# Single phase motor speed control



## nmackintosh (May 8, 2008)

Hey i got a single phase 208v 2hp motor controlled by a contactor. Customer is looking to control the speed of this motor... wondering what the most economical way to do that is? I know we could go to 3phase and a vfd but i was wondering if there is anyway to control the single phase motor. Thanks


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## gpop (May 14, 2018)

Before motor speed control became cheap they invented mechanical speed control. What does the motor do and why do you want speed control.


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## micromind (Aug 11, 2007)

As far as I know, the speed of a basic capacitor-start single phase motor cannot be controlled. 

If it's a standard NEMA frame (56, 56H, 143T or 145T) then replacing it with a 3Ø model and using a VFD with single phase input would work. 

Be advised though, the VFD will not be able tp provide as much starting, pull-up or breakdown torque as the single phase motor even if it's oversized.


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## MotoGP1199 (Aug 11, 2014)

As stated above I do not know of a way to control the speed with out a mechanical speed control or going to a VFD and 3 phase motor. Also if you go the 3 phase route size the drive 175% to 200% larger to feed it with 1 phase. I hear different numbers from other people but if I double the HP, I've never had a problem.


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## VELOCI3 (Aug 15, 2019)

Does 2-1/2 HP










Sent from my iPhone using Tapatalk


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## nmackintosh (May 8, 2008)

Controls an exhaust fan for a spray booth. Ya i thought u could control the speed of a single phase motor unless its designed too but i just wanted to double check


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## nmackintosh (May 8, 2008)

*couldn't


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## micromind (Aug 11, 2007)

VELOCI3 said:


> Does 2-1/2 HP
> 
> 
> 
> ...


That controller is for universal, shaded pole and PSC motors. The speed of the universal type will respond to changes in voltage and load, the speed of shared pole and PSC is dependent on V/HZ and load. 

If the Op has a capacitor-start motor, this controller won't work with it.


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## paulengr (Oct 8, 2017)

See previous comments about PSC motor control.


Speed control over capacitor start motors is POSSIBLE but I haven't seen one on the market yet. It would need to be a vector control drive right out of the box at a bare minimum.


And V/Hz microdrives are so crazy cheap (and most are even vector drives), with voltage doubling built in so that you can drive a 230 V three phase NEMA standard motor directly from a 115 V power supply, as are small 3 phase motors that it is really hard to justify doing it an other way.



If you have the change the motor anyway, there is a big reason why we use capacitor start or capacitor start/run motors: torque and efficiency. You lose both going to PSC.


https://isccompanies.com/wp-content/uploads/2016/04/Single-Phase-induction-Graph.png


So if you need to change the motor for speed control anyways, definitely it is worth your while to go to a three phase motor or maybe a permanent magnet motor.


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## gpop (May 14, 2018)

They make a single phase output vfd that can start and run motors with or with out a cap so that's the cheapest option. 
I have never personally tried one but the science behind it looks solid.


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## CMP (Oct 30, 2019)

*Spray booth air balancing.*



nmackintosh said:


> Controls an exhaust fan for a spray booth. Ya i thought u could control the speed of a single phase motor unless its designed too but i just wanted to double check



Your customer can control the motor and fan load, with a mechanical damper in the inlet or outlet duct of the exhaust fan. Closing off the airway slightly reduces the motor load and air delivery.



Fan motor speed would remain the same, but the load on the motor, and hence energy consumption, would be reduced, similar to a speed controlled motor.


This was often used to balance the airflow inside a paint booth, to accurately match the air delivery of the air makeup unit supplying the fresh air. Booth pressure should be slightly positive to keep the dirt out of the booth.


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## JRaef (Mar 23, 2009)

As said, you can control TWO TYPES of single phase motors; Shaded Pole and Permanent Split Capacitor (PSC) types. There are VFDs made for that, they are expensive and in many cases because 3 phase VFDs are so cheap now, they are MORE expensive than just changing to a 3 phase motor and using a VFD that puts out 3 phase from a single phase input.


I don't trust anything I see from that ATO company...


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## TechyWalla (Apr 6, 2020)

I agree.
It has been my experience that 1ph VFD control is a bit of a failure, particularly in the motor sizes above 3/4hp, they tend to drop out of run at low rpm/high load conditions.
This is the reason the 1ph to 3ph VFD is so popular now.
T.W.


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