On Tue, 16 Aug 2005 02:41:46 GMT, Ignoramus9179
wrote:

I built a self starting phase converter with a 10 HP idler and two 92
mF capacitors. It works great and starts in under 2 seconds.

I also have a very similar looking Century 7.5HP motor. They 10 HP and
7.5 HP look similar.

This 7.5 HP motor would NOT spin up from two other capacitors. If I
wire it with three capacitors (I have 5 total, 2 already used in the
main phase converter), it would spin up, but it takes it about 5
seconds to do so and it warms up appreciably. What gives?

Another tidbit is that the 7.5 HP motor starts just fine from 3 phase
output of my phase converter. I do not think that it is a bad
motor. Maybe some caps out of these three are in bad condition.

i


The minimum single phase starting capacitor value can vary a
lot between OK motors of similar HP but different design. This is
a parameter of pretty well zero significance in three phase motor
design so there's no incentive to control or to reduce it.

Older motors using poorer performance iron tend to need
rather more capacitance. Another factor is the initial bearing
friction and, to a lesser extent, armature inertia. The heat
input to the motor is disproportionately large at very low speeds
- just an extra second at near stall can make a big difference.

There's probably nothing wrong with your 7.5 HP setup but
if you're worried about the capacitors try the following test.

. Connect the capacitor in series with a high wattage lamp (100W
or more) across 110v supply WITH CAPACITOR INITIALLY SHORTED OUT
then remove the short.

No change in brightness or nil brightness - dud capacitor

Reduced brightness - functioning capacitor - compare
the brightness with a ditto test on a similar value capacitor.

Jim