# Troubleshooting 3 phase motor on vfd



## McClary’s Electrical (Feb 21, 2009)

If you’r e running an identical VFD and identical pump , there’s still a ton of programmable variables and parameters that could vary from drive to drive. Don’t base your conclusion on this alone. Please provide more info if possible


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## Peewee0413 (Oct 18, 2012)

Do you have a short in the wires to the motor?


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## gpop (May 14, 2018)

A vfd doesn't put out ac it puts out 8 to 12,000 pulses of dc which can confuse some brands of volt meter. Also at lower hertz we expect lower voltage as volt with out hertz equals heat. 

You will need to divide this problem as best you can with the tools available to you. 
You can do a diode and gate test on the drive
you can unhook the motor at the pecker head and drop the wires from the drive then meg phase to phase with disconnect open and closed. (wire nut ends of wires first)
If the tests pass then i would suggest trying a different motor. 

Was the original motor vfd rated?


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## Bluenose for rent (Nov 6, 2020)

Peewee0413 said:


> Do you have a short in the wires to the motor?


Nope meggered them they’re good


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## Bluenose for rent (Nov 6, 2020)

McClary’s Electrical said:


> If you’r e running an identical VFD and identical pump , there’s still a ton of programmable variables and parameters that could vary from drive to drive. Don’t base your conclusion on this alone. Please provide more info if possible


I have never programmed a vfd, what info would be helpful?


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## Bluenose for rent (Nov 6, 2020)

gpop said:


> A vfd doesn't put out ac it puts out 8 to 12,000 pulses of dc which can confuse some brands of volt meter. Also at lower hertz we expect lower voltage as volt with out hertz equals heat.
> 
> You will need to divide this problem as best you can with the tools available to you.
> You can do a diode and gate test on the drive
> ...


I meggerEd the cables they’re good.

I am not familiar with diode and gate test, might be too technical for me. lol

The motors and control cabinet came as a package from an equipment dealer, had technologist and engineer fingerprints all over it. They used rated pumps, but I actually didn’t check out the replacement, so I’ll check that.

I have only a basic understanding of how vfds actually work, as I understand it the pulses you mention mimic an ac sine wave. I’ve always been able to measure voltage off them with my meter before.


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## MotoGP1199 (Aug 11, 2014)

Since you said the the brewer changed the shaft and impeller:

- This can change the load on the motor and have good or bad consequences. 

You said the other VFD shuts off right away in overload: 

- Does the motor start to turn a little and then shut off?

-Does it not move at all? 

- Does it get up to full speed before shutting off?

On some pumps/loads you need to adjust settings to get it to ramp up faster or start at a certain speed/hertz(like 30) and then ramp up from there to get the motor spinning.


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## MotoGP1199 (Aug 11, 2014)

Bluenose for rent said:


> I meggerEd the cables they’re good.
> 
> I am not familiar with diode and gate test, might be too technical for me. lol
> 
> ...


Danfoss/Vacon has excellent customer service, sometimes there is a wait for technical support.

1-888-326-3677

If you have a meter that can test diodes they will walk you through it over the phone.


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## Bluenose for rent (Nov 6, 2020)

MotoGP1199 said:


> Since you said the the brewer changed the shaft and impeller:
> 
> - This can change the load on the motor and have good or bad consequences.
> 
> ...


It doesn’t move at all, instant fault

Maybe I’ll try it at a lower frequency and see what happens, I was trying it at 50 htz


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## Bluenose for rent (Nov 6, 2020)

MotoGP1199 said:


> Danfoss/Vacon has excellent customer service, sometimes there is a wait for technical support.
> 
> 1-888-326-3677
> 
> If you have a meter that can test diodes they will walk you through it over the phone.


Thanks, I’ll call


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## MotoGP1199 (Aug 11, 2014)

Bluenose for rent said:


> It doesn’t move at all, instant fault
> 
> Maybe I’ll try it at a lower frequency and see what happens, I was trying it at 50 htz



I didn't mean the setpoint. I'm talking about the starting hertz it starts to ramp up from. If you don't change any settings the drives start ramping from 1 hertz and works up to your setpoint of 50hz. There's a setting that will allow you to start the motor instantly from a higher hz and then ramp upto your setpoint. IE: it starts at 20hz and then ramps upto 50hz in 10 seconds(or what ever your ramp up time is) and skips 1-19hz completely. Sometimes just changing your ramp up time to a shorter time will help. If someone put in a ramp up time of 120 seconds it may not even start the motor before tripping in overload. Going to a 10 second or 30 second rampup time may allow it to start normally.

What model vacon do you have?


