OT multi motor/VFD startup inrush protection

[DickDV]

One other thing----- when you have more than three motors on a single drive, it would be good practice to provide fusing in each motor lead to limit current to safe levels. That drive could literally roast one motor and not even fault out with no fusing.

 

[mordred]

found out that the print I had was incorrect to whats actually in place they had marked down a drive that they had on a 4 motor circuit the drive size on the 8 motor circuit is 15 amp output. Dickdv your referring to the PWM raising the readings you would get even in true RMS correct?

 

[DickDV]

Yes, I have used numerous true RMS meters and none are accurate. Use the drive output. It has to be accurate for the drive to perform properly.

 

[AltaVoyager]

output on the drive is rated for 9.9 amps cat number 22b-E9p9n104 5.5kw/7.5 hp 3ph input 48-63 hz 13.7 amp. I'm wondering if extending the linear ramp will help or going to an s curve ramp style.

Your VFD defenetly undersized because designer didn't count filter. Filters add about 5-10% load. Besides filters screw up
output current readings.

When we started to use PF 700 with filters I couldn't start them at all. They failed on "HW overcurrent". Turns out there are two setpoints in PF700 506 and 507 freq and voltage angles.
By default they are set to 56 and 91. With filter they have to be set to zero.
Also in setpoint 56 bypass "long cable" and "transistor check" bits. I'm not familiar with PF 40, may be they have something similar.Hope it helps.

 

[OkiePC]

Have you tried extending the accel ramp and setting the boost to the lowest voltage? IIRC the default boost setting is not the lowest available in a PF40...

Also, what kind of overload devices are used and are they compatible with VFD output? Are they set up to open all three legs of each motor? Just WAG-ing...

 

[Freak]

A couple of simple things to check that are often overlooked

Are the motor's connected correctly according to the nameplate, i.e. star/delta? We have one motor onsite that needed to be connected in Delta even though it was on a Hz drive otherwise it pulled too much current.

Second thing I would test is isolate all but one motor run it and record the current (should be able to get of the drive's display) then test a different motor - and so on, just to make sure that all of the motor's are drawing the same.

Good Luck
Peter

 

[mordred]

I'll have to look into the overloads compatibility we've already been informed that they may be the wrong type and I've increased the ramp time as time isn't an issue in this application. We've manually lowered the boost so were not on default. Yes the motor wiring was confirmed. The Rockwell tech has also been on site looking over the system least this way we get better warrently on the drives LOL he also did not find much wrong. His recommendtations is changing the type of overloads and to look at wiring the VFD's closer to the motors. I'll have to look into the output filters and see if there are any setings involved there.
Thanks for the advise all helps to have the reassurance that most of what everyone is suggesting has already been looked into the confirmation is nice

 

[milldrone]

Mordred,

If you would not mind, can you clarify something for me. In your first post you said

I'm still detecting large inrush currents on motor startup and am looking for recommendations on how we can offer better protection as we have been experiencing output burnouts on the VFD's.

I'm interpeting this to mean you have been loosing drives.

they all start at the same time and they did not supply contact just overloads nor is there any extra outputs available on the PLC

I'm interpreting this to mean that the separate motor overloads stop the VFD if one of the motors OL sense an overload (real or imaginary)

We've manually lowered the boost so were not on default.

This boost thing is not intutitive. I have witnessed it working the opposite of how you would think.

The Rockwell tech has also been on site looking over the system least this way we get better warrently on the drives LOL he also did not find much wrong. His recommendtations is changing the type of overloads and to look at wiring the VFD's closer to the motors.

So he saw nothing wrong with the sizing?

 

[mordred]

The sizing I posted origianlly was a mistype on the electrical print the 9.9 amp was on a VFD with only 4 fans on that particualt circuit. Your correct on the overloads. Yes before we made some changes we were losing drives usually blown outputs. We haven't run this system after the provisions were made long enough yet to truly know if e have solved the problem or made it worse. There doesn't seem to be much info on the boost itself does anyone have a good article or link as to how to test whether or not the settings are correct for it?

 

[milldrone]

There doesn't seem to be much info on the boost itself does anyone have a good article or link as to how to test whether or not the settings are correct for it?

Mordred,

This is the best I could find. http://www.plctalk.net/qanda/showthread.php?t=20220

I think it is a try it and measure it. If it is better try a little more. If it's worse go the other way. Kind of deal

There is also it's sister thread
http://www.plctalk.net/qanda/showthread.php?t=20435&highlight=Motor+Equivalent+Circuit

Which has more to do with overspeed and overhauling loads than your problem.

 

[mordred]

Thanks Milldrone some good reading there

 

[Plc_User]

Two motors on one drive

Sorry this isn't a direct intervention in the current discussion, but what I experienced multiple times in configurations with two motors on one drive and with a thermal overload relais at the output of the drive (one for each motor) that always one motor drew a lot more current then the other, although the motors were connected by a shaft. The motors were identical. What I don't know if the motor cables have the same length for both motors. Could this be a reason? And do you use special thermal overload relais if they are place at the output of a VFD (PWM-signal)?
Thanks.

 

[milldrone]

Mordred, in post #2, rdrast asked this question

Also, if these are fans, is there any chance that they are being driven by the airstream? That could be bad.

Yor reply was

Sorry guess I wasn't clear enough its one VFD per 8 motors and air stream isn't a factor its for a dehumidification system

So from this answer I'm presuming there is no way the fans are running backwards, (because of air pressure at start up) correct? I know from experience this is a real problem on refigeration condenser fans. Just the normal a_m_b_i_e_n_t wind can make these fans run backward. The common cure for this problem is to use some DC injection braking before the start.

 

[timbo_uk]

Sorry this isn't a direct intervention in the current discussion, but what I experienced multiple times in configurations with two motors on one drive and with a thermal overload relais at the output of the drive (one for each motor) that always one motor drew a lot more current then the other, although the motors were connected by a shaft. The motors were identical. What I don't know if the motor cables have the same length for both motors. Could this be a reason? And do you use special thermal overload relais if they are place at the output of a VFD (PWM-signal)?
Thanks.

Surely this would have been caused by differing rotor positions in relasion to the drive output waveform. So one motor mangaged to "lock on" to the sine-wave, but (because of the mechanical link) the other was always trying to accellerate to the sinewave but never could?

 

[Vic]

timbo_uk said

Surely this would have been caused by differing rotor positions in relasion to the drive output waveform. So one motor mangaged to "lock on" to the sine-wave, but (because of the mechanical link) the other was always trying to accellerate to the sinewave but never could?

Induction Motors never "lock on" to the sine wave. They always run with some slip. Don't confuse Synchronous Motors, which "lock on" to the sine wave, with Induction Motors.

The different currents are probably caused by slight differences in the characteristics of the motors, even though they are the same make and model.