Motor overload protection

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kwd

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430.32 requires a "separate" overload device. Separate from what? The motor? The controller?
Can the motor current programmed into a variable speed drive be used as a "separate overload device"?
I thought I could program the drive with the motor fla (in this case 37a) and this would protect the motor, but an inspector tells me no, we must have a "separate" stand alone, overload device.
 
Re: Motor overload protection

I've always interpreted "separate" in this context to mean external to the motor, as opposed to "integral" which refers to overload devices built into the motor.

However, the inspector may still be correct. None of the VFDs that I have installed had built-in overload protection.

The manufacturers literature should specify whether a "separate" overload relay is required.

Ed

[ June 03, 2004, 10:32 AM: Message edited by: Ed MacLaren ]
 
Re: Motor overload protection

Ed,
A lot of the newer drives have listed motor overload protection built in. KWD will have to check the information that is provided with the drive.
Don
 
Re: Motor overload protection

A sub-rule like 430.32(A)(1) which you are actually referring to must always be interpreted in context of its primary rule; in this case, 430.32(A). It must also be interpreted in the context of its section; in this case overload protection, as opposed to short-circuit or ground-fault protection. Finally it must be interpreted in context of general rules such as 430.6(A)(1).

In this case the term ?separate? refers to WHAT is being monitored (responsive to motor current rather than short-circuit or ground fault currents) as opposed to HOW or WHERE it is being monitored.

The VFD setting you were describing meets the intent of this 430.32(A)(1).
 
Re: Motor overload protection

The VFD's I install are current limiting, IE the motor FLA is programmed in and the VFD will not allow its output current to exceed that value. I would interpert this as internal is part of the motor and seperate is external. The AHJ may not be familar with VFD technology and is looking for a overload relay.
 
Re: Motor overload protection

Tom,
I don't think that just having a current limiting function is enough. I think that the drive must have a listed built-in electronic overload system to meet the code rules. Most of the newer drives that I have used have this feature, but about 5 years ago, few drives had a listed built-in overload.
Don
 
Re: Motor overload protection

The context under discussion is Motor and Branch-Circuit Overload Protection, further it is for Continuous-Duty Motors, More Than 1 Horsepower. The NEC permits four options to accomplish motor and branch-circuit overload protection for continuous-duty motors that are more than 1 Horsepower.

One of those methods [430.32(A)(1)] is for a ??separate overload device that is responsive to motor current.? ?Separate? may be reasonably interpreted as either ?apart from the motor? since the other three methods require specific components that are integral to the motor or, as I believe, ?apart from other overcurrent functions? since it is specific to overload rather than short-circuit or ground fault.

In either case, the VFD meets the intent.

Recognizing the context of the entire NEC ??is the practical safeguarding of persons and property from hazards arising from the use of electricity,? does anyone reasonably believe there is risk of overloading the motor or branch circuits in the application as originally described?

And while we are at it, shocking as it may seem, while there are many listed ASD/VFDs that provide overload protection, "listing" itself is not an NEC issue here. Nor is it a FedOSHA one since it?s not on the ?List of Test Standards Recognized.? It may be a state or local issue ? that?s something I have to deal with regularly.

[ June 03, 2004, 04:49 PM: Message edited by: rbalex ]
 
Re: Motor overload protection

This Inspector is smoking some fat crack. If the VFD or Motor Soft Start controller has programmable overload protection I believe like most people I know, it meets the intent of the code. In fact, I would go as far as to say a motor protected by a Programmable drive has greater protection than a standard motor starter with separate overload's.

State Electrical Inspector
John Cosmo
 
All VFD's have overload protection. They TRIP when the current exceeds the value entered into the overload parameter. However, since this parameter in VFD's are sometimes accessible by unauthorized employees it may make sense to install an overload relay. But not an adjustable type. They can monkey with this too. Your inspector is wrong. But do what he says.
 
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