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## gpop (May 14, 2018)

A vfd likes to cause a burn mark on the coil winding especially if the motor is not vfd rated. This may or may not meg to ground and unfortunately you probably do not have a meter to test for this. (motor shop does). 

A instant overload fault is bad news (hardware overload). even a badly programmed vfd will try before it trips and you have been lucky so far that the vfd has not let go as they tend to get pissed on a phase to phase short. Honestly its never a good idea to test a suspect motor on a vfd.


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## Bluenose for rent (Nov 6, 2020)

I think I’m calling in the cavalry,(industrial guy I’ve worked for sometimes) thanks guys I’ll post when we get it figured out


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## just the cowboy (Sep 4, 2013)

Bluenose for rent said:


> *Unsteady voltage at the disconnect on line side with switch off* >100v phase to phase. 360v each per phase to ground, so I figured the vfd is cooked.


Back to basics, you explained something wrong.
The VFD will not like that unsteady voltage.
With the switch off line side is not effected by bad VFD, you need to look at line power.

Cowboy


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## Bluenose for rent (Nov 6, 2020)

just the cowboy said:


> Back to basics, you explained something wrong.
> The VFD will not like that unsteady voltage.
> With the switch off line side is not effected by bad VFD, you need to look at line power.
> 
> Cowboy


The switch is downstream from the vfd, vfd is in a control cabinet switch is right next to motor


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## just the cowboy (Sep 4, 2013)

Bluenose for rent said:


> The switch is downstream from the vfd, vfd is in a control cabinet switch is right next to motor


You said you megged the wires to the motor, but you did not say what the resistance of the windings were. 
T1-T2, T2-T3, T3-T1. 
Also you say the disconnect is after the drive, does it have aux contacts to stop the drive if turned off? 
Drives do not like to have the motor be started with a disconnect on the load side. If the owners do not know this and use the disconnect to start and stop it, it will shorten the drive life.


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## Bluenose for rent (Nov 6, 2020)

Resistance between the windings was less than an ohm Between any combination Of leads.

Good point about the switches shortening the life of the vfd. There are no aux contacts.
Around here lockable switches are mandatory for safety, but aren’t used for operation, this brewery is all operated by Touchscreen. The workers didn’t even know the switches were there.


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## JRaef (Mar 23, 2009)

If you tried measuring the output of the VFD with the motor side disconnect open, you will not get valid readings. The VFD output NEEDS the induction of the motor to complete the circuit and make it look like AC. With that disconnect open, it is just a wildly varying string of DC pulses that most meters that cost under $3k will not be able to interpret correctly.

As @just the cowboy said, a disconnect on the load side of a VFD is a potential reliability issue in that if the user opens that disconnect under load, it can damage the VFD output transistors. That's why you have to add an Aux contact to the disconnect that opens PRIOR to the main contacts opening, and wire than to the VFD as a "disable" circuit to the controller (most have that available). Without that, and with the likelihood of the end user having opened that disconnect under load, the VFD is probably toast now.


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## Bluenose for rent (Nov 6, 2020)

JRaef said:


> If you tried measuring the output of the VFD with the motor side disconnect open, you will not get valid readings. The VFD output NEEDS the induction of the motor to complete the circuit and make it look like AC. With that disconnect open, it is just a wildly varying string of DC pulses that most meters that cost under $3k will not be able to interpret correctly.
> 
> As @just the cowboy said, a disconnect on the load side of a VFD is a potential reliability issue in that if the user opens that disconnect under load, it can damage the VFD output transistors. That's why you have to add an Aux contact to the disconnect that opens PRIOR to the main contacts opening, and wire than to the VFD as a "disable" circuit to the controller (most have that available). Without that, and with the likelihood of the end user having opened that disconnect under load, the VFD is probably toast now.


I don’t think anybody did open it under load. But you and cowboy are right, there are 8 motors controlled by drives in that area and they all have conventional disconnects, no aux contacts, someone will switch one off under load eventually. I wonder why it wouldn’t have been in the plans I worked off.


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## paulengr (Oct 8, 2017)

Bluenose for rent said:


> A year or two ago I wired a microbrewery. I got called to hookup a replacement pump the other day, and the story was the pump had burnt out( I never checked it,was gone before I got there). I replaced the pump, gave it a bump to check rotation and away I go.
> Now a couple weeks later and I got a call the pump is not running. The brewer thought he killed the pump by playing with the shaft and impeller.
> 600v, 3ph, 1.5hp on a vacon vfd
> I meggered the motor leads to ground and got infinity, the shaft spins freely.
> ...


If you got infinity if you Megger Ed the motor either your Megger is bad or you didn’t connect properly. I just tested a very large 480 V motor, 900 HP today and it was 800 Megaohms. Very high but not infinity.

