Manual separate controls of V and F in VFD's 1

[edison123]

Is it possible to order VFD with separate and independent control of V and F with manual pots like controls like these?

I would like to control both V & F independently and manually (without programmed V/F). Capacity 100 KW.

Muthu

http://www.edison.co.in

 

[itsmoked]

I can't imagine that as being available. Maybe Jeff knows someone.

There may be partial ways you can command an OTS unit to approximate that.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

You may be able to achieve that by changing the parameter for motor voltage.
If there is a V/Hz parameter, changing that should do the trick.
Doing it on the fly with a running motor may not be an available feature.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Marke]

It would be very dangerous in the wrong hands!!
I have not seen anything like this.

Mark Empson
Advanced Motor Control Ltd

 

[RedSnake]

What is the purpose of this feature?
Meaning why do you need it?

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[edison123]

Thank you all.

This set up is to run medium and large MV motors decoupled (open shaft) in our shop, where I need to have higher V/Hz for the initial break away torque from the zero speed. Once the motor reaches about 1/3rd speed, I revert to rated V/Hz on the fly. At present, we use a AC generator-DC motor setup where I can control V and Hz separately to get the initial higher V/Hz and dial back to rated V/Hz, but this is a noisy set up. I was thinking of going for quieter VFD/Step up transformer route with independent V & Hz controls, if possible.

Muthu

http://www.edison.co.in

 

[RedSnake]

Most modern VFD drives has that built in as a premade option /function, that you can choose, often called fan or pump drive.

Best Regard A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[Drivesrock]

Hi Muthu.

What you describe as higher V/Hz at start sounds like the voltage boost function at 0Hz that most (if not all) VFDs have to enable the motor to 'break away' or overcome the winding resistance effects (voltage drop) at this low frequency and low applied voltage. There is fixed boost and maybe in some VFDs a load dependent voltage boost function. These functions are used in VF mode. The user only sets the amount of voltage at start instead of 0Volts at 0Hz. The voltage boost gradually reduces as the frequency increases and intersects with the normal V/F curve further up. In one drive I'm looking at here this intersection is at three quarters base frequency.
Are you familiar with this boost function?
You probably won't find a VFD that 'connects' the boost parameter (voltage) to an analogue input, but some may allow you to adjust via the keypad/HMI with the drive started. The troubles you will have are getting the right voltage and not too high currents that the VFD trips on overload. For this reacon I would suppose you want the potentiometer as a quick adjustment method?

Another way I thought before your last post, was to set the motor base frequency in the VFD much lower but maintain maximum rpm. The VFD will maintain constant V/f upto to the lower base frequency. Say for a 50Hz motor you set in the drive, 25Hz. So now you're doubling the V/f ratio up to 25Hz. Then, after 25Hz you'd be working in the field weakening range as the volts won't go higher but the frequency does. At 50Hz you would be back to the normal V/f ratio for that motor.

For sure, with the right VFD you should be able to start your MV motors as you described. I would also suggest you think about the Amps rating of the VFD and it's overload capability. Your generator set may allow for some high overloads that a VFD won't.

 

[edison123]

Hi Drivesrock

You nailed my requirements better than I did. I did not know of voltage boost function. Once I set the boost voltage, does the drive reduce the voltage automatically to the rated V/F at some point?

Your second method of setting the lower base frequency and fix the ceiling RPM also seems to be a good option. Once the motor reaches the 25 Hz RPM, can I set the drive to the rated V/Hz on the fly without stopping the drive?

Since all these motors are being run decoupled, basically the drive will be supplying only the no-load current.

As you rightly said, in some cases, I do overload the generator for a short time to get the beast moving to about 30% RPM, after which it's a smooth ramp up to rated speed. The whole exercise of getting up to to full speed takes from one to five minutes depending on the motor size. I have successfully run 11 KV, 10 MW motors with the present 1 MVA AC gen-250 KW DC motor set up.

Muthu

http://www.edison.co.in

 

[Drivesrock]

Hi Muthu.

The initial boost voltage stays on and may just slightly increase as the frequency increases but not at the same rate as would be the normal V/f ramp. So, at some frequency the boosted voltage will be at the same value as the normal V/f ramp voltage would be and then the normal curve takes over - say, at 75% of motor base frequency. VFD manuals should provide you with this info.

I don't think you'll find a VFD that'll let you change basic motor parameters - eg the motor base frequency - when it is running. You'd have to stop the VFD allowing the motor to coast (ie no ramp to zero speed), change the parameter and run the VFD again but use the fly-catch function at start to 'catch' the motor. Possibly too many parameters to change quickly while the motor is coasting down?

