Motor Control Circuit Voltage

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Nardie

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Recently I had a motor starter coil fail due to age, this starter has been in service since probably the 1950's. I contribute this failure due to age, however when performing an inspection of the cubicle I noticed that the control circuit voltage was 480 VAC. This is something that I am not use to, I am typically use to a motor control circuit voltage to be 120 VAC due to a control transformer. From reading the NEC it doesn't address the control voltage level, whether it be 480 VAC or 120 VAC. However, I believe the control circuit should be reconfigured with a control transformer with 120 VAC being used as the control voltage just due to safety considerations. I don't know of any code requirement to due as such, but would appreciate any opinions and/or suggestions on the subject. Thanks
 
I would find nothing unusual about a 480V coil on a motor starter. If everything is in good order then there shouldn't be a danger. If there is a fault on the control circuit the fuse or breaker should blow, just like it should if there were a fault on the circuit for the motor itself.
 
Is it local control (within the starter) or does the control circuit leave the starter ?
 
Nardie said:
However, I believe the control circuit should be reconfigured with a control transformer with 120 VAC being used as the control voltage just due to safety considerations.
Click to expand...

I do not see any safety improvement gained by changing things.
 
augie47 said:
Is it local control (within the starter) or does the control circuit leave the starter ?
Click to expand...

Good question. With little information provided by the OP I would take a guess that the pilot devices as well as the starter are contained within a common enclosure such as an MCC bucket would be which commonly is 480v control voltage.
 
iwire said:
I do not see any safety improvement gained by changing things.
Click to expand...

Other than the fact that one side of the coil more than likely has voltage on it all of the time which backfeeds through the winding of the coil and back to one side of the start stops.

I'm not a big fan of 240 or 480v controls myself but wouldn't go out of my way to change it.

JAP>
 
NFPA 79 provides a limitation.

9.1.2 Control Circuit Voltages.
9.1.2.1 AC Control Circuit Voltages. The ac
voltage for control
circuits shall not exceed 120 volts, ac single
phase.

Exception No. 1: Other voltages shall be permitted, where
necessary,
for the operation of electronic, precision, static, or similar
devices used
in the control circuit.

Exception No. 2: Any
electromechanical magnetic device having an
inrush current exceeding 20
amperes at 120 volts shall be permitted to
be energized above control voltage
through contactor or relay contacts.
The contactor or relay contacts shall
break both sides of the circuit
powering the magnetic device. The relay coil
shall be connected to the
control circuit.

9.1.2.2 DC Control Circuit
Voltages. DC control voltage shall
be 250 volts or less.

Interestingly, the standard also appears to require the use of a CPT presumably to limit the available short circuit current to less than 1kA to the control components.

9.1.1.1 Where control circuits are supplied from an ac source, control transformers shall be used for supplying the control circuits. [....] (too much to type)
 
it is pretty common to see 480V coils on larger NEMA starters. Like size 6 or 7. I rarely use them anymore and IEC starter coils tend to have a much lower current requirement.
 
petersonra said:
it is pretty common to see 480V coils on larger NEMA starters. Like size 6 or 7. I rarely use them anymore and IEC starter coils tend to have a much lower current requirement.
Click to expand...
You may find that those supposed 480ac coils are actually DC coils with a rectifiers for what it matters. AC in still the control power though.
 
Motor Control Circuit Voltage

Nardie said:
Recently I had a motor starter coil fail due to age, this starter has been in service since probably the 1950's. I contribute this failure due to age, however when performing an inspection of the cubicle I noticed that the control circuit voltage was 480 VAC. This is something that I am not use to, I am typically use to a motor control circuit voltage to be 120 VAC due to a control transformer. From reading the NEC it doesn't address the control voltage level, whether it be 480 VAC or 120 VAC. However, I believe the control circuit should be reconfigured with a control transformer with 120 VAC being used as the control voltage just due to safety considerations. I don't know of any code requirement to due as such, but would appreciate any opinions and/or suggestions on the subject. Thanks
Click to expand...

Based on some of the responses, I wanted to provide a more thorough description of the circuit. The circuit operates a chilled water pump motor with a Size 2 starter. The circuit originates in a MCC, it is tapped off the 480 VAC power feed, the control circuit then leaves the MCC to a rheostat, then to a pushbutton station to start and stop the pump. There are no fuses for the 480 VAC control circuit voltage, so the only protection is the power conductor fuses for the motor. The motor coil failed and began smoking and did not blow the power fuses. The MCC was de-energized to stop any further damage. The motor starter will have to be replaced, so the addition of a control transformer to lower the control voltage would not be that more work. In addition, we as a facility were not aware that the control circuit was 480 VAC. At this facility we require an arc flash analysis to be performed for all 480 VAC applications, otherwise we are not allowed to operate the equipment. I would appreciate any additional comments on this based on the additional information. If there is any other questions please let me know. Thanks.
 
I can't imagine it going to a rheostat but what do I know :). The no overcurrent protection appears to be a violation of Art 430.72
 
augie47 said:
I can't imagine it going to a rheostat but what do I know :). The no overcurrent protection appears to be a violation of Art 430.72
Click to expand...

My pained guess is that the rheostat is there to be adjusted so as to drop the 480V source current enough to allow a 120 or 240V relay coil to be used. :happysad:
The buttons and wiring would still have to be rated for 480.
 
GoldDigger said:
My pained guess is that the rheostat is there to be adjusted so as to drop the 480V source current enough to allow a 120 or 240V relay coil to be used. :happysad:
The buttons and wiring would still have to be rated for 480.
Click to expand...

Really?

JAP>
 
At the wastewater treatment plant where I worked years ago we had 480 volt push button control stations all over, some 500ft. or more from the starter bucket in the MCC. Never a problem. At least no control transformer to fail.
 
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