# Ac induction motor constant speed control

1. ## Ac induction motor constant speed control

Hi,
I want to run the AC induction Motor at 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 tachometer in my circuit. If the tachometer is available, so i can get the feedback from the Motor, then I can control speed of the motor at constant level. But my question is without tachometer is there any other technique for to run the motor at constant speed - with & without load. Please help me to solve this problem.

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2. ## Re: Ac induction motor constant speed control

Hi,

If you want constant RPM, you need to measure RPM somehow.
I' don't think there is a reliable sensorless prediction of RPM just by measuring V and I. ...especially with triac control.

Klaus

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3. ## Re: Ac induction motor constant speed control

Induction motors are NOT brushed DC motors where you can measure the back-EMF to estimate the speed the motor is turning.

You DO REQUIRE a tachometer. Can be optical, reluctance or magnetic.
Around induction motors I prefer optical, to avoid magnetic noise from stray fields.

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4. ## Re: Ac induction motor constant speed control

run the AC induction Motor...
Reliable speed control of Induction Motor IS difficult.

Most common are squirrel cage type (no electrical connection to the rotor) and if you want a wide speed range, you will need a variable frequency drive to have constant speed under different loads.

Some large induction motors have slip rings to connect to the rotor windings and speed control can be done by controlling the rotor current via external resistors.

Single PHASE induction motors are often capacitor start (and they cannot be reversed) - some smaller ones use shaded poles (they too cannot be reversed).

Most are used for constant load and constant AC power (and hence the torque) and without a sensor (tachometer) it will not be possible to control the speed of induction motors.

5. ## Re: Ac induction motor constant speed control

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 loads?

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6. ## Re: Ac induction motor constant speed control

Induction motor has almost load independent speed and doesn't need speed regulation in usual applications.

If precision speed regulation of induction motors is required for some reason , you'll use an inverter variable frequency drive (VFD), preferably with tachometer or - less accurate - with estimation of actual shaft speed.

Triac (phase angle) control for induction motors is only suited for special applications like fan drives, not for washing machines.

7. ## Re: Ac induction motor constant speed control

HI
I read at past you can attach a little dc motor to the rotor
and the voltage that generate by it , feed to op amp as a comparator
and do triac phase control to have nearly constant speed

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