×
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

Motor Trips During Voltage Sags

Motor Trips During Voltage Sags

Motor Trips During Voltage Sags

(OP)
We have a problem of motors trips during voltage sags although their contactors are equipped with ride through circuit or the control supply is fed from constant voltage transformers.  My question is "Has anyone experienced the same problem? Also, if yes, is there any way to test the time contactor holds?  The sag was of short duration (4 cycles) yet some motors tipped.

Your help will be appreciated.

Jaafar Al Sheef
Saudi Aramco
Saudi Arabia

RE: Motor Trips During Voltage Sags

You have to watch your overloads as well as the contactor control.  Some electronic overloads have internal power supplies which can dump the contactor during a sag.

To test the drop out time of the contactor you'll need a high speed recorder, watching the voltage and the contact parting time.  (Likely main contacts isolated for test equipement)

RE: Motor Trips During Voltage Sags

I agree with Gordon. I assume the OL didn't trip the motor in that short of time? As Gordon suggested, any electrical device that has the ability to shut the motor off could be the culprit. Most mfg's will have data showing the ride through time of each motor control device. If not, you will have to determine this by generating your own code within a microcontroller or buy something off the shelf. If it is the contactor, this info should be available from the mfg.

RE: Motor Trips During Voltage Sags

For the problem motors, can you reduce their mechanical load?  What is the motor Hp?  Can you detail the overload relay specs?  In the sags, does voltage AND frequency decay?  

Can you determine if the sag from transformer or cable impedance? Your only solution may be a stiffer circuit.  

RE: Motor Trips During Voltage Sags

Suggestion: There are two possible solutions:
1. To find the cause of voltage sags and eliminate them.
2. If 1. is not possible, then an alternate power supply will be needed, perhaps working in conjunction with a static transfer switch that can transfer the preferred power supply to the alternate power supply within a quarter of the cycle. Also, the DC controls could get some consideration.

RE: Motor Trips During Voltage Sags

Thermal overload devices are not able to respond so quickly(4 cycles). It seems that there is some problem with control supply. Contactors are dropping out at voltage sag, if voltage sag is less than drop out value of contactor, even for one cycle, contactor will drop out. Normal these type of critical controls are fed by UPS to avoid interruptions.

RE: Motor Trips During Voltage Sags

Good point electrifier. Are you sure that the trips were in fact directly tied to the sags. Is it possible that you simply have a loose connection. Ihave seen undervoltage relays which seem prone to dropping out on vibration due to presumed loose connection.

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