Rockwell 3ph motor won't start up spinning

[btrnuthatch]

I have a 12" disc sander powered by a Rockwell 220 3 phase motor (1 hp, 3.8 amps). I've wired it through a VFD for single phase; it's been fine up until recently.

Now, I have to spin the disc in order to get it going. I've done some research (I am not an electrician) and kinda sorta understand that this condition points to a capacitor problem. A couple questions:

1. Is the capacitor the only possible problem?
2. I've tried to look to see if I could spot some capacitor-looking thingy but I can't get the covers off to have an unobstructed view. I don't know how to take the covers off; I've loosened them but they seem to be pressed on the shaft(?).
3. Did the VFD introduce a problem even though it ran well for several months (occasional use)?

Any semi-simple recommended course of action short of taking it to a small electrical motor shop? I have a multimeter and can reasonably follow directions (at least enough to install the VFD successfully).

Thanks for any pointers/suggestions!

 

[matt_i]

There's no capacitors in 3 phase motors....so...

I would look into the parameters for the VFD, there is probably one that deals with the ramp-up, I would set that steeply. Also check out the current limit to make sure you can supply the FLA (full load amps) that's listed on the motor, or max for the VFD is another alternative. If the current limit is set too low you could have the described symptoms.

Also I recall working on a Lenze VFD which wouldn't start its conveyor motor at times, there was a V/R parameter that was essentially some type of boost (probably above the typical steady state current limit) to get the motor started under load. It was described as being a parameter designed for dealing with extra long motor cables but increasing that parameter fixed the issue and got reliable starting back.

Along those lines, there could be an increasing mechanical drag (like bearings going bad) that the VFD can handle near rated HP state (at 60 Hz synchronous speed), but not right off 0 rpms.

Hope this was helpful, a 3ph motor is usually a very robust device that can take a lot of abuse before it tanks. If you are able to get it running and can use it, despite having to hand-start it, I think the motor is in fine health.

 

[btrnuthatch]

Matt — Thank you very much for your thorough reply. I’ll check the VFD manual (Teco).

Met my quota of learning something new every day. No wonder I couldn’t find the capacitor.

 

[MattT]

Could be any number of things but it definitely ain't a motor cap.

What does the VFD do when you try to start the motor without giving it a kick start? If it faults the fault code would be a good place to start looking. If you can set the drive to display output voltage, frequency and current watching those while trying to start the motor may help figure out what is going on. And if your multimeter is good enough you can test the drive output to see if it matches what the VFD claims it's doing.

Wouldn't hurt to ohm out the motor leads. Power down the VFD for a few minutes before you disconnect them. Then ohm 1 to 2, 2 to 3, 3 to 1 and one of the leads to ground. You're looking for the lead to lead readings to be close to one another and no continuity to ground. I'm guessing the lead to lead readings should be high single digit or low double digit ohm readings but I haven't done much work with small 200v motors.

I agree it could be increased mechanical load which can be overcome with parameter changes. Possibly temperature related if it's in an unheated shop though in that case the problem should only be on a cold start. Best to get a better idea of what's going on before playing with parameters though.

Also post the VFD make and model #. Will be a lot easier for folks to help you with that info.

 

[btrnuthatch]

MattT -- Thanks for all that. I'll try to digest what you said.

Just in case it matters, it has been working fine until recently.

Teco L510

When switched on, the VFD's display starts at 60, goes to 0 then starts to ramp up to 60. I have been immediately spinning the shaft and then it runs as expected (and the variable speed control is also fine). This time, I let it get to 60 whereby the motor made intermittent noises (more than humming). When it got to 60, I couldn't spin the shaft. Is that a clue?

Thanks again (both Matts) for your analysis and recommendations.

I'll tackle troubleshooting tomorrow.

 

[MattT]

MattT -- Thanks for all that. I'll try to digest what you said.

Just in case it matters, it has been working fine until recently.

Teco L510

When switched on, the VFD's display starts at 60, goes to 0 then starts to ramp up to 60. I have been immediately spinning the shaft and then it runs as expected (and the variable speed control is also fine). This time, I let it get to 60 whereby the motor made intermittent noises (more than humming). When it got to 60, I couldn't spin the shaft. Is that a clue?

Thanks again (both Matts) for your analysis and recommendations.

I'll tackle troubleshooting tomorrow.

Sounds like it might be single phasing. I'd not attempt to run it any more and go straight to ohming out the motor leads. And check all the connections in the motor peckerhead even if it does ohm out OK.

 

[couch67]

I wonder if one phase is out on the motor, would it fail to start up on its own and also lock as indicated in your last post. MattT's advice should help you narrow it down if its a bad phase winding in the motor or a problem with the inverter.

 

[OccupantRJ]

Power down for at least 10 minutes to bleed off the unit, then check the wiring and torque of the T1,T2, T3 terminals. One may have broken or come loose. Also read the troubleshooting section of the manual for clues. Also look in the motor terminal box for a loose connection. If no manual, it is on the Teco website by model number. There are a few Teco specific VFD videos on YouTube that may help. There are a lot of parameters that can be set in that sucker. The parameters are divided by function in the manual. There is an operator’s manual and also a quick start manual.

 

[btrnuthatch]

I'm embarrassed to report that after starting to troubleshoot, I discovered the connection from the VFD to the motor was not so secure. After ensuring a secure connection, voila, it spun up!

What I don't understand but I bet is obvious to you all, is why with the bad connection it ran at all. Enough juice to spin after a nudge but not enough juice to start?

