Why must a motor be inverter duty for vari-speed applications? 5

[Lovison]

Is there a rule of thumb for just how low a motor
speed can be reduced before the motor itself stalls?

If you have a motor speed of 1775 RPM and you want to
slow it down - let's 50% what comes into play within
the motor? Does it pull more or less amps? Just what
takes place when the voltage is reduced? I suspect
overheating would take place because the fan is not
cooling the outer shell adequately? Can you still
maintain phase balance at lower speed ranges or does
it drift? Wayne E. Lovison
service-parts@naglepumps.com

 

[kspiroff]

The pulse width modulation used to simulate an AC source generates voltage spikes (ringing) and in certain cases standing waves can add up to high voltage values. Without damping of some kind phase to phase voltage differential can be as high as 1,600V.

Certain Annixter cable specifically for inverter fed motors uses 2kV insulation.

 

[dpc]

The minimum motor speed has more to do with the driven equipment than the motor. The motor can run at very low speed if the drive output is adequate. But it will be limited in the amount of torque it can produce. There can be cooling issues, but the motor is generally not producing nearly as much heat at slower rpm.

 

[electricpete]

As far as stalling goes, that would be more likely during an increased speed mode. Torque is proportional to (V/f)^2. Many drives adjust V in prop to speed f.

Amps is generally determined by the load power (at high loads) which will usually decrease as speed decreases.

 

[redtrumpet]

With a VFD, the speed-torque curve of the motor itself does not change significantly with frequency. The speed-torque curve at 30 Hz is more or less identical to the speed-torque curve at 60 Hz, only 30 Hz now represents 100% synchronous speed. The full-load current for rated torque should not change appreciably. The VFD itself will operate the motor in the linear portion of the speed-torque curve close to the 100% synchronous speed point. The motor will never approach the breakdown (stall) torque as it would if it was on constant-frequency supply and the load increased (unless the VFD is grossly oversized).

Many factors influence how low the motor speed can go. Ignoring motor heating, in theory a vector control drive with tach feedback could provide 100% rated torque at 0 speed.

 

[tmahan]

You are correct in your assesment regarding heating and lower fan spead. Motor makers cite widely different speed ranges for their motors ranging from 4:1 to 1000:1 at constant torque. Motor typically have two solutions for use at reduced speed ranges 1) use a higher temperature class of insulation so that heat build-up resulting from lower cooling fan speed does not result in motor damage or 2)use suplemental cooling for the motor. This is when you have a small (fractional hp) motor mounted on top of the primary motor. The smaller motor runs at constant speed providing airflow over the motor.


Either approach is valid. The key is to specify the speed range your motor will operate at under normal service conditions

 

[dpc]

Yes redtrumpet, I think I mis-spoke regarding the torque available - thanks for correcting me. But I believe the available horsepower is reduced as speed is reduced below 60Hz, correct?

dpc

 

[redtrumpet]

Correct, dpc; hp = torque*rpm/5252. A motor operating at half-speed, but with rated torque, is producing half of its rated power. The vector control drive with tach feedback can hold a load at zero speed but the motor is producing no power!

 

[electricuwe]

It is very important to distinguish two different types of speed control for induction motors:

Controlling the speed by a variation of the voltage using an transformer or a AC-Controller:

very cheap and only suitable for small motors with fan or pump loads, special motors with high resistance rotors required, for this type of drive stalling is an issue !

Controlling the speed by variation of voltage and frequency (using an inverter):

more expensive, but suitable in the range from a few W to several MW, can be used for all typs of loads, standard motors can be used in some applications

I guess all the other answers refered to the second type of drive, but I'm not sure if Lovison is really talking about an inverter drive.

 

[Lovison]

Gentlemen,

Can you run a non-inverter motor with a VFD? Some say
yes other's say you should buy a motor built for inverter duty.

If the non-inverter motor is used what are its chances
and limitations?

Many of you have really helped me out and I appreciate
the feedback. One other question and that is the old
style bubbler controls do they work under basically the
same principle as a VFD?




