Explanation: DC series motor at no load condition gives infinite speed ideally. Practically it will damage all the armature circuit. Thus, as the load is reduced speed of the motor will go on increasing rapidly. So, speed control is very poor in series motor.

Explanation: According to commutating fields of machine, speed and overall performance of a machine is determined. For low speeds of motor in a variable speed motor we need weaker commutating field.

Explanation: Speed of regulation of DC series motor is given by ratio of difference of no-load speed and full load speed with full load speed. Thus, in this case, speed regulation = (1200-1050)/1050= 14.28%.

Explanation: Diverter means adding a variable resistance in parallel with field winding. Tapped field control is the method where field ampere turns are adjusted in steps by varying the number of turns included in the circuit. In series parallel method field winding is connected so that it will form two parts.

Explanation: Field control method is not suited to applications needing speed reversal; since the only way to reverse speed is to disconnect the motor from the source and reverse the field/armature polarity. The field circuit being highly inductive, it is normally the armature which is reversed.

Explanation: For motors requiring a wide range of speed control, the field ampere-turns are much smaller than the armature ampere-turns at high speeds causing extreme distortion of the flux density in the air-gap. This leads to poor commutation. Compensating winding can be used to increase the speed range which can be 2 to 1 for large motors.

Explanation: Diverter is a small variable resistor, which is connected in parallel with field winding. By varying this resistance value, the field current and consequently the field ampere-turns can be varied and speed can be controlled.

Explanation: One precaution to be taken in this method in order to avoid oscillations in speed initiated by load changes is to use an inductively wound diverter resistor. Thus, speed controlled output will vary smoothly when load is changed.

Explanation: Speed-torque characteristic for decreasing values of diverter resistance or decreasing value of kd will lie above of the previous or lower value of diverter resistance. Thus, for higher diverter value curve will lie below the previous one.

Explanation: In tapped field control of the DC series motor a field winding is simply tapped at various points and many out pins taken, so that any out pin can be connected and particular part of field winding will be selected in the circuit, while other part is made open.

Explanation: In series-parallel method, field winding is equally divided in two parts which are connected in series and parallel with respect to each other. Thus, in series connection we’ll get one speed and in parallel mode we’ll get one speed. These 2 speeds are only possible.

Explanation: In parallel case, total ampere turns will be exactly half of the ampere turns of the series case. Since speed of any DC motor is inversely proportional to the MMF or flux or ampere turns developed, speed of the parallel control method will always large than the series one.

Explanation: Diverters are used in series motors for speed control, they are not used in shunt motors. Since, shunt field winding resistance is very high, if we connect a diverter across shunt field winding, total current will flow through diverter and it will almost short circuit the shunt field winding. This will increase the motor speed to a very high value.

Explanation: In a DC series motor, when the resistance increases due to rise in temperature, the IR drop in the circuit will increase, which reduces the effective voltage applied to the armature, hence speed decreases due to the direct proportionality.