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Speed control for AC motor

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olcal

Member
I need to replace the speed control of an AC motor for a sewing machine. It is a simple metal wiper moving across a wire winding on a heat resistant ceramic former. Its operated with a foot pedal. Rating is 70 watts. Its now falling apart.

I want to replace this with an electronic circuit. Are there any circuits which can do this ? Most of the circuits I've seen are for DC motors.

Help!!!
 

kinjalgp

Active Member
Light dimmers control the line voltage and lowering the voltage of AC motor to control its speed it not a recommended method. It works out for ceiling fans (which has constant load) but most motors don't like it. The speed equation of AC motor is

S = 120*F/P
Where,
F = Line Frequency
P = No. of Poles

So voltage does not influence the speed of motor. However due to reduction in voltage, there is some change in speed at a cost of increase in motor temperature and lowering the motor life.

Use Variable frequency Drive to control speed.
 

Phasor

Member
It works out for ceiling fans (which has constant load) but most motors don't like it.
A valid point. I neglected to mention that most dimmers don't work well with inductive loads, but some do.
 

stevez

Active Member
Isn't there a simple AC motor that does respond to "voltage" control? A friend who is an electrician was explaining that to me - most AC motors, with one exception don't like to see less voltage. Shaded pole comes to mind but that may be grossly incorrect.

Could the sewing machine motor be DC?

I do recall the rheostat (wiper) type of control but recall little about the motor.
 

kinjalgp

Active Member
Yes the motor can be DC. But high voltage DC motors are risky for this type of job. THere is always shock hazard due to insulation breakage. The chances of shock increases as motor becomes old. And DC is much more fatal than AC.
 

Sebi

Active Member
Most of household sewing machines have universal motors. (The rotor's commutator connected serial with stator winding) The motor structure same as drill-machine, and work fine with voltage control, and You have 75% chance for work with normal dimmer with TRIAC output. The TRIAC don't like the inductive load, but i think with small 70W motor no problem.
If You have problem with TRIAC, can change the output two antiparallel SCR-s(thyristor), it's better for inductive load.
 

olcal

Member
One point to note. Mains voltage here is 250V RMS.
Are there any IC's specifically designed for this purpose, that is, AC speed
control ?
 

Sebi

Active Member
I think, the simplest way to purcase a small drill-machine speed control
module. The biggest problem the mechanical driving of potentiometer
with paddle...
 

olcal

Member
What rating or parameter should I look for in such a speed control ?
In there anything specific that I should watch out for ?
 
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