VFD single phase to three phase conversion 4

[Wedoca]

alright guys .....here is the application ...

sewage pump station with on site generator.

the VFD located inside of a 230V panel,
the oringinal idea was to use the drive as a phase convertor, since the utility only provide us with single phase there...

pump infor .....

10.1 hp, 230V, 60 Hz, 2 pole, 3 phase , FLA 25A, service factor 1.15 ,
rated to pump 99GPM

VFD infor......

20 hp, Culter Hammer SVX drive ..

now I have set up the drive parameter to match the pump .... max RPM, Max Amp (25), Max Frequency (60)

Problem: When turn on the pump, the display on the VFD showed that the frequency would only go up to around 50Hz ....I measured the out put voltage and current and it was around 180 phase to phase(fluctuating), 135 phase to ground (fluctuating), and it was pulling about 27 amps ..... pump phase ground resistance was meggered and all three phase was above 999 Mega-ohms ....

now ...on the previous thread ....some of you (Thanks again) have mentioned that the lack of capacitance due to undersized VFD causeing low output voltage at the VFD (180V) which lead to over current (is that right??) and because of the current limit thats set on the drive ....therefore the drive was not able to get pass 50Hz...( please correct me if I am wrong)

I have checked the VFD and it is a 20HP unit, so it was oversized to compensate the DC ripple problem.... so that pretty much sets me back into the dark ...

ideas anyone????

 

[PeterKsiazek]

When I size a drive for single phase operation I take the motors FLA and double it... then find a drive with that current rating.

So in your case, you should be looking for a drive with a maximum output greater than 50 Amps (25A * 2). The drive you selected (assuming I'm looking at the right one, 20HP)states a current rating of 31A.

By doing this doubling of the FLA and not the HP you will ensure you are getting a VFD with a high enough DC bus capacity.

 

[jraef]

PeterKsiazek,
He never said a model number on the VFD. I think maybe you assumed he used a 25HP 460V drive (because you mentioned 31A), while I would hope Wedoca used a 25HP 230V drive, because he has 230V. The 20HP 230V drive should be more than adequately sized.

So Wedoca,
There are a number of possibilities here:
1) You did indeed pick out a 20HP 460V drive, in which case it is too small.

2) You fell into the trap of "Variable Torque" drive sizing, and by picking a 20HP 230V VT rated drive, the actual current rating is lower (more like a 15HP CT rated drive) and yes, you have run into the capacity problem.

3) You have neglected to change some sort of factory default setting that is limiting your output to 50Hz. Most VFDs designed and built for the European and Asian markets come with factory defaults at 50Hz; although most US sales organizations will either make the mfr change them or they change them before they are shipped. But every now and then one slips through and you must change them.

4) Your speed command signal is not telling the VFD to run at full speed. This itself can have a number of causes. Most VFDs allow you to view the speed command signal directly. if you are using the keypad to set the speed, this is not likely the cause, but if you are using a pot or an analog signal, it could very easily be the problem.



"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

 

[LionelHutz]

I wonder if the VFD is limiting the current to about 25A due to a parameter setting or it's overload capability.

Drawing full current at 50hz indicates to me that the motor is fully loaded (well, it's running at rated torque but not rated HP) and when you increase speed further the required torque to turn the pump faster would put the motor over it's rated torque.

 

[ScottyUK]

Lionel,

That depends on the V/Hz setting too - if the motor has less than rated volt/Hz at 50Hz then it will draw more current to compensate and slip will increase.

----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[DickDV]

Unless the voltage and current data you gave us comes from the drive keypad, I have no confidence that they are accurate.

I would first check the input voltage to the drive when the motor is running at the limiting speed and load. If the input voltage is holding up around 225-230V, then I would think that the drive sizing or programming is the problem.

If the input voltage is falling off as the load increases and, at the limiting speed it is below the above values, the drive is basically starving for voltage and has no choice but to starve the motor as well. That's a power supply problem, not a drive problem.

It would be useful to display the DC bus voltage on the drive display and then watch it as the motor runs up in speed to the limit. If the bus voltage drops below about 310 volts, you either have low input voltage or the drive rating is as jraef mentioned and you don't have enough bus capacitance to hold the voltage. In that case, your drive is one size too small for the application.

