I have a group project where I have to control the speed of a single-phase shaded pole induction motor. My plan was to use a Triac-based circuit however, all Triac circuits I have seen use a potentiometer to control the speed of the motor. In this project, the motor's speed is dependent on an external analog signal received from another unit. The 3 signals can either be high, medium, or low. How can that external analog signal be received and used to drive the motor at the specific high, medium, or low?

NOTE: No microcontrollers may be used

  • 3
    \$\begingroup\$ If you look closely at the circuits with potentiometers you will likely find that the wiper of the potentiometer provides a voltage that can be provided in other ways. You could even use three potentiometers with wipers selected by relays. \$\endgroup\$
    – user80875
    May 22, 2021 at 14:59
  • 1
    \$\begingroup\$ A shaded pole motor can't really be 'controlled' in speed. Once below near-synchronous speed (the normal operating region), the torque becomes very speed dependent and you may get large variations in speed with small variations in excitation. The exception is if you are driving a fan load, for which the speed/torque curve is so steep that it tames the motor's speed/torque curve somewhat. \$\endgroup\$
    – Neil_UK
    May 22, 2021 at 15:19
  • 3
    \$\begingroup\$ As the motor's speed gets lower the slip (difference between stator rotating field speed and rotor speed) becomes higher. This will increase the voltage induced in the rotor which will lead to higher rotor currents and possibly destruction of the whole motor from heat dissipation. If you want to properly control the speed of an asynchronous motor, in first approxiamtion you should vary the frequency of the voltage applied and keep the ratio stator voltage/frequency constant. E.g.: 240 V/60 Hz = 4 V/Hz. \$\endgroup\$
    – HarryH
    May 22, 2021 at 16:38
  • \$\begingroup\$ @HarryH Keeping the Webers constant. The motor's heavy iron would be pretty much designed around an expected operating level of volt-seconds. Makes sense. \$\endgroup\$
    – jonk
    May 22, 2021 at 21:51
  • \$\begingroup\$ @jonk It's the higher frequency in the stator (short circuit) winding. You have to lower the 'Webers' so the rotor generated voltage (linear with slip 'frequence') won't get too high. So it's the 'd/dt(Webers)' that causes the problem, as U=-d(phi)/dt. High slip means the rotor experiences a higher frequency of the magnetic field. If the field is the same as with low slip, a much higher voltage, hence short circuit curren, will result. \$\endgroup\$
    – HarryH
    May 23, 2021 at 1:27

2 Answers 2


> NOTE: No microcontrollers may be used

Define "microcontroller"? If you were to get a small SSR that provided phase angle firing from an analog input signal, is the IC inside of the SSR that controls the firing angle going to be considered a "microcontroller"? Because that would be the simplest way to do this. Analog signal to SSR controls the phase angle firing of the SSR to control the output voltage, SSR is then controlling the torque (and thus speed) of the SP motor. Note that I am being specific about phase angle control, because not all SSRs do this, you have to specifically look for that feature when using it for a motor.

And ignore the comment about not being able to change speed on an AC motor with voltage only, that is true for most AC induction motors, but not ALL types and Shaded Pole motors ARE one of the types that can be controlled this way.

  • \$\begingroup\$ The SSR would actually be perfectly acceptable for this project. Would I require 3 SSR's or just one? If possible, could you give me a brief explanation of how the phase angle triggering would work with an SSR? \$\endgroup\$ May 23, 2021 at 10:39

Here's one way to do it: the capacitor values are a guess. if the fan came with a speed controller use the capacitors it came with.

Capacitors should be mains capacitors, X1 rated or better. the relays are solid-state relays.


simulate this circuit – Schematic created using CircuitLab

R1 R2 R3 R4 form a potential divider making sample voltages at 1.25 2.5 and 3.75v

The three comparators each compare a sample voltage against the input signal. when the input it higher than the sample voltage to the corresponding comparator output goes to a low voltage allowing current to flow in the solid-state relay's input circuit.

So with 4V on the input all three relays are on and the fan motor sees full mains power voltage.

With 3V on the input the top relay SSR3 is off and so power to the motor has to flow through C2 this reduces the current available to the fan motor slowing it.

With 2V SSR2 and SSR3 are off and current has to flow through both C1 and C2 in series, this further reduces the current available and you get an even slower speed

With 1V the all three comparators are not conducting on their outputs so all three relays are off and thus there is no current to the motor

  • \$\begingroup\$ Thanks a ton for this! Just one question, why are there four comparators used? \$\endgroup\$ May 27, 2021 at 20:59
  • \$\begingroup\$ LM339 is a 4 comparator IC, you can use U1d for another purpose if you want. \$\endgroup\$ May 27, 2021 at 21:12
  • 1
    \$\begingroup\$ Would it be possible to give me brief explanation of this circuit? \$\endgroup\$ May 28, 2021 at 14:21
  • \$\begingroup\$ fair enough, all answers need an explanation. \$\endgroup\$ May 28, 2021 at 20:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.