Q:Why do AC supplied electric tools commonly have a brushed motor instead of an induction motor?

Cheaper? don't know about that one.. More compact, able to tolerate overload more easily, can be more easily/ cheaply speed- controlled.

Q:How large AC contactor and thermal relay should be used for the motor of 3KW? There are steps to calculate better.

1, Y-100L-4 type 3KW motor rated current is 6.4A, can also be obtained by formula: (power factor cos Phi about 0.7)I = P/ (root 3 * U * cos (3000/) = 1.732 x 380 x 0.7) = 6.5 (A)2 "AC contactor selection: general and motor current is available, but in actual use, the contact is easy to burn, choose more than the motor current double the more durable, should choose CJX2-1210 (12A).".3 "thermal relay setting current: choose regulating current range of 5 ~ 9A thermal relay, the setting current is 1.1 times the rated current of the motor (7A).

Q:An electrical cabinet is cooled by a 18000 BTU air conditioner. The heat that is transfered from inside the cabinet to the outside of the cabinet PLUS the heat generated by the A/C system componets = to the total power consumed by the A/C system ?

No, it most certainly does not. Some, hopefully most, of the electrical energy taken by the motor will be converted to mechanical energy. That's what a motor does. But, in the conversion process some of the energy, hopefully only a small amount, will be lost as heat.

Q:I have just received a Hurricane 2 horsepower electric motor. It was used as a pump for a pool. Is there any way i can re wrap this motor and make it run on either 24 or 36 volts DC?

I don't think so. Going from AC to DC is the hurdle (I'm assuming this is AC). It'd be more work than it's worth. If it's a DC motor then you can run on a lower voltage you just won't get nearly the output power.

Q:How does IGBT works in frequency inverter for ac motor control?

The IGBTs act as switches to provide a series of DC pulses to the motor. Since most AC motor frequency controls are for 3-phase motors, there are six IGBTs, two for each phase. One IGBT connects each motor terminal to the positive side of the DC supply and one connects each motor terminal to the negative side of the DC supply. In that way, each terminal to terminal or line to line voltage can be either positive or negative. By controlling the switching sequence of the IGBTs, the control provides a simulated 3-phase sine voltage with frequency and voltage control. The waveform is composed of DC pulses and doesn't look too much like a sine wave, but the effective value is a reasonably good simulation of a sine wave. Each IGBT has a diode in parallel but positioned to conduct current in the opposite direction. The anti-parallel diodes conduct the portion of the motor current waveform that lags the voltage. Look at the Wikipedia article for more details and diagrams.

Q:What is the limitation of a three phase AC induction motor?

speed control

Q:why DC motors are always started through starters?

Not only DC motors but also AC motors. Starter is required for only large size motors since large size motors take more time to gain speed than small motors. When motor is started it has low speed which does not produce back emf hence very large current is drawn by the motor which may destroy the coil inside. Since small motors gain speed very quickly they produce back emf very fast at start hence they do not require starter.

Q:How do I test an AC single phase electric motor with a meter to see if it is burnt?

Single Phase Motor Winding Checking

Q:is it possible to reduce rpm of AC motor with the help of rheostat?

Here's a link for store bought converters:

Q:Could someone explain the difference between a single phase AC motor and a multi (or three) phase motor. The info on the web is just confusing me. Thanks

single phase motor only has 1 phase wound on it's stator. These motors tend to run choppy. Three phase has all three phases wound on the stators ( + and - on opposite sides of the motor) all three phases are set 120 degrees apart around the stator (just like the phases of a 3 phase system). The function is not only to produce more torque but it make the motor run much smoother.