Motor De-rating and FLA

[MrJLH]

Not going to get into to too many details but here is what I have going on.

Recently a failed 200H.P. motor (460VAC, 1.15 SF, Design B, 400A MCP, size 5 starter, 350MCM feeder) was replaced with a manufactured 250H.P. motor (460VAC,SF 1.15, Design B).

The application is a PD boiler feed water pump. The replacement manufactured 250 H.P. motor has a name plate of 200H.P., per the vendor the motor “de-rated” from 250H.P. to 200H.P. Per the vendor I was assured that this is essentially a “200H.P.” motor.

What I’m having trouble grasping is:
Is the vendor correct on his statement of the de-rated motor?

Is the FLA/inrush of the "250H.P. manufactured" motor per a 250H.P. or 200H.P.?

Will the starter size still be 5? (cut off for 5 is 200H.P.)

Will calculations for feeder cable, MCP settings, and OL be calculated from a size of 200H.P. or 250H.P.?

Just having a little trouble getting my head around this.
Thanks a bunch

 

[Besoeker]

Is the FLA/inrush of the "250H.P. manufactured" motor per a 250H.P. or 200H.P.?

I think it unlikely.
Can you check the respective nameplate data?

 

[Jraef]

...The replacement manufactured 250 H.P. motor has a name plate of 200H.P., per the vendor the motor “de-rated” from 250H.P. to 200H.P. Per the vendor I was assured that this is essentially a “200H.P.” motor.

What the heck does that mean? De-rating of motors is done primarily to allow for increased duty cycles, or increased ambient temperature operation. But it's usually accomplished via just buying a larger motor than what the mechanical specs call for, not actually changing the nameplate of the motor. "HP" is a shorthand expression of torque at a given speed. So if the motor was built as a 250HP, but they put a name plate on it as 200HP, I fail to see how they could have LOWERED the torque capability of the motor.

Here's what I think. I think it sounds as though you may have stumbled across a motor importer that has taken a standard IEC 315kW 380V 50Hz motor and is telling you to use it at 460V 60Hz. 315kW = 235HP, but IEC motor specs have no concept of "Service Factor", so if you take a requirement for 200HP with a 1.15SF, that's 230HP; close enough in their mind, that extra 30HP is covered in the converted HP of that kW rated motor.

Why then are they calling is "250HP de-rated?" Because if you look at the TORQUE of a 315kW 50Hz motor and a 250HP 60Hz motor, they are closer to being the same. Torque = HP x 5250/RPM. Assuming (although not important) that this is a 4 pole motor, the 50Hz torque would be 235 x 5250/1455 = 847 ft-lbs, and a 250HP 60Hz motor would be 250 x 5250/1725 = 760 ft-lbs. The 315kW 50Hz motor is actually going to supply more torque than a 250HP 60Hz, but not enough to be called 300HP, so we jump from 250 to 300, they are calling it 250HP.

What's important to you? In this case actual motor nameplate FLA. If you look at the specs for a NEMA size 5 starter it is rated for 270A, which is actually a little higher than the NEC chart for 200HP (240A). This is because the NEMA rating has to cover lower pole counts (slower base speed) motors. That may work for you here in not requiring you to upgrade to a Size 6 starter. As to conductor sizing, I would interpret the NEC as saying you need to go with the 250HP aspect.

 

[Besoeker]

Here's what I think. I think it sounds as though you may have stumbled across a motor importer that has taken a standard IEC 315kW 380V 50Hz motor and is telling you to use it at 460V 60Hz. 315kW = 235HP,

You might want to review your numbers.

 

[Fnewman]

There are a number of possibilities here, but you might want to start by sending the serial number to the original motor manufacturer for verification of the rating.

 

[shortcircuit1]

Torque = HP x 5250/RPM. Assuming (although not important) that this is a 4 pole motor, the 50Hz torque would be 235 x 5250/1455 = 847 ft-lbs, and a 250HP 60Hz motor would be 250 x 5250/1725 = 760 ft-lbs. The 315kW 50Hz motor is actually going to supply more torque than a 250HP 60Hz, but not enough to be called 300HP, so we jump from 250 to 300, they are calling it 250HP.

