Minimum HP

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ptonsparky said:
AB states which overloads to use for the protection of motors on the load side of a drive. They had two types listed in the FAQ page I read this morning. One was a melting alloy type with a W heater for class 20 protection, the other was an IEC style, if I remember correctly. Electronic overloads were not to be used.
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In order to Comply with the NEC. AB will be happy to comply with it weather it actually provides the intended protection or not.

What I am saying that the only protection meaningful is the electronic overload protection WITHIN the drive for a single motor. When you have multiple motors on a single drive the only effective thermal protection is a winfing embedded thermistor with protective relay connected to it. It is actually an IEC requirement for hazardous area motors, even with a single motor/drive combination.
 
weressl said:
In order to Comply with the NEC. AB will be happy to comply with it weather it actually provides the intended protection or not.

What I am saying that the only protection meaningful is the electronic overload protection WITHIN the drive for a single motor. When you have multiple motors on a single drive the only effective thermal protection is a winfing embedded thermistor with protective relay connected to it. It is actually an IEC requirement for hazardous area motors, even with a single motor/drive combination.
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I think you make a very good point here, one that is, in my opinion, not adequately addressed in the NEC. But unfortunately, embedded thermistors are not as common in motors in the US as they are in other parts of the world. My suggestion to people who ask for advice in advance (rare as that is), is to order motors for VFD applications WITH embedded PTC thermistors. Almost all VFDs designed for worldwide use have a PTC input as standard now. It is a good 2nd line of defense. In the case of the multiple motors that this thread is addressing, you could probably just run them all in series (paying attention to the total circuit resistance) or use separate simple Thermistor Trip Relays then run their contacts in a series string to the VFD input.

Still, I don't think that an embedded thermistor and relay alone is going to fill the bill for the NEC, unless you can convince an inspector that it falls under the "other approved means" mentioned in the exception at the bottom of table 430-37. But you will also still need Short Circuit protection for each motor circuit per 430.40, hence the best practice I mentioned earlier regarding the IEC Motor Starter Protector devices.
 
Jraef said:
Still, I don't think that an embedded thermistor and relay alone is going to fill the bill for the NEC, unless you can convince an inspector that it falls under the "other approved means" mentioned in the exception at the bottom of table 430-37. But you will also still need Short Circuit protection for each motor circuit per 430.40, hence the best practice I mentioned earlier regarding the IEC Motor Starter Protector devices.
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Thanks JRaef,

The thermistor driven overload device would meet the NEC 430 Part III. 430.32(A) 2 requirements.

I just don't know HOW would you size the OCPD that it would be meaningful in any way. The drive itself would not be able to deliver the mangitude and duration of the current that would trip the OCPD.

So while meeting the 'letter of the law', it is a meaningless 'obligation'. But I guess that is why many consider the NEC the electricians Bible; "It is not ours to question or even to understand......":grin:
 
weressl said:
I just don't know HOW would you size the OCPD that it would be meaningful in any way. The drive itself would not be able to deliver the mangitude and duration of the current that would trip the OCPD.
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Given the system (drive sized to run some 9 motors), the drive would be able to supply enough current to trip a 'heater' style overload device between the drive and the motor. These devices use heating elements to somewhat model the thermal characteristics of the motor, and unless grossly oversized would eventually trip.

Granted the thermal model designed for 60Hz operation would be wrong for variable speed operation, but my hunch based thermally limited torque/speed curves on VSD operation is that the heater model would not be too far wrong; if the motor were 'derated' and heater trip level reduced by perhaps 20-40% from the nominal 60Hz value, then the overload would trip before the motor overheated. (Note: this is a hunch, not an engineered design.)

-Jon
 
winnie said:
Given the system (drive sized to run some 9 motors), the drive would be able to supply enough current to trip a 'heater' style overload device between the drive and the motor. These devices use heating elements to somewhat model the thermal characteristics of the motor, and unless grossly oversized would eventually trip.

Granted the thermal model designed for 60Hz operation would be wrong for variable speed operation, but my hunch based thermally limited torque/speed curves on VSD operation is that the heater model would not be too far wrong; if the motor were 'derated' and heater trip level reduced by perhaps 20-40% from the nominal 60Hz value, then the overload would trip before the motor overheated. (Note: this is a hunch, not an engineered design.)

