Question

State whether True or False :
If a DC motor is connected across an AC supply it may run.
A) True
B) False

Answer 1

A motor will only rotate if it experiences unidirectional torque.
But in case of AC supply the current is pulsating, i.e. it changes direction in every half cycle.
So, the torque developed due to this may be unidirectional or not depending upon the construction of the DC motor.

Complete step by step answer:
The answer to the question depends upon which type of dc motor is available.
So, mainly there are two types of DC motor :
1. Shunt wound DC motor: a shunt wound DC motor is a type of DC motor where the field windings are shunted to or are connected in parallel to the armature winding of the motor. Since they are connected in parallel, the armature and field windings are exposed to the same supply voltage.
2. Series wound DC motor: it gets its name from the fact that the field winding in this case is connected internally in series to the armature winding. Thus, the field winding is exposed to the entire armature current unlike in the case of a shunt motor.

Now, coming back to question:
A motor will only rotate if it experiences unidirectional torque.
1. For a series motor the field winding and the armature winding are connected in series. Hence, even if the voltage or current polarity changes due to AC supply it happens simultaneously for both the winding. Now the torque produced in the dc motor is given below (in linear region of BH curve)
$Torque \propto {I_{armature}} \times {I_{field}}$
Now, it is clear that even if both of these currents change their direction the torque remains positive.
So, in series motor unidirectional torque is maintained and the motor continues rotating, but with some problems like reduced torque due to reduced current because of inductance of field and armature windings; and heating of stator core (if it is not laminated) due to eddy loss and hysteresis loss.

2. However, in a shunt motor the impedance of these windings is different and being parallel connected in nature the current flowing through them is definitely going to be different with a finite phase difference in them. Thus, it depends on the phase difference. Depending upon the phase difference the torque can be positive, negative or zero, which doesn’t develop torque enough for starting rotatory motors.

Therefore, answer to the question is true i.e. A dc motor may run on AC supply.

Note:
DC motor works on the principle of torque generation due to current carrying conductor in a magnetic field, which is given by:
$\tau = \vec F \times d = I\left( {l \times \vec B} \right) \times d$
In practical application this magnetic field is provided with help of windings and not permanent magnets.