How do you increase and decrease the speed of an AC motor?

Answer 1

You really can't really "control" the speed of a the commonly available home-use 120 VAC fans.

The problem is that most of the motors on the "regular" fans are synchronous motors, and they want to sync with the line by virtue of frequency. Fans with different speeds have different windings in them that are specifically set up for the different speeds. A switch allows the different windings to be selected. Simply reducing the line voltage will only reduce the torque the motor will supply, but will not directly affect its speed. The lower torques will cause the motor to slow down, and that's true, but this is not necessarily a good thing as the fan was not designed to run like this.

Sophisticated electronic variable frequency drives (VFDs) are designed to provide AC power but at a frequency you can control. Your ability to control the frequency gives you the ability to control the speed of the fan directly. But these units cost. For the smaller non-commercial units, it is smarter (and cheaper) to just get another fan with multiple speeds than to try to fool around and control the speed by some "gizmo" or something else.

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I beg to differ. Most home fans are inexpensive single phase AC motors, either with a capacitor (PSC) or shaded pole. These are not synchronous motors. A simple light switch dimmer (aka phase controller or triac controller) can be used to control the fan. Caution: be careful to avoid fan stalling when setting at low speeds. The dimmer controls the speed by reducing the effective voltage to the fan. At low voltages (and resulting speeds) the fan torque is also reduced. Thus the likelihood of stalling is higher. If the fan stalls with the power on, the motor will overheat and possibly catch on fire.

VFDs are definitely not the right choice for a simple fan. These require motors (almost always 3 phase) designed for use with VFDs (aka, inverters) or a special output filter for the controller. Otherwise the bearings will pit within months to a year. Besides, VFDs with a single phase output is very rare.

Answer 2

With DC motors you use a DC motor drive which will adjust the out put voltage to adjust the speed of the motor. AC motors you would use an AC motor drive or VFD (Variable frequency Drive) and the most common ones will adjust the output voltage and frequency to keep the volts/ hertz ratio the same. There are many different types of AC motor drives and they are usually selected depending on the load (ie. constant torque, variable torque, constant power).

Answer 3

Speed control of AC motor:

First let us know the basic equation for speed.

N=120f/p

N= speed in rpm;

f= supply frequency;

p= number of magnetic poles;

1. Number of poles was decided during the design of motor so we cannot change this factor to controll the speed.

2. By inceasing or decreasing the supply voltage frequency motor speed can be changed accordingly.

3. Another method is by varying the supply voltage but this may affect the torque produced by the motor.

4. Combined variable voltage and frequency method(v/f method)

Answer 4

I assume you mean when driving and then using the gear box to help slow the car.

This is because when changing gears the speed will slow of the rotation of the motor and the flywheel. Then subsequent changes will slow a little bit more, also ever try to do 100 in first gear, most cars cant, each gear has a limit that it can reach for speed.

Answer 5

Use an auto transformer.