Weird issue with VFD hanging on acceleration. Any ideas?

eKretz · Jun 2, 2020

So I've finally got my Baldor VFD up and running to power my 5 HP lathe. Hooked it up a couple days ago and fired up the motor. First time fired right up. Second time it hung on acceleration at 2 Hz and spiked to almost 30 amps, like it was trying to accelerate but stuck. Sometimes it will fire right up, sometimes not and will hang there bouncing off 2 Hz at 29-30 amps. If I hit run and stop several times consecutively it will finally pick up and go. No-load current draw is about 6.5 amps. I tried disconnecting the motor and hitting run with no load and the VFD runs right up to 60 Hz every time, so I'm thinking something either with the motor or possibly the wire connections.

So... i guess step 1 would be check the resistance between the motor terminals and also each to the motor case. Then maybe terminal connections. Is there anything I missed?

This is a 25 HP Baldor 15J drive running off of 230VAC single phase. It's running in V/Hz mode and set for constant torque.

SomeoneSomewhere · Jun 2, 2020

Check current on all three phases is the same - it's not single phasing?

Does the shaft turn freely by hand - does it need much starting torque?

Try re-running the auto-tune.

eKretz · Jun 2, 2020

Will check current today. Motor shaft turns freely by hand, as does lathe input shaft. Starting torque doesn't seem to make much difference in terms of whether the lathe is in a high RPM gear or a low one, I've tried both ways. There is no auto-tune on this VFD, and it's unnecessary in V/Hz mode as I understand it.

markz528 · Jun 2, 2020

"It's running in V/Hz mode and set for constant torque"

That confuses me. I don't understand the set for constant torque - what does that mean? Do you have it set for speed or torque control? If torque control, then this could be screwing you up as torque control in V/Hz can be tricky down low.

Normally V/Hz is not a preferred mode in today's age, would like to run in sensorless vector mode, but does not look like this drive has sensorless vector mode (looked quickly through the manual) so you got what you got.

Is there a motor issue? Could be - but I would label it as a low probability. I have seen intermittent shorts in the windings where the drive phases back in hz in order to keep the amps in check, but this is very rare type of failure.

What is the motor full load amps? Around 11 or 12?

My suspicion is that your volts per hertz is out of whack at the low end. You may be going into magnetic saturation at the low end. I would do a few things one at a time:

- review your parameters again - make sure you have proper hp, amps and voltage entered in the drive - does it ask for power factor of the motor?
- remove the "constant torque" if you can
- reduce voltage boost to minimal levels
- see what your motor voltage is when it is stalled out

If it were mine that is where I would start. Without reviewing all the parameters, I don't know if there isn't a parameter setting that is getting you. I have good Baldor contacts and can probably find you someone to talk to if you need.

Jraef · Jun 2, 2020

The VFD is likely going into Current Limit, which functions by overriding the commanded speed in an attempt to shed load in order to keep current down. In your case, current is going high immediately, so the drive stops accelerating at 2Hz. That is usually indicative of a short in the Lead wires, the motor windings or a bad transistor inside of the drive.

markz528 · Jun 2, 2020

Jraef said:
The VFD is likely going into Current Limit, which functions by overriding the commanded speed in an attempt to shed load in order to keep current down. In your case, current is going high immediately, so the drive stops accelerating at 2Hz. That is usually indicative of a short in the Lead wires, the motor windings or a bad transistor inside of the drive.

The fact that it runs sometimes makes it much less likely that it is a motor or transistor issue. Yes - it is going into current limit but if you over-magnetize the motor it could be mostly reactive current. I actually ran into a similar situation one time and that time the motor design was so weird that the drive could not supply proper magnetizing current even in V/Hz mode. I am suspecting a V/Hz issue here but it is an educated guess.

Although possible, that would not be a typical motor failure. I have only seen it a couple of times in my career, and I have worked on many, many motors.

JST · Jun 2, 2020

You probably DO need to check the wiring carefully. There could be a reason for that "works sometimes", such as vibration that wiggles wires into a bad position and then out of it again.

