Having issues with 3 phase industrial motor... looking for ideas

[Corporate666]

A simple project is turning into a hassle.  A customer asked me to see if I can get a lathe going for them.  It was made in the UK and has a Jaguar CFD750 VFD connected to an ElectroDrives 7.5kW motor.  The machine is in the USA and was intended for the US market. 

The VFD was malfunctioning, so it was replaced with a Hitachi WJ200-075HF.  The machine is running on 380V 3-phase (measured right at 400V at the input).

The problem is that when the VFD is started, the motor turns at a few RPM and is incredibly jerky.  Then after several seconds, the VFD gives an overheat alarm and errors out. 

-I have checked resistance between all phases on the motor and they are 1.5 ohm and all phases open to ground
-I ran an insulation resistance test and got around 500 megaohms at 1000V, all phases are very close to one another in measurement
-I hooked up another 3-phase motor to the VFD (5hp, all I had) and it ran smoothly without problems
-I bypassed the VFD entirely and the 10hp motor in the lathe starts immediately and runs smoothly.  BUT.. when I checked with a clamp on amp meter, I was seeing 30 amps on each phase!  This is essentially with no load (in high-gear, a 1:1 connection thru to the motor). 
-I can turn the motor by hand easily when it's turned off, there are no grinding noises and it appears very smooth
-I eliminated all the circuitry possible to just the drive and motor, and the drive being activated only by the run switch on its panel and cycling through speed with the up-down buttons on the VFD.  Still does it (so it's not a bad speed control pot).

I first thought that perhaps the motor is not wound for 400V, causing the massively higher amp draw than expected, but the machine has been in service for over 25 years and used daily without issues... so if it was being massively overdriven, I'd expect it would have burned out long ago.

The input and output voltages on the old vs new VFD are identical.  There's no way the previous VFD was putting 30A to the motor.

I assumed the motor was 4-pole.  The nameplate is hidden where it's almost impossible to see, but had 1000 on there (couldn't read what that was in reference to).  I thought perhaps it's a 6-pole motor, so I changed the VFD settings and it ran much better, but had a very hard time accelerating and would usually end up petering out as RPM's rose and going back to very slow and jerky motion.  I will bring a tach and measure the no-load RPM tomorrow to correctly determine the # of poles.

The only two things I can think are that it was perhaps wound for UK voltage of 230 or so?  And running it at 400 is massively overdriving it.  But I can't imagine it lasting 25 years under such circumstances.  Nor would I expect the major manufacturer that sells these to make such a mistake.  Or perhaps there's something about a 50Hz motor I am missing that would lead to my results?

Any tips or pointers is appreciated.  I'm mostly at a loss for what to check next or where to go from here.  VFD works good on other motor.  Motor tests fine and runs fine direct from input power (but with high amp draw).  What to do/check next?

[NiHaoMike]

Could the motor be designed for more than 60Hz at its rated voltage? If possible, see if there's a way to extract the settings from the old VFD.

[Corporate666]

Could the motor be designed for more than 60Hz at its rated voltage? If possible, see if there's a way to extract the settings from the old VFD.

Unfortunately the old VFD is dead as a doornail and won't power up at all.  It supports up to 120Hz max drive frequency, but my understanding is that it's bad to run the motor above designed frequency.  But the inverter never manages to get more than a few Hz out because the motor won't accelerate... I could try increasing the frequency temporarily if it won't be detrimental to the motor?

[Circlotron]

-I bypassed the VFD entirely and the 10hp motor in the lathe starts immediately and runs smoothly.  BUT.. when I checked with a clamp on amp meter, I was seeing 30 amps on each phase!  This is essentially with no load (in high-gear, a 1:1 connection thru to the motor).

Is it possible that the motor is rated for 380V in star connection but is actually wired for 220V delta, and the old VFD never got to full frequency and voltage so it kinda scraped through? Newer more sophisticated VFD notices something is wrong and has a hissy fit. Direct mains connection under those circumstances would make it pull lots of amps.

[dom0]

There's no problem driving a 230 V delta motor from a VFD with 400 V bus voltage (besides somewhat lower efficiency). In U/f mode, just set U2/Umax to 230 V and f2/fmax to 50/60 Hz.

[richard.cs]

Agree with the above, the motor is probably wired in delta for 230ish V phase-phase leading to saturation and high current draw when connected directly and causing the VFD to throw errors. If so either re-wiring it for star or changing the V/f settings should fix it.

Many motors in the UK are wound for 230 V delta / 400 V star. Usually they're connected in delta when they're used with a VFD which has a low bus voltage, generally one which only has a single phase 230 V input. With a direct connection to 400 V phase-phase they're wired in star, for use with a VFD that has a high DC bus voltage they can be either but the VFD needs configuring correctly.

[rvalente]

Here some toughts:

Have you tried to auto-tune the VFD? Have you changed any paremetes for the control loop in the VFD? P o I values?
Any overtorque?

Have you tried telling the VFD the motor is 200ish volts and see how is runs?

