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i want to control the speed of induction motor single phase (about 1.5 kw / 2 hp \ 220 volt \ 50 hz and 1400rpm) using dimmer(triac) , i try this circuit i chang the value of C1 to get best control the circuit worked acceptably but not very well , not requested i Noticed that the range of control the speed with variable resistance R1 is tight and when i Reduce the Speed the motor have some noise 1-how i can solve the proplem of the thigt range of the control speed? 2-how i can solve the proplem of the noise in the motor? 3- what the Disadvantages and Damage of this kind of control on the motor? and by this kind of control As a result i reduce the voltage so the torque will be reduce , are i am right ? so what is the benefits?
for circuit Rs= 27 Ω ,5 watt and c1 = 300nf ,400volt and the load is the motor ant triac is bta41

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  • \$\begingroup\$ and i noticed there is Amount of heat in the motor \$\endgroup\$ – Mohamad Knight Dec 9 '16 at 6:02
  • \$\begingroup\$ Is this a wound-rotor motor where you have connections to the rotor via slip-rings, or a squirrel cage rotor motor where there are no connections to the rotor? \$\endgroup\$ – Ben Miller Dec 9 '16 at 8:25
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speed control of induction motor using dimmer

Induction motor speed control is done by varying the frequency of the applied voltage. Raising or lowering the voltage can have some speed control effects but these will be small, difficult to control, load-dependent, possibly cause the motor to overheat and, are not generally recommended.

This applies to three phase induction motors and single phase induction motors using a start/run capacitor.

People use VFDs to control induction motor speed but even these are not particularly helpful (a generalism of course) with single phase induction motors that use a capacitor.

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  • \$\begingroup\$ but I have SINGLE PHASE induction motor and VDFs only control three phase motor \$\endgroup\$ – Mohamad Knight Dec 9 '16 at 13:04
  • \$\begingroup\$ you mean the dimmer is not effective \$\endgroup\$ – Mohamad Knight Dec 9 '16 at 13:09
  • \$\begingroup\$ Yes. This means the dimmer can never be very effective. It has limited use with motors that have designs suited to that. Also the load must be suitable. It works best with fans and centrifugal pumps. \$\endgroup\$ – Charles Cowie Dec 9 '16 at 13:56
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The reason this didn't work well by the way is that in an induction motor, torque is a factor of the ratio between voltage and frequency, and a squirrel cage motor is designed for a particular ratio. So by using a dimmer, you are not changing the frequency, only the voltage and thereby affecting that ratio. Motor torque varies by the square of the applied voltage at a fixed frequency, so if you give that motor 50% voltage, it will only develop 25% of it's rated torque. That then means slip increases, the motor draws more current in an attempt to return to normal slip speed, but it can't, so the extra current just creates heat needlessly, until eventually the motor burns up.

A VFD works because it DOES change the voltage AND FREQUENCY together, maintaining the V/Hz ratio that the motor is expecting, so it delivers rated torque at any speed.

There are VFDs designed to be used with single phase motors, but not ALL single phase motors. Unlike 3 phase induction motors which are all built basically the same, there are numerous methods of making single phase induction motors function. This is because with single phase, there is no rotating magnetic field generated to make the motor begin spinning, it just cycles back and forth. Once a motor STARTS spinning however, it keeps spinning. So single phase motors must be designed with some "trick" to get that spin started, usually in the form of a temporary phase shift. Motors that use a separate winding (Split Phase) or a capacitor (Cap Start) to create that phase shift, but then use a centrifugal switch to remove it once the motor is spinning, cannot be used with any form of speed control because slowing them sown re-engages the starting method and it was not designed to be in the circuit continuously. There are however two forms of single phase induction motor that can be used with speed control, a Permanent Split capacitor (because the caps ARE designed to be in the circuit the entire time) and a Shaded Pole motor. Shaded pole motors are particulary well suited for simple voltage control via "dimmer" because the torque is poor anyway, so they are generally only used in applications that don't need a lot of torque, like small fans. PSC motors can also be used with dimmers but the torque issue is still there so it is very load dependent.

Bottom line, if yours is an induction motor but not Shaded Pole or PSC, you cannot use any form of speed control on it.

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  • \$\begingroup\$ How common are induction motors that are not PSC? I've never come across them - in my experience (air movement, machinery, etc) they are all PSC. There are PSC motors that have an extra capacitor for starting that is disconnected via a centrifugal switch - perhaps that is what you are talking about? \$\endgroup\$ – peter_mcc Jun 12 '18 at 5:12
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Except for small fan motors, single-phase motors are generally not suitable for speed control. Universal motors are DC-type motors with commutators that can be used for either AC or DC. They are suitable for simple dimmer-control type speed control.

The best and usually the only alternative for an AC motor is to buy a motor that is suitable for variable speed and a matching controller. There are VFDs rated up to a few horsepower that convert single-phase input power to three-phase output. Also available are commutator-type DC motors with either field windings or permanent-magnet fields. DC motors controllers, similar to dimmers, are available for both types. Brushless DC motors are also available. They use another type of controller.

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