Not too likely to be a motor issue, but it is kinda possible, it could be something similar, but inside the motor. Possibly some other odd factor having to do with the motor, like an end-wise shift of the rotor and shaft when magnetizing current comes on it that sticks in a jammed position sometimes.

eKretz · Jun 2, 2020

markz528 said:
"It's running in V/Hz mode and set for constant torque"

That confuses me. I don't understand the set for constant torque - what does that mean? Do you have it set for speed or torque control? If torque control, then this could be screwing you up as torque control in V/Hz can be tricky down low.

Normally V/Hz is not a preferred mode in today's age, would like to run in sensorless vector mode, but does not look like this drive has sensorless vector mode (looked quickly through the manual) so you got what you got.

Is there a motor issue? Could be - but I would label it as a low probability. I have seen intermittent shorts in the windings where the drive phases back in hz in order to keep the amps in check, but this is very rare type of failure.

What is the motor full load amps? Around 11 or 12?

My suspicion is that your volts per hertz is out of whack at the low end. You may be going into magnetic saturation at the low end. I would do a few things one at a time:

- review your parameters again - make sure you have proper hp, amps and voltage entered in the drive - does it ask for power factor of the motor?
- remove the "constant torque" if you can
- reduce voltage boost to minimal levels
- see what your motor voltage is when it is stalled out

If it were mine that is where I would start. Without reviewing all the parameters, I don't know if there isn't a parameter setting that is getting you. I have good Baldor contacts and can probably find you someone to talk to if you need.

There is a parameter setting with 4 options: 1.) Normal (2.5 KHz carrier frequency) Constant Torque; 2.) Normal (2.5 KHz) Variable Torque; 3.) Quiet (8 KHz) Constant Torque; 4.) Quiet (8 KHz) Variable Torque.

I have the drive set to option 1 since a machine tool is a constant torque demand as far as I'm aware. I wasn't able to get to checking on the motor today, hopefully will have time tomorrow.

The motor settings in the drive are all correct, double checked them more than once. I have set the full load current setting to 16 amps, but that's just a guess. The motor dataplate is missing. I'm running on the theory that it's a 5 HP motor, as it's about the right size. Also, this is a two speed (assuming 900/1,800) motor... I am running on the high speed windings/terminals. Not sure if that makes a difference. When the motor comes up to speed correctly, it's at 1,795 RPM at 60 Hz - checked the motor pulley with a tach.

johansen · Jun 3, 2020

either you are single phasing the motor or you have a volts per hz curve that is wildly incorrect.

i can think of no reason why you would see more than nameplate full load amps for your lathe, as normally by default you've got a reasonable acceleration of say, 10 to 30 hz per second set for the vfd frequency acceleration. you should only see a maximum of full load motor amps, on the order of 10 to 15.

your no load motor current once it gets up to speed is a little high but, sometimes those values are incorrect. my $67 2 hp vfd reports low on current draw.

vfds cannot deliver more volts than you deliver to the vfd so if the volts per hz is set correctly its not possible for the drive to deliver the motor 6.7 amps at 240vac no load at 60hz unless the voltage delivered by the vfd is low or if the waveform is particularly bad, or if your mains is below 200 volts.

no load amps should decrease with nominal voltage until you reach really low values.

i have a high efficiency 5 hp motor, its maximum efficincy point at no load is about 2 amps at 20 volts. only 44 watts needed to keep it running on single phase power. 40 watts if i remove the fan.

your lathe should absolutely not be sucking up 6.7 amps per phase, that heat has to go somewhere or it is being wasted in the motor windings via circulating current due to a shitty vfd or completely wrong volts per hertz.

motion guru · Jun 3, 2020

A couple of observations.

Constant torque vs variable torque settings are common parametric adjustments for drives and have to do with overload / continuous current / peak current protection algorithms in the drive. Fan / pump apps are variable torque applications, lathes are more typically constant torque. Generally only lower quality drives do not have these settings.

Many drives when operating in open loop V/Hz mode have a current limited frequency output that automatically reduces the output frequency when the drive exceeds the motor nameplate FLA parameter. Generally if this occurs with the motor shaft unloaded, as noted above, you either have an intermittent open circuit causing single phasing of the motor . . . OR, an intermittent high impedance short circuit somewhere in your motor or wiring. This is why we test every installation (motor, motor leads) with a megohm meter when installing a drive system.

