inverter - motor mismatch

[Pafi]

I have a problem regarding one client who ordered inverter and motor from different manufacturers and not matching characteristics. The inverter is for 315kW/ 3x400V, 50Hz asynchronous motor but the current capability (616A) and kVA rating are under what the supplied electric motor needs. It is an electric motor with 315kW but with cos PHI = 0.7 (rated speed 500rev/min, rated current 698A). They think to compensate the motor by adding a three phase capacitor in paralel at its terminals, but there is a problem with the inverter, it will not work (the distorted voltage of inverter, the inverter cannot have a capacitive load). I'm thinking to make a kind of filter by calculating an output reactor for the inverter and adding the capacitor in paralel to the motor. Which should be the resonant frequency of such filter? 50Hz ? Probably I would need to make a simulation?
Thanks for any clue.
Pafi

 

[mikekilroy]

Pafi, capacitors are wonderful things but they are not magic bullets. Adding a capacitor to the output of the vfd will do nothing to help this situation; in fact, if added across the vfd output it is a really bad thing - vfd's do NOT like capacitive loads.

Perhaps your customer thinks that since power factor capacitors can reduce the reading on a clamp on ammeter upstream of an induction motor across the line, that it will likewise perform the same on the vfd output: it will not.

Adding a tuned output filter to clean up harmonics on the current into the motor from the vfd will reduce losses - by about 1-5%! Nothing to help you.

In 95% of cases, folks do not use 100% of their motor rating: said another way, it may work fine as is - try it and see if you like it.

If they need all 698 amps they need to buy larger vfd. No way around it. Sorry.

http://www.KilroyWasHere.com

 

[jraef]

Forget what the cos Phi current of the motor is if it is calculated, that is irrelevant. All you need to know is the motor NAMEPLATE Full Load Current. If the nameplate FLC is 698, that means that the motor WINDINGS can handle being loaded to 698A without damage. What the motor power factor is at that point is completely irrelevant to the sizing of the VFD. Fixing the PF at the measurement point is, as Mike said, also completely irrelevant, because that does not change the HEATING effect that is taking place inside of the motor by virtue of the current is flowing through it.

If the VFD is only rated for 616A, you should be able to set the Current Limit in the VFD to 616A and leave it there. What will happen is that as you increase speed and thereby load on the motor, the drive will eventually hit the current limit setting and automatically override your speed command in order to decrease the load and thereby keep the motor current below the VFD's operating threshold. The result is that if the motor is fully loaded to ITS capacity, you will not be able to get full speed from it. Chances are however as Mike said, that you may never get to that point anyway because most of the time, a motor is slightly over sized due to standard manufacturer's available ratings. For example in the most simplistic of calculations, let's say your motor is now going to be restricted to 278kW maximum output (based on (616/696)*315 ). If what your machine designer needed was actually only 270kW, the next lower standard motor size is 250kW, so it was too small. He picked the 315kW even though the required LOAD is only 270, so when limited to 278kW you are still fine. Still, if that possibility of not getting full torque at full speed is unacceptable to the user, then the only alternative is to replace the drive with one rated for the motor nameplate FLC.

When I am teaching people how to select VFDs, lesson #1 is NEVER go by someone's arbitrary power rating, be it kW or HP. That is meaningful only in general terms. ALWAYS use the motor nameplate FLC. In your case you were burned by the fact that it is a 12 pole motor... This happens a lot more than you would think by the way.

"Will work for (the memory of) salami"

 

[Pafi]

Thanks for your advice!
From what I understood the problem was that they didn't oversized the motor - it is for a pump - and now they cannot fulfill the parameters, so they need the whole 315kW.
Anyway, my oppinion is (after reading YOUR oppinion) that there is no other solution but to change the inverter for a bigger one if they want 315kW from that motor.
regards,
Pafi

 

[jraef]

Correct. Limiting the speed to limit the current has a more dramatic effect on pump flow if it is a centrifugal pump.

But that said, if it IS a centrifugal pump, there are industry "tricks" in sizing inverter drives based on the load being variable torque. VFD output current ratings are based upon "worst case" scenarios under load. So for years, VFD mfrs have played a ratings game in giving larger inverters two output ratings, one for "constant torque" loads like conveyors, big machine drives etc., then a higher power rating for use on "variable torque" loads, meaning centrifugal machines like pumps and fans (note, not ALL pumps and fans are centrifugal however). The newer terminolgy for this is "Normal Duty" (ND), meaning the variable torque rating, and "Heavy Duty" (HD), the constant torque rating.

The difference is in the overload capability of the drive. In the HD rating, the drive is expected to be able to handle an overload condition of 150% of its rated current for a minimum of 60 seconds, upward of 200% for 3-10 seconds (varies by mfr). The same drive, if used on a ND load, is only expected to deliver 110% overload for 30 seconds, 125-150% for 2-3 seconds. This is essentially an acknowledgement of the fact that if you sized the motor correctly for a centrifugal load, you should be essentially unable to cause an overload condition other than in the case of some sort of failure, in which case you would want to shut down anyway. As a general rule, the difference is usually one motor size. So if your VFD is sized as 616A for a HD application, the rating for a ND application may be significantly higher, possibly well above the required 698A.

If however the pump is NOT centrifugal, and/or the drive is ALREADY rated at 616A under ND operation, you are back to square one.

If you are not sure, post the mfr and model number of the VFD, many of us can tell you right away.

"Will work for (the memory of) salami"