators, and power supplies.
Explanation:
- The M-3 type controlled converter is a three-phase converter that uses three SCRs (Silicon Controlled Rectifiers) in each phase.
- The term "three-pulse" indicates that each phase of the converter uses three SCRs.
- In a three-phase system, there are three phases: phase A, phase B, and phase C. Each phase requires three SCRs to control the conversion process effectively.
- The SCRs are semiconductor devices that can control the flow of current in a circuit by acting as switches. They are turned on and off using gate signals to control the power flow in the converter.
Advantages of using three SCRs:
- The use of three SCRs in each phase of the converter allows for better control and regulation of the output voltage and current.
- The three SCRs are fired at specific angles during each half-cycle of the input AC waveform to control the output power.
- By controlling the firing angles of the SCRs, the converter can regulate the output voltage and current, resulting in better power quality and efficiency.
Other types of controlled converters:
- Apart from the M-3 type controlled converter, there are other types of controlled converters, such as the M-1, M-2, and M-6 types.
- The M-1 type controlled converter uses one SCR in each phase, the M-2 type uses two SCRs in each phase, and the M-6 type uses six SCRs in each phase.
- The choice of the type of controlled converter depends on the specific application and the desired level of control and regulation required.
Conclusion:
In summary, a three-phase, three-pulse, M-3 type controlled converter uses three SCRs in each phase. The use of three SCRs allows for better control and regulation of the output voltage and current, resulting in improved power quality and efficiency.
The PIV experienced by each SCRs in M-3 converter is __________ times that in a 3-phase full converter having the same output voltage.- a)0.5
- b)1
- c)2
- d)1.5
Correct answer is option 'C'. Can you explain this answer?
The PIV experienced by each SCRs in M-3 converter is __________ times that in a 3-phase full converter having the same output voltage.
a)
0.5
b)
1
c)
2
d)
1.5
|
Om Saini answered |
In case of M-3 type of connection, the devices have to handle more peak inverse voltage (PIV) than the 3-phase full converter which has 6 SCRs.
In a three-phase, three-pulse, M-3 type controlled converter T1 starts to conduct at 30 + n°. At what angles do T2 and T3 start to conduct? Assume that the conduction sequence is T1-T2-T3.- a)2n° and 3n°
- b)150 + n° and 270 + n°
- c)n° each
- d)30 + n° and 60 + n°
Correct answer is option 'B'. Can you explain this answer?
In a three-phase, three-pulse, M-3 type controlled converter T1 starts to conduct at 30 + n°. At what angles do T2 and T3 start to conduct? Assume that the conduction sequence is T1-T2-T3.
a)
2n° and 3n°
b)
150 + n° and 270 + n°
c)
n° each
d)
30 + n° and 60 + n°
|
|
Athul Banerjee answered |
In three-phase three pulse converter the conduction can start only after 30°. As each SCR conducts from 120°, T2 would conduct on 30+120+n° = 150+n° and so on.
A three-phase, three-pulse, M-3 type controlled converter has firing angle for one of the SCRs set as 15°. This SCR would start conducting at- a)0°
- b)15°
- c)30°
- d)45°
Correct answer is option 'D'. Can you explain this answer?
A three-phase, three-pulse, M-3 type controlled converter has firing angle for one of the SCRs set as 15°. This SCR would start conducting at
a)
0°
b)
15°
c)
30°
d)
45°
|
|
Raj Desai answered |
In a three phase controller, the actually conduction starts at 30° + α. Hence, ωt = 30+15 = 45°.
For a three phase full controlled converter, with 3 thyristors in the upper or positive group and 3 thyristors in the lower or negative group, at any given time- a)two thyristors are conducting from each group
- b)one thyristor is conducting from each group
- c)one thyristor is conducting from either of the groups
- d)all 6 thyristors are conducting at a time
Correct answer is option 'B'. Can you explain this answer?
