On Tue, 11 Apr 2017 21:28:41 -0000 (UTC), Cursitor Doom
wrote:

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?
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