Under frequency test

[Gyo]

Hi,

Could a 6.6kV 500kW 4pole 50Hz motor be tested at 47.5Hz using a VFD or a generator set? How does each of these work?

 

[jraef]

Yes, it could be tested with either. But either way that is an expensive test bench for 6.6kV! Why don't you fill in the details? Why do you need to test a motor at a lower frequency? If the V/Hz ratio going to remain the same, as a VFD would do it, or is the voltage going to remain constant and you want to test performance on a frequency droop? That might be easier to do with a VFD than a generator.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

The VFD will drop the voltage in proportion to the frequency to maintain the Volts/Hertz ratio.
The generator will hold the voltage up until the frequency drops about 3 Hertz. Below about 47 Hertz it will maintain the Volts/Hertz ratio based on rated voltage and 47 Hertz.
You may adjust either the VFD or the Generator to either maintain the rated voltage at 47.5 Hz or drop the voltage to maintain the Volts/Hertz ratio.
Depends on the reason for the test.
A generator may be easier and cheaper to locate and connect than a VFD in that size range. Renting a 1000 kW generator and possibly a transformer should be fairly routine.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I totally agree that if renting is the best option for you, you can easily rent a generator but I doubt anyone will rent a VFD.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

To get that motor started across the line you may need as much as 1500kW of generator capacity.
If the motor is direct connected to the generator before the diesel engine is stared the 100 kW generator should start it. If the Automatic Voltage Regulator (AVR) is powered by the generator output you may have to power the AVR from a separate source such as a UPS. A Permanent Magnet Generator (PMG) powered AVR will probably not need special treatment.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gyo]

Thanks for the replies. I do not quite understand how the generator can be adjusted to 47.5Hz. Has it got to do with adjusting the excitation voltage of the generator or it is akin to pressing the car accelerator to accelerate/decelerate the car?

A customer queried if a pump performance test can be done at 47.5Hz without giving any details of the purpose. I supposed it has to do with measuring the flow at 47.5Hz or could be the customer's site power supply is uneven.

 

[waross]

Something like pressing the accelerator, more like adjusting the cruise control.
The speed and of course the frequency of the engine is controlled by the governor.
You adjust the governor for the speed that you want.
You need 1500RPM x 47.5 Hz / 50Hz. = 1425RPM.
A 50 Hz set will typically be set at 1545RPM, dropping to 1500 RPM at full load. If your motor draw is 1/3 of the set capacity try a no-load speed of 1440RPM. That should drop to 1425RPM/47.5Hz at full load.
It's late, Ive been driving through a storm. Just stopped for something to eat. Hope to be home in 2 hours, 11:30PM. Beware of typos.

Bill
--------------------
"Why not the best?"
Jimmy Carter