Motor Trips During Voltage Sags 1

[sheefja]

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

 

[gordonl]

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)

 

[buzzp]

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.

 

[busbar]

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.

 

[jbartos]

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.

 

[Electrifier]

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.

 

[electricpete]

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.