Lastly, the amount of current in the windings when running an unloaded motor will be associated with the V/Hz mapping set up in the drive. Normally if set up correctly, this current will be constant from just above the slip speed of the motor all the way to full speed and is associated with the magnetizing current (inducing magnetic fields in the rotor). I would fully expect nominally 6.8 amps for a 230V, 5HP motor. See below for example nameplate for a 5HP inverter duty motor.

johansen said:
i have a high efficiency 5 hp motor, its maximum efficincy point at no load is about 2 amps at 20 volts. only 44 watts needed to keep it running on single phase power. 40 watts if i remove the fan.

your lathe should absolutely not be sucking up 6.7 amps per phase, that heat has to go somewhere or it is being wasted in the motor windings via circulating current due to a shitty vfd or completely wrong volts per hertz.

2 Amps at the line connection perhaps, but if 2 Amps at the motor T-leads, then your motor is running severely under fluxed and you need to correct your V/Hz settings and/or motor cos/phi setting if the drive is so equipped.

jim rozen · Jun 3, 2020

eKretz said:
Also, this is a two speed (assuming 900/1,800) motor... I am running on the high speed windings/terminals. Not sure if that makes a difference. When the motor comes up to speed correctly, it's at 1,795 RPM at 60 Hz - checked the motor pulley with a tach.

Wait, back up a step.

You were running the motor on utility three phase (or via a rotary converter maybe) and it ran fine, correct?
Now you have hard wired the motor to the high speed and tied to the Baldor drive.

1) how many wires does the motor have, is there any possibility it is wired for the wrong voltage? If six wires then it would be consequent
pole, and single voltage. Lack of a nameplate here does hamper things.

2) if you have wired the motor direct to the Baldor drive, be sure the wiring agrees with the contator/switch setup usually used in the machine.
For consequent pole motors, in one speed three of the wires are shorted together, and the others go to line. In the other speed range, three
wires are left open, and the other three go to line. Point being it might be easy to get this subtly wrong, and the motor might run - sort of.
Maybe double check the wiring.

3) if you are relying on the lathe's original controls, powered by the VFD, then something is not going to work well at all there.

4) consider hooking this baldor drive to another, known good, single speed motor and test it there.

5) again, measure the motor winding resistances and isolation from ground. A DVM that reads 20 megohm is probably good enough for the isolation.
If the motor is consquent pole, when you write down the matrix of winding resistances (1 to 2, 1 to 3, etc all the way up to 5 to 6) then you should
see three distinct values. If consquent pole, imagine the windings are in a ring with six equally spaced taps. The resistances will be a couple
of ohms so you need to account for the lead resistances of the meter, most meters can zero those out.

eKretz · Jun 3, 2020

Thanks for the comments and additional info. One thing I do recall now that it was mentioned: the amperage actually rose as speed was decreased when the drive managed to bring the motor up to speed correctly. So at 60 Hz the no-load current was 6.5ish amps, and when the motor speed was brought down the amperage rose. Not much until it got to really low frequencies down around 20 Hz or lower - say maybe up to 7.2 or something amps and then way higher into the 15-20 range at very low speeds but with the motor still turning. I remember that because it struck me as very odd.

eKretz · Jun 3, 2020

jim rozen said:
Wait, back up a step.

You were running the motor on utility three phase (or via a rotary converter maybe) and it ran fine, correct?
Now you have hard wired the motor to the high speed and tied to the Baldor drive.

1) how many wires does the motor have, is there any possibility it is wired for the wrong voltage? If six wires then it would be consequent
pole, and single voltage. Lack of a nameplate here does hamper things.

2) if you have wired the motor direct to the Baldor drive, be sure the wiring agrees with the contator/switch setup usually used in the machine.
For consequent pole motors, in one speed three of the wires are shorted together, and the others go to line. In the other speed range, three
wires are left open, and the other three go to line. Point being it might be easy to get this subtly wrong, and the motor might run - sort of.
Maybe double check the wiring.

3) if you are relying on the lathe's original controls, powered by the VFD, then something is not going to work well at all there.

4) consider hooking this baldor drive to another, known good, single speed motor and test it there.

