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A d.c. chopper circuit connected to a 100 V d.c. source supplies an inductive load having 40 mH in series with a resistance of 5 Ω A freewheeling diode is placed across the load. The load current varies between the limits of 10 A and 12 A. The time ratio (T_{on}/T_{off}) of the chopper is
The average value of load current
The maximum value of load current
Now, the average value of the voltage is
Also,
or,
or,
or,
A stepup chopper is used to deliver load voltage of 500 V from a 220 V d.c. source. If the blocking period of the thyristor is 80 μs, the required pulse width is
Given, \/_{s} = 220V, V_{0} = 500V
and T_{off} = 80 μs
For a stepup chopper, output voltage is
or,
or,
or,
or, 25 T_{on} = 14 T_{on} + 14 T_{off}
or, 11 T_{on} = 14 T_{off}
or,
A 220 V dc shunt motor runs at 1500 rpm at noload. The motor is fed through a typeA chopper. If the armature resistance is 1 Ω and duty cycle of chopper is 60%, the speed of the motor when it draw a current of 20 A would be approximately equal to
The output voltage,
V_{0} = αV_{s }= 0.6 x 220  132 V
∴ Back emf
At no load, N_{1}, = 1500 rpm and E_{b1} = 220 V Now, E_{b} α N
∴
or,
or,
If the motor in the previous problem is driven by an active load at 1710 rpm, then the type of chopper suitable for speed control and its duty cycle for an input voltage of 250 V will be respectively
As the motor is running at 1710 rpm its back emf is
Since therefore current flows in the reverse direction and hence second quadrant chopper will be used with the motor.
Now,
= 230.8 volt
Also, V_{0} = (1  α) V_{s}
or, 230.8 = (1  α) x 250
or, 1  α = 0.9232 or α = 0.0768
or, α = 7.68% = 7.7%
A fourquadrant chopper is driving a separately excited dc motor load. The motor parameters are R = 0.1 Ω, L = 10 mH. The supply voltage is 200 V d.c. If the rated current of the motor is 10 A with E_{b} = 150 V and motor is driving the rated torque, then it is operating under
For a fourquadrant chopper, the average voltage in all the fourmodes is given by
V_{0} = 2 V_{s} (α  0.5)
The average curreni,
or,
or, α = 0.877
Since α > 0.5, the motor is operating undi
forward motorina mode.
A singlequadrant type A chopper is operating with the following specifications:
Ideal battery of 220 V;
ontime t_{on}  1 ms;
offtime f_{off} = 1.5 ms
The ripple and form factors will be respectively given by
For a typeA chopper, average output voltage
Match List1 (Chopper circuit) with ListII (Characteristic) and select the correct answer using the codes given below the lists:
Codes:
• TypeA chopper is also called “stepdown chopper” as average output voltage V_{0} is always less than the input dc voltage V_{s}.
• TypeB chopper is also called “stepup chopper” as load voltage
which is more than source voltage Vs.
• TypeC chopper configuration is suitable for motoring and degenerative braking of dc motors.
• TypeD chopper characteristic is shown below
Consider the following statements:
1. DC chopper can be used in both dc and ac drives.
2. For fourquadrant operation dual converter is required.
3. Output voltage from a chopper circuit depends both on load current and duty cycle
4. In a stepdown source, current can be discontinuous if duty cycle is low.
Which of the statements given above are correct?
• DC choppers can be used in only dc drives.
• Output voltage from a chopper circuit depends only on duty cycle and is independent of load current.
Hence, statements 1 and 3 are not correct.
For the typeA chopper shown below, the voltage drop across the chopper when it is on is 2 volt. If the duty cycle is 0.4, then the chopper efficiency would be approximately equal to
Given, V_{s} = 230 V
Load resistance, R_{L }= 10Ω α = 0.4
When chopper is ON,
output voltage = (V_{s } 2) volt
When chopper is OFF, output voltage = 0 volt
Average output voltage
Rms output voltage,
Power output delivered to load is
= 2079.364 Watt
Power input to chopper;
Chopper efficiency
A boostregulator has an input voltage of 5 V and the average output voltage of 15 V, The duty cycle is
Boostregulator is a stepup chopper.
Output voltage,
or,
or,
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