One Drive Two Motors

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augie47

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Had an E/C call today and ask how the Code addresses the following:
He has (1) 20 HP Drive feeding (2) 10 HP motors. He says the equipment manufacturer calls for that arrangement. The drive has only the one output.
THe design calls foe a fusible disconnect ahead of each motor (on the load side of the VFD).

It seems to me that he would need to protect the drive input per nameplate. (I believe it calls for a 40 map circuit).
In addition he needs overload protection for each motor.
This leads to my first question: Can he accomplish overload protection by sizing the fuses in his disconnect per 430.32(A)(1) ?
Second question: Would over=temperature protection be required for each motor per 430.126(B) ?
Third question: Will the fuses in his disconnect suffice for motor shot-circuit protection ?
 

BAHTAH

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One Drive Two Motors

For ASD's where there are multiple motors operating from a single drive, OL protection shall be provided for each motor in accordance with Article 430 Part III (430.124 Overload Protection). Yes you can use the fuses in the disconnect to provide the OL protection where the rating provides the protection required by 430.32 (430.55 Combined Overcurrent Protection). Making an assumption that the motors mentioned are rated to operate at the nameplate rated current over the speed range required by the application I would say no additional overtemperature protection would be required. Another assumption is that the equipment manufacture is specifying a specific drive for the application? Not all drives can have their secondary opened under load, so if both motor disconnects were to be opened there could be a problem unless the correct drive is used.
 

augie47

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Thanks

I did recommend a auxiliary switch in the disconnect to notify the drive if the switch is open.
 

Besoeker

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FWIW I wouldn't put fuses in the load side of a VFD even with two motors.
Fuses rated to take the motor current generally need multiples of that rating. At that, the VSD would trip rather than the fuses blowing.
As I noted in another thread, I would use thermal overload units for the individual units.

And I agree with Augie47's point about having the auxiliary contact in the disconnect. It's something we routinely do when a disconnect by the motor/driven plant is required for safety reasons.
 

Jraef

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I'lll chime in too.

Fuses on the output, bad idea. If one fuse blows, it creates a severe current imbalance on the output of the drive. Most drives will pick that up and shut down, but not all and you may end up single phasing the motor. I would never recommend fuses.

A VFD that is UL listed as a motor controller* now is required to provide the Short Circuit protection for the load side, because the VFD is actually more like a new source, and SC protection on the line side may be too slow to react to a short on the load side. For many drives, especially when you have just two motors that are 1/2 the rating of the drive, this built-in SC protection may still fall into the acceptable range of what the NEC requires for the individual motors, using the concept from an IT breaker trip setting or even the fuse sizing requirement. Think of it as if it is an electronic fuse. This is not specifically stated in the NEC, but I have yet to come across an AHJ that will not accept the argument. The VFD mfr data sheet should tell you what the output SC protection level is.

That said, then the only absolute requirement is separate running OL protection, because the VFD cannot tell the difference between the two motors. So at a minimum, you need two OL relays, one for each motor. But if instead of OL relays you use the IEC style Motor Protection Switches that have the OL and SC protection built-in, you cover all bases. That to me is the right way to accomplish this, especially considering the following:

* Not all VFDs that are UL listed, are UL listed as "motor controllers". Some of the cheap Asian drives flooding E-Bay right now are either not UL listed at all, or they are listed as "power conversion equipment", just like solar inverters, and are thus not required to provide the motor SC protection mentioned above.
 

Besoeker

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UK
I'lll chime in too.

Fuses on the output, bad idea. If one fuse blows, it creates a severe current imbalance on the output of the drive. Most drives will pick that up and shut down, but not all and you may end up single phasing the motor.
I think we are in agreement that it is not a good thing. But maybe for different reasons. As I said above, a fuse will take multiple times its rating to rupture under fault conditions. The VSD would trip first rendering fuses unnecessary.
 

Jraef

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...The VSD would trip first rendering fuses unnecessary.
One would hope. But of late there have been a couple of real bottom feeder VFD mfrs from China dumping product at unbelievable prices on Ebay, and they are notorious for not having the kind of features that we have come to expect from VFDs now. Seems almost as if they stole some plans and specs for drives from 20 years ago and modernized them just enough to look current. But when people use them, they discover that age-old proverb, "You get what you pay for".
 
I think we are in agreement that it is not a good thing. But maybe for different reasons. As I said above, a fuse will take multiple times its rating to rupture under fault conditions. The VSD would trip first rendering fuses unnecessary.

Agreed. Furthermore the self protecting feature of most reputable ASD's would be lower and faster than any fuse or other OC device you can put on the individual motor's fro 'branch circuit' protection AND it's not subject to single phasing.
 

Besoeker

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UK
One would hope. But of late there have been a couple of real bottom feeder VFD mfrs from China dumping product at unbelievable prices on Ebay, and they are notorious for not having the kind of features that we have come to expect from VFDs now. Seems almost as if they stole some plans and specs for drives from 20 years ago and modernized them just enough to look current. But when people use them, they discover that age-old proverb, "You get what you pay for".
I take your point. I get regular marketing emails from China, mainly the Guangdong province, with companies wanting to partner them or be agents or some other commercial arrangement. I always decline. This is not based on product quality or features. For all I know, they may be making products for the mainstream suppliers in some cases. Badge engineering is not altogether uncommon. On the other hand, the kit might be cheap and nasty. I just haven't pursued it. I would find it very difficult to sell a system using drives from a source unknown to our customer. Apart from concerns about quality it would raise questions about documentation, spares, and local support where the system is to be installed. Digression - apologies for that.

Back to the issue of output fusing. I don't think it would help on the cheap drives either. If they don't have modern protection features such as de-sat, the IGBTs would fail long before you got to a current that would blow the fuse. Failed IGBTs generally make a bit of a mess.....outside the SOA, the semiconductor gets a bit hot and makes a bid for freedom. The ensuing carnage usually takes out the incoming supply over current protection.
 

GoldDigger

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outside the SOA, the semiconductor gets a bit hot and makes a bid for freedom. The ensuing carnage usually takes out the incoming supply over current protection.
The carnage results from all of the electrons that follow the semiconductor in its bid for freedom.....
 
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