Speed control confusion of an induction motor.

Thread Starter

stevo776

Joined Nov 1, 2017
1
Hey all,

I have been handed over a project at work where I am building a machine that requires a motor to be controlled from 0 to full rpm. Previously, this motor had been bought with this speed controller.

https://www.princessauto.com/en/detail/1-2-hp-totally-enclosed-fan-cooled-electric-motor/A-p8513541e
single phase induction motor, capacitor start
https://www.ebay.com/itm/For-Router...c-Motor-Rheostat-AC-120V-Newest-/232233941242
Variable Rheostat.

The rheostat was able to vary the speed of the motor slightly, but not as much as is required. After turning the potentiometer down in the back of the speed controller I was able to run the motor very slowly, however the fuse blew in the speed controller. I then bought this rheostat thinking that the issue was too much current running through the circuit.

https://www.mlcswoodworking.com/shopsite_sc/store/html/smarthtml/pages/speed_control.html
20A rated version

The motor wouldn't turn over and would just chatter. After talking with the seller, they explained that their speed controllers wont work on an induction motor and are really only designed for universal router motors. They told me that to control my motor I would require a more expensive controller, like a Variable frequency drive (VFD).

I talked to a local motor supply company about their VFDs, they explained to me that my motor isn't able to be speed controlled and that it would shut off after reaching a certain minimum RPM.

Could you guys help me figure out what is going on with the motor? If the motor is unable to be speed controlled, or if it requires a VFD, then why was I able to control it by varying the voltage with the 15A rheostat? (albeit with the fuse blowing). If it is the case that my motor is not able to be speed controlled, then what kind of motor will allow me to speed control it and what would be the best way to do so?

Thanks for all your help in advance.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
The problem with that motor which is a cap start/cap run motor is that they will not control via the Triac controller as you have found out.
Also VFD's for 1ph induction motors are rare as they tend to drop out of run at low rpm and/or on load.
You can either use a DC brushed motor (TreadMill type etc) with that controller by using a bridge rectifier or buy a 3h induction motor and use a VFD, they can be ran off of 1ph 240v supply.
Max.
 

crutschow

Joined Mar 14, 2008
34,280
then why was I able to control it by varying the voltage with the 15A rheostat? (albeit with the fuse blowing)
You can control the speed by varying the voltage of an induction motor, but it will have very poor speed vs. load characteristics, as you are operating it with a high slip frequency.
Induction motors basically are designed to operate at near synchronous speed, and behave poorly much below that speed.
 
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