-Jon
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If it is a thermal element, a heater style overload with actual motor current in it, frequency doesn't matter.
 
winnie said:
Given the system (drive sized to run some 9 motors), the drive would be able to supply enough current to trip a 'heater' style overload device between the drive and the motor. These devices use heating elements to somewhat model the thermal characteristics of the motor, and unless grossly oversized would eventually trip.

Granted the thermal model designed for 60Hz operation would be wrong for variable speed operation, but my hunch based thermally limited torque/speed curves on VSD operation is that the heater model would not be too far wrong; if the motor were 'derated' and heater trip level reduced by perhaps 20-40% from the nominal 60Hz value, then the overload would trip before the motor overheated. (Note: this is a hunch, not an engineered design.)

-Jon
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My hunch and experience is the opposite. We did have a drive shut down on overcurrent when we were driving two 50HP motors with a 100HP drive and one of the motors suffered an in-hase winding failure. (Thgis would be represented by a single pohase overcurrent.) The thermal overload never blinked the drive OC beat it to the task, eg. still offered superior protection even though it was not designed to do so. In the case of a phase-to-phase it would still be faster than a thermal overload.
 
Besoeker said:
If it is a thermal element, a heater style overload with actual motor current in it, frequency doesn't matter.
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As I described above, the heater will respond correctly to current over a wide frequency range; probably down to well below 1Hz.

However the motor's thermal response to the same rms current at different frequencies and under different loading conditions is not the same as its response to that current if sourced at 60Hz full voltage.

If the drive is putting out properly related V to F for the motors, then the heat produced by a given RMS value of terminal current will be pretty well constant over the frequency range. In this case the difference between motor temperature and overheating will be primarily caused by differences in cooling at different shaft speeds. Such a motor, properly protected by the heaters at 60Hz might overheat at low speed because of impaired cooling.

If the V to F relationship is not correct, then all bets are off, and you could get considerably more than expected heating with the same RMS current flow.

-Jon
 
weressl said:
We did have a drive shut down on overcurrent when we were driving two 50HP motors with a 100HP drive and one of the motors suffered an in-hase winding failure. (Thgis would be represented by a single pohase overcurrent.)
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Okay, I see that, and revise my assessment to say that the drive would probably respond to electrical faults far faster than any thermal overload device.

I guess that I was envisioning some sort of mechanical overload placed on the motor, where current would creep up and the motor would heat up, but not with an electrical fault.

Also consider that what goes first probably changes with 1 drive to 2 motors versus 1 drive to 10 motors.

-Jon
 
winnie said:
Okay, I see that, and revise my assessment to say that the drive would probably respond to electrical faults far faster than any thermal overload device.
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This magnitude of the current was in the range of the oveload setting as we were able to see from the fault data log.

winnie said:
I guess that I was envisioning some sort of mechanical overload placed on the motor, where current would creep up and the motor would heat up, but not with an electrical fault.
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You can have overload at reduced speed where the reduced cooling - due to reduced speed - would overheat the winding WITHOUT exceeding the 125% FLA setting of the thermal overload. That does NOT happen in the case of single motor/drive combination because the software ADJUSTS the trip current setting along with the speed change

winnie said:
Also consider that what goes first probably changes with 1 drive to 2 motors versus 1 drive to 10 motors. -Jon
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May change. As I said earlier multiple drive and combinations of impedances and resistances totally screw up the motor model in multiple motor applications so there is no (easy) way to predict how the drive would predict.
 
weressl said:
My hunch and experience is the opposite. We did have a drive shut down on overcurrent when we were driving two 50HP motors with a 100HP drive and one of the motors suffered an in-hase winding failure. (Thgis would be represented by a single pohase overcurrent.) The thermal overload never blinked the drive OC beat it to the task, eg. still offered superior protection even though it was not designed to do so. In the case of a phase-to-phase it would still be faster than a thermal overload.
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For that particular circumstance, I agree. A thermal relay wouldn't protect against a catastrophic fault on one of the two motors.
Have 80 or so and it's a different protection scenario.
 
winnie said:
If the drive is putting out properly related V to F for the motors, then the heat produced by a given RMS value of terminal current will be pretty well constant over the frequency range. -Jon
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That's a good point.
You need to consider load characteristics too.
Sure, constant V/f will give (about) constant flux and the capability of the drive producing rated torque at any speed.
For many loads (fans and pumps) power drops off as the cube of the speed reduction and, thus, torque with the square of the speed reduction.
Shaft fans on the NDE can normally cope with this regime.
Separately powered fans are often used for applications requiring constant torque over a wide speed range.
 
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