I was tryting to calculate RPM which you have it as 1455 and 1725 RPM i used this formula..N=120*f/P...
For 50Hz motor with 4 poles doesnt its RPM=120*50/4=1500 RPM?
And for 60Hz motor with 4 poles RPM=120*60/4=1800 RPM?

Am i missing something?

 

[kwired]

I was tryting to calculate RPM which you have it as 1455 and 1725 RPM i used this formula..N=120*f/P...
For 50Hz motor with 4 poles doesnt its RPM=120*50/4=1500 RPM?
And for 60Hz motor with 4 poles RPM=120*60/4=1800 RPM?

Am i missing something?

You calculated synchronous speed. There is going to be some slip and a lesser rotor speed in a simple induction motor.

 

[shortcircuit1]

You calculated synchronous speed. There is going to be some slip and a lesser rotor speed in a simple induction motor.

Is there any formula which includes that slip as well?

 

[Besoeker]

Is there any formula which includes that slip as well?

The nameplate will normally give you rated speed and rated frequency.From that you can, as you have done, calculate synchronous speed. The difference between that and nameplate speed is the slip. It is usually expressed as a percentage of synch speed. Values of 2-3% are typical. Sometimes less for larger motors.
There is no simple formula that I know of.

Can you calculate it?
You can, or at least get close.
But it's not simple.

 

[kwired]

The RPM on the nameplate should only be correct when motor has rated voltage, frequency, and load applied to it, as is other data that might be on the plate, like current, power factor, efficiency.

Changing voltage, frequency or load will result in changes to any or all of actual running current, power factor, or efficiency.

 

[Hope4Today9]

De-Rating a motor is done to adjust for the altitude as the thinner air effects the cooling of the motor. The higher the altitude the less cooling you will have.

Duty as in Mill duty is a method by which a large motors nameplate is lowered to make it beefier in an attempt to make it last longer and as a cost saving method incase more motor is needed you can squeezes more out of the motor with out having to reorder, and reinstall. Just because you can make something turn, doesn't mean you can stop it the way someone thought you could.
When the motor name plate is changed in this manor the application always involves a DRIVE. This way you can control the torque. By lowering the current limits, operating in the field weaken range, and or limiting the voltage. Motor frame size "generally" determines the current and torque capabilities, while the insulation level and applied voltage determined the horsepower and "generally" speed capabilities. So without a DRIVE controlling the motor you could have enough torque at full speed (60Hz) to shear off a shaft or have fatigue happen over time. Just depends on the machine, man!

When a 60Hz motor is used on a 50Hz line, the motor will run 20% slower and run hotter as it's fixed speed fan turns slower. also the V/Hz ratio will increase so that during part of it's cycle it would be in an overload condition. So, one would need to tap down a transformer (as long as any other equipment would be fine operating at a slightly lower voltage) to compensate.

Running a 50Hz motor on a 60 Hz line, the motor would run ~20% faster causing it to try to operate in it's field weaken range. (I'm sure you know this, however others mint not) When a motor does this HP remains constant and Torque drops. As your application is a pump it has an exponential load curve meaning the faster you turn it the more load you get an the more flow you get too. May be what the one guy was saying, is what you have happening (a larger HP to counter operating in the field weaken range) ... just know no matter what someone changes to nameplate too; more speed equals more flow and higher pressures drops. ( no you don't have to do any thing with a transformer in this case )

Kind Regards

 

[Besoeker]

When a 60Hz motor is used on a 50Hz line, the motor will run 20% slower and run hotter as it's fixed speed fan turns slower. also the V/Hz ratio will increase so that during part of it's cycle it would be in an overload condition. So, one would need to tap down a transformer (as long as any other equipment would be fine operating at a slightly lower voltage) to compensate.

A motor designed for 50Hz is likely to be rated for 400V. That's the same V/Hz as 480V 60Hz.