VFD Motor and Drive compatibility 4

[PDS68]

All - I am very new to VFD drives and VFD type motors. I am trying to help someone solve a problem with both. He is using ABB ACH 400 Drives with dv/dt filters installed. each vfd has 8 motors attached. He thought that the motors he is using were "vfd rated" or "inverter compatible" but it turns out that they are standard motors. They have had about 10% failure rate of these motors mostly on startup. The motors are .5 HP 200-230/460 3 phase with class B insulation. We have talked with some motor experts and they say they do not show the "typical" signs of VFD compatibiltiy failures. His customer is insisting the he change to VFD rated motor to fix the problem. In reading this forum I am not convinced that the rating of the motor is the issue. Any advice?

Thanks.
P

 

[Skogsgurra]

PDS68,

It can sometimes be difficult to find inverter rated motors in small sizes. The extra insulation simply takes too much place. I am not sure if that is a problem with 1/2 hp motors - it definitely is with 1/4 hp.

Eight motors on one VFD. That means that you have to run the inverter in scalar (U/f) mode. Not vector. It also means that your motor protection has to be outside the inverter (one thermal for each motor). The built-in protection cannot protect the eight motors individually.

Your dV/dt filter should be alright, but make sure that there are damping resistors parallel to the reactor or you can (will) get bad ringing between reactors and cable capacitance. This is not the reflected wave phenomenon, but pure LC ringing, you can easily see it with any oscilloscope - the frequency is usually not more than ten or twenty kHz. It depeds on the total cable capacitance, how long are the cables, BTW?

The ringing does not only heat the motors, it heats the reactors in the dV/dt filter as well. If you measure the core and the coils, you will find that the core is hotter than the coils. It is the eddy currents in the core that produces this unexpected effect.

Also, try to run with the lowest possible carrier (switching) frequency. It will reduce any adverse effects that the switcing can have on the motors. If the drive has DTC you cannot choose frequency, but you can select "long", which - in a way - is the same thing.

 

[PDS68]

skogsgurra,

Thanks for your quick response. I will check on the cable lengths and see what I can find out.

Regarding the motors. Yes, We have had a difficult time finding a VFD rated motor in 1/2 HP. but, if I read your reply correctly you inidicate that the motors might not be the issue as well. Is that correct?


Do you think we are pretty safe running those motors with a dv/dt filter?
Thanks
PDS

 

[Skogsgurra]

PDS68,

I am really not able to say if the motors are the problem or not. But I know that a dV/dt fiter - if correctly sized - will take the steep edges down to levels that usually do not hurt the motor insulation.

The PWM pattern can still (the dV/dt filter does not help here) heat the motors more than is healthy for them. Do they run extremely hot? If that is the reason you should increase switching frequency (in spite of what I said about that earlier).

There is also the possibility that there is too much slip. Measure speed with a stroboscope and compare to the rated speed. If there is too much slip at nominal load then your voltage/frequency ratio may be on the low side and should be adjusted.

A vector drive can also do things to the motors if there are several connected to the inverter. The inverter simply cannot tell what the motors are doing since it is used to seeing one motor and keep track of the goings on in that motor. More motors make the vector algorithm confused. Especially if the motors are working under different loads. A U/f drive can handle several motors easily. But you must, of course, do the right adjustments (nominal frequency, nominal voltage and correct boost).

The thermal protection becomes important when there are several motors connected to one inverter. The inverter has to supply power to all motors and cannot tell if one single motor is working too hard. You need thermal protection for each individual motor. Either a "bimetal" or a "klixon" or other device.

 

[jbartos]

Suggestions to PDS68 (Electrical) Feb 24, 2004 marked ///\\I am trying to help someone solve a problem with both. He is using ABB ACH 400 Drives with dv/dt filters installed. each vfd has 8 motors attached.
///Please, would you elaborate how 8 motors are "attached" to the vfd output in terms of description of the motor feeders from the vfd to the motor terminal boxes. State if there are any individual motor overload relays, thermal cutouts, disconnect or control switches, whether all motors are started simultaneously or some of them only and others added at a particular speed of others running, etc. what may help to find the solution.\\ He thought that the motors he is using were "vfd rated" or "inverter compatible" but it turns out that they are standard motors. They have had about 10% failure rate of these motors mostly on startup.
///This may imply a deteriorated motor power supply quality at the motor startups.\\ The motors are .5 HP 200-230/460 3 phase with class B insulation. We have talked with some motor experts and they say they do not show the "typical" signs of VFD compatibiltiy failures.
///Yes, this is possible, e.g. due to overloads, low motor terminal voltage, etc.\\ His customer is insisting the he change to VFD rated motor to fix the problem.
///This may or may not solve the problem.\\ In reading this forum I am not convinced that the rating of the motor is the issue.
///The rating of the motor may be a culprit. However, this remains to be seen.\\ Any advice?
///Please, describe the circuitry on the vfd output as requested above.\\\

 

[Laotze]

PDS68,

I am no expert but certainly, there are no quick answers to this question.
A proper analysis must first be done on the root cause of the motor failure/s. (Root cause and contributory cause to clearly established). Insulation punch through will indicate problems with voltage spikes (overvoltages) while insulation 'burn out' indicates problems with harmonic currents (overheating). (Of course, bearing failure may indicate eddy currents as a result of harmonics). Each phenomenon has different time windows associated with it. Insulation punch through occurs more quickly (often instantaneously) while overheating is a longer term problem.

