Electric air compressor help?

If you ever watch a cap start motor coast down you're going to notice a sudden drop in speed when the switch closes. This is do to the cap hitting the coil with a DC jolt causing a stationary magnetic field.

 

The motor winding resistance and ground detection is really easy.
Just check resistance between the spades on the plug on lowest ohm setting then switch to next highest setting 400 and see if the reading changes.
Check either of the spades to the ground prong on 400 setting and keep turning until the highest setting or a reading is achieved

 

Is that how battery powered drivers stop so quick?

 

Sort of the same concept except reversed as the stator windings have a fixed magnetic polarity the rotor/armature if you've ever noticed the slots/grooves where the brushes contact is points the armature winding segments are terminated. As you increase/decrease voltage through the trigger the motor is always trying to stop itself as the stator is trying to pull it to magnetic normal +\- and vice versa.

In the single phase ac motor it won't spin unless there is a phase shift in the ac sine wave- that's where the start cap comes in. It essentially makes the 120ac two phase 220ac until the motor reaches speed and the centrifugal switch opens. At that point thee is enough slip between the sine waves to keep the motor running. When the voltage is removed from the stator nothing is stopping the squirrel cage rotor from slowing so it free wheels to a stop.

 

A lot of battery tools just short the motor leads when the trigger is released. The DC motor acts like a generator when power is removed so a electrical load becomes a mechanical load causing braking. Not to be confused with the effect that gravity has on tools causing breaking.

 

Ok, I thought the test was done inside the motor housing. To the spades was reading zero.

Each spade to ground prong was Open Loop all the up to 40m. So if I am understanding this correctly, I have good continuity and no shorted parts?

Thanks again for the help guys. Some knowledge has been gained.

 

Yeah you have it right the motor winding spade-spade should be close to zero indicating no high resistance contacts, no bad plugs, switches or open in the winding.

The spade-ground with a simple resistance meter is the only feasible way to check winding to ground resistance unless you have access to an insulation tester, commonly referred to as a meggar, which induces voltage in the winding and measures leakage to ground and gives a more in service reading of insulation resistance. It should be at least 1Mohm or more for a in service motor