For a three phase full controlled converter, with 3 thyristors in the upper or positive group and 3 thyristors in the lower or negative group, at any given time
a)
two thyristors are conducting from each group
b)
one thyristor is conducting from each group
c)
one thyristor is conducting from either of the groups
d)
all 6 thyristors are conducting at a time
|
Raghav Nambiar answered |
Let’s say T1, T3 and T5 belong to the positive group and T2, T4 and T6 to the negative group. At any given time one SCR from each group conducts. e.g. T1 and T6 or T1 and T2.
A three-phase semi-converter circuit is given a supply of 400 V. It produces at the output terminals an average voltage of 381 V. Find the rectification efficiency of the converter circuit.- a)99.65 %
- b)95.25 %
- c)91 %
- d)86.5 %
Correct answer is option 'B'. Can you explain this answer?
A three-phase semi-converter circuit is given a supply of 400 V. It produces at the output terminals an average voltage of 381 V. Find the rectification efficiency of the converter circuit.
a)
99.65 %
b)
95.25 %
c)
91 %
d)
86.5 %
|
Aaditya Choudhary answered |
Rectification efficiency = Pdc/Pac
Pdc = Vo x Io . . . (both average values of output current and voltage)
Pac = Vrms x Irms . . . (both rms values of input current and voltage)
For a semi-converter Irms = Io
Therefore, Rectification efficiency = 381/400 = 95.25 %.
Pdc = Vo x Io . . . (both average values of output current and voltage)
Pac = Vrms x Irms . . . (both rms values of input current and voltage)
For a semi-converter Irms = Io
Therefore, Rectification efficiency = 381/400 = 95.25 %.
In a 3-phase semi-converter, for firing angle less than 60° the freewheeling diode conducts for- a)30°
- b)60°
- c)120°
- d)0
Correct answer is option 'D'. Can you explain this answer?
In a 3-phase semi-converter, for firing angle less than 60° the freewheeling diode conducts for
a)
30°
b)
60°
c)
120°
d)
0
|
|
Om Saini answered |
In case of a semi-converter operating with α < 60°, FD does not comes into play, as the voltage never falls to zero and gives no chance for the inductor to discharge.
Which of the following converter circuits would require a neutral point?- a)3-phase semi-converter
- b)3-phase full converter
- c)3-phase full converter with freewheeling diode
- d)3-phase half wave converter
Correct answer is option 'D'. Can you explain this answer?
Which of the following converter circuits would require a neutral point?
a)
3-phase semi-converter
b)
3-phase full converter
c)
3-phase full converter with freewheeling diode
d)
3-phase half wave converter
|
|
Arya Mukherjee answered |
Answer:
In a converter circuit, a neutral point is required when there is a need to connect the common or neutral terminal of the load or source to a reference potential. This reference potential is usually the earth or ground potential.
Among the given options, the converter circuit that requires a neutral point is the 3-phase half wave converter (option D).
Explanation:
A half wave converter is a type of converter circuit that converts AC voltage to DC voltage. It is commonly used in various applications such as motor control, power supplies, and battery charging.
In a 3-phase half wave converter, the AC input is a 3-phase supply with three phases: Phase A, Phase B, and Phase C. The AC input is connected to a diode bridge rectifier circuit consisting of six diodes. The output of the rectifier circuit is a pulsating DC voltage.
Reason for requiring a neutral point:
The reason why a neutral point is required in a 3-phase half wave converter is because the load or source may have a common or neutral terminal that needs to be connected to a reference potential. In many practical applications, the neutral point is connected to the ground potential.
Importance of neutral point:
1. Ground reference: The neutral point provides a reference potential for the load or source. By connecting the neutral point to the ground potential, it ensures that the voltage levels are measured and controlled with respect to the ground.
2. Safety: The neutral point helps in ensuring the safety of the system. By grounding the neutral point, any leakage current or fault current can be effectively diverted to the ground, preventing potential hazards such as electric shock.
