Hi Folks,

Can anyone tell me about the typical operation of start motor windings of a tumble dryer motor? My timer is broken, and the dryer can start rotating in either direction, the problem being it will work (ie dry the clothes) in one direction, and not in the other. Once it starts in a particular direction, it can be very difficult to get it to change.

It has been suggested to me in the appliances section that dryer motors have start windings,
And that I ask here in the electrical section about how these operate. Can anyone tell me more about them, or about how the specific direction of rotation of an AC motor (my assumption) is electrically determined. Many thanks.

Cheers
Rissole

AC motors work based on the change in the electromagnetic field generated by the coil. As the current switches back and forth, the EM field changes direction, causing an alternating push and pull on the magnetically active parts of the rotor, causing its motion to synchronize with the oscillations of the current. For single phase motors (what you have in your house), the synchronization is bi-directional... that is, it can work in either direction. The EM field is balanced, so it's basically a coin toss whether it will work clockwise or counterclockwise at startup.

To ensure that the motor starts up in the correct direction, a secondary coil with a capacitive load is added to the motor (the capacitive load kicks this coil out of phase with the main coil), to unbalance the EM field just enough to force the motor to start in a specific direction. Once the motor starts, this secondary coil is deactivated and the balanced EM field is restored.

It sounds like you need a new capacitor for your dryer motor.

Quote:

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Originally Posted by Nosnosna

AC motors work based on the change in the electromagnetic field generated by the coil. As the current switches back and forth, the EM field changes direction, causing an alternating push and pull on the magnetically active parts of the rotor, causing its motion to synchronize with the oscillations of the current. For single phase motors (what you have in your house), the synchronization is bi-directional... that is, it can work in either direction. The EM field is balanced, so it's basically a coin toss whether it will work clockwise or counterclockwise at startup.

To ensure that the motor starts up in the correct direction, a secondary coil with a capacitive load is added to the motor (the capacitive load kicks this coil out of phase with the main coil), to unbalance the EM field just enough to force the motor to start in a specific direction. Once the motor starts, this secondary coil is deactivated and the balanced EM field is restored.

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Hi Nosanna,

This is very useful and a good clear explanation of how these motors can be forced in one direction. However, the dryer motor (when the timer is working properly) will occasionally reverse direction for a couple of minutes (to avoid creasing and untangling etc), so this motor will also have a way of reversing direction on demand. It may have three coils I suppose, or some other clever way of kicking it off in the other direction. Any thoughts?

Anyway, I think the next stage is take the machine apart and look at the motor for clues, and measure a few voltages.

Best regards
Russell

I would try to track down the circuit that reveres the direction in response to the timer. There should be a schematic that shows all the wiring somewhere in the dryer. Find the circuit, and you should be able to control it.

Note, you can do a rough check of a large capacitor with an ohmmeter. Discharge it first, then connect the ohmmeter across the terminals. It should show a resistance at first, and then slowly climb off scale.

Quote:

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Originally Posted by rissole

Hi Nosanna,

This is very useful and a good clear explanation of how these motors can be forced in one direction. However, the dryer motor (when the timer is working properly) will occasionally reverse direction for a couple of minutes (to avoid creasing and untangling etc), so this motor will also have a way of reversing direction on demand. It may have three coils I suppose, or some other clever way of kicking it off in the other direction. Any thoughts?

Anyway, I think the next stage is take the machine apart and look at the motor for clues, and measure a few voltages.

Best regards
Russell

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In order to switch directions, it would likely use a second starter winding that is designed to push the motor in the other direction. There are a number of ways to do this, but the implementation is probably unimportant.

Since it's having problems with both the initial startup and failing to switch directions, it's likely the switch that energizes the starter coils. There will either be a three-state switch (off, coil one, coil two) or two two-state switches (off, starter coil; coil one, coil two) to do this... if it's the latter, the first switch is the one you should be interested in... if it were the second switch, it would always rotate in the same direction. It could also be the capacitive load, if the two starter coils share a capacitor. I'd look at the switches first, as they are moving components.

Quote:

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Originally Posted by Nosnosna

In order to switch directions, it would likely use a second starter winding that is designed to push the motor in the other direction. There are a number of ways to do this, but the implementation is probably unimportant.

Since it's having problems with both the initial startup and failing to switch directions, it's likely the switch that energizes the starter coils. There will either be a three-state switch (off, coil one, coil two) or two two-state switches (off, starter coil; coil one, coil two) to do this... if it's the latter, the first switch is the one you should be interested in... if it were the second switch, it would always rotate in the same direction. It could also be the capacitive load, if the two starter coils share a capacitor. I'd look at the switches first, as they are moving components.

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Ok I've had a look at the timer and measured the following wrt earth.

Wire colour Clockwise rotation Anticlockwise rotation

Blue 0V 0V
Brown 390V 0V
Yellow 0V 390V
Orange 230V 230V

Clearly the brown and yellow are switched to provide a direction of rotation, but has anyone an explanation for a reading of the apparently high voltages?

Cheers
Russell

If you put electricity in, it will energise the electromagnets and spin the motor
If you spin the motor (with the electromagnets activated), you will get electricity as per a generator

It does not matter if the cause of the spin was the electricity you put in or not - so, the generator effect adds to the voltages you put in, hence the 390v.

The technical answer is more involved than this, but this is the guts of it.

I sat here and replied an answer to this, and didn't even realize the dates.

Gheeze (embarrased)

Your'e two years to late werafa!

Hello Folks,

I'm delighted to see everyone is as eager as ever to help me out.:)
The key thing was I found which supply where controlled motor direction, and since one way worked, and the other didn't, I have hard wired the supply in the direction that works. The tumble drier has since been in daily use for the last couple of years.

Best regards
Rissole