# AC Motor Speed Control



## just the cowboy (Sep 4, 2013)

You are going to find it almost impossible to find something for a standard motor.
The plug in one run tool/fan motors because they are universal motors.
A standard motor don't work the same way. If the motor can be replaced with a three phase motor they do make drives that go 120 volt single phase in 240 3 phase out.


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## joesparky28 (Mar 12, 2015)

Thanks. I might have to get a phase converter and go up to a 3 phase motor.


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## paulengr (Oct 8, 2017)

Single phase speed control is an oxymoron. It exists but it’s overpriced and not worth it. The first thing you do for variable speed control is convert the incoming power (single or 3 phase) to DC. Then you either use it as is in a permanent magnet DC motor (highest power for the size) which is what servo motors are all about, or a stepper motor, or convert back to 3 phase AC to operate an induction motor. A single phase motor is practically junk and only used because single phase across the line operation barely works and it’s cheap. There are a number of other exotic designs (BLDC, switched reluctance, electronically commutated AC and DC) that work too but the ones I mentioned are the most common.


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## gpop (May 14, 2018)

There are a lot of problems with electric speed control in general. With out knowing the problem we can only give you a generic answer.

With a gearbox generally half the speed double the torque. 
With a Vfd half the speed and you loss half the HP. Starting torque etc is also different depending on how you deiced to control the speed.


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## JRaef (Mar 23, 2009)

joesparky28 said:


> Thanks. I might have to get a phase converter and go up to a 3 phase motor.



No, that's not what *just the cowboy* was meaning. VFDs can take in single phase power and give you three phase out, it's inherent in the design, so long as they are properly sized. 20A at 120V suggests that this is a 1-1/2HP motor. There are VFDs that accept a 120V input and produce 230V 3 phase output (a technology called a Voltage Boost Converter). They generally stop at 1HP because the parts get too expensive for the boost converter, but there are a few mfrs that make them up to 1-1/2HP; 24A at 120V input, 6A at 230V 3 phase output. It would mean having to change your motor, but you do not have to get a phase converter, the VFD will do that for you.


The main problem with trying to change speeds on single phase motors is that most of them use a starting scheme that involves a centrifugal switch and if you lower the speed, the switch re-engages and the motor loses torque and/or damages the drive. The only single phase motors that can have their speed changed are PSC (Permanent Split Capacitor), Shaded Pole type, both of which are generally low torque motors used for simple fans or some pumps, and Universal motors, such as are used inside of portable power tools and small kitchen appliances. The vast majority of commercial off-the-shelf single phase motors are Capacitor Start or Split Phase, both of which use the centrifugal switch method, so they cannot have speed control.


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