Why can’t an AC voltage be applied to a DC motor?
It is generally not possible to provide AC voltage to a DC motor for a number of reasons, primary among them being the design and principles of operation of DC motors:
Brushes and Commutator
Typically, DC motors use brushes and a commutator to sustain unidirectional torque. In order to maintain a constant current direction in the armature windings with respect to the magnetic field, the commutator mechanically rectifies the current. The alternating current in the windings would reverse direction if an AC voltage was applied, which cause severe sparking and quick wear on the brushes and commutator. There would be overheating and possible failure of the motor.
Magnetic Field and Armature Windings
The armature windings of a DC motor are made to handle direct current, which creates a constant magnetic field that interacts with permanent magnets or field windings. When AC electricity is applied, the magnetic field changes quickly, producing unpredictable torque. The motor might make noise, vibrate, and not create a mechanical output that is helpful.
Insufficient Synchronization
DC motors can only provide smooth, continuous torque when there is a constant magnetic field & current flow. When AC voltage is used, the torque produced by an unsteady magnetic field would oscillate, making the operation unstable. The motor would not operate at all, let alone effectively or efficiently.
Construction and Insulation
Generally, DC motors are constructed with insulation & construction requirements appropriate for DC operation. The motor windings and other components may be harmed by insulation breakdown and overheating brought on by the fluctuating voltage & current associated with AC.
Failure to have Voltage Rectification
Standard DC motors lack integrated rectification components, in opposition to AC motors which utilize solid-state rectifiers (or) other methods to convert AC to DC for particular applications. They are therefore unable to convert AC to DC, which is necessary for them to function.
Problems with Efficiency and Performance
The motor would perform poorly even if it could theoretically run on AC power (even if very inefficiently and with limited capability). The torque would vary, & the motor might not produce mechanical output that could be used, leading to ineffective energy conversion and possible motor damage.