I have a machine tool with a 1 HP reversible motor. I am looking at implementing speed control by using a zero crossing detector and phase angle controlling circuit. I am planning on using a micro controller such as Arduino to generate the pulses for phase angle control.

The issue is the motor is capacitor start with a centrifugal switch - at lower speeds the centrifugal switch will re-engage.

I am wondering if it is possible, or if there are already designs, to remove the capacitor and centrifugal switch, and replace with another TRIAC to control the start winding?

This could either be controlled by time from the microprocessor, or motor speed if I add another circuit to monitor this (which I may do to display RPM and control an external cooling fan anyway.)

  • 1
    \$\begingroup\$ Triac phase angle control doesn’t work well with AC induction motors. \$\endgroup\$
    – John D
    Commented Nov 26, 2020 at 3:03
  • 1
    \$\begingroup\$ To reliably control induction ac motors You need an inverter. \$\endgroup\$
    – fifi_22
    Commented Nov 26, 2020 at 4:36
  • \$\begingroup\$ Okay, maybe I will have to rethink but why doesn't it work well? I have seen a fair few circuits out there. \$\endgroup\$
    – Dave022
    Commented Nov 26, 2020 at 5:11

2 Answers 2


The triac voltage control method works to some extent to control the speed of an induction motor that is driving a fan or centrifugal pump. It works because the torque required to drive those types of loads is low to virtually zero at zero speed and rises in proportional to the square of speed. That means that reducing the motor's torque capability allows the natural torque requirement of the load to overcome the motor and force it to a lower speed.

Look at this question on this site: Speed control for PSC induction motor

With a machine tool, the torque is determined by the workpiece material, the size of the cutting tool and the rate of movement of the tool into the material. Any variation of torque requirement will result is speed change. The desired torque at slower speeds is likely to be higher rather than lower than for higher speeds.

As mentioned in the answer by @Kartman, the best alternative is probably to use a three-phase motor with a variable frequency drive (VFD).

A universal motor is a commutator motor with the armature connected in series with the field. A universal motors can be used with a triac or thyrister (SCR) speed controller, but a 1 Hp universal motor may not be easy to find.


Induction motors are locked to the mains frequency by design. If the rotor speed moves outside the required speed range, then the motor becomes very lossy - ie it draws more current and gets very hot.

Phase control works well with series wound motors like in your power drill - these are not induction motors.

You can find cheap 3phase induction motors with variable speed drives that will work from single phase quite cheaply. It is an off-the-shelf solution that just works.

Or scavenge a motor from a washing machine (if the motor has brushes it is most likely a series wound type) and then you can control the speed via phase control.

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    \$\begingroup\$ Commutator motors that are AC powered are series wound, not shunt wound. \$\endgroup\$
    – user80875
    Commented Nov 26, 2020 at 14:08
  • \$\begingroup\$ These are great answers; thank you. Was seeing if I can work with what I have rather than buy 3 phase and VFD (I have done that on my lathe) as I may well it in a bit. Will have to see if it’s worth it. Many thanks once again \$\endgroup\$
    – Dave022
    Commented Nov 26, 2020 at 18:52
  • \$\begingroup\$ @Charles Cowie - yes, i had the series image in mind but wrote shunt :(. \$\endgroup\$
    – Kartman
    Commented Nov 26, 2020 at 21:00
  • \$\begingroup\$ @Dave022. if you found the answers useful, upvote them. \$\endgroup\$
    – Kartman
    Commented Nov 27, 2020 at 11:07

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