Thanks matt i, MattT and OccupantRJ for taking the time and effort to help me. It never occurred to me that a bad connection at the VFD would cause this problem. Because it ran after a nudge, I suspected something else and went down the wrong path (other capacitor embarrassment). I jumped to the wrong conclusion probably violating an electrician's first principle -- check the connections dummy!

On the plus side, my "learn something new everyday" quota is met.

Thanks again!

 

[MattT]

Good job finding the problem Most electricians would either replace the motor, blame the VFD and call me, or nuke the VFD with a megger

As to why it ran with a kick start. Very simplistically a three phase motor is driven by three electro-magnets spaced 120* apart and when you loose a phase you loose one of the magnets. So down to two magnets 120* apart the rotor (motor shaft) will just oscillate back and forth between them instead of rotating. When you kick start it then it can then ride thru' the missing magnet.

The missing phase causes an increase in current on the other two. Since your VFD didn't fault now would be a good time to check the motor FLA (full load amps) setting in the VFD matches the motor nameplate. Though it's possible that is already set correctly and the VFD just has really crappy motor protection.

 

[OccupantRJ]

You are welcome. Thank you for reporting back what you found so that others can learn from it. Unlike a vfd, a static phase converter uses the principles MattT wrote about to run a 3 phase motor by using a capacitor as a kick starter. The motor then runs on 2 of the three phases.

 

[wyliesdiesels]

was gonna come on here and say single phasing when i read the title and first post but i see others covered it.

Glad you figured it out.

 

[TRWham]

Good job finding the problem Most electricians would either replace the motor, blame the VFD and call me, or nuke the VFD with a megger

As to why it ran with a kick start. Very simplistically a three phase motor is driven by three electro-magnets spaced 120* apart and when you loose a phase you loose one of the magnets. So down to two magnets 120* apart the rotor (motor shaft) will just oscillate back and forth between them instead of rotating. When you kick start it then it can then ride thru' the missing magnet.

The missing phase causes an increase in current on the other two. Since your VFD didn't fault now would be a good time to check the motor FLA (full load amps) setting in the VFD matches the motor nameplate. Though it's possible that is already set correctly and the VFD just has really crappy motor protection.

If you drop one line to a 3 phase motor, it just becomes a single phase motor with no means of self starting. When you lose one line of 3, you really lose 2 phases. Because, a phase is the difference between 2 lines (or line and neutral), you can only have one phase from 2 lines. Depending on how the motor is wired (wye or delta), when a line drops, you will have 2 windings in series (if wye), or only one winding (if delta) so the motor will not self start, pull less total current and produce less power.

 

[MattT]

If you drop one line to a 3 phase motor, it just becomes a single phase motor with no means of self starting. When you lose one line of 3, you really lose 2 phases. Because, a phase is the difference between 2 lines (or line and neutral), you can only have one phase from 2 lines. Depending on how the motor is wired (wye or delta), when a line drops, you will have 2 windings in series (if wye), or only one winding (if delta) so the motor will not self start, pull less total current and produce less power.

Yes I was referring to a Wye connected motor. Using Delta as an example would've needlessly complicated things and it's also highly unlikely the OPs motor is Delta connected.

Beyond that you're pretty much wrong. Loosing a phase doesn't drop a delta connected motor down to one winding. Current does increase on the two remaining phases with either connection type.

 

[TRWham]

Let's drill into this for a minute. Remember we are talking about losing an incoming line, not losing a winding. If you only have 2 lines, you can only have one phase. That is simple physics. If we have three lines, A, B and C, then the three phases are AB, BC and AC. If we lose one, say A, then we can only have BC carrying current.

To simplify, let's consider a three lead motor. If you lose one line on a wye motor, then one winding is connected to nothing on one end, so no current can flow through that one. The other 2 are each connected on one end to a line and the other end to each other. The only path for the current is through one winding to the other, then back out the second winding to the other line. Thus we have 2 windings in series and effectively a single phase motor. The impedance is now doubled, and the voltage is the same, so the current must be reduced. The other winding is just sitting there watching helplessly. Under load, the motor might try to maintain speed and increase current draw to offset the slip.

In a delta motor, 2 windings would be connected to nothing on one end and only one has an active line connected to both ends, thus only one winding carries current. One winding can only carry one third the current of three, so the total current is reduced.

The only scenario where we might have a 3 phase motor run on 2 phases is if you burn one winding open on a delta motor but maintain all 3 incoming lines. Then two windings will carry current but the open one will carry nothing.

 

[MattT]

The other winding is just sitting there watching helplessly. Under load, the motor might try to maintain speed and increase current draw to offset the slip.

On wye yes one winding is dead. And current does increase on the other two to compensate.

In a delta motor, 2 windings would be connected to nothing on one end and only one has an active line connected to both ends, thus only one winding carries current. One winding can only carry one third the current of three, so the total current is reduced.

On delta the two windings you think are dead are connected to each other which provides a series path between the two live phases. And as with wye current increases on the two live phases.

And if by "total current" you mean the sum of all three phases that's irrelevant. The motor protection should be monitoring each phase individually.

 

[TRWham]

You are correct that if the motor is running when the line drops, the current will increase in the remaining windings. i was thinking more about a motor trying to start with a missing line. Also, you are correct that 2 windings of a delta motor would be in series and carry some current with the third carrying more. Regardless, it is only a single phase if only 2 lines are present.