01․ In a DC Motor 3 point starter is used to
Three point starter is used to limit the starting current and to provide voltage protection by 'NO VOLT RELEASE' coil and overload protection by 'HOLD-ON' coil.
02․ If starter is not used with large DC motor for starting, it will
Large DC motor without starter will draw high inrush current at the time of starting and may cause -
- Heavy sparking at the commutator and even flash over.
- Damage to armature winding either by heat developed in the windings or by mechanical forces set up by electromagnetic action.
- Large voltage drop in the supply voltage.
03․ Direct-on-line (DOL) starters are not suitable for starting large DC motors, because
Direct-on-line (DOL) starting are not used for large DC motor because the starting current will be enormously high which will cause large voltage drop in the supply mains and armature may get damaged due to excessive heating. It is used for starting of induction motor of ratings up to 3.75 KW.
04․ Which of the following DC Motor uses 4-point starter?
The armature current of DC shunt motor . At the time of starting, Eb = 0. If the full supply voltage is applied to the motor it will draw a large current due to low armature resistance. A variable resistance should be connected in series with the armature resistance to limit the starting current. Hence a 4-point starter is used to start and control speed of DC shunt motor.
05․ DC motors are stopped by opening line switches and not by forcing the handle of the motor starter back to OFF position. This is due to occurance of
Generally to stop the motor, the starter handle should never be pulled back, this would result in burning the starter contacts. When there is heavy sparking at the first stud of starting resistance steps, the motor is to be stopped by opening the main line switches.
06․ High inrush of current during a starting of DC motor can efficiently be restricted to a safer value by increasing the _________________ resistance.
The DC motor draws high current at the starting instant due to absence of back emf. In order to prevent this high inrush of current at start, a resistance is connected is series with the armature. It increases the voltage drop in series, so current is reduced.
07․ The current drawn by a DC motor at the time of starting is
When the motor is at rest the speed of DC motor is zero, therefore back emf is zero and if a DC motor is connected directly to the supply mains a heavy current (about 10-12.5 times of rated current) will be drawn because from emf equation of armature circuit , armature current at starting is given as Ia = Vt / Ra and armature resistance Ra being very small, so current drawn is very high.
08․ In a seperately excited DC Motor if field current is increased, it will
For a large field current, a higher value of flux will result in low operating speed and in higher motor torque for a particular value of starting current because motor torque is proportional to the product of flux per pole and armature current. Thus for given load torque, the motor will accelerate quickly and take less time to reach the lower operating speed from the starting instant. This will result in less heating of armature during starting.
09․ A DC series motor is running with a divertor connected across its field winding. If the divertor resistance is reduced then the speed of the motor will
The diverter of series motor is connected in parallel with series field winding to reduce the current flowing through the field winding. This reduces the flux and increases the speed. If the resistance of diverter is reduced then the current flowing through the series field is increased and therefore flux is also increased and thereby the speed will be decreased.
10․ In which of the following speed control of DC motor the speed obtained is below than the rated speed?
In armature resistance control, the field is directly connected across the supply. Hence the voltage drop across the armature drops as the current passes through the series resistance and the remaining voltage applied to the armature is lower than the line voltage. Thus the speed is reduced in direct proportion to this voltage drop at the armature terminals.