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Tumble Dryer

Hi all,

I finally got around to taking a look at our tumble dryer, which has been out of action for a while (SWMBO's nagging finally paid off for her). In order to get it spinning round, you have to give the drum an initial shove, after which it rotates totally fine. But it cannot revolve without that artificial boost at start-up. I don't deal with repairing this kind of gear, but still fixed it within

5 minutes anyway. The 7uF/400V cap that lies strapped to the motor was faulty (had gone completely open-circuit). It was the first thing I checked. Nevertheless, I'm still unsure what exactly this cap's purpose is. AIR, they are often referred to as 'motor-start caps' or 'motor-run caps' but why are they needed? I'm guessing if they weren't used then manufacturers would have to fit more powerful motors simply to overcome the start-up inertia and a cap is cheaper than a heftier motor. But that's just a guess. Can some kind soul disabuse me of my ignorance here, please?
Reply to
Cursitor Doom
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Typical motors don't have to torque to start turning on their own. The start winding in series with the start capacitor provides the torque to start the motor. There's a switch to disconnect the capacitor after the motor comes up to speed.

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Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com
Reply to
Foxs Mercantile

A single phase induction motor won't spin up without a push. This is because until the motor is spinning there is no ratating magnetic field. There are many ways to do this. One way is to use a starting capacitor. This cap is connected to a different winding, called the start winding, than the run winding and causes a phase shift, about

90 electrical degrees to the run winding. This gives the rotor a direction to start spinning. There is a centrifugal switch that takes the cap out of the circuit once the motor is about 85% of rated speed. If left in circuit the start winding would overheat. Another cap scheme is to have a cap that starts the motor and stays in circuit. This scheme doesn't provide as much starting torque but does have the advantage of simplicity and making the motor run smoother. There are also motors with switched start caps and always in circuit run caps. Eric
Reply to
etpm

Most dryers I've seen have a "Run" cap only. I.e it stays in circuit all the time, much smaller rating than a "Start" cap and typically 7uF.

Reply to
Rheilly Phoull

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