purpose of capacitor

[jgb123]

looks like i opened up a can of worms. i appreciate all the information though.

[fat eddy]

We would all like to see the question answered by an electrical engineer before putting it to rest.

[framehvac]

Were is NormChris I have not seen him but he could tell us.

[fat eddy]

I asked for him earlier in the post he seems to be pretty detailed with that stuff, I'll take his explanation.

[bornriding]

Amswered your quetsion thouroughly on previous post
been electrician for 30+ years
Now I teach it, you can believe me on this one

[James 3528]

We normally keep all the junk on one thread around here.

[a/c-harris]

gives the motor a three phase effect. takes start winding out of phase with run and common.

[robbie]

http://www.engin.umich.edu/labs/csdl...ase/index.html

Here is a web site explaining the differant types of electric motor's and how they are started. To bad it want come out as a link

It did come out as a link even better

[resolutetech]

Ok the run capacitor acts as a limit on the current entering the start winding. This is why it there.

If you connect the start winding across the 240 volts with no capacitor the winding will burn up due to excess current.

Yes there is phase change due to the capacitor, which aids starting, but the current flowing in the start winding is also limited by the capacitor.

Here is why, inside the capacitor are two plates of aluminum foil separated by an insulating material. There is oil also to help dissipate heat from the capacitor charging and discharging. Not motor heat.

Ok here is what happens as the motor runs. Current flows through the start windings of the motor (here is the capacitor next to it (in series with it) so the same current flows through it). The current builds on the capacitors' plates and charges it. As it charges(electrons build on each plate) its voltage increases, and matches the line voltage. It can't hold any more charge on the aluminum plates because its voltage matches line voltage. So it limits the current in the start winding in this way.

When the capacitor has the same voltage as line no current will flow and the windings don’t burn.

Now don’t forget current direction changes in alternating current and so does voltage. With a reversal in voltage, the charge (current) on the plates can now run the opposite diretion through the start winding.

The voltage changes direction and so does the current. Well now the charge built on the capacitor flows through the start windings in the opposite direction till the new and opposite voltage builds on the plates and again limits current.


Here is why matching capacitance is important.

If you install too large a capacitor more current flows through start windings because the aluminum plates are larger and hold more charge. There can be enough excess current to burn the windings.

If the capacotor is too small not enough current will flow through the start winding and it will contribute reduced power to the motor.

The start capacitor is there allowing extra current(stored on its plates) for start up and then quickly removed from the circuit before the start windings burn from the extra current stored on the start capacitors plates.

So if you think about it, it is in parallel with the run capacitor so it adds capatance to the run capacitor and hence more current to the start winding for the short starting period and then its removed leaving only the capacitance of the run cap.

[Edited by resolutetech on 02-27-2005 at 06:51 PM]

[bornriding]

Please explain why a RSIR motor does not use a capacitor. The winding does not burn up.
I can't believe what I read here sometimes!!

I have done many service calls where, either, the compressor run cap or the fan run cap is bad, but the motor is not burned up. Replace the cap and the motor works fine. Please explain ??

[resolutetech]

the winding is not burned up because if the capacitor is fried no current flows throught the winding.

[resolutetech]

What is a RSIR motor?>

START WINDINGS BURN IF 240 VOLTS REMAIN ACROSS THEM !

[DJC]

Hey guys, The Capacitor can have two different uses depending on the application. When used in the motor start circuit, the cap is used to provide a phase change to the incoming power. This increases the motor's starting torque by providing a stronger rotating magnetic field than a split phase motor. The start cap can have the effect of reducing current in the start winding, but if that were the only purpose, a higher resistance winding would accomplish the same thing.

A run cap is used primarily for power factor correction. An electric motor is an extremely inductive load that can cause the change in current to lag behind the change in voltage as the power line voltage alternates. Adding the capacitor neutralizes the inductance and puts the power back in phase. This results in a cleaner power line.

DJC