Strange Motor / VFD Behavior

How are you measuring the output voltage and current? Are you reading it from the VFD display or are you using separate metering devices?

Hi

It would seem as tho the voltage would have to drop with the frequency, else the motor would draw to much current. Note the much higher current at 30 Hz vrs 50 hrz.

re the rise in motor current - It seems as tho you may have a partial winding to winding short somewhere in the motor. This would NOT show up to ground. One or more turns of the motor windings are shorting together somewhere. This can be hard to prove from the outside of the motor. If you have any kind of impedance bridge available, this might show up a significant difference in one set of windings compared to another.

Might we ask what other symptoms brought you to take all these readings?

Was the system as a whole not performing correctly?

The current is still at the name tag rating - 28 Amps.

Also - if the load on the motor is minimal, would a smaller motor serve? and save a bit on power?

Hi,

this situation is not at all usual nor ok.

Voltage shall be roughly proportional to frequency to overcome the inductive part of the resistance.

But is your motor does not produce any significant output torque this may be different.(?)

If you are sure that the motor is not matched to the needs of the process than I would change the motor.

Please try to measure all voltage and current waveforms, not only rms values.

Is there a filter between the VFD and the motor?

RHABE

I think the problem may be mechanical rather than electrical.

With the motor OFF- disconnect wiring or turn off feed breaker- try spinning the cutting blade. If all is well, it should spin fairly fast and for a reasonable time.

If- as I expect- you have a bearing problem, the spin will be slow and it will stop fairly quickly. If the bearings are the problem, your best choice is likely to replace the motor. You will also benefit from improved power efficiency.

Make sure that the new motor has easy-access lube ports. You may want to install something like a zerk fitting and use the "pressure" fitting to periodically (about every 6 months) lube the motor when it is running without load (at about 50% speed) using the recommended lightweight motor oil (likely not grease). The pressure lube approach will flush any stone dust from around the bearings- I think it is too late for these.

Hello Rudy,

seems like you're wondering if its the VFD or the motor/saw that's causing current / voltage to rise.

If the apparent load increase is real, then input current will rise accordingly to represent the extra power needed to drive the motor, so 33Hz at 100%+ current will be 70%+ power compared to rated input.

If created by abberrant calcs etc in the VFD, the motor will have a poor pf and efficiency but active power will not increase much.

I'm surprised the motor is not getting hotter than usual - is the motor current balanced.

If in doubt, revert to V/f and output the usual at 33Hz, if lower voltage the current goes well down, VFD errors; if at lower Volts the current goes up, mechanical problems??

Your meter should measure the input current OK so you should get an idea of power comparing to VFD rated input for comparison.

It sounds like VFD errors to me as current goes down as you increase speed to 50Hz and I suppose voltage can only stay at about your input level 380V.

Otherwise, if the saw blade is the same, why are you running slower as cutting speed may affect your load characteristics etc??

Worn bearings most definitely do not require replacing the motor. Bearing replacement is simple and easy, but should not be done by do-it-yourselvers. Motor replacement should be considered only in the most extreme cases, where bearing damage has progressed to the point where the motor shaft and armature are also damaged. Otherwise just relpace the bearings and provided no other damage is present, the motor will be as good as new.

I agree that bearings CAN be replaced fairly easily BUT- as you so aptly stated- it should not be done by amateurs.

My thought on recommending motor replacement was based on the fact that- by the time you pay someone to replace the bearings- and the fact that the motor will likely be sent/taken to their shop for that to happen- you could just install a new motor for likely less NET cost (especially considering the lost production) AND have a new, high efficiency, longer life, lower operating cost motor designed to run with a VFD within an hour.

My thoughts also tends to lean toward bearing problems, especially since during initial run your amperage is lower. Once the worn bearings heat up, they can alter the rotor positions enough to overwork the VFD while it is attempting to monitor and control the motor as efficiently as possible. Frequency run drives can be more temperamental than mechanical starters, but they will inform you when failures are forthcoming. If you are coming up with overvoltage faults, it may be you are experiencing a regenerating condition due to motor slip.

RE Bearings in this beast - this is a 20 HP motor. think big. Shaft size of what? 1.5" to 1.75" or so? 4 or 5 cm? And it is used to cut stone? If that bearing goes dry, it will blow in far less than one day. Just fry itself. Balls that look like burned Peas in a black pot.

20 HP is not going to just warm up a bearing of this size. It's gonna blow it big time.

I'm still leaning toward shorted windings.

I do hope we get to find out what the actual problem was.

Without data on the motor- it is possible that it is a sleeve bearing. Since it is used to cut stone, it is very likely that some sub-micron dust has gotten into the bearing oil and causing the added load.

My suggestion for a non-powered spin test will tell if it is a bearing problem or not.

As stated earlier- if it is a bearing problem, a new motor, modified to have the ability to be periodically "flushed" with clean oil should eliminate the problem forever.

If the problem is NOT the bearings- known within 15 seconds into the test- then some other cause may be possible. However- the megger testing definitely did NOT indicate any level of internal short- both phase to phase and phase to ground were reasonable values.

Good point

"it is possible that it is a sleeve bearing"

- must admit I did not consider that. I would still tend to think that a bearing load that sucked up a third more power would get darn hot darn fast.

I totally agree on the issue of temperature. But- unfortunately- that issue has never been generated in the discussion.

Pragmatically- as a sleeve bearing (and its oil) got warmed up by the friction, the oil would thin out and the bearing would "grow" a bit, likely minimizing overall temp rise. The likely oil/dust slurry would still offer reasonable resistance to the spin so the load would still be present.