5) again, measure the motor winding resistances and isolation from ground. A DVM that reads 20 megohm is probably good enough for the isolation.
If the motor is consquent pole, when you write down the matrix of winding resistances (1 to 2, 1 to 3, etc all the way up to 5 to 6) then you should
see three distinct values. If consquent pole, imagine the windings are in a ring with six equally spaced taps. The resistances will be a couple
of ohms so you need to account for the lead resistances of the meter, most meters can zero those out.

The motor was being run at the place I bought the lathe on 230VAC line three phase. I was not able to test it there but they said it ran fine... No guarantee of that of course. Where the machine resides three phase is not available so the input to the drive is 230VAC (about 234 last time I checked) single phase converted to three phase by the Baldor drive. The motor has 6 terminals with 6 wires - one to each terminal and the drive is wired directly to the motor, nothing in between. The terminals are labeled U4, V4, W4 and U8, V8, W8. I am using U8, V8, and W8 terminals, the other three are open. Not using original controls at all. Thanks for the additional info about the dual speed consequent pole motors... The lack of nameplate/dataplate was indeed worrisome to me. The only thing I've got is a formed maker name which appears to be "Tatung." Hopefully I can check the motor and wiring closer today.

eKretz · Jun 3, 2020

So after reading Jim's comment I did a little google sleuthing about two-speed consequent pole motors. It seems that since I'm using the high speed on the motor I should have U4, V4, and W4 connected. After checking that those resistances correlate with the diagram I will give that a try today and see what happens.

jim rozen · Jun 3, 2020

The resistances will be pretty low. If you can find a DVM that gives you one or two decimal places that's good. I have an old analog simpson meter specially designed
for low resistance measurements that I use.

Most DVMs allow you to 1) short the probes together, and 2) zero the display again. This drops the lead resistance out of the measurement, which can be almost an
ohm or so.

eKretz · Jun 3, 2020

I've got a milliohmmeter. That will do the trick.

markz528 · Jun 3, 2020

I don't know anything about 2 speed motors so not much help in that aspect.

But I have been to both the Taiwan and Chinese Tatung factories (not sure they are still making motors in Taiwan or not) and must admit they were making some of the better "Asian" motors at that time. Been a few years, but they had a good tech guy in the states - if you continue to have problems might be worth to try to track him or a counterpart down - maybe they can shed some light on what you have.

eKretz · Jun 3, 2020

Ok. So I checked all the winding resistances via the terminals and definitely getting three distinct numbers. Low was 0.59 ohm, middle was 0.95 ohm, high was 1.06 ohms. Measuring resistance of terminals to ground was outside my DVM's ability to measure (over 50 MOhms). Tried shorting the other three terminals and that resulted in the motor not starting at all. I'm wondering if this winding configuration is not constant torque... Found this other winding configuration (variable torque) on the same site as the first. There also seems to be a "constant horsepower" winding config with the same horsepower in both low and high speeds.

Switched back to my original wiring orientation and tried a few more parameter changes. Remembering Mark's comment about the V/Hz profile possibly being out of whack, I decided to try some changes there. After a few tweaks on the 3Pt. V/Hz Profile parameter, the issue seems to have gone away. I'll update in the next few days as I have had time to play with it a little more. If it really is an intermittent problem it should reappear before long. I meant to test the wire lead running from the drive to the motor for any high resistance shorts or bad wires also but spaced it.

eKretz · Jun 5, 2020

Well, I guess it must have been a saturation or "pullout" issue. Found a white paper from Yaskawa describing the pullout thing, they say either one can cause a big current draw and no-start condition. I played with the 3 point thing a bit more today and the motor started just fine every single time with setting of 10% and 2 Hz. I think that increases the voltage at low frequency, but not entirely sure, heh. On acceleration it doesn't draw more than 12 amps and settles out at 6.3 just spinning the transmission and empty chuck. Setting the V/Hz parameter back to linear results in the intermittent hang on acceleration again. Setting it back to 3 Pt. cures it and it starts right up every time.

eKretz · Jun 8, 2020

Ran the machine for a few hours today with no issues, guess this one can be marked solved.

Weird issue with VFD hanging on acceleration. Any ideas?

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