Contactor engaging and disengaging like a slow heartbeat

[dmc1701a]

I have an exhaust fan starter that I'm having troubles with not staying engaged, rather it cuts in and out like a slow heartbeat. with this EF there is a motorized backdraft damper that is operated by a 24v actuator . To get this 24volts I have installed a 480 to 24v 40VA transformer which is tapped off the 480v's from the line side of the contactor same as the 120v control transformer that came with the starter. on the damper actuator there is an end switch that will make once damper turns to at least 80 percent open(proof that EF will be sucking air). attached is a rough sketch of what I have wired. any thoughts on what might be the cause?

 

[George Stolz]

I'm not giving your diagrams full review, but it sounds like you're connected to the NC of the damper aux contacts, not NO. That would produce the symptom you describe.

 

[hurk27]

Or the 40va transformer is not large enough to power the damper actuator and the voltage drop off every time the damper goes full open and RIB 2 energizes thus damper closes and the cycle starts over again.

 

[gar]

140827-0814 EDT

dmc1701a:

Why ask us to conjecture what is happening when you should use your voltmeter to investigate the circuit operation? You might answer your own question. I can not really read your diagram. What is the label on the coil "Rib relay coil#" (that is what I think it is labeled). Is that relay # 1?

Why are you switching the neutral? Generally troubleshooting is much easier if one side of all coils are tied to common (neutral, ground).

Does this erroneous operation occur in hand? I will assume it does. Does "Rib relay #" remain energized?

Provide more information by making measurements to see how the circuit is actually operating.

.

 

[Jraef]

Or the 40va transformer is not large enough to power the damper actuator and the voltage drop off every time the damper goes full open and RIB 2 energizes thus damper closes and the cycle starts over again.

That sounds the most plausible to me too. That would explain the "heartbeat" like repetition. Most of the time those damper motors are spring return. So motor energizes, but is taxing that 40VA transformer to the edge of its capacity. Then when the limit switch is hit, RIB2 coil is energized, it overloads the transformer, voltage collapses and the damper turns off, backs off because of the spring, drops out the limit switch and thus RIB2 coil, so transformer is once again capable of running the damper, it opens again, trips the switch, energizes RIB2 again, repeat ad nauseum.