575V/50Hz to 690/60Hz Variance

Hi all,

I have a client that is trying to save money on generation costs. They have decided to run their generator on different speeds and so our supply voltage will vary between 575V/50Hz and 690V/60Hz. This causes problems for our pumps and more important, burner. Air to Oil ratios are dependent on Oil Pump and Air Fan speeds.


Now.. I was thinking to install a frequency converter at our supply that would keep the voltage and frequency at a steady level.

Anybody has experience with that, and what equipment would one use? A regular Danfoss FC?

As long as the only loads on the VFD output are induction motors and you can start and stop them all at the same time from the VFD, it could be made to work as long as you pick a VFD that will tolerate the input voltage range without faulting.

You will need to program the drive output for the minimum expected input voltage and use motors at that minimum voltage rating. This is due to the drive not being able to increase voltage input-to-output.

I would also recommend a 5% line reactor in the drive output leads to reduce the voltage of the PWM pulses when the input rises to 690V. And be sure to use motors with an MG1 Part 31 endorsement to protect the insulation.

The concept of letting a generator speed fluctuate like that is just plain stupid by the way. I understand that you are not responsible for that and are just looking for a way to overcome the challenge as presented to you for your equipment, but your customer is in serious need of help here.

So just to clarify what Dick was saying there, the input FREQUENCY to a VFD is basically irrelevant, because it is just rectified to DC anyway. So he's right, pick a motor based on the lowest end of the range and program the VFD to put out that voltage even if the incoming line voltage is higher. I would also use at least a load reactor as well as a line reactor too by the way. The PEAK of the DC pulses is still going to be based on the maximum input voltage, it is only the RMS voltage that the drive can adjust with it's PWM pattern. So if for example you pick a 480V motor because that is the lowest possible swing, that motor will be subject to added stresses when the DC bus climbs to over 975VDC while the input AC is at 690V. The load reactor will help mitigate any damage from that.

However, there might be another issue with SOME drives. In most VFD designs, power for the electronics, i.e. the PC boards, is derived with a Power Supply Unit (PSU) tapped off of the DC bus with a DC/DC converter, in which case no problem here. In some designs, the VFD engineer will have a totally separate AC/DC PSU that taps off of the incoming AC line ahead of the rectifier. If that PSU board is sensitive to line frequency and voltage swings, it might give you problems. it may be difficult to find someone at your favorite supplier who truly knows that detail, but it's extra important in your case, so don't cut a PO until you get it in writing that it will work.

But again, 1 VFD for 1 motor, not as a power supply for everything on a machine.

Awesome, thank you guys!

And yes you are right, I get to deal with the consequences and have no say on the cause .

what type of generator they use?

others things to look the power cable and instrument around the generator

I don't know. All I know is that I get a varying voltage in.

Lets assume that we could live with the frequency variance, do you guys know any "off-the-shelf" products that could maintain a static voltage with a varying supply voltage.

A constant voltage transformer?

Yes, that would account for a sliding supply between 575-690V and output a static frequency.

Autotransformer comes to mind

Any make you would suggest?

jkerekes said:

A constant voltage transformer?

Click to expand...

A ferroresonant CVT would not be a good idea for supply source for motors, they will "flat top" the input sine wave to current limit and will interfere with accelerating torque unless significantly over sized, in which case you could be looking at an efficiency loss of up to 30% through the CVT.

danatomega said:

Autotransformer comes to mind

Click to expand...

An Autotransformer by definition is just a transformer that is not an isolation transformer, it does not adjust anything. Maybe you meant a variable transformer (variac) with a motorized automatic voltage control? It would likely cost a lot more than the VFD, but it would be good for multiple loads. Staco is the most widely known automatic variac supplier.
http://variac.com/staco_480v_3ph_y.htm

Wont change the frequency however, what comes in goes out.

Thanks all!

I am in contact with some suppliers, who seem to be able to deliver exactly what I need. I am waiting for information, but what he explained to me sounds like it is a device similar to a UPS but without the batteries.

I will let you know what I end up with!

Thanks again, I appreciate you guys borrowing me those big heads

@JRef : Yes, my mistake - I was thinking from way back when we used a motorized variac and the term autotransformer was used in the description.