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DC Motor Starting and Protection

DC Motor Starting and Protection

DC Motor Starting and Protection

(OP)
We've seen a recent DC motor failure on a large hydro turbine shutoff valve actuator. The DC motor is 125VDC, 6 Hp, long-shunt compounded. The controls for it use a simple reversing contactor to reverse the armature connections. After the failure we found a few issues with wires sizes but the root failure cause is unknown. Our suspicion is that the actuator torque limit stopped the motor early in travel and it went through several stop/starts as the external PLC kept starting it back up to get the status indication. That is what we think caused the failure but there are two other items that contributed to it. First, the motor is started directly without a series resistance. We're waiting for the motor to return from the rewind shop to measure armature and series winding resistance but we fully expect starting current is very high, especially when it's being started against the valve stop. Second, there is no separate thermal protection for the motor. The upstream thermal/mag breaker is the only protection. Thus, we have two questions:

1. Is it common to start DC motors without a starting resistance? All DC motor starters we've designed of similar size have a starting resistor.

2. The NEC allows the use of the thermal/mag breaker with no separate thermal protection if the design is such that it naturally limits motor operating time. Since this is a valve actuator, one can say it is naturally limited to the time it takes to travel open or closed (granted, one could continuously open and close the valve but that would not be expected). As such, the NEC allows the breaker to be 150% of the FLA and can be the next size up where 150% doesn't correspond to a standard rating. The motor FLA is about 30A and the breaker is 50A. This appears to meet code, though a separate overload would have done a much better job of protecting the motor. For DC motor actuators, is it typical to not provide separate thermal protection?

As always, we appreciate the experience you guys share. Thanks!

Marty

RE: DC Motor Starting and Protection

There used to be DC motors that were designed to start DOL with no resistors. They were common on old ships(weak DC source) but I haven't seen such a motor for a very long time.

Put a standard starting resistor in the circuit and install a thermal protection no need to discuss such an obvious improvement. There may also be something done in the PLC program. Like outputting an alarm or a warning if it starts cycling or doesn't reach end position in a reasonable time.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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