 

[Wedoca]

I did set the VFD max current output to 25A, and the motor is under the full load condition ...but still it should not have been pulling more than 25A agree???

more Drive Infor....

its a Culter Hammer SVX drive model number SVX 020A1-2A1B1

its a 20HP 230V Drive Max Current Input is 61A if I am not mistaken therefore the max out put current should be around 35A ...which based on PeterKsiazek mention on the forum.... ideally max current output needs to be double of the FLA of the load... I guess what I am saying is ..is my drive too small to do the job??? if it is ...how do I verify it?? .... and also ...the output voltage flucuate so much 180 is just a "rough" guess ...is that a sign of cause of a problem ...or it just a by product of whats really screwing things???

 

[jraef]

You shouldn't have any capacity problem with that drive, it is a 20HP CT rated drive so it is capable of the full 61A output. I'm not sure where you are coming from with the 35A issue, but the derate factor would say that this drive is good for about a 30.5A INPUT current without problem.

So now I'm curious as to what you mean by "I did set the VFD max current output to 25A," in the above statement and in your original post. Did you set the OVERLOAD setting for 25A, or did you set the MAX AMPs of the drive to 25A? There is a difference here. The Overload setting is just to protect the motor. The Max Amp setting is to protect the drive and/or power source; think of it as a "Current Limit" setting.

If what you set was the Max Amp setting, that may explain the reaction by the drive; it is getting to 25A, but the load is demanding more so the VFD artificially pushes down the output frequency in an attempt to limit the current to 25A, overriding the commanded speed. In that case, current will fluctuate as the drive attempts to keep in under control, and the 27A you are seeing is basically the overshoot. Voltage will be all over the map as well, because the drive is modulating on the fly to try to maintain the limited current. I would set the Max Amp setting to 30.5A (because of the phase conversion) but then make sure the OL setting is for 25A. It should allow for brief forays into higher amps that way, but still protect the motor.

If your motor continually attempts to draw more than 25A, you have a motor that is too small for the job you are asking it to do. Either replace the motor or change the task.


"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

 

[Wedoca]

Jraef

what I meant by " I did set the VFD max current output to 25A" was that I set the parameter for max current to 25A not the overload setting.... I understand that you are saying the drive is trying to maintan the current ouput thus forcing the frequency to remain at 50 Hz ...and all that is because the motor is pulling too much current...
but my question is ...why??? why would the motor pull more than the FLA ???? it was designed to run at the giving condition and it was tested at the factor, winding resistance seem to be fine ( all Phase to Ground ressistance are over 999 Meg-Ohm) the measurement was taken on site after the motor had been exposed to the enviorment. could it be the inrush thats causing the motor to pull more than FLA??? and since it can't never pass that current limit. Therefore the motor never reachs that stable stage and instead, stuck at the max VFD current setting thus causing the frequency to drop and the input voltage to drop and flucuating????

 

[LionelHutz]

An induction motor will pull whatever current is necessary to turn the connected load and you put too large a load on the motor. The VFD gets to 25A at 50Hz and stops because the motor requires more than 25A to accelerate further.

If you increase the current limit to 30A, I'm betting the motor will accelerate to a higher frequency until it's running at 30A.

FYI, a motor connected to a VFD would not have an inrush current. Inrush current is caused by a motor having a high slip.

 

[Wedoca]

lets say the I adjust the max current setting on the VFD to 30A and now the frequency is up to 60Hz ...but how about the voltage??? I think one of the reason for low voltage is because of low frequency and as result over current ...but if the frequency go back to normal so should the voltage right? then wouldn't it drop the load current ??? since now the voltage is back to 230V instead of 180V ( refer to the top for situation stutas) ??

 

[LionelHutz]

Yes, the lower frequency is the cause of the lower voltage. The VFD operates using a Volts/Hz ratio. 230V/60Hz = 3.83. So, for every 1Hz increase in frequency, the voltage increases 3.8V. This is how a VFD works. At 50Hz, the output voltage should be about 192V.

No, the current will not go down as the frequency goes up unless the load on the motor (the torque required to turn the pump) is reduced.

 

[jraef]

I had a brilliant response, the website faulted on it. Now LionelHutz said a lot of it (in fewer words). Here is what's left.

