Voltage Readings

[cyoung120]

Good Morning... Please bear with me as I am new to this. Last night I was called to a agitator that when tried to start it would jerk a little and then just take off running, almost like it was single phasing at start up. The motor is controled by a AB powerflex 700 drive. The drive recieves a speed reference thru a remote I/O. When I started troubleshooting, I checked for incoming voltage, had 480 on all three line terminals, now where I get confused is that when I measured the voltage on the motor terminals I got roughly 180 volts...and to even confused me more was that the drive was running at -22.0 Hz. I always assumed that 480 in 480 out and that Hz was always a positive number. Can someone please shed some light on to this for me...Thanks

 

[bphgravity]

My three guesses:

Bad Drive?

Bad Program?

Miscalibration?

 

[cyoung120]

I probably should have put this in OP, but after looking at the drive/motor causing the headache, I went to two other sets of motors/drives that are the same and used for that same application. They where both displaying that they were running at -22 Hz and they both had less than line voltage at the motor terminals. All three are programmed alike. I guess whats really confusing me is the -22 Hz. I was leaning toward the drive being shot untill I started comparing voltage readings from the other two....Once again please bear with me..I am new to this and I am trying to give you all the facts...

 

[ptonsparky]

Measuring output voltage of a Vfd is an exercise in futility. There are to many other things going on in drive.

 

[ptonsparky]

If the drive is not displaying any fault codes I would suspect an input (wiring ) or parameter problem.

 

[billsnuff]

why would 22Hz on a variable frequency drive be unusual?

 

[mclassen]

Most vfd's have fwd and rev for applications requiring that. In your case the display may show negative for what you think should be positive, swapping motor leads may solve the negative hertz but some reprogramming would be required.

 

[don_resqcapt19]

Most drives keep the volts per hertz ratio so you should expect 176 volts on the motor when running at 22 hertz.

 

[mxslick]

Most vfd's have fwd and rev for applications requiring that. In your case the display may show negative for what you think should be positive, swapping motor leads may solve the negative hertz but some reprogramming would be required.

Warning: Swapping motor leads will reverse the rotation of the motor! Make absolute certain the mechanical load can handle reverse rotation or you can cause some very expensive damage!

If the other two are working ok with the negative hertz display then the problem lies elsewhere.

As for the problem drive, check the ramp up times and settings, they are either changed from the other two or the drive in question is bad.

 

[kwired]

If drive parameters are ok don't rule out possible mechanical problems. I bet at least half of my service calls for motor problems (for a motor that does run but trips overloads, or other performance problems) end up being mechanical problem in the driven load, bad bearings, worn gearboxes, too much load, etc.

 

[glene77is]

Warning: Swapping motor leads will reverse the rotation of the motor! Make absolute certain the mechanical load can handle reverse rotation or you can cause some very expensive damage!

MX,
Swap the meter leads, not motor leads.

OP,
Try using an O'scope for true Voltage readings in a complex waveform.

 

[cyoung120]

Good Morning Gentlemen...wanted to give an up date...tonight we are going to download the parameters from all three drives and compare, as I am told that sometimes the parameters change "all by themselves" If all three are the same, we will remove the motor leads from the problem drive and hook them to one of the drives that is working ok to rule out mechanical issues. As for the negative hz. I guess I am just curious how a negative hz. is obtained. These agitators can be driven forward or reverse via the HMI at the location. Regardless of what direction they are spining they still show a negative value. Thanks again for all of your input..will keep you posted...

 

[Jraef]

Warning: Swapping motor leads will reverse the rotation of the motor! Make absolute certain the mechanical load can handle reverse rotation or you can cause some very expensive damage!

I interpreted that mclassen was just trying to explain to the OP that because a VFD recreates an output that can have any rotation direction, what it thinks is + or - rotation of the output is irrelevant, it's only what the motor responds to that is significant. So in making the motor spin the correct direction it means the output of the VFD is -, then so be it.

 

[mbeatty]

Good Morning... Please bear with me as I am new to this. Last night I was called to a agitator that when tried to start it would jerk a little and then just take off running, almost like it was single phasing at start up. The motor is controled by a AB powerflex 700 drive. The drive recieves a speed reference thru a remote I/O. When I started troubleshooting, I checked for incoming voltage, had 480 on all three line terminals, now where I get confused is that when I measured the voltage on the motor terminals I got roughly 180 volts...and to even confused me more was that the drive was running at -22.0 Hz. I always assumed that 480 in 480 out and that Hz was always a positive number. Can someone please shed some light on to this for me...Thanks

