Controller vs. Adjustable speed drive

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Jay Reynolds

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Is an adjustable speed drive considered a controller? There are two different definitions but a controller is required to have a disconnect while an adjustable speed drive says it is "permisable" to include a disconnect in the line side.
 
In my opinion the adjustable speed drive is a controller. I have no idea what the intent of the wording in 430.128 is.
 
Variable speed drives can be ordered with disconnects as part of the package, otherwise we install a disconnect.
 
After I studied the definitions more closley I find that an adjustable speed drive isn't a controller because it does not "make or break the motor current" it manipulates it instead. Voltage is always present at the motor, running or not, when it is connected to an adjustable speed drive. Now the question is will a seperate disconnect be required for each drive in a multiple drive branch circuit or will the one breaker that feeds all of the drives in a circuit suffice even if it is not in sight?
 
Are you talking about something like a VFD?

I don't know whether or not there is voltage at the motor leads with a VFD or not... I've never thought about it ....now that you've put it in my head, I will check it out , though.

I know that synchronous motors often have "holding" voltage when the motor is not turning.

Seems to me that 430.128 says the disconnect on the line side of the VFD is good enough. If the VFD or synchornous motor drive has no voltage, the motor won't either.

I think that VFD manufacturers want you to stop and start the motor with the VFD and NOT with some sort of controller or disconnect between the drive and the motor. I would think synchronous would be the same way. I don't think you'd want anything between the drive and the motor.

Interrupting the motor current between the drive and a synchronous motor could even be a safety hazard, because of the "holding" current . The machinery being held by the motor could fall or move, and the brake in the motor is often energized by a seperate source that would likely be disengaging the brake while the motor is supposed to be holding it.

So far as the single disconnect for multiple motors is concerned, I would think 430.102 would apply.
 
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Too slow for the 10 minute edit rule...

I meant to say I'm don't know if there is voltage at the motor leads when the motor is not running or not...


IMO the drive would be a controller. If the motor becomes overloaded the drive is capable of stopping it... I'm not sure what the definition of " interrupting" is.
 
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Adj Speed Drives

Adj Speed Drives

I have seen older Carter pumps with a mechanical, adjustable speed gear box. The gear box is driven by a motor and a start-stop contactor. Is this what you are referring to? If so, a disconnect is required.

If by Adj speed drive - you mean VFD, a disconnect MUST be installed.

VFDs output "idle" current full-time during stop mode, even when not spooling the motor into rotation. Idle currents allow the internal VFD bus to stay ready and monitor the VFD bus.
 
Jay Reynolds said:
After I studied the definitions more closley I find that an adjustable speed drive isn't a controller because it does not "make or break the motor current" it manipulates it instead. Voltage is always present at the motor, running or not, when it is connected to an adjustable speed drive. Now the question is will a seperate disconnect be required for each drive in a multiple drive branch circuit or will the one breaker that feeds all of the drives in a circuit suffice even if it is not in sight?
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First I tend to agree with your statement about the definition of the ASD, however if the ASD is not a controller then it is not even within the scope of Article 430. The definition is faulty, but the intent is the same as for any other motor controller....you must have a disconnect within sight of the controller.
 
Unfortunatley, I have first hand knowlege that there is voltage at the motor even though the drive is at full stop with no output. I'm not sure what the let thru voltage is, but I yelped. I am refering to VFD's, though I think the code should make a distinction between a VFD, a reduced voltage starter and a soft start. They all perform similarly but function differently. My specific circuit has a 30 amp breaker feeding four VFD's with each having an input rating of 5.7 amps. The breaker is lockable per 430.102 and IMO a motor disconnect runs the risk of damaging the drive as well as introduces personnel risks as well. I tend to think that a 3 pole motor switch ahead of the drive will be the best solution as long as that doesn't foul me up somewhere else in 430.
 
Control Guy said:
I have seen older Carter pumps with a mechanical, adjustable speed gear box. The gear box is driven by a motor and a start-stop contactor. Is this what you are referring to? If so, a disconnect is required.
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NFPA 79 contains the same definition of "Adjustable Speed Drive" as the NEC, and also includes in its Annex A (Explanatory material) the following clarification:

A.3.3.4 Adjustable Speed Drive. This includes ac and dc voltage
modes and frequency mode controls. Belt, chain, or roller
shifting controllers are not included.

According to the instructor at an NFPA 79 seminar I attended a while back, an adjustable speed drive IS a motor controller.

As realolman points out, the intent of NEC 430.128 is to allow the disconnect (if required) to be on the line side of the inverter, not between the inverter and the motor.

Note that Exception 2 to 430.102 says you don't need individual disconnecting means for all motors if all the motors are part of a single machine or piece of apparatus.

Does your application more closely resemble a self-contained multi-motor machine tool, or a conveyer system spread throughout the building?
 
No, thes are four individual VFD/motors that drive large ceiling mounted fans. They are stand alone and not tied together for control except for fire alarm shut down.
 
The circuit is for four individual vfd / motor sets all fed from one branch circuit. None are interlocked so they cannot be considered one machine or apparatus.
 
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