shaded pole motor control

"Ian Macmillan" skrev i meddelelsen news:492d4026$ snipped-for-privacy@dnews.tpgi.com.au...

Try a standard TRIAC light dimmer (Check & Make sure that the motor gets AC though, a bulb won't mind but the motor will!)! Shaded-pole motors are often speed controlled by regulating the voltage down.

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AC

ten

Shaded pole induction motors can only be reduced in speed slightly by lowing the voltage. On something where the load increases rapidly with speed, like a pump, you may have a little luck. The torque of the motor decreases when the speed falls below the "pull out speed". This makes it suddenly go down to a very slow speed as the voltage is decreased.

Decreasing the frequency and voltage together is the way to go if you need a significant decrease.

Yes, I tried it many years back. It doesn't work.

oo

Can you cycle it on and off?

How about running a pipe back around the pump with a valve in it to dump a lot of the water back around to the input.

You can use a stepwise sine wave like shape to do it. If works a lot better if you place an extra inductance in series with the motor to keep the sharp edges from causing current spikes.

My brother made a system that would power up an induction motor using a square wave drive. He was using a transformer to step up from a 12V battery to mains voltage. The motor got fairly hot but it did work for an hour without serious overheating. He was working with a 1/4 horse motor.

A company called "Flytronics"(SP) used to make a system for making a sine like power with a trick that you can borrow. What they did was used a transformer with many taps on the primary. The primary was driven by transistors with rectifiers in series with them. It worked very nicely but they charged an awful lot for them. I have my own version I have long wanted to try and will offer it here:

Imagine 3 binary weighted transformers. I am thinking of center tapped "primaries". The actual transformers would be 12V, 24V and 48V step down transformers run backwards. The "secondaries" would be in series. The transformers would be way over rated for the intended power but low cost ones.

The drive on the "primaries" would be transistors that are bypassed with rectifiers so that current can flow back onto the input power. The transformers are never allowed to see a high impedance.

If you want more points and more complexity without adding more transformers, you can go with this idea:

Imagine that the transformers are base 3 weighted and driven by full bridges. By turning on the two lower transistors of the bridge, you can short out the transformers. This gives you minus, zero, plus as the three points.

I suppose you could try the way the big boys do it and build a variable frequency drive.

1. An ordinary light dimmer can work over a limited range. Sometimes you have to swap the leads to kill an oscillation.
2. Put a plain light bulb in series. Start with 25Watt.

Mike Monett

I don't have actual experience doing it, but I recently read this article and found it interesting (all those emails I've sign up for sometimes have articles I actually want to read). The controller described may be overkill for your app, but it describes the characteristics of such motors very well, and that may help you figure out what you can reasonably do:

From that article I get the impression you can't control the speed of such a motor very well without feedback of its actual speed to measure the 'slip.' These things appear to be near-constant torque when slowed down below their peak torque. That works well on fans which have increasing load with speed, and may also work with your app, presuming the pump similarly gives an increasing load with speed.

Use my variable inductance controller. Carry out this simple experiment to see how it works. Get a 100VA transformer, something like 240V to 40V. Put the 240V windings in series with the motor and connect to the mains - nothing happens. Short the 40V windings - motor runs at nearly full speed but you are still working with AC.

Put a bridge rectifier on the 40V windings and short the DC output. Motor runs at nearly the full speed it did before but now you are working with DC. Put a transistor (I normally use a darlington - TIP121 for a 40V secondary and add simple control electronics. You can normally get up to about 90% full speed.

Approximately 10% losses occur in the transformer, bridge rectifier and darlington. This could be reduced by using a higher voltage secondary and a MOSFET when the losses could be 5% or lower. The control electronics side is isolated by the transformer so you are working with low voltage DC.

Have fun experimenting but beware of the 240V on the transformer. Peter

--------------------------------------- Posted through

Shaded pole motors need variable voltage to control their speed, Buck the voltage as described above.

Cheers

If you don't care about efficiency and don't need variable speed a power resistor in series with the motor is the simplest solution. Experiment with a variac to see what voltage you need on the motor and determine the ohms needed. Or just leave the variac in place. Art

Interesting! What kind of (stable) speed range can you get? ...Jim Thompson

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Gosh that is only about 40 years old. Over 75 if you count tubes.

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You make it sound like an old design is not as good as something more recent. I'd never heard of this so it is 'new' to me.

George H.

AKA Magnetic Amplifier, or Saturable Reactor.

Not the same thing! In a saturable reactor, a DC current saturates the core of an inductor, reducing its inductance. In the arrangement described above, there is no DC magnetization, and the core does not saturate.

Jeroen Belleman

That's not a mag amp. Mag amps use DC to control AC.

Cheers

Phil Hobbs

And it's not saturating.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

One way to control speed is to add series resistance. The technique varies the impedance seen looking into the primary of the transformer.

Cheers

True.

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

Conceded. Stupid of me.