Voltage Drop on Motor Circuits

Yeah I use battery backups for these momentary voltage drops...cheap, reliable, surge protection....

The coil of your contactor actually keeps the contacts closed(?) due to the magnitude of the current flowing through the coil and not the voltage applied to the coil. (Ampere and Biot-Savart Law) If you cannot get your power provider to correct this frequent but brief under power problem, the elegant solution would be to add a snubber like network to these coils that maintained this current during these brief brownout conditions.

To design such a network one must know many parameters of the contactor, motors and the overall system. Some of these parameters will probably have to be empirically discovered by testing, like the contactor coil induction, resistance, pull-in and drop-out current along with delay times. In short, this is possible but probably not practical.

A capacitor would help with a DC control circuit. With an AC control circuit, the supply which is falling to 80%, will just pull down any capacitor voltage.

I assume that your starters bypass the start button with an auxiliary "hold-in" contact of the 3 phase power contactor when they close. Once that auxiliary contact opens during a brown-out it "cuts the throat" of the control circuit and you are off.

Maybe a slow-release (>0.2 second) auxiliary relay following the contactor auxiliary would work.

But the necessary wiring/relay seems complicated compared to changing to a low drop-out volt contactor. Is it possible to swap the contactor coil rather than whole contactor?

Thank you, I will replace these contactors with the latest contactors and overloads, expensive but worth it.

With some experimenting you can find out how much DC current (voltage= IR) required to pick up the coil. Compare that with the AC rms to the coil. Rectify the ac and use a dropping resistor to limit the DC current. With a free wheeling diode or a capacitor you can time the drop out.

If you want a fast pickup apply full voltage rectified through a NO contact shorting a "holding" resistor - as the contactor pulls in it automatically limits the current to the holding level.

If the drop out time is too long, then experiment with additional components to absorb the discharge current.

Be aware that breaking dc current in an un-suppressed inductor gives a bit of a voltage spike.

Immediate restart function is available in the modern numerical relays available for LV motors from many of the LV MCC manufacturers.

These are cheap and can ensure short duration voltage drops don't affect motor running.

They also have delayed restart function in case of critical duty motors to restart automatically on restoration of supply beyond 200ms.

Alternatively, simple restart relays are also available in the market that can be connected in the motor control circuit to serve the same purpose.

In olden days, we used to employ pneumatic timers (Off-delay) in the circuit to hold the contact in closing circuit for a defined period after power loss so that the subject critical duty motor can automatically restart.

I was looking at electronic off-delay timer, for an automatic restart but wasn't sure if we have a dozen 200KW motors starting at the same time produce a high inrush current if the delay was less than 200ms.

One might reasonably expect <...brown outs...> on the motor power cables of the order of <...200ms...> simultaneously as well, and these must be considered in the context of whatever the motors are driving to determine the potential impact on the process.

There is nothing to stop a several-pronged approach and commercial threats to the utility supply organisation need to be considered in addition to the technical ones within the operation.

There are devices that you can put on critical coil circuits, generically called PORT devices (Power Loss Ride Through).

Here's an example:

http://pqsi.com/