[capt bullshot]

The symptoms you describe point into the direction (as other said before): The VFD settings (voltage over frequency, ramp up speed) do not match the motor.
For a VFD running an induction motor, IMO these are the most important parameters:
- ramp up speed (so the motor won't slip/jerk when accelerating)
- voltage over frequency (the output voltage of the VFD should match the motor nominal voltage at the frequency). Required voltage is usually proportional to frequency and has some boost at low frequencies.

Both parameters (if set wrongly) can cause the VFD to trip on overcurrent and the motor to jerk.

[Andy Watson]

Is it possible that in re-wiring the motor for the new VFD that one of the phases has had its polarity reversed?

[dom0]

Then it would only turn the wrong way

[Andy Watson]

Then it would only turn the wrong way

It might turn the wrong way - very inefficiently.

[duak]

I think the motor is most likely not wired correctly and the previous VFD was programmed to make it work that way.  If it was configured delta, the V/Hz could have been turned down but to get full power, the current limit could have been turned up perhaps leading to its demise.  A lathe can be a hard load to drive because of the inertia of the spindle & chuck when in the highest or fastest speed.

Just to make sure we're on the same length of wave:

1.) is the motor NEMA or IEC and what voltage is it wired for?  Full specs would be handy but a shot of the data plate is good enough.

2.) is the line input single or three phase?  I expect three because of the power ratings.

3.) how are you measuring the line voltage - leg to leg or leg to neutral?  380 V is not common in the US (or Canada).

Most four pole motors can usually tolerate a 50% speed increase as manufacturers use a similar rotor design in their two pole motors.  On the other hand, the lathe gearing and bearings may not be up to an increase.  Lathe chucks are often cast and have a definite RPM limit; they do indeed come apart.  If they don't disintegrate, they can release the workpiece being held due to being expanded by the radial forces on the jaws and body.

[jmelson]

The machine is running on 380V 3-phase (measured right at 400V at the input).

The problem is that when the VFD is started, the motor turns at a few RPM and is incredibly jerky.  Then after several seconds, the VFD gives an overheat alarm and errors out. 


-I bypassed the VFD entirely and the 10hp motor in the lathe starts immediately and runs smoothly.  BUT.. when I checked with a clamp on amp meter, I was seeing 30 amps on each phase!  This is essentially with no load (in high-gear, a 1:1 connection thru to the motor). 

Well, this is not far out of line.  A 10 HP 240 V motor would draw about 30 A per line at idle, then the phase angle changes from nearly 90 degrees to close to zero as the load increases to full load.  This is the typical characteristic of an induction motor, they draw near full-load current but at 90 degrees lagging at idle.

Are you sure the motor is a 380 V motor?  Is it a dual-voltage motor that is now wired for the wrong voltage?

If it is a 240 V motor, then the VFD should be able to be programmed for a different V/F curve, so it gets 240 V at 60 Hz.

What is the current limit setting in the VFD?  It might be set to a much lower default value, causing it to trip on what may be a normal load for a 10 Hp motor.

Jon

[Kalin]

Important information needed. Is it a 3 lead motor connection in the peckerhead or possibly 9 lead or 12 lead? I have seen the behaviour in improperly connected 9/12 lead 3 phase motors. I could help you troubleshoot this but more info is needed.

[Corporate666]

Thanks for the replies.  I didn't get much of a chance to look more at this today, however I am pretty confident that the issue is the motor is indeed wired for 240V instead of 380V.  It would explain everything that I have observed.  I initially thought the motor may have been wired in delta instead of wye, but didn't pursue this avenue too much, since I had checked the VFD and it was a 380Vin/380Vout vector drive unit, and because the motor is buried in the machine with the wiring on the back side (of course) and the motor plate wedged against the casting, so almost impossible to check either without removing the motor.

The previous drive was most likely set up to operate in V/F mode, whereas the new unit I was operating in sensorless vector mode.   I'll have to dig into the settings a bit to see how to limit the V/F settings, if I can't just re-wire the motor to operate on 380 (which I would prefer).  The new drive does not have a simple Vmax setting I can see, rather it allows adjustment of the V/F curve through numerous parameters... but either way I think I'll try to rewire the motor if feasible instead.

I'll keep you all posted, thanks again!

[tautech]

One more consideration for a lathe motor, until the headstock oil warms with a little running the load on the motor can be higher than normal especially it the headstock gears are set to high speeds.
In this case selecting lower gearbox speeds overcomes startup issues.

[Gregg]

I have seen the same symptoms caused by a bad connection to one of the phases.  The motor will run OK without the VFD because the grid doesn't care if the load is imbalanced, but the VFD sees it as a problem.  What I have seen several times is poor crimp connections to the ring terminals of the motor leads, where the crimps were covered by the lead numbers at the factory. 

[dom0]

I have seen the same symptoms caused by a bad connection to one of the phases.  The motor will run OK without the VFD because the grid doesn't care if the load is imbalanced, but the VFD sees it as a problem.

With VFD the VFD itself provides the necessary motor circuit breaker functionality, but without VFD it would be unwise to connect a big motor without MCB to mains. MCB will also trip on unbalanced load, since a three phase motor missing a phase may not start properly and at zero speed current is very high.