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.

Larger, older-style motors operate at a lower flux density and need more
capacitance per horsepower than modern TEFC motors. But you say the
motors are similar in design?

From my experience an unloaded motor will spin up with a wide range of
capacitor values. I would expect a 7.5 hp motor to spin up with a single
92 uF capacitor.

In what arrangement are you connecting the capacitors? If you're
connecting them in parallel, I reckon you most likely have two bad
capacitors or a motor with a dodgy connection somewhere.

Best wishes,

Chris