Point taken..matching VFD to such small motors is generally a problem.

The dv/dt filter/s will assist in reducing the affect of steep voltage wavefronts, so in my opinion will also work for PWM inverters.

Cable lengths are generally a concern, as impedance mismatching (cable vs motor) can cause significant voltage peaks (reflected waves - voltage doubling). Small motors will be more vulnerable to insulation failure (depending on their insulation rating).

 

[ozmosis]

PDS68
As skogsgurra pointed out, the total cable length with 8 motors involved will possibly be quite long, this needs checking but also confirm if you are using screened cable or not. If you are, there are practical limitations to screened motor cable lengths due to the increased high capacitance effect. In some cases the capacitance effect can cause problems with motors or the actual drive itself but if you have a dv/dt filter then this should minimise the effect.
It might be useful to know what is actually happening with the motors. Is it the windings or bearings?

 

[jbartos]

Comments: Marginal interactions among motors should not be ruled out since each motor is manufactured within certain manufacturing tolerance, which impact the motor parameters. If different motor parameters exist, there will be marginal currents flowing among motors. This is necessary to treat as a system of interacting loads on the VFD output.

 

[PDS68]

All - Thanks very much for the feed back. Please bare with me on my questions/Answers due to my inexperiance.

JBartos - connections. - I think what you are asking is type of cables etc. I will have to find out. Is there a specific cable/feeder I should be looking for?


Disconnects - There are disconnects at for each motor.

thermal protection - The motor spec. does not show this so I will have to check.

Starting - All motors are started together.

SED2developer:
The motors are new and fail on startup or with less than 2 weeks run time. We are still doing F/A on them but so far there have been bad lead connections, and one burned out motor coil. We have only inspected a few to date.

I will pass along the other information as soon as I get it.

Again...Thank you very much for your time!

 

[jbartos]

Suggestion: The output filtering needs to be improved.

http://www.transcoil.com/

For:
1. dV/dTGuard KLC Filter Motor-Protecting Output Filter
The dV/dTGuard product family has been designed
as an engineered solution for motor failures due to
the reflected wave phenomenon. Designed to be
installed at the output terminals of the Drive,
dV/dTGuard Output Filters provide effortless
installation, convenient accessibility and enhanced
motor performance and durability. Available in a
UL/CUL version or in our original version, these
products provide the outstanding performance that
our customers have come to expect from TCI..
2. The MotorGuard High Performance Output Filter has been designed to be installed at the output terminals of PWM devices. The KMG filter converts the PWM wave form to a near sinusoidal wave form, allowing sensitive applications to take advantage of the efficiencies and savings that PWM output power supplies offer

 

[jcarrr]

I may have missed the information, but are these motors operating on 240 or 480 volts? We have many pumps running on drives with leads up to 1000 feet long. I know all about the trade offs of voltage drop and the cost of drives and cable, but we have much better luck with 240 volt configurations when the installation allows us to afford it. The pulses from the 240 volt drive only stress the insulation about half as much as the 480 volt pulses. We are in an area that is plagued with over voltage supply

 

[macmckim]

If your are looking for a true inverter duty motor in small HP's, two companies come to mind.
a) Marathon Electric - Black Max VFD duty motors
b) Bodine Electric VFD duty mtrs.

I know Marathon has 1/2HP mtrs sizes, and can operate on constant torque applications at 1000:1 speed range, without any auxiliary cooling.

 

[jbartos]

Suggestion to (Electrical) Feb 28, 2004 marked ///\\The pulses from the 240-volt drive only stress the insulation about half as much as the 480-volt pulses. We are in an area that is plagued with over voltage supply
///The 480V cables and conductors can come with better insulations. The normal category is low voltage (1000VAC and under) insulation rating. However, if voltage spikes or voltage swell are frequent a higher insulation rating will be beneficial. 240V cables and conductors require large cross-sectional areas. These are more expensive.\\

 

[aolalde]

I understood jcarrr recommendation is to work with 240 Volts when the motor is already manufactured as dual voltage (240/480 V) and the motor insulation (already built) will be less stressed by the spikes developed on 240 V mode as compared with those at 480 V.