AC motor questions (1 Viewer)

[muramasa]

How to I explain the difference between an AC and DC generator other than the fact that AC has a slip ring?
Also, why can a universal motor use either AC or DC without any adaptation of its structure and why will an AC motor spin at a constant speed?

 

[anomalousdecay]

AC generator generates an alternating current (which flows in both directions) when it undergoes generation. The direction changes every half turn.

In a universal motor, the polarities of the field winding and armature winding both reverse as they are in series with each other, which causes the torque applied to be in the same direction as before being reversed. You should not worry about this too much as I don't think it has anything to do with the syllabus.

AC motors have a constant current flowing through them as the terminals always keep contact with the slip rings, which always produces the same current flow through it. This will provide a constant torque and hence a constant angular velocity.

 

[ProtoStar]

AC motors have a constant current flowing through them as the terminals always keep contact with the slip rings, which always produces the same current flow through it. This will provide a constant torque and hence a constant angular velocity.

Constant torque doesn't result in constant angular velocity, just as constant linear force doesn't result in constant velocity. The reason AC motors must maintain a constant speed is because the current alternates at a specific rate. The motor must complete a revolution as the AC completes one cycle. Otherwise, the torque will not be in the same direction as the motor's spinning. For example, if you plug an AC motor into a source with too high a frequency for its spinning to match, it'll just vibrate (Source: did this once with the AC generator I made for school).

 

[anomalousdecay]

Constant torque doesn't result in constant angular velocity, just as constant linear force doesn't result in constant velocity. The reason AC motors must maintain a constant speed is because the current alternates at a specific rate. The motor must complete a revolution as the AC completes one cycle. Otherwise, the torque will not be in the same direction as the motor's spinning. For example, if you plug an AC motor into a source with too high a frequency for its spinning to match, it'll just vibrate (Source: did this once with the AC generator I made for school).

Because we frequently deal with phasor voltages and phasor currents in HSC......

Its assumed constant frequency. If it weren't constant frequency at a rate which is used everyday (say 50 Hz) then yes the torque applied would not be sufficient to keep it rotating at the same speed.

Is this you bleakarcher?