24vac control circuit for a motor starter

[minionsboy]

I got a 24vac circuit with some start/stops on it that are suppose to pull in a 3phase 480v motor.

Was wondering why the voltage to ground measures at 13volts after I push and hold in the start button. Measures 24vac on one side of start button before but after once the circuit closes measures 13 volts, and so does the stop bitton since tje corcuit is closed all the way. My problem is the starter ain't pulling in. Will operate if i manually pish it in, but i have to hold in.

 

[augie47]

Welcome...
1st thought is that your 24 v source does not have the needed capacity.
What is the 24v source and how many items (loads) are trying to start ?

 

[Barndog]

How far is this control circuit and what size wire are you using. we had a lighting contactor do this once. we put an Ice cube relay in the contactor box and had the push buttons control the relay that controled the contactor. 13 volts is not enough to pull in a 24VAC starter.

 

[dicklaxt]

You are dropping minimun voltage across the meter so you are seeing the source voltage but when you close the start button the coil wattage is pulling the circuit voltage down because of voltage drop..you are dropping nearly 50% amd the coils typical will drop out at 60 to 65%(some even higher) and as Barndog states 13VAC is not enough to pick that coil up.I don't have a clue as to why it picks up when you hold it in tho

dick

 

[minionsboy]

they pulled in 18gauge wire for the circuit.

It is trying to start just one 3phase motor and there are about 5 start stop buttons on the circuit at various locations. Checked all the wiring and all the starts and stops are wired correctly. Didn't change the original wiring at all since the system was installed 12 years ago and has been working ever since.

I'm thinking the control transformer has gone bad.

 

[Cow]

What size is the control transformer?

What's the coil VA(inrush/holding) for the motor starter? If that's not known, what kind and size of starter is it?

 

[Barndog]

Is that Transformer just used to pull in that one starter or is there multiple starters using the trans. Was there anything added on the system that would cause the transformer to be over its rating like Augie said in post 2. and when you saying manual pull in are you pushing in the contacts on the starter its self or jumpering out the start switch.

 

[gar]

101019-1156 EDT

minionsboy:

You have provided insufficient information and not clearly defined your circuit.

At the output terminals of the transformer what is the voltage between the terminals with no load, and then when you apply power to the contactor coil? Do not use ground as a reference point.

By implication this terminal voltage is 24 V before you push the start button, but you made your voltage measurement to ground. If the transformer secondary terminals voltage is much different than 24 V after you press the start button, then you are overloading the transformer, or a transformer problem, or you need to look for a substantial voltage change at the transformer primary terminals.

Assume the transformer secondary voltage is relatively stable. Meaning maybe above 20 V when you press the start button. This would mean there is a high impedance in the circuit from the transformer to the contactor coil. To find where the problem is connect a long wire to the common side of the transformer at the transformer terminal. The common terminal will be the one with the least number of switches or contacts in series between the transformer and the contactor coil. This long wire is now to be one of your voltmeter test leads. Use the other meter test lead to check various points in the circuit between the transformer and the contactor coil to look for one or more high impedance points.

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