×
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

Abnormal PSC Motor Current Draw?

Abnormal PSC Motor Current Draw?

Abnormal PSC Motor Current Draw?

(OP)
We use 3/4 horsepower PSC motors operating at 120VAC. Normal current draw is 8 amps. Locked rotor current is 13 amps. We have a circuit that monitors the motor current and shuts off AC power if it exceeds 12 amps for a fraction of a second. The PSC capacitor is 90uF.

With an average frequency of once every year or two our motor controls are getting fried by motor current. We think a rare and short term spike of motor current is killing our drive triacs. (Our drive electronics use triacs so that we can select motor shaft rotation direction under software control.)

I have tested these our electronic controls extensively. They are bulletproof for 15 amps continuous current and 50+ amps of short term current. The over current detection circuit works reliably, but takes tens of AC line cycles to respond and shut off current. We have protections in software that prevent reversing motor drive from clockwise to counter-clockwise while the shaft is spinning.

Question to readers: Can a PSC motor's stator core get saturated and/or left with residual magnetic field when the motor turns off? Would this residual field in the core cause the motor to draw super-large current during start?

Another observation: Very occasionally the motor's load causes the shaft to stop spinning for about 10 AC line cycles, thereafter releasing it to spin freely with little or no load. When this occurs, it is common for the motor shaft to "rebound" from lock rotor with spin opposite to the applied AC drive current. Helping the spin reversal phenomenon is that the mechanism that causes brief locked rotor has some spring wind up to it. When the lock rotor scenario ends, the load spring has a tendency to spin the motor backwards for a fraction of a second. The dynamics of a PSC motor are able to sustain backward spin if there isn't much load on the motor.

Does this description (possible core saturation or locked rotor plus reversed spin) hint at a reason for brief high motor current that exceeds locked rotor current?

Thanks in advance for any comments you might offer on this.

RE: Abnormal PSC Motor Current Draw?

In my experience, solid state control and single phase motors with capacitors are incompatible. To a triac, the charging current of a motor the capacitor looks like a short circuit. In a PSC motor this is dampened by the fact that the cap is in series with the winding, so of the single phase motors with capacitors it is the “least unfriendly” to electronic control, but still not ideal and it’s usually a race to see what fails first; the controller or the motor. Under the right conditions however, such as the phase angle at the moment the triac fires for the first time or in your case maybe a reverse command while the motor is still spinning, the dI/dt seen by the triac as the capacitor charges can be high enough to cause it to short.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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! Already a Member? Login


Resources

Research Report - How Engineers are Using Remote Access
Remote access enables engineers to work from anywhere provided they have an internet connection. We surveyed our audience of engineers, designers and product managers to learn how they use remote access within their organizations. We wanted to know which industries have adopted remote access, which software they are using, and what features matter most. Download Now
eBook - Managing the Context of Product Complexity Using the Digital Twin
Keeping track of changes to complex products is difficult—think Aerospace & Defense equipment, new generations of commercial aircraft, and software-based automobiles. A new way to managing the digital context of the physical product is required and the answer is the Digital Twin. This ebook explores the opportunity available for Operations and Maintenance for the Digital Twin. Download Now
White Paper - Trends in Industrial Filtration
Substantial progress has been made in filtration technologies in recent years. New filter media materials, designs and processes have led to filters that are more efficient, reliable, compact and longer lasting. This white paper will discuss the various trends that are impacting operational responsibilities of MROs today and the resources that are available for staying up-to-date on the latest filtration solutions. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close