400 VAC Motor - Conductor sizing

[nhee2]

We have a pump motor to be installed, nameplate 400 VAC (at 200 Hz) to be supplied by a VFD.

As this doesn't fall under table 430.250, what is the correct method to size branch circuit conductors (between VFD and motor). Does it fall under 430.6(C), which would just tell us to use the nameplate amp values?

 

[Jraef]

Does the motor nameplate show a HP rating at that speed and voltage? It only matters if they custom built that motor. If it is a standard motor that is being over speeded via the VFD, once you get above the nameplate base speed (which is what the HP is rated on), the motor is in "Constant HP mode", meaning the actual nameplate HP is no longer increasing with speed, so the torque is dropping and current remains the same. So the conductor sizing rules actually would not change.

But once you get into machines doing things with higher frequencies on the motors, there are all sorts of "tricks" they can use to give the machine what it needs. You really need to know the actual nameplate data and what your feed is before making any decisions. Post it and we can help you if this is a new concept for you, it can be confusing the first time you run into it.

 

[nhee2]

Does the motor nameplate show a HP rating at that speed and voltage? It only matters if they custom built that motor. If it is a standard motor that is being over speeded via the VFD, once you get above the nameplate base speed (which is what the HP is rated on), the motor is in "Constant HP mode", meaning the actual nameplate HP is no longer increasing with speed, so the torque is dropping and current remains the same. So the conductor sizing rules actually would not change.

But once you get into machines doing things with higher frequencies on the motors, there are all sorts of "tricks" they can use to give the machine what it needs. You really need to know the actual nameplate data and what your feed is before making any decisions. Post it and we can help you if this is a new concept for you, it can be confusing the first time you run into it.

Thanks.

Actually I was less concerned about the 200 Hz part and more questioning the 400 VAC part. As it does not fall in the 460 VAC column, or the 230 VAC column, I assume I would base the conductor ampacity as 125% of the nameplate amps.

 

[Jraef]

The conductors feeding the VFD must be sized at 125% of the VFD max input amps at whenever your supply voltage is. On the output side, just go with 125% of the motor nameplate FLA, unless, like I said, they are doing something funky.

As an example of funkiness, often times to get around the torque/speed issues, they will use a dual voltage motor, in this case 400V IEC motors can often be internally connected in Delta instead of Wye and run at 230V. So if you use a 400V maximum output of the VFD, but wire the motor for 230V, then tell the VFD it is a 230V 50Hz motor, the VFD puts out 230V at 50Hz so that the motor gets its proper V/Hz ratio and can develop full rated torque, but can continue to increase the speed at that V/Hz ratio until they get to 400V, which would be 87Hz before the motor enters the Constant Power operating range. So they don't start losing torque until you go over 87Hz, rather that beginning at 50Hz. The thing about that is, the motor power is actually going to be higher by the same ratio. So if it was 10kW at 230V 50Hz, it is now going to be 17.3kW at 87Hz, and you would pick the conductor size based on 17.3kW 230V. 1HP = .746kW, so that motor went from 7-1/2HP to 13HP, so I would base the conductor size as if it was 15HP 230V.

That's why I said the details matter.

 

[nhee2]

The conductors feeding the VFD must be sized at 125% of the VFD max input amps at whenever your supply voltage is. On the output side, just go with 125% of the motor nameplate FLA, unless, like I said, they are doing something funky.

As an example of funkiness, often times to get around the torque/speed issues, they will use a dual voltage motor, in this case 400V IEC motors can often be internally connected in Delta instead of Wye and run at 230V. So if you use a 400V maximum output of the VFD, but wire the motor for 230V, then tell the VFD it is a 230V 50Hz motor, the VFD puts out 230V at 50Hz so that the motor gets its proper V/Hz ratio and can develop full rated torque, but can continue to increase the speed at that V/Hz ratio until they get to 400V, which would be 87Hz before the motor enters the Constant Power operating range. So they don't start losing torque until you go over 87Hz, rather that beginning at 50Hz. The thing about that is, the motor power is actually going to be higher by the same ratio. So if it was 10kW at 230V 50Hz, it is now going to be 17.3kW at 87Hz, and you would pick the conductor size based on 17.3kW 230V. 1HP = .746kW, so that motor went from 7-1/2HP to 13HP, so I would base the conductor size as if it was 15HP 230V.

That's why I said the details matter.

Thanks for the explanation. This one is 200 HP. So the impact on conductor sizing will have a big impact on cost. I believe in this application they will be configuring similar to what you describe, however the details are still being developed.

 

[kwired]

Thanks.

Actually I was less concerned about the 200 Hz part and more questioning the 400 VAC part. As it does not fall in the 460 VAC column, or the 230 VAC column, I assume I would base the conductor ampacity as 125% of the nameplate amps.

430.6 is what sends us to the tables at the end of 430 to determine motor FLA. It has some exceptions, but isn't all that clear what to do if your motor doesn't fit or even come close to fitting into any place in the tables that are there.

there is no table that covers a motor rated 400 volts. There is also a note with the table for three phase motors that says "The following values of full-load currents are typical for motors running at speeds usual for belted motors and motors with normal torque characteristics." Which I believe would also exclude your application from using table values.

If the motor is not covered by one of the tables, I don't see where it says to use the nameplate for your situation, but don't have any clue what else NEC would expect you would use.

 

[nhee2]

The thing about that is, the motor power is actually going to be higher by the same ratio. So if it was 10kW at 230V 50Hz, it is now going to be 17.3kW at 87Hz, and you would pick the conductor size based on 17.3kW 230V. 1HP = .746kW, so that motor went from 7-1/2HP to 13HP, so I would base the conductor size as if it was 15HP 230V.

In this case, isn't basing on 230 VAC going to result in oversized conductors? Because the motor is achieving the 13 HP at 400 VAC, not 230 VAC? If the motor (or pump assembly in this case) came with a nameplate that said "13 HP/400VAC/35A", would you be sizing on nameplate or on the 230 VAC / 15 HP value in Table 430.250? IN this case it probably does not matter much, in the larger motor it might.

 

[nhee2]

If the motor is not covered by one of the tables, I don't see where it says to use the nameplate for your situation, but don't have any clue what else NEC would expect you would use.

430.6(C) applies to Alternating-Current Adjustable Voltage motors. I did not know if this would fall under that section.

 

[Jraef]

In this case, isn't basing on 230 VAC going to result in oversized conductors? Because the motor is achieving the 13 HP at 400 VAC, not 230 VAC? If the motor (or pump assembly in this case) came with a nameplate that said "13 HP/400VAC/35A", would you be sizing on nameplate or on the 230 VAC / 15 HP value in Table 430.250? IN this case it probably does not matter much, in the larger motor it might.

Ah, no, you're right, I messed up. The current will be based on it being configured AS a 230V motor and run at more than base speed, but at the point where the HP is higher, it's at the highest voltage. So your motor is likely 150kW 230/400V 50Hz. It will be strapped as a 230V motor and the VFD will use it as if it is a 230V motor, so it develops full torque at 50Hz, then can maintain is at full torque all the way to 400V/87Hz. Because that changes the HP rating to it being essentially 350HP, the current will be higher than it would have been as a 200HP 400V 50Hz motor, but it is going to be based on 400V 350HP, not 350HP at 230V.

Which, by the way, equates to about 300HP at 480V from a current standpoint.

 

[Besoeker]

Thanks for the explanation. This one is 200 HP. So the impact on conductor sizing will have a big impact on cost. I believe in this application they will be configuring similar to what you describe, however the details are still being developed.

200HP at 400V is typically around 260A.
Dunno if that helps.