Why do 1ph ACA sirens need start/run capacitors?

[sirendude2012]

I understand that single-phase ACA sirens have caps to help the motors start and run, but what makes their motors different enough from Federal's single phase motors that ACA needed capacitors and Federal didn't? I mean, what would happen if you ran an ACA siren with no caps in the circuit?

[holler]

First off, a single phase power will NOT start a motor turning. A single phase motor always has to have some sort of start mechanism, whether it be repulsion start, shaded pole, split phase, capacitor start, etc.

Single phase federal motors are repulsion start, they use the repulsion force to start the motor. They have two brushes shorted together and an armature. Voltage is induced into the armature by the field windings in the field and the magnetic "repulsion" of the two brushes (poles) shorted together makes the motor start spinning. They are known for BRUTAL startup torque and low current draw during startup. But power and efficiency drop off sharply as the motor gets closer to it's rated RPM. This is the flaw inherent in single phase power, it sucks for motors.

ACA motors are capacitor start/capacitor run. They have a start winding and a run winding. The start winding is a high resistance winding that can produce great torque for a very short amount of time. The start capacitors provide enough of a phase shift to get the motor turning. Once it's almost up to full speed the start caps drop out and the run capacitors take over. The run caps improve the power factor and running efficiency of the motor by altering the phase angles. Without them the motor would run very hot and have very little power. Cap start/run motors are generally more efficient than repulsion induction motors at the rated RPM but lack the brutal breakaway torque of a rep. induction. They also consume a HUGE amount of power during startup. I often compare locked rotor current of a large single phae motor to a bottomless pit.

Basically, if you take away the start capacitors, it won't start. If you take away the run capacitors it won't have any power and will overheat. If the start capacitors don't drop out of the circuit after the motor hits full speed you'll let the smoke out of both the motor and capacitors.

[Andys Live WX]

Does the single phase motor's brushes lift off the commutator once its running almost full speed? That would save the brush life greatly.

[FSThunderboltfan1000]

No they do not.

[holler]

On some motors they do, but not on siren motors. Brush life is greatly reduced by excess arcing and high loads.