3. Stability: The neutral point helps in stabilizing the system. By connecting the neutral point to the ground potential, it provides a stable reference point for voltage measurements and control, minimizing fluctuations and ensuring stable operation.
4. Common mode voltage control: The neutral point helps in controlling the common mode voltage in the system. By connecting the neutral point to the ground potential, it helps in reducing common mode noise and interference.
Therefore, in a 3-phase half wave converter, a neutral point is required to provide a reference potential for the load or source and to ensure safety, stability, and common mode voltage control.
In a converter circuit, a neutral point is required when there is a need to connect the common or neutral terminal of the load or source to a reference potential. This reference potential is usually the earth or ground potential.
Among the given options, the converter circuit that requires a neutral point is the 3-phase half wave converter (option D).
Explanation:
A half wave converter is a type of converter circuit that converts AC voltage to DC voltage. It is commonly used in various applications such as motor control, power supplies, and battery charging.
In a 3-phase half wave converter, the AC input is a 3-phase supply with three phases: Phase A, Phase B, and Phase C. The AC input is connected to a diode bridge rectifier circuit consisting of six diodes. The output of the rectifier circuit is a pulsating DC voltage.
Reason for requiring a neutral point:
The reason why a neutral point is required in a 3-phase half wave converter is because the load or source may have a common or neutral terminal that needs to be connected to a reference potential. In many practical applications, the neutral point is connected to the ground potential.
Importance of neutral point:
1. Ground reference: The neutral point provides a reference potential for the load or source. By connecting the neutral point to the ground potential, it ensures that the voltage levels are measured and controlled with respect to the ground.
2. Safety: The neutral point helps in ensuring the safety of the system. By grounding the neutral point, any leakage current or fault current can be effectively diverted to the ground, preventing potential hazards such as electric shock.
3. Stability: The neutral point helps in stabilizing the system. By connecting the neutral point to the ground potential, it provides a stable reference point for voltage measurements and control, minimizing fluctuations and ensuring stable operation.
4. Common mode voltage control: The neutral point helps in controlling the common mode voltage in the system. By connecting the neutral point to the ground potential, it helps in reducing common mode noise and interference.
Therefore, in a 3-phase half wave converter, a neutral point is required to provide a reference potential for the load or source and to ensure safety, stability, and common mode voltage control.
A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation.
Find the value of 2nd harmonic source current amplitutue.- a)11.25 A
- b)0.256 A
- c)2.69 sin (ωt – 45) A
- d)0 A
Correct answer is option 'D'. Can you explain this answer?
A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation.
Find the value of 2nd harmonic source current amplitutue.
a)
11.25 A
b)
0.256 A
c)
2.69 sin (ωt – 45) A
d)
0 A
|
|
Gargi Basak answered |
2nd harmonics are absent in a 3-phase full converter, it has only odd number of harmonics i.e. 3rd, 5th etc.
A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation.
Find the fundamental component of the source current amplitude.- a)11.03
- b)2.205
- c)11.03 sin 45
- d)46.98
Correct answer is option 'A'. Can you explain this answer?
A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation.
Find the fundamental component of the source current amplitude.
Find the fundamental component of the source current amplitude.
a)
11.03
b)
2.205
c)
11.03 sin 45
d)
46.98
|
|
Snehal Rane answered |
Put n = 1 (fundamental component) and the rest of the given values in the above given equation.
In the circuit shown below, SCR T1 conducts first. If T1 is fired at an angle of α > 30°, then T1 would conduct from
- a)α to 180°
- b)30 + α to 180°
- c)30 + α to 150°
- d)30 + α to 120°
Correct answer is option 'B'. Can you explain this answer?
