ac motors

Hello
Can someone explain to me what a capacitor is used for on an ac motor?

And if it were blowen how it would it effect the motor.

Thanks

On single phase AC motors, usually 10HP and less, the capacitor assists with the starting of the motor.

If it goes bad the motor usually doesn't start and trips its C.B. or fuse after a short while.

Sometimes with small motors you can get it going by spinning it by hand, then the motor will take over. This is obviously dangerous in many circumstances so be careful.

Thanks I'll have to give it a try. Any ideas why the capacitor would melt? Is the capacitor there to soften the start up?

The cap is to fake a phase untill you get to speed. Your motor load is high (bad bearing, high load). It will also melt if the cap voltage rating is lower than the one applied.

The capacitor does not soften the motor start up. Put simply, on a single pole motor the capacitor causes one of the poles to lag slightly when the motor is first started. This determines the direction that the motor will rotate. Without it the direction the motor turns would be random. Once the motor has started turning, a centrifugal switch will remove the capacitor from the motor winding circuit. Some smaller sinlge pole motors will have a shaded pole instead of a capacitor - this is usually a copper bar that shorts a portion of the laminated core. This is called, naturally, a shaded pole motor. Shaded pole motors are cheaper to manufacture, but are less efficient and weaker than a capcitor start motor.

Sometimes the capacitors just go bad, but I recommend that you check the operation of the centrifugal switch.

On three phase motors (you didn't specify) capacitors are used to improve the power factor of the motor.

If on a single phase motor; if the capacitor was a "start" capacitor and of the incorrect voltage, say 250v rather than 400v - would that melt it?

I will have to refresh on my theory notes from years back!

Mark provides some good information here: http://www.lmphotonics.com/single_phase_m.htm

If you overvoltage an Electrolytic capacitor, it could actually blow up. Paper o ceramic capacitors could simply short out, which would cause them to pop, too.

Tom i think he meant a cap not caps

Any ideas why the capacitor would melt?

Click to expand...

If the start switch inside the motor doesn`t open it will blow
your start capacitor up. Most single phase motors have a rotating
centrifical switch inside the motor. This switch is called the governor and as the motor comes up to speed this governor opens a set of contacts on the stationary start switch in your motor. This opens the circuit to your start winding and just leaves the run winding on. If this doesn`t happen all the smoke will come out of your capacitor before you can say heck. This is can be checked by taking your motor apart and inspecting the parts. With the motor apart the points on your stationary switch should be open then make sure your governor will move to open and close the points on this switch.

Many of the replies are right and wrong at the same time. I am writing a reply with pictures but it will take some time. My link above is not available at this time but Pego's is and has relevant information.

I have never heard of a centrifugal switch being called a governor, a centrifugal switch just breaks a circuit when a motor reaches approximately 75% of rated speed. NOTE that when the motor slows down the switch re-engages to prevent burnouts etc. It should also be noted that not all capacitor start motors use a centrifugal switch, PSC (permanent split capacitor) motors do not. A split phase motor (without a capacitor) will use a centrifugal switch.

Alaric starts out ok but faded on the stretch, wiring connections determine the rotation (technically) with the capacitor providing torque (for start or run..depending) at lower current inputs. I assume he was rushed because his posts usually supplies much more detail.

The basic concept creates a 2 phase motor with an "emulated" second phase provided by a capacitor, inductor and possibly a switch in parallel to the run winding. Without this parallel circuity an AC motor on start up could rotate either direction at random. The capacitor as you know builds up to supply voltage, that means on every negative cycle the capacitor will discharge...ie current will flow and a stronger magnetic field will be retained...this in turn develops more torque....this could be used for starting or running.

There are at least 4 types of capacitor start and/or run single phase motors.

AC Motors call for Capacitors to assist them with the running process... Stepper Motors have proven to be highly advanced and can overexceed in performance matters of various other type of motors.