Two possible scenarios where starting current could be getting you in trouble with your current limit:
1) You may have failed to mention that the VFD is acting as a phase converter, but it is feeding another FVNR (DOL) starter downstream. In other words, you just used it to give you 3 phase but there was an existing controller that is coming on and off independently of the VFD. You can't do that. The drive MUST be the ONLY controlling power device.

2) If the VFD is the only controlling device, you (or someone) has the ramp time set too low. The factory default is 1 second, that may be too low to avoid a starting current problem. Someone might even have set it to the minimum, 0.1 seconds. Sometimes, if the pump technician set it up, they get concerned about damage to the pump rotor because the mfr says it has to be at full speed within 1 or 2 seconds. But this refers to Across-the-Line starting, not VFD starting. It should probably be around 10 seconds.


"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

 

[LionelHutz]

Good thought. The VFD must be connected to the motor and then a start command given to the VFD. Don't switch a motor on and off of a running VFD.

I'm not sure a short ramp time would cause an acceleration problem like this. The VFD would just keep "pausing" the acceleration to allow the motor to catch up and the current to fall before continuing. I've seen VFD's show current limit as they accelerated and it didn't cause a problem, except for the acceleration time being lengthened a bit.

 

[Wedoca]

Jraef

The VFD is connected straight to the motor ..but also the VFD is controled by other device ...we got a device thats monitoring the water level and turn on and off the pump by it...can you elabrate hows that a problem ????

my next question is ..... with the same "load" if motor is pulling 27A @ 190V would it pull less current if now its operate at 230V?? cause 27A @190V ...thats about 5.1KW right?? and with the same load, power consumption should remain constant therefore at 230V now we should be pulling about 22A yeah??? ( plesae correct me if I am wrong)

 

[itsmoked]

Wedoca; Your voltage verse current statement is correct! But! Only for a non-VFD controlled motor.
A conventional across-the-line hookup would do exactly what you described. With a VFD it would just limit the motor speed or throw a fault.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

No, it is valid for a motor on a drive too: that's what I was trying to say in my previous post. If the volt/Hz ratio is low, i.e the voltage is too low for the frequency the drive is generating then it is analogous to line volts being low on a fixed speed motor. The slip will increase and the motor will draw a higher current until the motor stalls or the drive goes into current limit.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

The drive doesn't just hold at the lower frequency?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ozmosis]

It may be possible the VFD may need some settings internally to indicate you are running on 1 phase.
I don't know the drive all that well but our VFD has a setting if you are going to operate the 3 phase unit on a single phase supply. The settings are: ignore phase loss detection, detect phase loss but automatically derate to function but not trip, trip on phase loss detection.
This size of drive will not as standard, I would guess, be suitable for operating on 1 phase input when you pull it out of the box so parameter settings would normally be made to tell it.
If you haven't made any paramter settings to tell the drive that you are only using a 1 phase input, you should have. Otherwise, if you were using a 3 phase input and you lost a phase then why hasn't the VFD tripped?

 

[LionelHutz]

Could you do everyone a favour and go spend some time with Google?

Try to find some info describing the basic operation of a VFD's and the way the V/Hz ratio is used. Apparently, we can post here that the drive should maintain a constant V/Hz ratio but you're not following so hopefully another source might help. In theory, the output voltage should be 190V @ 50Hz so in practice measuring 180V isn't that far off. Many drives will do what is called field weakening, especially when set to variable torque. Basically, the VFD expects the load to drop quickly as the speed reduces, so it will lower the voltage a little extra to save energy.

If you increased the voltage to 230V while the VFD is running at 50Hz, the magnetic circuit (the steel) in the motor would likely saturate and the current would rapidly increase. Try it. There will be a setting such as base frequency or motor rated frequency which will be set to 60Hz. Set it to 50Hz instead and you will get the 230V at 50Hz you so badly desire. Just don't blame me if you destroy the motor or the VFD.

Also, at the same time, search out the info on the load profile a centrifugal pump will present to a motor. You should learn that with a centrifugal pump you're not going to increase the frequency by 10hz and reduce the load at the same time. You would also have to do something to reduce the flow (hint-partially close a valve) if you want to reduce the load the pump is putting on the motor. The load could increase by 30% to 40% going from 50Hz to 60Hz.