The direct-wound motor is the kind you make at elementary school science class. Because of the rotor commutator it runs on AC or DC in the same fashion.
The direct-wound motor can be use AC or DC, but not in the same fashion.
Its rotational direction can be reversed by simply switching the stator coil terminals (a nice feature for automotive use).
In a DC motor brushes, you can't control rotational direction by switching the stator coil terminals with two brush on equal side. Think about it as water falling to a wheel to turn it. The higher the water pressure (voltage) the faster it goes. The water volume (current) is pipe size of drinking straw. The wheel is spinning faster cause of the pressure (volage). I could stop the wheel cause the water volume is so small. If you change the water volume of a oil pipe line. I would unable to stop the wheel cause of heavy weight of that water.
If you look at the watermill near the river. The wheel is slowly turning casue little water pressure is pushng the wheel. I sure can't stop the wheel cause of trying to stop large volume of water going thru the wheel. To control rotational direction on the DC motor would be an a third brush near one of the two brush that created more current on one side or a gear the force one way to turn. In AC motor is very different cause frequency is now play with voltage and current as well. There is too many things happen in electrons effect.
You control its speed and torque by changing the voltage.
In a DC motor, control speed is votlage. In AC motor is somwhat fix. In DC motor, control torque is to change amount of current. In some AC motor, current is Torque, not speed.
Speed actually is load-dependant. On a very light load, the power/load equilibrium may make the speed so high that motor destructs itself. Torque (current flow) is highest when the motor is not running, which could cause the motor to fry.
yes, I agree. It can be on AC or DC motor, but it does not appy all motor.
Speed of induction AC motor is frequency-dependant. Varying voltage does not change its speed (to a point).
I agree with that.
You use PWM to control torque (therefore, speed) of DC motor rather than rheostat because a resistor in series means waste of energy.
Wrong, In DC motor in current would control the torque, not speed. When electric go thur a resistor that opposes an electric current by producing a voltage drop between its terminals. When voltage drop it lose it energy. If there is no votage then there is no energy. You never loses electron.
So if you add, Pulse-width modulation (PWM) of a signal or power source involves the modulation of its duty cycle, to control the amount of power sent to a load. It would add current to the DC motor as an on/off switch. It would add more current to the motor give a heavy push when it needed.
PWM for AC motor controls the torque while the frequency determines the speed.
Yeah, I agree. In AC motor, it a huge waste when it get a high speed. The current doesn't drop as much like a DC motor. It doesn't need that much current. So it send a short amount of time in burst of current for a same load as when a motor just started. The frequency of AC is controlling the speed. I add with the PWM as an on and off switch to the AC motor. The AC motor is going on and off so fast that it all most like the AC motor is always on. Like I said, it fool the AC motor like it getting a full load on.