In the circuit shown below, SCR T1 conducts first. If T1 is fired at an angle of α > 30°, then T1 would conduct from
a)
α to 180°
b)
30 + α to 180°
c)
30 + α to 150°
d)
30 + α to 120°
|
|
Kunal Sharma answered |
When firing angle is more than 30°, T1 would conduct from 30 + α to 180°. Irrespective of the firing angle, T1 will be turned on at 180° because it conducts first which means it is connected to the R phase and the phase sequence is R-Y-B. As R starts at 0° its value is 0 at 180° which reverse biases the SCR T1.
Find the expression for average output voltage for the given circuit if firing angle is greater than 30°. Take Vmp = secondary side maximum value of phase voltage.
- a)(3√3Vmp/2π) x cosα
- b)(3√3Vmp/2π) x (1+cosα)
- c)(3√3Vmp/2π) x [1+cos(30+α)].
- d)(3√3Vmp/2π) x [3+cos(30+α)].
Correct answer is option 'C'. Can you explain this answer?
Find the expression for average output voltage for the given circuit if firing angle is greater than 30°. Take Vmp = secondary side maximum value of phase voltage.
a)
(3√3Vmp/2π) x cosα
b)
(3√3Vmp/2π) x (1+cosα)
c)
(3√3Vmp/2π) x [1+cos(30+α)].
d)
(3√3Vmp/2π) x [3+cos(30+α)].
|
|
Raj Choudhary answered |
Vo = 3 x [ 1/2π ∫ Vmp sinωt d(ωt) ] Where, the integration runs from α+π/6 to π. Because conduction takes place from 30 + α to 180° for T1 and than the waveform is symmetrical for all other SCRs.
Vo = (3√3/2π) x Vmp x [1+cos(30+α)].
Vo = (3√3/2π) x Vmp x [1+cos(30+α)].
Find the expression for average output voltage at R for the below given configuration. Take firing angle as α = 15°, transformer ratio as 1:1:1 and Vmp as the maximum value of phase voltage at the supply.
- a)(3Vmp/2π) x cosα
- b)(3√3Vmp/2π) x cosα
- c)(3√3Vmp/2π) x sinα
- d)(3Vmp/2π) x sinα
Correct answer is option 'B'. Can you explain this answer?
Find the expression for average output voltage at R for the below given configuration. Take firing angle as α = 15°, transformer ratio as 1:1:1 and Vmp as the maximum value of phase voltage at the supply.
a)
(3Vmp/2π) x cosα
b)
(3√3Vmp/2π) x cosα
c)
(3√3Vmp/2π) x sinα
d)
(3Vmp/2π) x sinα
|
|
Nilanjan Saini answered |
The circuit is that of a three-pulse M-3 connection. The firing angle is less than 30°. Therefore, each device conducts for an angle of 120°.
Vo = 3 x [ 1/2π ∫ Vmp sinωt d(ωt) ] Where, the integration runs from α+π/6 to α+5π/6.
Vo = (3√3/2π) x Vmp x cosα.
Vo = 3 x [ 1/2π ∫ Vmp sinωt d(ωt) ] Where, the integration runs from α+π/6 to α+5π/6.
Vo = (3√3/2π) x Vmp x cosα.
Name the below given circuit.
- a)Full controlled, bridge converter
- b)Full controlled, semi converter
- c)Bridge type semi-converter
- d)Half controlled, full converter
Correct answer is option 'C'. Can you explain this answer?
Name the below given circuit.
a)
Full controlled, bridge converter
b)
Full controlled, semi converter
c)
Bridge type semi-converter
d)
Half controlled, full converter
|
|
Om Saini answered |
It uses 3 SCRs and 3 diodes, hence it is a semi-converter. Option (b) and (d) make no sense, because there can be no full controlled semi-converter.
In the below given circuit, __ and __ conduct along with T2.
- a)T1, T3
- b)D1, D2
- c)D1, D3
- d)T1, T2
Correct answer is option 'C'. Can you explain this answer?
