Double speed in AC motor (second part)

This will work on a "universal" motor, that is a motor that has brushes and is commonly used in domestic appliances. This will definitely not work with an induction motor!!

Best regards,
Mark Empson
http:www.LMPhotonics.com | http://www.LMPForum.com

What do u want to mean with "will not work", It truly works with my motor, but I want to know if it will endure

Specs of my motor:

http://www.ebmpapst.com/index/sid/36CD1116-0E7B-4D71-984D-17FF9A8A05FE/nid/88CDC5AE-67E8-4AC5-A49A-F6FD3EB916C2/produkte_detail.asp?pid=19861&overview_action=%2E%2E%2F88CDC5AE%2D67E8%2D4AC5%2DA49A%2DF6FD3EB916C2%2Fprodukte%5Fliste%2Easp%3Fback%3Dtrue%26amp%3Bparent%5Fid%3D

This links to a three phase motor, not a single phase motor.

Putting a diode in series with the supply phase to a single phase induction motor, will cause DC current to flow into the motor. The current will then be limited by the resistance of the motor only and this would result in a high current flow and the motor will overheat and fail.

If the motor is a universal motor, (with brushes), the motor will slow down but not be damaged.

Best regards,
Mark.

Grrr,. I dislike generalisation like this..

put a diode in series with the motor will make DC flow into the motor? ummm, not quite, you'll get 1/2 wave, not DC

put a FULL Bridge in the motor circuit and you'll get DC flowing in, motor will not run then.

at 50Hz, you'll get 10mS of zero input, and 10mS of AC, or 1/2 wave, not quite DC, the motor possibly wont like it, for the same load, you'll be running about twice the current, reducing the life of the motor, with more current flow more heat.

According your point of view DC does not exist because of the switching on and off... The real DC would be ethernal...

Actually, you do get DC current flowing, it is discontinuous DC. It does not reverse in polarity. To be more correct, it can be analysed as an AC current superimposed on a DC current, but it is certainly not AC current on its own.

This technique is commonly used to stop induction motors. It is called DC injection braking, except that it is done with only a small part of a half waveform because with a full half wave applied to the induction motor, the current is much too high and damage results.

If you applied a full half wave circuit with full voltage to an induction motor, the current is limited by the resistance of the motor and this is very small, hence the current is very high.

If the motor was stationary when the voltage was applied, the stationary torque due to the DC field is much higher than the rotation torque due to the AC field and the motor will not spin.

If you apply this to a motor spinning at synchronous speed, it may lock into the AC component and continue to rotate at synchronous speed with high motor losses.
If you apply this to a motor spinning at less than synchronous speed, the stationary field will win and the motor will come to a rapid stop.

Best regards,
Mark

If you've got the motor turning then it would seem to work...

The only problem you might have is overheating. You might want to check the current with a clamp meter, this will give you an idea if you are stressing the wire in the rotor. Beyond that you're good.

Hope that answers your question.

Thanks Ca1cocat,

short answer but the most accurate.I'll check it what you are saying

WOW!

IMHO this is an irresponsible answer. As Marke said, there is no way that this is actually changing the speed of the motor in any safe manner. To allude to it being "OK" is to invite a disaster, possibly even a fire that might get someone killed.

jpbohorquez,
From reading the older thread, you have a 2 pole AC capacitor start induction motor. There is NO WAY that putting a diode in one part of the circuit is going to change the speed. What you are doing it likely causing the DC flow that Marke mentioned, which is creating negative torque in the rotor, EFECTIVELY slowing down the shaft but at the expense of the motor itself. You have two torque components fighting each other, one is stronger so the motor continues to spin, just slower than before. An added likelihood is that because of the slower speed, your start switch is not taking the starting capacitors out of the circuit. Bad bad bad...

This is a fire waiting to happen. STOP IT NOW.

This would seem to be the difference between theory and the bench test. So far nobody has worked the equations to prove that this won't work. The fellow has said himself that it IS working, so there must be something that is being overlooked. As has been said, one major downside is the risk of overheating. If it can be shown by measurement and observation that overheating is not occuring then the system should be good.

I did make an assumption that thermal protection and circuit breakers are present. But I don't see this as being the risk that others make it out to be. If somebody wants to run the circuit and equations and show me why I'm wrong I'll agree with the analysis. Anything else, especially as this fellow has the motor turning in spite of it being "impossible," is bunk.

difference between theory and the bench test

The long term bench test may well prove the theory.

If the motor was a universal motor, then there would be no issue at all.
If the motor is an induction motor, and there is every indication that it is, then there will be problems. If it is a standard induction motor then I very much doubt that it would appear to work at all, however, if it is a special high slip induction motor, that is a motor designed for torque control by variable voltage, then the diode would certainly increase the slip by reducing the AC flux, and further increase the slip by adding DC flux, and the motor would slow down if it was driving a variable torque load such as a fan or a pump.
The problem remains however, that the current would be higher than it should be due to the DC component of the current flowing in the stator, plus there would be a major offset in the hysteresis of the stator increasing the iron losses in the motor.

If the diode was replaced with a triac providing a voltage reduction with a symmetrical waveform, then is would work provided that it was a high slip motor.

High slip motors are sometimes supplied as an integral part of a small fan assembly. They are designed to provide a measure of speed control by variable voltage. Typically, the rotor is on the outside of the stator so that it gets higher cooling than usual.

Although the diode may appear to work in practice, I would strongly suggest that IMO, the continued use of the diode would cause a very premature failure of the motor.

Best regards,

Mark

SORRY , ONE OF THE PROBLEMS WITH SOME POSTINGS IS A PHRASE LIKE TWO POLE AC MOTOR . THAT IS SOME VERY GOOD INFORMATION , BUT NOT QUITE ENOUGHT .

I CAN TRULY SAY A LITTLE BIT OF INFORMATION CAN BE DANGEROUS.

PLEASE TRY TO LOOK UP MOTOR WIRING DIAGRAMS AND BE VERY SPIFICE OF ALL THE " W " QUESTIONS WHAT BOOK WHO WHERE ETC.

iN MY CAREEAR I HAVE SEEN $100,000 OF MOTORS TURNED INTO JUNK AND MILLIONS OF DOLLARS OF PRODUCTION LOST OR SCRAPED BY WRONG APPLICATION OF KINDS AND TYPES OF MOTORS AND SPEED CONTROLERS AND MISS USED DC MOTOR RATED AT X HOPSEPOWER HAS ALMOST NO HORSE POWER AT VERY SLOW SPEEDS AND THE SMOKE CURELS. THEY ARE NOT STEAM ENGINES.

Please dont shout

No problem at all. My agent does a nice line in replacement pump motors. Just fell off the back of a Space Shuttle. Can he have your credit card details and a delivery address for the containers-full that he needs to send you to stay in business? <Chuckle>