Most smaller, single phase motors usually have a permanent magnet armature that is pushed / pulled around by the rotating inductive field produced by the stator (outside) windings. The inductive field rotates simply as a result of the positive / negative alternations of the 60HZ AC current flowing through the windings. The problem is that when the voltage is applied, the 60HZ is applied immediately, the rotation of the field through the windings begins immediately, and the armature has no chance to react (or catch up, as it were) to the field.Originally posted by Iodem_Asakura@May 19 2005, 06:11 AM
Can someone explain me what is the function of the Start and Run Capacitor used for motors?
The run cap and aux. windings never drop out of the circuit in a cap-start, cap-run motor. The current to voltage lag is always present, which makes the motor act like a two phase motor. The advantage of a cap-start, cap-run motor over a cap-start, induction-run motor is that cap-run motors operate at a higher power factor than induction-run. Single cap-start / run motors use only one cap and are for lower torque applications. Two value cap-start / run motors use two caps, a higer value cap for starting, and lower value for running. The centrifugal switch switches from the high cap to the low cap, but the aux windings never leave the circuit. Two-cap motors are for higher torque applications.Originally posted by Iodem_Asakura@May 20 2005, 05:55 AM
Wow that was a really good explanation. Thanks man.
But there's something you forgot or maybe you didn't noticed it. I asked about the run capacitor too. I don't understand what is its function. I don't know if that is the correct name, but is the capacitor used when the motor is on march. Maybe the correct name is march capacitor..... Well, i don't know how it works.
Do you have some information about it?
Sorry: Should have said that it is a cap-run setup.Originally posted by AlanM@Jul 23 2005, 09:45 PM
I'd like to extend this topic, if there's anyone still following it.
I have a 350 Watt (230 V, 50 Hz) pump with a motor which would not start. As you say, I could start it by flicking the impeller. The 8 uF capacitor had failed. I didnt have an 8 uF available, so used a 5 uF instead.
What effect will this have on the motor and pump ?
How is the capacitor size chosen ?
Thanks for any advice.
In a purely capacitive circuit the current will lead the voltage by 90 degrees. In a purely inductive circuit the current will lag the voltage by 90 degrees. Of course, there is always some resistance in any circuit, so whether inductive or capacitive, the lead / lag relationship between current and voltage will be somewhere between 0 and 90 degrees. I say all that to say this; the net effect of the difference in the capacitor is that it will change the angle of lead / lag between voltage and current in your motor.Originally posted by AlanM@Jul 23 2005, 02:51 PM
Sorry: Should have said that it is a cap-run setup.
Yes. The rotation direction of the magnetic field, and thus the motor rotation direction, is determined by which winding has the capacitor.Don't capacitors also determine the direction of rotation in a single-phase AC motor? I know that usually when we have one go bad, when the motor just sits and hums, you can start it rotating in either direction.
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