Wayne E. Lovison
service-parts@naglepumps.com

 

[resqcapt19]

We have over a 100 motors at this site on inverter drives. They range in size from 1 hp to 400hp. None of the motors are rated for inverter duty and most of motor leads are over 150' in length. We have no indication of an excessive motor failure rate on the inverter powered motors. Only three of the inverters have input and output reactors, and this was too provide some additional protection to 200 hp submersible deep well pump motors. One of these pump motors has a total lead length of 1300'. The other motors are connected directly to the inverter output using standard wiring methods.
I know these installations do not meet the recomendations from the drive and motor manufacturers, but we haven't had any trouble with them. Maybe we've just been very lucky.
Don(resqcapt19)

 

[redtrumpet]

I think you should look at the NEMA application guide for AC adjustable speed drive systems. It contains a lot of useful information, and is available for free download in .pdf form from the NEMA website

http://www.nema.org/index_nema.cfm/1260/

Type in "adjustable speed drive" in the keyword search and it should bring it up.

 

[dpc]

redtrumpet,

If you actually try to download this, it doesn't seem to be free anymore. Cost is $70. Or at least that is what happened to me.

dpc

 

[GordS]

We have had VFD's at our facility since about 1985. We never used to do anything special with the motors and never had any VFD related failures. We did find that as soon as technology started using IGBT switches, we did have to take greater care in our applications. Our first IGBT drive was feeding 15 hp motors. The motors sufferred insulation failures within about 2 weeks ( 2 out of 3 motors in the installation failed). We now, with the newer drives either specify inverter duty motors ( as per NEMA MG-1-31) or add output inductors or filters onto the drives. We have not had VFD related motor failures since.

 

[redtrumpet]

dpc - I tried the web link I gave in my previous posting. It opens up a search screen. On the right-hand side, under "Standards Search Tips", it says "if the electronic price says $0.00, click on the title to obtain the complimentary document."

Then I entered "adjustable speed drive" in the "keyword or phrase" box and clicked search. The first document that pops up is the Application Guide. Price is $0.00 (electronic copy), $70.00 (hardcopy). I click on the title as per the instructions under "Standards Search Tips". I scroll down the screen to "Complimentary Documents". There is the Application Guide, a Motor FAQ, and a Technical FAQ. Right-click on the Application Guide, save to your chosen location, and voila - free Application Guide.

 

[electricuwe]

I also tried to load the application guide and got it free. But if you do not follow the instuctions dpc gave exactly you also can end at a place where the download costs 70$, the same as the printed version.

 

[dpc]

Redtrumpet,

Thanks, I got it. I guess this is why I do power engineering and not computer science

dpc

 

[dydt]

to answer you main (posted) question...

an inverter duty motor should be use for all VFD on the market today. one main reason is that almost all modern VFD uses IGBT which switches from zero to peak in microseconds...the fast voltage rise (dv/dt) per unit time cannot be handled by a standard motor...inverter duty motors have more insulation on phase to phase or slot to slot..although you can technically use a standard motor premature breakdown is likely to happen...and for few extra money why not go to the right motor for the job reducing downtime and labor of replacing it on the job site.

dydt

 

[electricuwe]

I do not aggree with dydt:

There are lots of applications where using a standard motor is possible wihtout risk. See the NEMA application guide for dealing with the voltage transient problem.

Using an inverter duty motor in can also cause some problems:

Availabilty
Additional cabling an protection for the fan
Cost (of course, but it's not the most important aspect)
Spare part inventory
Upgrading existing DOL drives with VFDs gets much more expensive, if the motor has to be replaced

And additionaly: If the voltage transient problem is the only reason for using an inverter rated motor alternative solutions, like a dv/dt filter should be considered. dv/dt can cause other problems than motor insulation failure, e.g. if small drives are used with long motor lines, dv/dt can cause current spikes. These spikes might trip the internal short circuit protection of the drive. In this case the dv/dt filter solves both problems associated with dv/dt whereas the inverter duty motos solves only one.

It would be nice if GordS could give furhter details of the application where the problem with standard motors on IGBT VFDs occured.

 

[twowire]

DYDT is correct. Although using a standard motor is still common practice amoung many industries, The duty cycle of the motors is much shorter than an 'inverter motor" due to the dv/dt of the switching voltage. Although standard duty motors work, the life cycle is cut down. Once a motor is rewound you can really see the diffrence in reliability.
Twowire