Single-Phase Induction Motor Question

Thread Starter

Do0gra

Joined Jul 30, 2008
3
Hi all. I have a question when i connect a capacitor (7.96uF) across the power supply (230V 50Hz) and before the ammeter into the motor, the ammeter reads 2A. But when i change the capacitor's place parallel to the primary winding and after the ammeter. It read 1.5A. Can anyone tell me why? Thanks.
 

thingmaker3

Joined May 16, 2005
5,084
Try looking at it less from the perspective of the capacitor and more from the perspective of the meter. In one instance, you put the meter in the tank, measuring tank current in addition to circuit current. In the other instance, you put the meter outside the tank, and measured only circuit current.
 

mik3

Joined Feb 4, 2008
4,843
Hi all. I have a question when i connect a capacitor (7.96uF) across the power supply (230V 50Hz) and before the ammeter into the motor, the ammeter reads 2A. But when i change the capacitor's place parallel to the primary winding and after the ammeter. It read 1.5A. Can anyone tell me why? Thanks.
The capacitor is used to correct the power factor of the motor. When you put a capacitor to correct the power factor essentially you reduce the current drawn by the supply but the total current supplied to the motor is the same as the current before you use the capacitor. In your case, the motor needs 2 amps to work. When you measure the 2 amps current you measure the current without power factor correction (the current the motor needs to work, is the same if you remove the capacitor and measure the current, it will be the same). When you measure the 1.5 amps you measure the current with the power factor corrected (the another 0.5 amp is supplied by the capacitor).
 

Thread Starter

Do0gra

Joined Jul 30, 2008
3
Oh i get it now. That means the capacitor supplies current as well to the motor(depending on the reactance) and it is working at 2A not 1.5A. Thanks!
 

mik3

Joined Feb 4, 2008
4,843
Yes you are right. The motor works at 2 Amps but you draw only 1.5 Amps from the main's supply, so you pay less and the electrical company is happier because they need to install lighter wires for the power distribution. Thats the reason they will penalise factories with bad power factors.
 
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