VFD's can operate at a certain volts per hertz ratio (constant torque). It looks like your drives may indeed be at a set volts/Hertz ratio. Constant torque is maintained as long as the volts/Hz ratio remains constant. Based on your previous statements, your 180V, 22Hz readings are within the range for discussion. I wouldn't worry about the negative frequency meter reading, it's the 22.0Hz that counts. Take the 180 (drive output voltage) and divide by 22.0 (drive output frequency). This gives you a volts/hertz ratio of about 8.18. At 60Hz, your voltage would calculate to about 491V. Normally, for constant volts/Hz at a 460V motor, the ratio would be 7.67 V/Hz and for a 230V motor it would be 3.8 V/Hz. Your drives may not be set up this way. Just my 2 cents worth.
Regards,
Mark

 

[billyzee]

V/Hz

V/Hz

Snuffy said it all. The volts per hertz ratio (V/f) is relatively linear for VFD.

In other words when the motor is going fast, the output frequency will be high ~60Hz and voltage will also be high ~480V.

But when the motor is going slow the frequency will be low (22 hz) and voltage will be low (180V)

I suggest this is not your problem.

 

[mxslick]

MX,
Swap the meter leads, not motor leads.

OP,
Try using an O'scope for true Voltage readings in a complex waveform.

Please read what I quoted before you make a statement, as meter is NOT what he said::roll:

Most vfd's have fwd and rev for applications requiring that. In your case the display may show negative for what you think should be positive, swapping motor leads may solve the negative hertz but some reprogramming would be required.

Perhaps he meant meter but I was responding to what was written, and stand by my cautionary post.

I interpreted that mclassen was just trying to explain to the OP that because a VFD recreates an output that can have any rotation direction, what it thinks is + or - rotation of the output is irrelevant, it's only what the motor responds to that is significant. So in making the motor spin the correct direction it means the output of the VFD is -, then so be it.

Again I repeat my caution, as you are very correct, but at this point we are presuming that the motor IS running in the correct direction, even if the VFD is indicating negative Hz. Swapping motor leads WILL reverse the direction of the motor, which the mechanical load may not be able to handle.

 

[Jraef]

Good Morning Gentlemen...wanted to give an up date...tonight we are going to download the parameters from all three drives and compare, as I am told that sometimes the parameters change "all by themselves"

I love this one. I hear that a lot, yet I have NEVER seen a drive (or PLC) where this actually happens. Miraculously, the cure for me has ALWAYS been to lock out the programming panel access, and the "phantom code changes" cease. Keep in mind that if you have a remote HMI and a PLC, chances are (especially if it's an A-B PLC) that the engineering workstation PC likely has all of the available programming software on it. So some goofball could be reprogramming the VFDs over a network connection without ever actually being in front of them. If so, you need to get to the bottom of that and establish better security protocols.

If all three are the same, we will remove the motor leads from the problem drive and hook them to one of the drives that is working ok to rule out mechanical issues. As for the negative hz. I guess I am just curious how a negative hz. is obtained.

Sounds like a good plan to me.

These agitators can be driven forward or reverse via the HMI at the location. Regardless of what direction they are spining they still show a negative value. Thanks again for all of your input..will keep you posted...

I'm not familiar enough with the PF700, but might you be misinterpreting a symbol used on the A-B display to mean "negative" when in fact it's just displaying something that means "Running"? Or is the "-" sign in the HMI display? If so, that might just be some programmer error or misinterpretation, no way to know from afar.

 

[Tarbaby]

The speed out of the variable frequency drive will be proportional to the frequency out until you reach the normal frequency. i.e at 30hz the voltage will be 240 volts, 90 hz would only be 480 volts.
When the frequency is less than the normal the impedance of the motor will be less and the current could be much greater. Above the normal frequency, the voltage is limited by the voltage rating of the motor.

So 180 at 22hz sounds about right. Cannot explain the -22hz.

 

[Cow]

Good Morning... Please bear with me as I am new to this. Last night I was called to a agitator that when tried to start it would jerk a little and then just take off running, almost like it was single phasing at start up. The motor is controled by a AB powerflex 700 drive. The drive recieves a speed reference thru a remote I/O. When I started troubleshooting, I checked for incoming voltage, had 480 on all three line terminals, now where I get confused is that when I measured the voltage on the motor terminals I got roughly 180 volts...and to even confused me more was that the drive was running at -22.0 Hz. I always assumed that 480 in 480 out and that Hz was always a positive number. Can someone please shed some light on to this for me...Thanks

I think your voltage measurements and -22 hz are red herrings.

It seems the only problem your having is it jerks, then ramps to speed immediately? I'd verify rotation is correct and then have a look at the accel/decel times.

 

[billsnuff]

when tried to start it would jerk a little and then just take off running

the mechanical resistance to rotation is where I'm putting my money. Bad bearing in motor/pump, etc. When all you have is a hammer, every problem seems to be a nail. At least don't overlook it.