In the below given circuit, __ and __ conduct along with T2.
a)
T1, T3
b)
D1, D2
c)
D1, D3
d)
T1, T2
|
|
Prasenjit Yadav answered |
When one SCR conducts, a diode conducts along with it at a time to provide the path of current flow. . For example, if T2 starts conducting at 90° it will conduct till 90+120 = 210°. But while T2 is conducting, half of the time i.e. from 90 to 150 D1 is conducting and another half of the time D3 is conducting. T2 and D2 cannot conduct together as it will cause a short circuit. Hence, T2-D1 conduct for 60° and then T2-D3 conduct for another 60°.
For the below given circuit, α = 60°. T2 will start conduction at ωt = __________ Assume the inductor L value to be negligible.
- a)60°
- b)120°
- c)90°
- d)150°
Correct answer is option 'D'. Can you explain this answer?
For the below given circuit, α = 60°. T2 will start conduction at ωt = __________ Assume the inductor L value to be negligible.
a)
60°
b)
120°
c)
90°
d)
150°
|
Krish Saini answered |
Assuming the phase sequence is R-Y-B. T1 would start conducting at 30+60 = 90°, T2 at 90+210/2 = 150°. This is because after T1, T3 would conduct from the upper group, as T2 belongs to the lower group it will start to conduct exactly between T1 and T3 i.e. between 90 and 210(90+120) which is 150°.
For the below given circuit, the conduction sequence for the negative group of SCRs is
- a)T4-T6-T2
- b)T1-T2-T3
- c)T2-T6-1
- d)T2-T4-T6
Correct answer is option 'D'. Can you explain this answer?
For the below given circuit, the conduction sequence for the negative group of SCRs is
a)
T4-T6-T2
b)
T1-T2-T3
c)
T2-T6-1
d)
T2-T4-T6
|
|
Ashutosh Majumdar answered |
The negative group of SCRs has T2, T4 and T6. The conduct as T2-T4-T6, as T2 is connected to the B phase, T4 to the R phase and like-wise.
In the below given circuit, each SCR and diode conduct for
- a)60° and 120° respectively
- b)120° and 60° respectively
- c)120°
- d)60°
Correct answer is option 'C'. Can you explain this answer?
In the below given circuit, each SCR and diode conduct for
a)
60° and 120° respectively
b)
120° and 60° respectively
c)
120°
d)
60°
|
|
Mahesh Datta answered |
At any given time, one SCR and one diode is conducting, each conduct for 120° per cycle.
What is the value of voltage at the output terminal when the freewheeling diode (FD) is conducting?
- a)Zero
- b)Maximum
- c)E
- d)It could be anything depending on α
Correct answer is option 'A'. Can you explain this answer?
What is the value of voltage at the output terminal when the freewheeling diode (FD) is conducting?
a)
Zero
b)
Maximum
c)
E
d)
It could be anything depending on α
|
|
Rounak Rane answered |
When FD is conducting it will short circuit the load terminal resulting in zero voltage. It won’t be E because the terminals are shorted. It can be E when none of the devices are conducting (This can happen only when α > 120°).
Chapter doubts & questions for Three Phase Line Commutated Converter - Power Electronics 2025 is part of Electrical Engineering (EE) exam preparation. The chapters have been prepared according to the Electrical Engineering (EE) exam syllabus. The Chapter doubts & questions, notes, tests & MCQs are made for Electrical Engineering (EE) 2025 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests here.
Chapter doubts & questions of Three Phase Line Commutated Converter - Power Electronics in English & Hindi are available as part of Electrical Engineering (EE) exam.
Download more important topics, notes, lectures and mock test series for Electrical Engineering (EE) Exam by signing up for free.
Power Electronics
5 videos|52 docs|46 tests
|
Signup to see your scores go up within 7 days!
Study with 1000+ FREE Docs, Videos & Tests
10M+ students study on EduRev
|
© EduRev
|
Education Revolution
|
|
Signup on EduRev and stay on top of your study goals
10M+ students crushing their study goals daily