Sizing transformer

[Dansos]

Hey guys,

we we are hooking up a 15 HP severe duty motor that will run on a VFD that the company paired with the motor. The VFD is calling for 9.3A @ 480V. The building is only fed with a 240D 3 PH electric drop. What are the factors for sizing the needed transformer? I understand the demand that’s needed but does my current voltage and amperage calculate into the sizing of the transformer?

 

[cpickett]

Is the VFD rated for 15HP? NEC FLA table shows 21Amps for 15HP, 460V 3phase. Converting 15HP to watts (11.2kW), ignoring efficiency losses, and assuming 1.0 PF, gives 13.5A at 480V

Either way, I would take the max input current draw and convert that to kVA and round up to the next standard size transformer. The VFD should limit the inrush so you don't have to factor in high startup currents.

480V x 9.3A x 1.73 = 7.7kVA, next standard size may be 10 or 15 kVA. Though I assume if you need the full 15HP that you'd be looking at a 25kVA transformer minimum.

 

[Jraef]

Safe bet rule-of-thumb for sizing a transformer to feed a VFD is to use the 2x the HP value as the kVA, so in this case a 30kVA transformer. The FLC on that transformer at the 480V side will be 36A, which will allow room for the VFD to pull 150% current if it needs to when providing for maximum torque from the motor (150% for 60 seconds is a typical VFD Constant Torque overload capability). The next size smaller transformer is 25kVA and although, at 10.8A it is capable of the motor FLA, it leaves you with not enough overhead so if the VFD wants to let the motor deliver full torque it will cause a voltage drop.

Use a transformer with a 480/277V secondary and ground the neutral point on the 480V side, your VFD will probably require that, or at least be best served that way. No need to run the N wire to anything though, it's just to establish the ground reference.

Oops, I didn't even check the 15HP vs 9.8A issue... so if you read my first version, forget it...

 

[Besoeker]

Hey guys,

we we are hooking up a 15 HP severe duty motor that will run on a VFD that the company paired with the motor. The VFD is calling for 9.3A @ 480V. The building is only fed with a 240D 3 PH electric drop. What are the factors for sizing the needed transformer? I understand the demand that’s needed but does my current voltage and amperage calculate into the sizing of the transformer?

Is that 9.3A input or output current?

 

[Dansos]

9.3A @ 480V input
9.4/8.2A 0-400/480 output
is on the VFD nameplate. I originally sized it for a 10Kv transformer but now that I understand what you guys have said a bigger transformer will be needed I guess

 

[Besoeker]

9.3A @ 480V input
9.4/8.2A 0-400/480 output
is on the VFD nameplate. I originally sized it for a 10Kv transformer but now that I understand what you guys have said a bigger transformer will be needed I guess

Assume you mean 10 kVA?
I would have thought that adequate for a 9.3A input VFD.

 

[Jraef]

Assume you mean 10 kVA?
I would have thought that adequate for a 9.3A input VFD.

The point though is that the motor is 15HP, so it will be 22A FLC. What the VFD actually draws under the expected load is irrelevant in terms of sizing components. In a motor circuit, you must size devices and components for the size of the motor. You can power a motor with an under-rated VFD in a pinch to get something running as you await the right parts, but it would not be code compliant and you can't design a piece of equipment that way on purpose.

 

[Besoeker]

The point though is that the motor is 15HP, so it will be 22A FLC. What the VFD actually draws under the expected load is irrelevant in terms of sizing components. In a motor circuit, you must size devices and components for the size of the motor. You can power a motor with an under-rated VFD in a pinch to get something running as you await the right parts, but it would not be code compliant and you can't design a piece of equipment that way on purpose.

The transformer is supplying the VFD, not the motor.

 

[Dansos]

That is where I am getting confused. I never used a transformer for a Motor AND a VFD. I guess I’m getting confused as to what spec I am sizing the transformer to. I ASSUMED it would be the motor but first termination point AFTER the transformer is the VFD so I thought that would be the equipment I use for sizing the Transfomrer

 

[retirede]

Is there a reason that a 230 volt motor / drive wasn’t selected in the first place thus eliminating the transformer?

 

[buffalonymann]

The transformer is supplying the VFD, not the motor.

No, the transformer is supplying the motor.

 

[Besoeker]

No, the transformer is supplying the motor.

The transformer is supplying the VFD, the VFD is supplying the motor.

 

[cpickett]

That is where I am getting confused. I never used a transformer for a Motor AND a VFD. I guess I’m getting confused as to what spec I am sizing the transformer to. I ASSUMED it would be the
motor but first termination point AFTER the transformer is the VFD so I thought that would be the equipment I use for sizing the Transfomrer

You're correct, the 10kVA transformer would work electrically with the 9.3A VFD, as the VFD will limit the output current to the motor, but you won't get the full 15HP out of the motor.

What we're suggesting is that you confirm with the customer that they have selected the proper VFD for the motor, or if they need to get a bigger VFD.

 

[Besoeker]

You're correct, the 10kVA transformer would work electrically with the 9.3A VFD, as the VFD will limit the output current to the motor, but you won't get the full 15HP out of the motor.

What we're suggesting is that you confirm with the customer that they have selected the proper VFD for the motor, or if they need to get a bigger VFD.

Yes, I agree. The VFD and motor seem to be ill matched. The VFD can supply not a lot more than the typical no load current of that size motor.
But how do you tell the customer the VFD that they sized is not suited to their application? Particularly a heavy duty application.
Diplomacy required.

To the OP, be careful about what you take responsibility for.