help explaination for induction motor and phase-controled ..

[nguyennam]

Hi,
Could someone help me explained why induction motor needs a sinewave voltage, but its speed control remote using the phase control thyristor/triac?

nguyennam

 

[powersys]

Permanent magnet brushless AC motor (PMBLAC) and permanent magnet brushless DC motor (PMBLDC) are same physically. For PMBLAC, the supply is a sinewave, whilst for PMBLDC, the supply is trapezoidal wave. However, with the trapezoidal wave supply, the output torque tends to have more ripples.

For a 3-phase induction motor, with a 3-phase sinewave ac supply, the torque you get will be smooth or constant. In my opinion, it's not necessarily a sinewave voltage to drive an induction motor. The main idea is to supply a voltage to the induction motor so that it can produce a rotating magnetic field. If the rotating magnetic is constant, then you will get constant torque. If the rotating magnetic is not constant (like in single-phase induction motor), then the torque is not constant. At high speed, the vibration caused by torque ripple is filtered out by the mechanical load.

 

[electricpete]

Re: help explaination for induction motor and phase-controle

I agree with everything above.

The most important part imo was the comment about a rotating field. That is what the stator has to produce in order to make the rotor rotate. The easiest ways to do that involve supplying it with a (approximate) sinusoidal voltage.

Since the stator windings are stationary, obviously a dc voltage wouldn't produce a rotating field. If we install a single-phase winding with ac sinusoidal excitation, it can mathematically be shown to be the equiavalent of a forward-rotating field and a reverse rotating field. Once it gets started moving, only one of those fields remains relevant. For three phase motor, the reverse rotating fields associated with the three phases cancel.

 

[nguyennam]

Re: help explaination for induction motor and phase-controle

Thank you all for your kind replies.

So, do you think we can replace the inductor-speed-control-switch of a ceiling fan by a phase controlled triac?

nguyennam

 

[powersys]

Re: help explaination for induction motor and phase-controle

Thank you all for your kind replies.

So, do you think we can replace the inductor-speed-control-switch of a ceiling fan by a phase controlled triac?

nguyennam

Yes... u can do that... modern controller is using solid state switch.