There are many small motors in use that utilize a low capacitance value
capacitor. The most common type smaller than 1/8 HP is the PSC permanent
split capacitor type motor.
PSC motors have 2 windings connected in series (3 connections or wires), and
don't have centrifugal switches for starting.
PSC motors are commonly labeled with; thermal and/or impedance protection.

The method used to determine if a motor is a PSC, is to measure the
resistance of the windings. A PSC motor's windings resistances will be
nearly identical for boh windings.

The value of the capacitor affects the torque and speed of the motor. I don'
know of any formula for determining the (best) value of the capacitor.

Years ago, there was a reference to a capacitor value selection chart,
related to PSC motors used in HVAC servicing, IIRC.
The chart suggested capacitor values for different sized motors, for
increasing torque and/or speed.

Many of the small PSC motors that I'm familiar with will operate with
different capacitor values, and a couple of them use different capacitor
values to make the motors operate at different speeds (low-medium-high).

One PSC motor I was using for a machine application actually ran much cooler
when the cap value was changed to a higher value (about 3x the
manufacturer's specified value).

I'm surprised that your motor didn't run with just one 5uF cap connected.
Marked values on caps can vary as much as 20% from the actual measured
value, but 10% is probably a much more likely worst case situation.

The best indicator from a standpoint of harm, is probably going to be the
motor temperature. If the motor doesn't run excessively hot, there wouldn't
be any reason that the cap's value could harm the motor.

--
Cheers,
WB
..............


"Andy" wrote in message
...
I have a small single phase blower motor which requires a capacitor
for running (not starting). The motor nameplate says that I need 7.5
microfarads. I have a pair of 5 microfarad caps in the shop. I
hooked up the motor with just a single 5 uF and it didn't start. Then
I put the pair in parallel and the motor did start. If I use 10 uF I
will draw more current into the directional winding of the motor. So
my question is, how closely are these caps sized? Is the difference
between 7.5 and 10 uF enough to do damage?

thanks