×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Effects of Too Much Capacitance (Leading Power Factor) on a Motor

Effects of Too Much Capacitance (Leading Power Factor) on a Motor

Effects of Too Much Capacitance (Leading Power Factor) on a Motor

(OP)
If the power factor correction capacitors are oversized for a 50 hp motor causing the power factor at the motor to be .85 or .9 leading, what would be the effect on the motor itself if any?

RE: Effects of Too Much Capacitance (Leading Power Factor) on a Motor

Hello vobprecis

There will be no effect on the motor provided the capacitors are never connected to the motor during the run down period.

If the capacitors remain connected to the motor after the supply is removed, (main contactor is opened) there will be major problems.

When a motor is disconnected from the supply (switched OFF), it will act as a generator for a period of time. The frequency of the voltage generated is speed dependent, so it reduces as the motor slows.
If a motor is under corrected (as it should be) there is a resonant frequency between the inductance of the motor and the capacitors which is above line frequency. A slowing motor will not pass through resonance. If the motor is critically corrected, (corrected to 1.0) then the moment that the contactor is opened, the motor is at resonance with the capacitors and it will generate a very high voltage potentially causing capacitor failure and insulation breakdown. There can be major mechanical damage as well due to the torque transients associated with the insulation breakdown.
If the motor is over corrected, the motor will pass through resonance as it slows and can cause major damage.
So the problem is not while the motor is running, rather it is when the motor is shut down.

In order to prevent such problems, you should only correct to 80% of the magnetising current of the motor, or use a separate contactor to control the capacitors such that it must disconnect the capacitors when the motor is disconnected from the supply.

When the motor is running, over correction may increase the voltage on the motor terminals but should not cause issues other than supply resonance issues, but these are not related to over correction.
see http://www.lmphotonics.com/pwrfact.htm

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Effects of Too Much Capacitance (Leading Power Factor) on a Motor

(OP)
Mark, Appreciate the response. Makes sense and was helpful.

Vince

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources