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Old 05-29-2008, 07:25 PM   #1
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Default Speed Control Switch for Motor

I have a friend who wants me to install a variable speed switch on his squirell cage motor. The motor is 120volt and pulls 8 amps(No nameplate on motor). I installed the switch in series with the line and it works, however the motor is very noisy when switch is turned to "low". Is this a motor problem? Can only certain motors have their speed controlled? The motor is equipped with impedence protection(capacitor).

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Old 05-29-2008, 07:38 PM   #2
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did you install a variable frequency drive or did you install a voltage regulator (dimmer switch)

Either way yes it is normal for the motor to be noisy, either A. because it is running at a frequency that is not native to the motor or B. because it is running at too low of a voltage.

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Old 05-29-2008, 08:25 PM   #3
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What type of motor are you talking about? Shaded pole, Split Phase, Capacitor start, Universal…..
Motor nameplate information would be helpful in order to answer you question.
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Old 05-29-2008, 11:04 PM   #4
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the motor is an old blower motor out of an A/C unit. I installed a motor rated dimmer switch(8amp)
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Old 05-29-2008, 11:52 PM   #5
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dimmer switches are not motor rated, you are running the motor at a voltage below its spec be ready to replace it very soon.
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Old 05-30-2008, 12:01 AM   #6
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This is a permanent split capacitor type of motor. In a nutshell, it's a 3 phase motor that's designed to run on single phase power. The capacitor is used to generate the third phase, much like a static-type phase converter. They're designed to operate at a specific load and voltage, and if either one is off, the phase angle of the generated phase will wander as well. Thus the noise.

Changing the value of the capacitor will help to some degree, but at the expense of higher speed performance. Sort of a trade-off.

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Old 05-30-2008, 11:07 AM   #7
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Quote:
Originally Posted by Absolutetruthz View Post
I have a friend who wants me to install a variable speed switch on his squirell cage motor. The motor is 120volt and pulls 8 amps(No nameplate on motor). I installed the switch in series with the line and it works, however the motor is very noisy when switch is turned to "low". Is this a motor problem? Can only certain motors have their speed controlled? The motor is equipped with impedence protection(capacitor).
That motor is a single phase /split phase motor and is not a candidate for speed control. the capacitor is there to provide a phase shift between the start and run windings for starting purpose only and is removed from the circuit by the centrifugal switch after start has been achieved. in general the only way to vary the speed of an AC motor is by varying the frequency. you are learning in the best possible way by experimentation. Good for you! BillW.
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Old 05-30-2008, 02:43 PM   #8
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Quote:
Originally Posted by Absolutetruthz View Post
I have a friend who wants me to install a variable speed switch on his squirell cage motor. The motor is 120volt and pulls 8 amps(No nameplate on motor). I installed the switch in series with the line and it works, however the motor is very noisy when switch is turned to "low". Is this a motor problem? Can only certain motors have their speed controlled? The motor is equipped with impedence protection(capacitor).
Even the motor rated speed controls output 'chunky power', and chunky power equates to 'noise' at low speed.

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Old 05-30-2008, 03:50 PM   #9
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Its not a dimmer switch, It IS motor rated, this switch is specifically for fan motors. You are right about it being a split phase motor, there are other leads for dual voltage. Thanks for the input!
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Old 05-30-2008, 09:40 PM   #10
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When the motor is running at full speed, and you remove power to it, do you hear a click as it is slowing down? If you do, then it's a capacitor start induction run type. If you don't, then it's a permanent split capacitor type, and the other leads are usually for different speeds.

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Old 05-31-2008, 09:29 AM   #11
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Permanent-split capacitor motor

One way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current displaced 90o in time. This is called a permanent-split capacitor motor in Figure below.

Permanent-split capacitor induction motor.
This type of motor suffers increased current magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to minimize losses. The losses are less than for a shaded pole motor. This motor configuration works well up to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor, described elsewhere is this chapter.

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