VSD operation on a 15 kW scroll chiller

[jayaeros]

Hi All,



I have a scroll chiller that can be operated with and without the VSD. While running at full load without the, the power meter registered 4.5 kW with line voltage of 415 V and 10.7 Ampere.


On the other hand, while operating in the VSD mode at 50 Hz, the line voltage before the VSD is 415 V and the ampere is 8.3 A (instead of 10.7 A when operating without VSD).
But after the VSD, the connection to the compressor motor indicated that Voltage is 450 V and the ampere is 11.2 A.


Please refer to the attached file on the VSD connection.


It can be seen that power difference between the inverter inlet and outlet is around (4.8 -4.5 = 0.3 kW). I assumed that this is the losses in the VSD. My major worry now is on the input voltage to the compressor which is around 450 V. Since the compressor is 415 V, is this input will damage the compressor? I have also used another clamp meter to measure the input voltage to the compressor, but it indicates around 465 V.



Please help.


Regards,
jaya

[Yuri B.]

At what frequency come those 450 volts to the motor?
If about 60Hz, then no worry about motor's winding (because ratio V/Hz stays the same). If unit's nameplate states 400V 50Hz/ 460V60Hz, then no worry about the compressor as well.

[jayaeros]

Hi,

The output frequency was set at 50 Hz. I dont really understand on what you meant by "If unit's nameplate states 400V 50Hz/ 460V60Hz, then no worry about the compressor as well"

Please help

[Yuri B.]

Hi.
The motor/compressor has a nameplate with el parameters?
VSD is original to or a is "modernization'' to the chiller ?

[jayaeros]

hi,

The system does not come with VSD. I installed it separately.

The electrical parameters stated on the motor are 420 V (50 HZ) and max Ampere of 15 A.

[Yuri B.]

You have not measured (did not mention at least) what power it draws when with the VSD. If no more than 4.5kW (which it does without VSD) - at the same load of course - then no reason to worry. No excess in power consumed = no excess heat in windings = no danger to the motor.
Your measurement after VSD may turn out to be simply unnprecise (harmonics voltages add).

[jayaeros]

It can be seen that power difference between the inverter inlet and outlet is around (4.8 -4.5 = 0.3 kW). I assumed that this is the losses in the VSD.


As stated earlier, the power consumption while operating using the VSD is around 4.8 kW and without VSD is 4.5 KW.

Is this damaging to the compressor? I think that excess .3 KW is the power consumption of the VSD itself.

Please advice

[Peter_1]

You measure this probably with an electronic voltmeter. This meter measures RMS (which comes from a perfect sinusoid) I suppose and it is therefore calibrated for a perfect sinusoid which isn't generated at all by a VFD.
Measure once with an older analog meter and I'm almost sure you will see smaller deflections. This meter 'flattens' out more your readings

Are you also sure the parameter on your VFD - if applicable -is set to the correct inlet voltage?

A good source I found for this http://www.opamp-electronics.com/tutorials/measurements_of_ac_magnitude_2_01_03.htm

[jayaeros]

Yes, i have measured it using digital rms multimeter. About the inlet voltage setting of the VSD, im sure that the setting is correct as i have to chose the available option on the voltage rather than typing in the value. I will get a anolog meter to test it soon.

Thanks for the reply and the great link.

[NoNickName]

Peter has got the point. A multimeter cannot properly measure the PWM waveform output from the VSD, therefore the 450V reading is false.

[Y L Lai]

Just wondering what was the pressure, hi and low when you measure?

[jayaeros]

the refrigeration side measurements do not differ much while running with and without vsd.

[Electrocoolman]

Measuring the output current on a VSD with a meter will be inaccurate. High frequency leakage currents affect the readings and can burn out ammeters.

[jayaeros]

Hi,

Can you please advice on how the measurement can be done apart from using the multi meters?

[SteinarN]

I think it is necesarry to use expensive equipment to measure input and output amps and voltage of a VFD with any degree of accuracy.

The most important thing is to be sure you have configured the VFD correctly. If you have set up the V/Hz parameters to suit your motor you should be safe at that point at least. Dont the VFD have a display where you can do a readout of the suplied volts, Hz and amps?

Edit: I saw you have a DRO on the VFD. I trust this DRO to be more acurate than your measurment with a multimeter. Many VFD's can suply a slightly higher output voltage to the motor than the input voltage to the VFD.

[ktmtragic]

The drive has to be at the same frequency as your local supply for example 50 HZ in Australia when the reading is taken. A VSD puts out a DC voltage which is then chopped by thyristors to give you (dirty) AC. That is why even the best meters never get it right. If the VSD parameters are correct just let it work.

[jayaeros]

Dear All,

Thanks for the all the replies. Really appreciate it.
I have another question regarding the operation of VSD.

It seems that if VSD is operated at full speed(50 HZ), the chiller consumes more power (4.8 kW) if compared to the operation of the chiller without the VSD (P=4.5 kW). The additional power consumption, 0.3 kW is used by the VSD itself.

Thus, can i claim that it is very inefficent to use VSD at full load?

Regards,
jaya

[El Padre]

Hi,
Interesting topic, the portable oscilloscopes that I have seen are very expensive, I think that it is possible to hire them here in the UK, regarding the VSD at full load I am not surprised by your findings, when VSD started to become popular over here, there were concerns about their reliability when used in constant load applications.

Cheers

[ktmtragic]

This is correct the additional load is that consumed by the control and drive thyristors

[Noori]

Dear jaya
Are u sure about the setting values for the used vfd , if so , don’t worry if the compressor running well without any increasing in temperature.
Can u send me the type of the used vfd.
best regard

[jayaeros]

Hi Noori,

Sorry for the late reply. Im using Danfoss VLT 2875

[Y L Lai]

Hi Noori,

Sorry for the late reply. Im using Danfoss VLT 2875

Jaya,

I don't have experience running 2800 on compressor but many in fan application; I never have this problem.

According to Danfoss AKD manual, parameter 101 Torque characteristic (I don't know bout VLT and it might defers):

If Compressor CT (1) is selected, a load-dependent U/f characteristic is obtained, in which output voltage and output frequency are increased at increasing loads in order to maintain constant magnetization of the motor.

Maybe this is why you get higher power output and voltage when compare to running w/o VSD.

Hope this help. If I'm wrong, someone please correct me.

[goshen]

hi there u need a true rms meter !!
most vfd allow u to monitor the voltage and amperage supplied to the motor on screen.
powering a compressor via a vfd alowes u not to install pf capacitores
the net sees pf=1 !!!
in most vfd u also have a kw meter so u can calculate ecxactly what your sys preformance is
if u would like more info mail me at
gill@gosheneng.co.il i have more info regarding vfd & compressores