Electrical Question - 24V Transformer - Dust Collector Control Stations

[Rob Will]

Today, we ran low voltage wire about the shop for DC control stations. The magnetic starter for the 7.5 hp 3 ph blower motor is equipped with a 24V AC coil.

I bought a small 24V transformer at the electric supply house. Unfortunately, when we try to energize the starter's coil, the contactor just hums and fails to close. At first we suspected voltage drop in our control station wiring..... but the coil fails to close even if we bypass all that and hook it up direct. In a no load situation, the transformer shows the correct 24V. When exposed to the load of the starter coil, the transformer will only hold 14V. If we manually engage the contactor, the transformer apparently has enough voltage to "run hold", it just does not have enough "umphhh" to close the contacts on its own.

The starter is a Cutler Hammer in a NEMA size 1. We checked the coil and it is correct (24V AC). Anybody had experience with a transformer that could not carry a motor starter? I can get a better transformer Monday at the motor shop but any other opinions are welcome.

Rob

[Kyle Kraft]

Here is a thought, the inrush current to a coil/solenoid is usually much higher than the current required to hold it in. Perhaps the transformer you are using can't put out enough current to feed the required inrush. On the machinery I work with, most coils are 24v DC. There may also be a factor of the AC vs. DC thing. Is is possible that a low DC voltage can push more current than an AC source at the same voltage?

I guess the bottom line is that your 24 volt transformer may be undersized for the starter's coil.

[Bill Brady]

I would agree, the transformer is undersized. Is this just a doorbell transformer? You will need to pickup a general purpose control transformer. The size willl need to be 50 or 100 VA, talk to the people at your electrical supplier.

[Matt Meiser]

I've found FactoryMation to be a good inexpensive source for this kind of stuff if you find you are suprised at the price from your electrical supplier.

http://www.factorymation.com/s.nl/sc.2/category.1295/.f

[Ken Garlock]

Given that you know the name and model of the starter switch, get in touch with the manufacturer and find out what the coil current requirement is for the relay. Then find a transformer that is rated at 150% of the relay coil.