Operating a DC Shunt Motor on AC Power
What will happen if you connect a dc motor to an ac source? When the ac source changes polarity, the current in the motor will reverse. But the reversal will occur in both the armature winding and the field winding. Therefore, the torque on the armature will remain in the same direction. However, there are certain problems in running the DC motor on AC power. One problem is the effect of induction in the windings.
Induction is not very important in the DC motor, because the current remains constant after the motor starts. But the ac power supply operates at a frequency of 50 or 60 Hz. That is, the potential difference across the motor and therefore the current in the windings changes at a rate of 50 or 60 cycles per second. Each cycle consists of two changes in direction, from positive to negative and back again.
The field winding of a DC shunt motor has many turns of fine wire. The large number of turns is required so that the motor can develop a strong enough magnetic field to produce the required torque on the armature.
If ac power is applied to the field winding, the inductive reactance causes high impedance. This impedance in an ac circuit has the same effect as a high resistance. The result is a low current in the winding producing a weak magnetic field and low torque on the armature. As a result, the motor will not run efficiently.
DC shunt motor do not operate on AC power
Another reason DC shunt motor do not operate well on ac power has to do with the difference in phase between the field winding and the armature. The field winding has a higher inductive reactance than the armature winding. Therefore, the current in the field winding lags farther behind the applied potential difference than does the current in the armature winding. The problem of phase difference is shown in below image.
The angle θ is the lag of the current (IF) in the field winding behind the applied potential difference (E). The angle φ is the lag of the current (IA) in the armature winding behind the applied potential difference.
The difference between θ and φ causes a problem. The torque of the motor depends on the force exerted on the current in the armature winding by the magnetic field produced by the current in the field winding. Increasing either one increases the motor’s torque. But if the current and the magnetic field are out of phase, the torque of the motor is reduced.
DC Series Motors Operated on AC Power
The field winding of a dc series motor has fewer turns and larger wire than the field winding of the dc shunt motor. Therefore, its inductive reactance is less. The strength of the magnetic field is about the same as when the motor runs on dc power.
The field winding and the armature winding are connected series. Therefore, the current lags behind the potential difference by the same amount in both windings. As a result, the current in the two windings is in phase. The current in the armature winding is in phase with the magnetic field produced by the field winding.
However, a dc series motor does not operate well on ac power. The motor has low efficiency power factor, and runs with considerable sparking a low the brushes. The low efficiency is caused by high hysteresis loss and high eddy current loss. Power factor is caused by the inductive reactance of the field and armature windings.
Read more about Motor work on both AC & DC
Hysteresis and Eddy-Current Losses
1. Hysteresis loss
Hysteresis loss occurs because of constantly changing magnetic field in the iron core of the armature and of the field winding. The continual changing of the magnetization of the iron cores uses up power. Hysteresis loss is directly proportional the field strength and the speed of the motor. Laminations in the core do not reduce the hysteresis loss.
2. Eddy Current Loss
Eddy current loss occurs because the iron of the armature core is a conducting material moving a magnetic field. This motion induces eddy currents in the iron. Eddy currents vary with the strength of the magnetic field and with the speed of the motor. The laminations of the armature core reduce the magnitude of the eddy currents.
The amount of power lost equals the product of the current squared and the resistance of the iron (P=I2x R). The current varies in direct proportion to the speed and the magnetic field strength. Therefore, the power loss varies with the square of the motor speed and with the magnetic field strength. To overcome these disadvantages, an ac series motor frame is made from laminated iron.
The field winding is made with fewer turns than the field winding of a dc series motor. The field pole area is increased to reduce the concentration of the magnetic field at these places.
These measures reduce the eddy current losses and the inductive reactance. In addition, the armature winding is made with more conductors to obtain the required torque with a lower magnetic field strength.
Induction in the coil undergoing commutation causes brush sparking in a dc motor. As this coil is short-circuited by the brushes, a potential difference is induced in the coil.
This induction is minimized by winding the coils with fewer turns. More coils having fewer turns are required on an ac armature to produce sufficient torque.
More commutator segments are therefore required, which is another characteristic of the ac series motor.
3. Armature Reaction AC Series Motor
Armature reaction is more severe in the ac series motor than in an equivalent dc motor. The reason is that there are more conductors in the armature, causing a greater induced potential difference. A compensating winding added to the field winding can reduce the effects of armature reaction. This winding is set in the stator slots 90 electrical degrees from the main field winding. Figure is showing how such a winding can be included in the motor circuit.
The compensating winding can be connected in two ways. It can be connected in series with the armature and field windings. It can be a short-circuited winding.
In both cases, the magnetic field of the compensating winding is proportional to the armature current. The short-circuited winding is said to be “inductively coupled” to the motor circuit. That is, it performs its function because of currents induced in it.
A motor having an inductively coupled compensating winding is used only on ac power. The compensating winding is lined up with the axis of the brushes. Therefore, the alternating magnetic field of the armature induces a current in the short-circuited winding. This magnetic field opposes the magnetic field of the armature. Thus, it opposes the armature reaction.