Yes, the VFD will need to supply mainly the mag current so you'll have to calculate what that'll be at the low voltage side (approx 30% of motor FLC?), plus, of course, the real current to accelerate the motor and for the losses. Or you may have these values from motors already tested with the generator set up?

 

[LionelHutz]

I think you're over thinking things.

A VFD is a power converter. It will only draw the real power required to turn the shaft from the line. You could probably run that 10MW motor on about 100kW of source power with a VFD, possibly even less. We sold a shop a VFD after showing them a VFD running a 700hp, 600V motor at speed while drawing <10A from the line.

A uncoupled shaft motor should not need a voltage boost to start with a VFD. The motor rated V/Hz will start it just fine.

If you use a step up transformer after a VFD then you need to take care of it too. You can't voltage boost a transformer unless you purposely get a transformer rated to do that. It will just saturate and draw excessive current otherwise.

If you are going to run the motor at less than rated voltage, then you could use a lower base frequency which would follow a V/Hz ramp up to the rated VFD output voltage. Past that frequency, the output voltage would be flat and only the frequency would increase. Once again, if you're using a step-up transformer then you'd have to it built for this use. More or less rate the transformer for the lower knee point frequency you're going to use. We have used 480V VFDs to jog 5kV medium voltage motors by setting the motor V/Hz ratio in the VFD and then running at 6.9Hz or less.

If you expect to run the motor at rated V/Hz then you have to setup the VFD and step-up transformer to be capable of supplying the expected no-load motor current, plus a little extra safety margin. That 10MVA motor might be what, 200A idle draw? This means it would draw this current at all frequencies and ramp to drawing 4MVA at rated voltage and frequency. The VFD and step up transformer would have to be capable of at least 4MVA as well.

 

[Gr8blu]

edison123: The magnetics of the motor are designed for a maximum flux density - which can be directly correlated to volt/hertz. If you juggle parameters to increase the slope (more volts for same hertz, or less hertz for same volts) you WILL have a problem. Other than the built-in "voltage boost" function on most modern drives, there is no good way to accomplish the slope change while the machine is moving - if you want it operate correctly afterward. Most drives also have either a "timer" or a "comparator" function tied to the voltage boost profile so that the switchover to the regular ramp occurs when there will be minimum disturbance in the run-up.

The drive should be capable of providing the no-load loss for the machine (friction, windage, copper loss, iron loss) - which should be fairly low for an MV machine rated for the power rating you envision. Then add a bit more, because somewhere down the road you will likely want to expand your capabilities to include a larger test article. Also recall that your acceleration time is a result of torque applied against system inertia. If you keep the current low, it will take forever to accelerate. The trade-off is that higher (than nameplate) currents create extra heat, which ultimately shortens the life of the windings and other components. No issue if the current limit is set below nameplate, though.

Converting energy to motion for more than half a century

 

[waross]

I am asking here:
I was under the impression that torque boost was accomplished by initially using a setting above the normal acceleration ramp, so that the slip frequency was higher so that the current and resulting torque were higher.
Increasing the output frequency should also increase the output voltage in the optimum V/Hz ratio.
Increasing the voltage without increasing the frequency will shortly drive the motor into saturation. Running in saturation will be wasteful and counter productive.
If you are stepping up through a transformer, transformer saturation may reduce the amout of over-voltage reaching the motor.

Will this work:
Set the short time current limit to the maximum.
Ramp up the V/Hz curve. Set the drive parameters so that the current limit is reduced once the motor starts turning.
Isn't that what torque boost does basically?

Bottom line: If the motor will not turn with the maximum current available from the drive at the motors V/Hz characteristic, then you may not be able to start the motor on that drive.

If you still want to try over-voltage, then set the V/Hz parameter higher and try it.
See the comments above concerning saturation.

Suggestion: Do you have a smaller drive available? say 5 or 10 HP?
Try various settings with a large motor to get a feel for the expected action of a drive starting a large, unloaded motor.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

We need an explanation of why there has to be a big torque boost up to 1/3 motor speed.

I personally haven't seen an unloaded motor that needs this yet. It seems rather pointless to solve a problem that likely doesn't exist.

You don't emulate how you use the genset with a VFD, you use a VFD how a VFD is supposed to be used.

 

[jraef]

Muthu,
Are you using a MV VFD, or is the plan to try to turn the MV motor using a LV VFD?


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