When a DC motor is connected to an AC supply, the outcome significantly depends on the type of DC motor. While some types might attempt to run, others will simply vibrate or suffer damage due to the alternating current flow.
Behavior of a DC Series Motor on AC Supply
A DC series motor is constructed with the field winding connected in series with the armature winding. This configuration is key to its behavior when connected to an AC supply.
- Why it Runs: In a DC series motor, the current flowing through the field winding and the armature winding is the same. When connected to AC, the current through both windings alternates simultaneously and is in phase. The magnetic field produced by the field winding and the magnetic field interacting with the armature conductors both reverse polarity at the same time. This ensures that the torque produced by the motor remains in the same direction (positive or negative, causing continuous rotation in one direction) despite the alternating current.
- Performance Issues: Although a DC series motor can run on AC, its performance is not ideal. As stated in the reference: "If a DC series motor is connected to a ac supply same current will flow through the series field and armature field after being rectified by the commutator. So, the machine will run but with high sparking at brush, poor pf, low speed and low efficiency."
- High Sparking at Brushes: The rapid changes in current direction due to AC, combined with the inductive nature of the motor windings, induce high voltages in the armature coils undergoing commutation. This leads to excessive sparking at the brushes.
- Poor Power Factor (pf): The field and armature windings are highly inductive. This inductance causes the current to lag significantly behind the voltage, resulting in a poor power factor.
- Low Speed: The average torque produced by the motor on AC is lower than on DC for the same voltage magnitude due to the pulsating torque and inductance. This leads to a lower running speed.
- Low Efficiency: Increased losses occur due to eddy currents and hysteresis in the iron core (which are minimized in DC-only designs but significant on AC) and the increased sparking at the brushes.
This principle is utilized in the design of universal motors, which are specially constructed DC series motors capable of operating on both AC and DC power. Universal motors have laminated cores to reduce eddy current losses and often include compensating windings or interpoles to improve commutation and reduce sparking.
Behavior of Other DC Motor Types on AC Supply
Other common types of DC motors, such as shunt and compound motors, are generally not designed to run on an AC supply.
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DC Shunt Motor: In a shunt motor, the field winding is connected in parallel (shunt) with the armature winding.
- Why it Fails on AC: When connected to AC, the alternating current will flow through both the shunt field and the armature. However, the inductance of the shunt field winding is very high compared to the armature. This causes the current in the field winding to lag significantly behind the applied voltage and also lag considerably behind the current in the armature (which has much lower inductance).
- Outcome: Due to the phase difference between the armature magnetic field and the main field produced by the shunt winding, the torque produced is pulsating and often reversing over each cycle of the AC waveform. The motor will typically just vibrate violently and fail to rotate, or it might rotate erratically at very low speeds if the average torque is slightly positive, but it will likely overheat and be damaged.
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DC Compound Motor: A compound motor has both series and shunt field windings.
- Why it Fails on AC: The behavior is a combination of series and shunt characteristics. The series field will act similarly to a series motor, but the shunt field's reaction to AC (large phase lag, pulsating field) will dominate the overall performance, preventing stable rotation for the same reasons as the shunt motor.
Summary of DC Motor Behavior on AC
| DC Motor Type | Behavior on AC Supply | Key Issues (if it runs) | Suitability for AC Operation |
|---|---|---|---|
| Series Motor | Runs (but poorly) | High Sparking, Poor Power Factor, Low Speed, Low Efficiency | Not Ideal (unless 'Universal') |
| Shunt Motor | Vibrates, Fails to Start, Potential Damage | No stable rotation | Unsuitable |
| Compound Motor | Vibrates, Fails to Start, Potential Damage | No stable rotation | Unsuitable |
In conclusion, connecting a DC motor to an AC supply is generally not recommended. Only the series wound configuration has the potential to run on AC, and even then, it performs poorly unless specifically designed as a universal motor.
