A starting resistance is inserted at the starting in an induction motor as well as dc motor.
For both the machines the purpose is different.
Considering a human handed control system for the dc motor speed control, if the resistance wire cut out too slowly, then
Due to the slow cut off the resistance the current will be high for a large amount of time and it will cause the machine to burn.
A 100 hp, 250 V, 350 A shunt dc motor with an armature resistance of 0.05 ohms. To limit maximum starting current to twice the rated of its value, what will be the number of stages of starting resistances?
n= log(Ra/Rt)/log(Imin/Imax) ; Rt = Vt/Imax= 250/700 = 0.357 ohms
= 3 (approx).
Following are the observations due to large starting current in dc motor.
(i) Sparking at brushes
(ii) Mechanical shock at shaft
(iii) Source current fluctuations
(iv) Burning of the field winding
All the observations can be seen due to large starting current in dc motor.
The shunt motor starters that can be used is/are
Both 3-point and 4-point starters can be used.
The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit.
The direct-on-line starter can not limit the speed by limiting the current.
Thyristor controlled starter is preferred over DOL starter due to
A thyristor controlled method is more efficient as it has lesser resistance and losses while operating.
For a 7.46 kW, 200 V dc shunt motor with full load efficiency of 85% has armature resistance of 0.25 ohms. Calculate the value of starting resistance in ohms for a current 1.5 times of the full load current.
Full load current = 7460/(200*0.85) = 43.88 A
Starting current = 1.5*43.88 = 65.883 A
R = V/I = 200/65.883
= 3.038 ohms
Starting resistance = 3.038 – 0.25 = 2.788 ohms.
The effect of fringing increases as we
Fringing is the effect in which the magnetic flux lines bulge out of the the flux. and it is introduced at the air gaps.
The post effects of the armature reaction is
There is no such post effect of armature reaction . The armature reaction phenomena affects the working condition only.