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I want to run the AC induction Motor at a constant speed with LOAD also without Load. So I am controlling motor speed by TRIAC. I can control the motor speed, but my problem is:- I don't have a tachometer in my circuit. If the tachometer is available, so I can get the feedback from the Motor, then I can control the speed of the motor at a constant level. But my question is without tachometer is there any other technique to run the motor at a constant speed - with & without load. Please help me to solve this problem.

For an Example: I want to run the Washing Machine - Wash motor, in the Wash motor there is no RPM feedback. Only the Supply line three wires are present in the motor. I want to run the motor at the same RPM whatever given User Load. In this scenario what is the solution to run the motor at same speed with different load?

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    \$\begingroup\$ you need some kind of feedback, no way around that afaik... \$\endgroup\$ – vicatcu Nov 1 '19 at 13:06
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    \$\begingroup\$ Decent and reliable induction motor speed control needs something more than just a triac. \$\endgroup\$ – Andy aka Nov 1 '19 at 13:23
  • \$\begingroup\$ How constant do you need the speed to be? \$\endgroup\$ – Kevin White Nov 1 '19 at 14:07
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    \$\begingroup\$ Omitting the tachometer? You have omitted the entire VFD. You need one just to drive AC motors at variable speed. Triacs won't work. AC motors aren't just DC motors that run off AC instead. Amplitude AND frequency affects speed. \$\endgroup\$ – DKNguyen Nov 1 '19 at 15:03
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    \$\begingroup\$ @DKNguyen is talking about induction machines, specifically. There are motors (called universal motors, or shunt-wound brushed motors) that can be controlled with a triac -- but if you really do have an induction motor, you really don't want to try to control its speed by varying its input voltage. \$\endgroup\$ – TimWescott Nov 1 '19 at 18:13
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You can't get there from here :-).

You need to specify what you mean by speed control.
Unless the range of speeds is within a very small range below synchronous speed then you cannot reasonably open-loop control the speed of an induction motor by varying the drive power. And you can only closed loop control it in a reasonable manner withing a relatively limited range. For any sort of "decent" speed control the motor rotor must track the driving AC frequency with a limited amount of "slip".

An induction motor works by using the difference frequency between the applied frequency and the rotor frequency to induce low voltages at high currents in the rotor structure. It is inherently self regulating speed wise within certain limits as increasing slip angle increases power input which works to decrease slip angle.

TRIAC control works by reducing the power level to below that required to maintain 'proper' induction/slip angle control. It is essentially a chaotic situation - somewhat akin to surfing a wave - and has too much variability to be able to be open loop controlled.


There are numerous stack excahnge answers over the years relating to speed control of AC induction motors. I recommend that you read them - much of interest even when of variable applicability.

From Whiskeysip69s good answer here is this curve which shows fan load versus AC inductioj motor torque curves with varying voltage. A fan load varies with speed in such a way that it "sort of works" as you vary motor voltage. Other loads are not so obliging. Near synchronous speed the motor has a negative slope speed/torque curve - as speed drops torque increases so that a constant power load or a linear power with speed load tends to self regulate speed wise. . When you get on the positive slope slower speed portions you get (the more usually experienced in life) decreasing power and torque with decreasing speed curve where the slower you go the slower you go ... .

enter image description here

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  • \$\begingroup\$ For an Example: I want to run the Washing Machine - Wash motor, in the Wash motor there is no RPM feedback. Only the Supply line three wires are present in the motor. I want to run the motor at the same RPM whatever given User Load. In this scenario what is the solution to run the motor at same speed with different load? \$\endgroup\$ – Nishanth Nov 2 '19 at 14:27
  • \$\begingroup\$ @Nishanth You need a VFD. \$\endgroup\$ – DKNguyen Nov 2 '19 at 20:31
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    \$\begingroup\$ @Nishanth The fact that you "want" it does not mean that it is reasonable to expect that you will be able to get it in a reasonable manner. IF single phase (or 3 phase) induction motors were usefully speed controllable by simple open loop voltage variation then it would almost certainly be the preferred method of motor variable speed control (due at least to the low cost). It is not used for almost any purpose. Fans are a special case due to their power:RPM cubed power law. Washing machines do not "look" like fans load wise. \$\endgroup\$ – Russell McMahon Nov 3 '19 at 2:52

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