Breaker sizing for de-rated motor?

[emiller233]

We have a project that needs CE. And our motor vendor (WEG) told us they don’t have a 60HP version, so we have to get a 75 HP and derate it down to 60HP.
How do you handle the over current protection and wire sizing for this. Based off a 60HP or 75HP…?

 

[Rock86]

Did the vendor give you nameplate data for the derated motor or is he telling you to use a control system to limit the output of the motor?

 

[kwired]

You still have a 75 HP motor when it comes to starting current, I'd size short circuit/ground fault protection for a 75 regardless.

Probably do need to size conductors for the 75

Motor overload you can set anywhere you want below the nameplate rating if you wish.

 

[kwired]

Did the vendor give you nameplate data for the derated motor or is he telling you to use a control system to limit the output of the motor?

Motor is only going to draw what the driven load demands from it.

 

[emiller233]

Motor is only going to draw what the driven load demands from it.

Wouldn’t this be tru for startup current also?

I’ll size the conductors larger for sure. There is a 3rd party involved and they will be supplying the circuit breaker/starter so just want to make sure they don’t undersize it and have it trip when they try to start the motor

 

[emiller233]

Did the vendor give you nameplate data for the derated motor or is he telling you to use a control system to limit the output of the motor?

we requested it, don't have it yet tho since its a 'custom' motor

 

[topgone]

Wouldn’t this be tru for startup current also?

I’ll size the conductors larger for sure. There is a 3rd party involved and they will be supplying the circuit breaker/starter so just want to make sure they don’t undersize it and have it trip when they try to start the motor

Your TOR should have a range where it is possible to adjust the current protection level. Set your overload protection setting equal to the full current of the 75 HP motor--> that's what is being protected! Your short-time trip settings should be set a bit higher than the starting current/ locked-rotor draw when starting the 75 HP motor! There is no harm sizing your conductors to the 75HP level, IMO.

 

[kwired]

Wouldn’t this be tru for startup current also?

I’ll size the conductors larger for sure. There is a 3rd party involved and they will be supplying the circuit breaker/starter so just want to make sure they don’t undersize it and have it trip when they try to start the motor

No. different sized windings in a 75 than in a 60 HP motor is going to mean different overall impedance particularly at initial energization.

The 75 HP should have more torque ability and would accelerate load at higher rate - more current but for lesser time. Running current will be similar but again different impedance will mean different power factor and different current so real power is what is close to the same. Motor efficiency does factor in also.

A 75 generally should have a heavier rotor - it should take more power just to spin that shaft even with no load coupled to it.

 

[petersonra]

It's a 75 horsepower motor. The application doesn't matter. You have to size the ampacity, overloads, and overcurrent protection device to a 75 horsepower motor.

 

[winnie]

Think about it this way: NEMA motors are often rated with a 'service factor'. A 100 Hp motor might have a SF of 1.15, and could be safely run with a 115 Hp mechanical load as long as you had sufficient conductor ampacity. Your overloads could be safely set for the SF current. But if you only needed 100 Hp you would not be required to wire for the potential 115 Hp that the motor could supply.

It seems to me that the customer has a mechanical load that requires 60Hp. Their supplier can only provide a 75 Hp motor. Say this motor were a NEMA motor rated 75 Hp with 1.0 SF.

If the supplier were willing to slap a different nameplate on the exact same motor design which said '60Hp 1.25SF' and you only needed 60Hp, how would you wire things?

Now I understand that there is probably a limit to this game; you couldn't take a 100 Hp motor and label it '1Hp 100.0 SF' because of things like starting current, but different motors with the same nominal rating can have huge differences in starting current, and IMHO a 60Hp 1.25 SF motor would not be significantly different from a 60Hp 1.0 SF motor in terms of starting.

-Jon

 

[emiller233]

so i did some digging and the part number Weg quoted us is this part # 07518ET3E365TC-W22-CE . so i went to their website and was able to find this motor if i took the "-CE" off of the end of it and use just the 07518ET3E365TC-W22.

search for it and then select the '60HP @380V (IE3)' option and it gives you this info which i'm ASSUMING will be the same as the motor we're actually going to get...? still waiting on them to confirm

http://catalog.wegelectric.com/produtoInformacao.asp
07518ET3E365TC-W22

Characteristics Output: 60 HP Poles: 4 Frequency: 50 Hz Torque: 210 lb.ft Voltage: 380 V Frame: 364/5T RPM: 1480 Full Load Amps: 88.5 A Efficiency (100%): 94.2 Power factor (100%): 0.82 Insulation: F Noise: 63 dB(A) No load current: 33.0 A Locked rotor current (II/In): 7.0

Catalog Number: 07518ET3E365TC-W22 WEG id: 12076589 Price: $9,148

Output HP	Frame	Full Load Amps	Locked rotor current (II/In)	Locked Rotor Torque	Break Down Torque
60 HP	364/5T	88.5 A	7.0	210 %	250 %

Efficiency (%)	Power factor (%)	Service Factor	Inertia	L.R. Time	Weight
50	75	100	50	75	100
93.7	94.2	94.2	0.68	0.78	0.82	1.00	23.251 sq.ft.lb	10 s	919 lb

 

[winnie]

That motor has multiple ratings depending upon supply voltage, frequency, and efficiency rating.

It is considered an IE3 motor (which I think means premium efficiency) at 60Hp and _50Hz_ supply, but an IE2 motor at 75Hp. This sounds lots like the NEMA service factor.

In your list I only see one 60Hz value; your question is what sort of rating would the motor have at 60Hz 60Hp. But it is clear that they happily create different motor nameplates for the same motor and different applications.

-Jon

 

[kwired]

If you stall this motor it is still going to draw locked rotor current typical of a 75 HP with same design characteristics and not a 60.

 

[paulengr]

We have a project that needs CE. And our motor vendor (WEG) told us they don’t have a 60HP version, so we have to get a 75 HP and derate it down to 60HP.
How do you handle the over current protection and wire sizing for this. Based off a 60HP or 75HP…?

Starting current will be higher than normal and the no load current will be a small amount larger but that’s only about 10-15% of FLC so 3% higher overall. That’s inside the overload accuracy and typical motor variation so the overload doesn’t change...rate for 60 HP. Same with the wiring. That leaves the short circuit protection which again will be marginally higher so I wouldn’t get concerned. At worst depending on the device used you could set it based on LRC rules. Small motors run up to 10xFLC on LRC and code accommodates these where at 75 HP even a crusher duty motor will rarely exceed 9xFLC.

This sort of thing happens more than you realize with oddball sizes.

 

[topgone]

Starting current will be higher than normal and the no load current will be a small amount larger but that’s only about 10-15% of FLC so 3% higher overall. That’s inside the overload accuracy and typical motor variation so the overload doesn’t change...rate for 60 HP. Same with the wiring. That leaves the short circuit protection which again will be marginally higher so I wouldn’t get concerned. At worst depending on the device used you could set it based on LRC rules. Small motors run up to 10xFLC on LRC and code accommodates these where at 75 HP even a crusher duty motor will rarely exceed 9xFLC.

This sort of thing happens more than you realize with oddball sizes.

We had that problem before. Our techs replaced a motor with a bigger-sized motor as their was no spare available. The motor controller tripped on starting. When I investigated, the short time setting of the CB was below the starting current of the new motor causing the tripping. Although the load of the equipment was more or less the same, the starting current was higher especially as the replacement motor was a high-efficiency one! They should have known better.

 

[petersonra]

You are not derating the motor. It is still a 75 hp motor and had to be treated that way.