3 Phase motor on AHU

[normanton]

Hi everyone. Have a problem iv not come across before. So the motor has stopped running on the AHU in our building. It's controlled by an inverter. First thing I done is check the inverter was running (which it was), then checked for voltage at the motor terminals. To my surprise (and probably not yours), I have around 309V between phases and around 300V phase to earth. This seems strange to me. Anyone know if this is right or wrong? At the moment my guess it there's a fault on the output voltage of the inverter. But I'm sure you guys will correct me... Haha . Thanks

 

[Knobhead]

What tests did you do on the motor?

 

[normanton]

What tests did you do on the motor?

Ive checked between windings and checked the windings to earth. All readings are pretty much the same... Does that voltage seem normal to you? I disconnected the cables from the motor and still had 309V between phases and 300V phase to earth..

 

[Marvo]

There's a good chance your tester won't give you an accurate voltage or current reading on an inverter output. The supply is pulse width modulated and you'll need a high-end tester to measure it accurately.

I'd suggest you run the motor DOL if you can and test it that way. If it runs fine then check the VFD for error codes and run the usual reset procedures. If that fails the remove and return to agents for bench test.

*edit* If this motor is on an HVACR system maybe there's other inputs to the drive that's causing the problem ie BMS interlock, pressure transducer etc

 

[Darkwood]

The inverter may have speed control either fixed or variable through a control panel ... if the speed request is not for full speed then the voltage will drop proportionately with frequency - its likely the motor is set for 75% speed if your seeing 300v between phases but like Marvo says your tester may not be able to read the output of the inverter due to the nature of how the inverter works, and as Tony has asked already what testing has been done at the motor.... and the big question does it smell a bit fishy, other posssibilities is you may have a phase down again this comes down to the question of what other tests have you done including inverter supply. We can only speculate with wild guesses here as you provide limited info hence all our questions..

 

[normanton]

Great feed back guys, thanks a lot. Will try what you both just said and keep you updated. This is new to me so I'm just gonna have to go about it one step at the time I think. Thanks again!

 

[Darkwood]

please tread carefully on this one these systems are carefully designed and set-up and unless you find its a straight forward issue i would advice to leave to an engineer who's fully competent with AHU set-ups ive seen these systems totally messed up because joe bloggs Sparky had a tamper around to see if they could increase air flow.

 

[normanton]

please tread carefully on this one these systems are carefully designed and set-up and unless you find its a straight forward issue i would advice to leave to an engineer who's fully competent with AHU set-ups ive seen these systems totally messed up because joe bloggs Sparky had a tamper around to see if they could increase air flow.

Joe bloggs? Cheers for that mate lol

 

[SirKit Breaker]

If your motor is balanced Ok, and IR tests out Ok, then its probably OK, but this could depend on the type of motor. I am not the worlds expert on motors, but when i have done the above tests the motor has always been OK.

 

[LeeH]

You need a true RMS meter for speed controllers, they are not expensive.

Is the drive showing 50hz? Is there run or a fwd/rev light?

If there is there is it won't be any sort of interlock but I'm surprised if the drive is running and the motor is not turning, it hasn't tripped out on the built in thermal overload.

 

[LeeH]

If your motor is balanced Ok, and IR tests out Ok, then its probably OK, but this could depend on the type of motor. I am not the worlds expert on motors, but when i have done the above tests the motor has always been OK.

There is nothing else to check apart from a manual rotation check to make sure it spins freely.

 

[Darkwood]

Joe bloggs - old hat for random person same as John doe or Jane Doe when name is purposely concealed or identity not known....

 

[Darkwood]

If the motor has power it could be running OP hasn't explained fully what are the symptoms -

No air flow? - ( In a AHU many things can account for lack of air flow not just issues with motors
Are you positive the motor isn't turning? - does it still feel like it is but the impeller doesn't rotate... (loose impeller, sheared shaft etc)
Do you have the user/set-up manual for inverter you could change the readout to current monitor and see what the motor is doing
The inverter does well if set up ok should monitor the motor and trip if any issues arise - check logs for recent alarms.


Again so little info wild guesses are our best resort..

 

[LeeH]

I would suggest you leave the parameters well alone, even changing the monitoring mode can be in the set up parameters.

If you have no working knowledge on AC drivers/3PH motors tread carefully.

 

[Marvo]

You need a true RMS meter for speed controllers, they are not expensive.....

A true RMS clamp meter should give a good indication of the current but the problem with a PWM output is measuring the voltage. In the OP he was worried by the 309v and 300v readings but they were most likely inaccurate/meaningless results if a standard tester was used.

 

[Knobhead]

Great feed back guys, thanks a lot. Will try what you both just said and keep you updated. This is new to me so I'm just gonna have to go about it one step at the time I think. Thanks again! 

Here we go again, I’ve taken on a job I know nothing about.

Please help!

It doesn’t help when you just say the motor is on, an AHU. AHU covers a multitude of things.
An industrial electrician has a mental check list when it comes to motor faults. What is your background?

I don’t work on guess work, I’m out of here

 

[Stuuk1]

Assuming the installation is fairly large, I'd be more tempted to check my frost stats etc rather than diving right in and changing motor wiring configurations..

 

[normanton]

Turns out it was simply the bearings had gone on the motor. What threw me was the voltages I was getting. Was unaware an inverter altered voltages as well as the frequency . Thanks everyone

 

[Marvo]

Turns out it was simply the bearings had gone on the motor. What threw me was the voltages I was getting. Was unaware an inverter altered voltages as well as the frequency . Thanks everyone

Thanks for letting us know and glad it got sorted out. This thread is a good reminder that any modulated outputs can be confusing to measure with standard testers including even the humble lighting dimmer.

Just out of interest did you post-mortem the bearings that failed. Variable frequency drives in some circumstances can cause the rotor to develop a voltage with respect to earth. This rotor voltage causes pulses through the bearings as it discharges to ground. The current pulses leave a very small pit on the bearing race and ball every time it happens which eventually causes the surface to become rough and also lubricant contamination. The end result is the bearing suffers premature catastrophic failure.

I'd suggest it's always worth checking for rotor voltage wrt earth when you get bearing failure and there's a VFD and it's always worth cutting the bearing open and examining for signs of pitting on the raceway faces. If voltage is evident then there are rotor earthing kits available for most motors.

 

[normanton]

That's a good theory, I'll be sure to check that out actually. Interesting. Thanks again