If the motor is burnt as in overloaded the slot insulation is destroyed and it Meggers 0. Otherwise as an example of one last week the end turns were smoked. It looks like reflected wave. Inductance and resistance readings were both bad but it Meggered around 300+ Megaohms even though it failed. Megger test only detects failed motors about 80%+ of the time. It’s a great quick test but not the only test.

Your other test tells the story. The motor is shorted. As to why, you stopped too early. On the first motor change considering the last one lasted a while it sounds like maybe you got a defective motor. Could be overload or other things. For instance if the line length between motor and drive is too long (starting around 50+ feet) the drive will easily destroy motors without a filter such as a dv/dt filter.

To do a full analysis takes experience. As a first crack take the bell end off the motor and look at how it failed visually. If you have access to a Baker tester run that before disassembly. Those two steps give you most of what you need to troubleshoot a motor in depth.


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## Peewee0413 (Oct 18, 2012)

paulengr said:


> If you got infinity if you Megger Ed the motor either your Megger is bad or you didn’t connect properly. I just tested a very large 480 V motor, 900 HP today and it was 800 Megaohms. Very high but not infinity.
> 
> If the motor is burnt as in overloaded the slot insulation is destroyed and it Meggers 0. Otherwise as an example of one last week the end turns were smoked. It looks like reflected wave. Inductance and resistance readings were both bad but it Meggered around 300+ Megaohms even though it failed. Megger test only detects failed motors about 80%+ of the time. It’s a great quick test but not the only test.
> 
> ...


I would bet the motor is bad too, but I'll just wait for the update... Instantly tripping the drive for OC 9/10 times the motor is smoked. The other would be bad wires.


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## SouthernCanuck (Jul 30, 2021)

Bluenose for rent said:


> A year or two ago I wired a microbrewery. I got called to hookup a replacement pump the other day, and the story was the pump had burnt out( I never checked it,was gone before I got there). I replaced the pump, gave it a bump to check rotation and away I go.
> Now a couple weeks later and I got a call the pump is not running. The brewer thought he killed the pump by playing with the shaft and impeller.
> 600v, 3ph, 1.5hp on a vacon vfd
> I meggered the motor leads to ground and got infinity, the shaft spins freely.
> ...


I gotta ask, did you disconnect the motor leads from the output of the VFD before you meggered the motor? Hopefully you did.

If the drive is going into over current fault right away you should try to uncouple the pump from the motor and run the motor with no load. See if it runs without tripping. If the impeller size was increased that can add a much larger load on the motor, 1.5hp is pretty small. The drive will see the increased load as locked rotor current and trip right away. If the VFD trips with the pump uncoupled, try to go right back to the VFD output and disconnect the motor and see if the drive even runs without the motor connected. 

Double check the motor is VFD rated. A VFD has a lot of parameters that can be configured as well as increasing torque on startup, ramp up time to set point and so on. 

Good luck, and update us with your findings.


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## paulengr (Oct 8, 2017)

VFD rated motor (“inverter duty”) is NOT necessary. Most of them are not. An inverter duty motor means it has phase papers, higher voltage insulation, and often some cooling upgrades. However the phase papers and upgraded insulation is so cheap most motor manufacturers are offering it as standard.

If you are operating a motor between about 100 and 250 foot cable length from the VFD without an output filter the inverter duty motor is less likely to be damaged. Above about 250 feet NO motor is rated for it.

On centrifugal pump applications all motors can go down to 6 Hz minimum speed. On other applications worst case is 30 Hz minimum but most applications can get to 15 Hz. With extra cooling you can go lower. With a blower cooled motor (not an inverter duty option per day) you can go to 0 Hz. Inverter duty motors may allow 10:1 or 100:1 turndown so you can go lower on your minimum speed settings. But this is OPTIONAL and something you can order, inverter duty or not. The motor manufacturer just puts a bigger fan on it and changes the efficiency (lower).

Many drive manufacturers promulgate the myth that a motor MUST be inverter duty rated because they private label motors with exorbitant markups (like 50-100%) and make huge profits scamming customers on something they don’t need. The reality is that if reflected wave is a problem the filter is cheaper than the inverter duty rating and you have zero chance of someone putting the wrong motor on if you just put in the filter.


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## Bluenose for rent (Nov 6, 2020)

Turned out it was a programming issue. Drive was fine, motor was fine. There was some problem with the run signal from the Touchscreen, a buddy who is an experienced industrial electrician fixed it, he also adjusted the ramp up time. Done in 30 mins, I was talking to the customer about something else the whole time. It’s good to have friends.


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