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Variable Speed For Soft Start Induction Motor?


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Is there such a thing?


Seems to me it would have to be an intelligent circuit which allows full power on start up, then adjusts speed by modifying the line frequency.


Is this assumption correct, and is there such a controller? I know there's lots of different types of induction motors (slip ring, cap start, etc), but I don't know much about them.


I'd like to have variable speed for my power tools (bandsaw, etc), but don't want to replace the existing motors.



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Hello Desert1st


Welcome to the forum.

You seem to have some confusion in terminology, there are soft starters for induction motor, and there are induction motors. I am not aware of Soft Start Induction motors.


You can apply a soft starter to an induction motor to reduce the start current and torque, or you can apply and inverter speed controller to an induction motor to vary the speed of the motor.

You would not combine the two as there is no advantage. The inverter is able to provide a slow start and low current so it achieves what the soft starter does during start.

Modern inverters can be of the vector type and provide full torque from almost zero speed.


Variable speed control of power tolls can be more difficult to apply, as often these are single phase, and speed control of single phase motors, although not technically impossible, is difficult. Speed control of single phase universal motors can be achieved by variable voltage control. This is not particularly great as you are controlling the torque, not the speed. The speed varies as a result of the reduction in torque.

Voltage control of single phase induction motors as an attempt to control the speed is not recommended as major damage can be done to the motor unless the motor is purpose designed for this operation.


Best regards,

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Hello Mark,


Thanks for the reply. I did purchase a universal motor type controller (SCR based) and tried it on several motors:


o shop vac - is somewhat controlled the speed, but not very well.


o bench style disc sander - no speed control, motor violently stalled, then buzzed


o bench style bandsaw - same effect as with disc sander


o house vacuum - motor ran at reduced speed when set to full, when control set to reduced speed the motor stopped


So the light dimmer style controller doesn't work really at all! I looked inside at the electronics; it's just an SCR with a simple control circuit consisting of pot, diodes, resistors, etc. It's just chopping the AC, is it not?


Is there something more sophisticated using an IC that works on a single phase induction motor? And if so, does the soft start circuit need to be bypassed?


Or as you said before, is it best to bite the bullet and purchase a 3 phase motor with a variable frequency drive (sounds expensive)?


What about DC motors with controller?


Cost is 1st criteria, 2nd is hassle (changing out motors, etc). The sander and bandsaw cost less than $150 each.


Thanks again!

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Hello dessert1st


SCR based controllers vary the voltage only, not the frequency. These can be used on universal motors (motors with brushes) but not induction motors.

From your tests, I would expect that your sander and bandsaw are both using induction motors. The only real answer is to replace the single phase motors with three phase motors and then use a single phase input three phase output inverter. These are relatively cheap today. I understand your comment about the cost of the units, but that is the way that it is.


The cost of a single phase inverter to drive a single phase motor would probably be higher than a three phase inverter due to volume of manufacture. I do not see this as a practical solution.


Best regards,

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Hello again Mark,


Okay, thanks for all the great input. I certainly appreciate it. It's starting to make sense now, that the norm for speed control of AC motors is to use a 3 phase frequency drive. I suppose the reason for a 3 phase motor vs 1 phase is better accuracy at low speeds.


One last question...


Are induction motors the same as brushless?


Thanks again,



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Hello dessert1st


Have a look at our page on single phase motors. A true single phase motor can not actual start, so it is wound as a two phase motor and the second phase is created by the use of a capacitor or inductive phase shift. This prevents it from being used on a single phase variable frequency inverter. You would need a special two phase inverter. The three phase option is actually a cheaper option due to the numbers produced.


Induction motors are brushless, but there are other motors that are also brushless. The induction motor has a short circuited winding on the rotor, commonly known as a squirrel cage.


Best regards,

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  • 1 month later...

Don't mean to confuse the issue at hand at all, just wanted to comment on the 'soft start induction motor'.


There are some companies now that manufacture soft starters that fit into the terminal box of a standard induction motor and there are others that market induction motors with in-built soft starters.


Although not required for this discussion thread, I am prepared to provide URL's for anyone interested in 'soft start induction motors'.




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