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  #1  
Old 05-19-2005, 11:11 AM
Iodem_Asakura Iodem_Asakura is offline
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Hi everybody.

Can someone explain me what is the function of the Start and Run Capacitor used for motors?
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  #2  
Old 05-20-2005, 03:44 AM
Erin G. Erin G. is offline
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Quote:
Originally posted by Iodem_Asakura@May 19 2005, 06:11 AM
Hi everybody.

Can someone explain me what is the function of the Start and Run Capacitor used for motors?
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.

The start cap provides that electrical "push" to get the motor rotation started. It does this by creating a current to voltage lag in the seperate start windings of the motor. Since this current builds up slower, the armature has time to react to the rotating field as it builds up, and to begin rotating with the field. Once the motor is very close to it's rated speed, a centrifugal switch disconnects the start cap and start windings from the circuit. Watching a single phase motor starting you can see that this all happens very quickly.

Without a start cap (such as when one burns up) when the voltage is applied, the motor will just sit and hum. But if you were to grab the shaft and give it a spin, the motor would (usually) start and run normally.

hope this helps,...
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Old 05-20-2005, 10:55 AM
Iodem_Asakura Iodem_Asakura is offline
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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?
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  #4  
Old 05-21-2005, 03:03 PM
Erin G. Erin G. is offline
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Quote:
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?
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.
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  #5  
Old 07-23-2005, 07:45 PM
AlanM AlanM is offline
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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 didnít 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.
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  #6  
Old 07-23-2005, 07:51 PM
AlanM AlanM is offline
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Quote:
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 didnít 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.
Sorry: Should have said that it is a cap-run setup.
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  #7  
Old 07-27-2005, 02:24 AM
Erin G. Erin G. is offline
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Quote:
Originally posted by AlanM@Jul 23 2005, 02:51 PM
Sorry: Should have said that it is a cap-run setup.
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.

Chances are that your motor is starting slower than it used to, but the change would be so small, you will not notice without an o-scope. There is a small chance that the electric motor may have a reduction in it's life-span, but it's probably negligible. Nothing will come of this that has an effect on the pump itself.

The capacitor in an AC circuit is constantly charging and discharching, creating the electrical effect of another phase by intrucing another lag in current / voltage. Capacitor size is is based on the amount of charge difference needed to start or run a motor within the motor's rated capacity. It's always best to go with the manufacturer's recommended values for voltage, and as close as possible to the uF rating.
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  #8  
Old 07-30-2005, 05:13 PM
AlanM AlanM is offline
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Thanks Erin,
I've been out of town for a few days; just got back.

I'll leave the pump as it is. It seems as though there will not be excessive overheating, or any other sinister effects. If the starting is any slower, I can't hear it.

Thanks for your time,
Alan
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  #9  
Old 06-26-2008, 10:48 PM
kbradley kbradley is offline
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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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  #10  
Old 06-26-2008, 10:59 PM
jpanhalt jpanhalt is offline
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Here's (http://en.wikipedia.org/wiki/AC_motor ) a good source and description of AC motors. AAC also has a very good section on them: http://www.allaboutcircuits.com/vol_2/chpt_13/1.html

John
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