Input side power factor of a VFD driving decoupled motors (no-load run) 3

[edison123]

In my repair shop, we test run motors from tens of HP to thousands of HP on no-load, decoupled condition. The no-load PF of these motors will be very low varying from 0.01 to 0.05 (less than 0.1).

I know the AC drives typically have nearly unit power factor on their input side when the motors running on load. What will be the PF at the VFD input side when the motors are running decoupled (no-load) with such very low PF? Will it still be close to UPF?

I plan to buy a 415 V, 250 KW, 6 pulse VFD without front end sine wave filter. Can this single VFD be used for various ratings of motors (from tens of HP to thousands of HP) for no-load testing? Will I have to 'tune' the VFD for each type of motor or will it auto tune?

Thanks in advance for your answers.


Muthu

http://www.edison.co.in

 

[FacEngrPE]

On the input side of a rectifier the issue is no longer power factor, it's harmonics, and how your billing meter interprets the harmonic content. You can get a significant improvement in harmonic content with a choke, if that is not enough I would consider an isolation transformer. In my location the issue we need to be concerned with is the utility harmonic tariff content limit.
No need to order the drive with the chokes installed, but i would make sure their is space to install them later if needed.
ABB Technical Guide No. 6 Guide to Harmonics with AC Drives has some useful information.

https://files.engineering.com/getfi...Technical_guide_No_6_3AFE64292714_RevF_EN.pdf

Also check your utility tariff to see how harmonic content matters.

Some drives can accept a wide range of inputs, but no off the shelf drive will cover your entire range of sizes.

This is a quote from Schnyder's Altivar 61, Altivar 71 documentation
[li]An autotune is not needed for most applications, and a bad autotune (because of a loose connection or a warm motor) can cause more problems than it's worth. We suggest doing an Autotune if it is a high performance application requiring low speed torque or closed loop control.[/li]

I suggest keeping a computer connected to a drive used for motor testing, the data available from the drive may be useful for troubleshooting and customer reports. It is also easier to load parameters from a computer. Some drive support programs allow uploading stored drive parameters.

Fred

 

[jraef]

Thousands probably not. But you can probably test up to 1250HP or so. The motor PF does not change, so the unloaded current remains the same, on large motors usually around 30% of FLA. don’t autotune, leave it in V/Hz mode.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

Thanks, Fred and Jeff, for confirming the drive can cover a good range and there is no need to auto tune.

What about the power factor at the drive input end? Will it be UPF even if the motor PF is less than 0.1?

Muthu

http://www.edison.co.in

 

[LionelHutz]

I wouldn't bother with the filter. You will likely find you don't need it because the current draw will be so low it's not required. Something to note is that the input current will only be the real current required to spin the motor. You might only see something like 5A or 10A being drawn from the source when running a 250kW, 415V motor.

The distortion power factor will be quite poor because the drive will only be pulling small blips of current from the source. But none of the traditional passive filtering methods will work to filter the drive input because of the lower current draw.

I'm doubting you can test motors that are 1000's of HP with a 250kW drive.

 

[edison123]

Thanks, Lionel.

Below are some high HP/KW motors tested via 0-415 V auto transformer (variac) with PFC caps in parallel. The auto transformer input current is kept under 10 to 50 Amps with an input pf of around 0.95 to 1.0.

690 V, 700 KW, 3000.3 / 3532 RPM, 50.2/59.1 Hz 670/665 Amps
No load test Stator 420.6 V/43.2 A

207-690 V, 27-1000 KW, 899.2/3002.7/3242 RPM, 15/50.2/54.2 Hz 245/930/925 Amps
No load test Stator 390 V / 46 A

440 V, 1250 HP, 743 RPM, 50 Hz
Stator Volts/Amps-440 V/1605 A Rotor Volts/Amps - 885 V / 645
No load test Stator 120 V/169.3 A
No load test Rotor 440 V/143 A

587 V, 1150 HP, 800 RPM, 1120 Amps
No load test Stator 445 V/185.2 A

587 V, 1150 HP, 800 RPM, 1120 Amps
No load test Stator 250.9 V/102.4 A

So no-load testing motors up to this capacity with a 250 KW VFD shouldn't be a problem?

Will the VFD input current be in this range of 10 to 50 Amps i.e. PF compensated ?

Muthu

http://www.edison.co.in

 

[LionelHutz]

The input current will be the real current only, so yes you could say the VFD will power factor correct the motor internally.

As for your test data, it's useless to evaluate if the VFD will work. The VFD must supply the no-load current of the motor, not the corrected current.

 

[edison123]

Lionel - The currents posted are actual motor no load currents, not the PFC compensated currents at the auto primary. Thanks for confirming the VFD will correct the PF internally.

Muthu

http://www.edison.co.in

 

[LionelHutz]

OK, but it still doesn't help much unless you are going to fudge the V/Hz ratio in the VFD to starve the motor of voltage.