Large NEMA device will not remain energized at starting
New, just installed large NEMA devices energize and immediately drop out even though proper control voltage remains on the coil
NEMA Classes 85##, 86##, 87## and 89##
Sizes 5, 6 and 7 or equivalent size lighting contactors
Incoming line power has insufficient regulation or capacity to prevent a voltage sag during load starting surge. The NEMA size 5, 6 and 7 share a common circuit design called an anti-telegraphing circuit. On the size 5, the circuitry is now housed within the coil housing. The older design of the 6 and 7 uses an external coil control module, see the attachments for details.
The typical sequence of events is that nominal voltage is present for operation. When the contactor is energized, the starting current for the load causes the voltage to sag below a threshold. This sag causes the contactor to release, removing the cause of the sag and normal voltage returns. The issue is that this sequence can be quick enough to trigger the anti-telegraph circuit. This circuit will then lock out the contactor to prevent chattering or telegraphing from potentially damaging the contactor and motor. When this occurs, coil voltage will remain present on the terminals of the size 5 or the terminals on the control module on the 6 and 7.
With the class 8606 autotransformer starters, this pick-up and drop-out may only occur when the Run starter engages. The starter may work fine up until the transition to Run occurs. This is due to the nature of an autotransformer starter. The autotransformer reduces the line current during the Start phase. At the point of transition, full line current is drawn. See the Attachment " Theory of Operation - Autotransformer Starter" for more details.
A sure method of determining if this is the cause is to remove the load from the contactor or starter. If it then energizes normally without the load but picks up and immediately drops out with the load connected, a voltage sag is occurring and causing the anti-telegraph circuit to engage.
NOTE: Size 6 and 7 devices that have been installed and operating for some time may show a similar symptom with the exception that they will not remain energized even with the load disconnected. In this case the first step to return the device to proper function is to replace ( not just check) all the control circuit fuses.
The stability of the voltage being supplied to the coils must be improved. There are no alternate coils or device enhancements that will provide a better ride-thru of the voltage sag. Suggestions include checking the security of all the electrical connections ahead of the device, improving the mains service, supplying the coils from an independent supply and providing a UPS for the control circuit.
In the case of the 8606 Autotransformer starter another option may help. As the voltage sag occurs at the time of transition, adjusting this time further out will reduce the current spike and resulting voltage dip. However, the proper timing for this transition is more dependent on the motor being able to correctly start the load but if this transition occurs too early, the current spike will be greater.