VFD Amp anomalies

[Tom Jenkins]

I just finished analyzing some blower test results. This uses a high-speed permanent magnet synchronous motor (25,000 rpm) controlled by a VFD at 417 Hz. The Amp draw on the line side of the VFD measured with a quality power analyzer was 412 A. The VFD display showed the current on the load side of the VFD as 460 A. The VFD display showed a power output of 300 kW (400 hp). Assuming a 95% efficiency the motor shaft hp would be ≈ 380 hp. The test load exceeded the actual field load, and the projected field hp draw would be 8% lower, ≈ 380 A and ≈ 350 hp.

I've never looked at this data before, and it raises two questions.

1) Why is the current to the motor higher than the current into the VFD?

2) Should the conductors and branch circuit protection to the package be based on 412 A or on 380 hp or 350 hp?

 

[widelto]

Tom that happened to me several years ago with allen bradley equipment, I was supposed to use that data to charge Kwh, I asked AB people the reason why and they told me that they had problems with hall effect sensors. Since then I never use drives to measure current and voltage. Maybe other brands don´t have this problem.

 

[jraef]

I just finished analyzing some blower test results. This uses a high-speed permanent magnet synchronous motor (25,000 rpm) controlled by a VFD at 417 Hz. The Amp draw on the line side of the VFD measured with a quality power analyzer was 412 A. The VFD display showed the current on the load side of the VFD as 460 A. The VFD display showed a power output of 300 kW (400 hp). Assuming a 95% efficiency the motor shaft hp would be ≈ 380 hp. The test load exceeded the actual field load, and the projected field hp draw would be 8% lower, ≈ 380 A and ≈ 350 hp.

I've never looked at this data before, and it raises two questions.

1) Why is the current to the motor higher than the current into the VFD?

2) Should the conductors and branch circuit protection to the package be based on 412 A or on 380 hp or 350 hp?

Current on the motor side includes the reactive current, i.e. the motor's actual Power Factor. Current on the line side is at a relatively consistent .95 PF because of the DC bus capacitors.

Line side conductors are to be sized at 125% of the rated INPUT current of the VFD, per 430.122, partly for this reason, but also because some Asian drive mfrs base their larger drives on kW rated motors, so they end up over sized for HP rated motors and the rated input current is actually higher.

 

[lfe]

On the load side, you are measuring the apparent intensity, which is the vector sum of the active and reactive currents, and therefore greater than the active one, which is the one that really does work.

I thing that the cables on the load side should be sized for the apparent current, that is, for 460A x 1.25 margin = 575A

On the line side, with a 25% margin they would have to be rated for about 500A

 

[Gene Bond]

It also depends on the output voltage to the motor. If the Input is 460v, and the output is 400v for example, then the output current can be 15% higher... This always throws my customers off when they are running below base speed

 

[Tom Jenkins]

Thanks guys!