MULTIPLE MOTORS ON AC DRIVE PACKAGE

[Farkel]

CUSTOMER HAS EXPERIENCED MOTOR FAILURE ON A 20HP AC DRIVE, WHICH POWERS 15 3/4HP 460VAC MOTORS IN A PARALLEL CIRCUIT, OVER A DISTANCE OF SOME 150 FEET. THE MOTORS ARE NOT INVERTER DUTY. WHAT IS THE CONCENSUS OF EVERYONE UPON PUTTING LINE REACTORS IN FRONT OF EACH MOTOR, OR A COUPLE OF REACTORS IN DIFFERENT PARTS OF THE LINE FOR WINDING PROTECTION. HAVE ASKED EASA HOUSE TO VERIFY, THAT MOTORS ARE NOT FAILING ON OVERLOAD, SINCE THEY ARE POWERING GEARBOXES, OR CONSTANT TORQUE LOAD.

 

[rym]

Farkel
you didn't say what kind motor failure you're having.
eg burnt out windings or bearing problems.

I understand that motors have to be inverter rated When operated by inverters or the insulation of the motor has to be class F. This is because inverters produce spikes that can damage the insulation , due to the high switching speeds of IGBT.
Line reactors are use to prevent reflected voltages appearing at the motor terminals.
However,in your application, you have 15 motors.I would think one properly rated line reactor would be sufficient

 

[GusD]

Hi Farkel

You don't mention what type of drive you have.You did say that your motors are non-inverter duty,so I assume that you do have a VFD driving these units.You also don't mention what was the type of failure on these motors.Line reactors or filters would help if the problem was caused from inverter related problems.The cable length may be too long as well,however;it is hard to speculate on a cure, unless you know what is causing your motors to fail.
A little investigation in this direction would go a long ways.

GusD

 

[CB2]

Generally speaking, I would put a dv/dt output filter on this application at the get go. Motors less than 10 Hp are very susceptible to wire length problems (standing wave) even if inverter duty rated. This may not be your problem (yet) but the dv/dt filter is very cheap insurance.

Caution on using output line reactors, they can get squirrely at >100' of wire.

CB2

 

[HarryDampers]

Farkel , If I assume that you have a seperate cable feeding each Motor , then your poor VSD is 'seeing' 15 x 150 feet of cable . Sounds like you have about 700 meters of cable on this poor little VSD . I am suprised that the customer even gets these motors to start.

First step should be to install a single Motor reactor at the drive's output (in the panel) , the manufacturer should advise you , but it should be between 2 and 3% .

There are other things you can do but the installation of a Motor reactor is cheap, practical, carries little risk and provides a cure in 99,9% of these situations.

I would also suggest that you check the Voltage boost setting to ensure adequate compensation for cable volt drop(especially after adding the reactor in the circuit)........and if the drive allows it , keep your switching frequency at its lowest setting .

 

[tmahan]

Prior to blaming the drive or the motor's lack of inverter duty rating I would check the simple and in-expensive stuff.

In an installation such as this I think that each motor should have its own overload protection. Is that the case here?

 

[jbartos]

Suggestion: The AC Drive may need to check its output current and voltage waveforms. As recommended above dv/dt filters and harmonic filters may be needed.
There are two different phenomena:
1. dv/dt is caused by fast switching devices, e.g. IGBT
(sometimes called ringing).
2. Harmonic distortion, which is in different frequency range for different frequencies. It is caused by the DC-AC conversion, i.e. its topology.

 

[mdonner]

You mentioned contant torque application.
If motors are fan cooled, going under 50% frequency may cause motor overheat to burnout.

 

[jOmega]

Most likely the motor failures are due to dv/dt breakover occurring on the end turns. Nothing new about this. A lot has been written on this subject.

The long motor leads are contributory to setting up standing waves on the wires that get reinforced by the next PWM pulse coming down the wire and the amplitude of the voltage delivered to the motor with a fast rise time causes a corona discharge typically in the end-turns .... 3/4 HP motors are particularly succeptible because of their impedance and also because of the severity of the radius of the end turns. Also chances are the failed motors do not have phase papers and slot paper insulation which also makes these motors more succeptible to this type of failure.

You can certainly replace the motors with Inverter Duty Rated ... but that could be expensive. A more economical solution would be something like TCI's KLC filter installed at the output of the VFD.

See ...

http://www.transcoil.com/#klc

http://www.transcoil.com/documents/Klc-brochure.pdf

 

[jOmega]

.... and to mdonner's point about heat,

There is an inverse relationship between heat and corona inception voltage. As temperature rises within the motor, a lowering of the corona inception voltage occurs. This results in the breakover occurring at lower values of dv/dt.

 

[Farkel]

Sorry Fellows, I have no new information to report. End customer went to another source for solution to problem, and the EASA shop I was dealing with, did not get motor to inspect, nor any additional information. If things change, I will notify you of outcome.
Thanks!!!

 

[jOmega]

Sorry to hear that, Farkel.

F Y I

See:

http://www.baldor.com/pdf/manuals/780-1002.pdf

("Fundamentals of Inverter-Fed Motors" by Baldor)
and picture on page 2 therein for a reasonable portrayal of the type of damage caused by dv/dt ...


jOmega