VFD Application - Motor operating temp. + Harmonic Distortion

[mishlove]

Application: Centrifugal Pump (Variable Torque)
Motor: 200HP 1200RPM (FL RPM = 1185) 3/60/575V
447T TEFC

Some history: Motor/Pump operated approx. 4 years without problems with Direct on Line starting and running at full load RPM. Routine motor maintenance and observation had the motors running at approximate surface temperature of 60^C - 70^C.

Company decided to retrofit application with a Variable Frequency Drive for process control and operating cost savings. Motor rewound with Spike Resistant wire for application and VFD includes load reactors.

Immediate observation, with the motor running at 1000RPM, found motor surface temperature anywhere between 124^C and 143^C. With isolated area's of the motor at 170^C. (External blowers are keeping the motor operating)

VFD company validated the drive and gave it a clean bill of health. Power Quality Analysis found voltage transients at 2047.50V (In excess of NEMA but tolerable for spike wire in the short term - issue to be addressed)and THD of 10.153%, with maximum current total harmonic distortion of 14.49%.

Question: Do the above harmonic distortion value's fall within spec or not (IEEE 519-1992). And from the pools vast experience and suggestions any other observations would be welcome.

Cheers

 

[RadarRay]

Wow, what is your ambient? I realize its the same for both but that is a big difference. What happens if you run the motor at 1200RPM? Does it cool down? It is fan cooled so you are losing some cooling at 1000.

What frequency is the carrier set for? And what currents are you getting now compared to before? If the volts/hertz curve set correctly? You don't say how much current the motor is drawing but do you have before and after data?

Sorry for all the questions but this is an interesting problem.

 

[jOmega]

IEEE 519-1992 is for...... line side harmonics.... not load side from a VFD... It applies to the harmonics contributed by the VFD to the mains power distribution system; not to the harmonics introduced to the motor by the VFD.

What you have not told us, is what the running current was before the change (when operated on mains power) .. and what the current is at 1000 rpm .

Also, what other changes in process have they made ?

At what frequency output of the VFD is this 1000 RPM occuring at ?

 

[rbulsara]

This may have to do with our cable lengths. You may wish to keep the cable lengths between the VDF and motor under 50 feet.

By the way, current THD of 16 % is not a problem or it is expected, it’s the voltage THD that matters.


Please check out this link:

http://www.gorhamschaffler.com/vfds.htm

An excerpt is copied below.

"The use of IGBT's, while offering significant cost and performance benefits, has led to motor failures. The problem that has been occurring is a breakdown in the motor insulation, which causes the motor to go to ground or across windings. This is caused by the high voltage spikes that occur when the transistor in a VFD switches on. When the transistor switches on, there is a high voltage spike (as high as 1600 volts) for a very small period of time. The switching speed of IGBTs is very high, typically 8khz or even 16khz. This results in a much smoother output sine wave and significantly lower audible noise of the motor. This increase in switching speed has also increased the rate of voltage change over time, or the dv/dt. These high voltage peaks and short voltage rise times can cause premature breakdown of the motor insulation if it is not selected for these conditions. NEMA (National Electrical Manufacturers Association) has issued a new standard for VFD motors. This standard is NEMA MG1-1993, Part 31.40.4.2. This standard states that the motor should be designed for a peak voltage of 1600 volts and a minimal rise time of 0.1 microseconds for motors rated less than 600 volts. This standard should always be specified when using motors with VFD' which utilize IGBT's.
Another factor that is causing motor insulation to break down is motor lead length (the length of wire between the VFD and the motor). This is a problem because if the impedance of the motor is much larger than the impedance of the wiring a reflected wave can be established. When this reflected wave is added to the transmitted wave from the VFD, the voltage spike can exceed what the motor can withstand and motor insulation breakdown will occur. Because the impedance of motors is larger in smaller motors, this problem is most likely to occur in smaller motors. All VFD manufacturers should have published data on the allowable motor lead lengths
for their VFD's."

 

[rbulsara]

Since the motor is rewound, you may wish to run the motor direct on line (if you have bypass arrangement) and see if behaves differently than the original motor.

In any case the temp rise you mentioned seems high.

 

[jbartos]

Suggestion to mishlove (Electrical) Sep 1, 2003
Application: Centrifugal Pump (Variable Torque)
Motor: 200HP 1200RPM (FL RPM = 1185) 3/60/575V
447T TEFC

Some history: Motor/Pump operated approx. 4 years without problems with Direct on Line starting and running at full load RPM. Routine motor maintenance and observation had the motors running at approximate surface temperature of 60^C - 70^C.

Company decided to retrofit application with a Variable Frequency Drive for process control and operating cost savings. Motor rewound with Spike Resistant wire for application and VFD includes load reactors.
///The load reactors alone may or may not suffice. There may be need for harmonic filters instead of the reactor that is the simplest harmonic mitigator.\\
Immediate observation, with the motor running at 1000RPM, found motor surface temperature anywhere between 124^C and 143^C. With isolated area's of the motor at 170^C. (External blowers are keeping the motor operating)
///The mentioned temperature would require the motor very high insulation rating, e.g. Class H or higher.\\VFD company validated the drive and gave it a clean bill of health. Power Quality Analysis found voltage transients at 2047.50V (In excess of NEMA but tolerable for spike wire in the short term - issue to be addressed)and THD of 10.153%,
///The IEEE 519-1992 has voltage THD<=5%.\\ with maximum current total harmonic distortion of 14.49%.

Question: Do the above harmonic distortion value's fall within spec or not (IEEE 519-1992).
///The IEEE 519-1992 has voltage THD<=5%. This is for the VFD input side. First, suppress the voltage THD and wait what will current THD be. This might also be simulated by suitable harmonic analysis software, e.g.

http://www.skm.com

http://www.etap.com

etc.\\ And from the pools vast experience and suggestions any other observations would be welcome.

 

[jOmega]

rbulsara:

I checked out the link you posted.

I can't recall ever having read a more egregious collection of misinformation about VFDs.
(I could cite a couple that come close, though.)

 

[jOmega]

JB:

&quot;///The load reactors alone may or may not suffice. There may be need for harmonic filters instead of the reactor that is the simplest harmonic mitigator.\\\&quot;

Hogwash !

JB, there is no such thing as Harmonic Filters for the output of VFDs.

Suggestion: go back and read the information on TCI filters in a previous thread !

There are Sine Wave filters (Low Pass Filters) that change the PWM voltage pulses to near sine wave.... and there are dv/dt filters for application between the output of the VFD and the motor that soften the edges of the rectangular PWM voltage pulses.

There are no harmonic trap filters for the output of the VFD. And no self respecting VFD manufacturer would recommend such folly.

The reactor used in the VFD output is not there for harmonic mitigation.

Most VFDs don't have output reactors until they get up around 100 HP (75 kW) and above.

Here are a couple of reasons for output reactors:

1. Output reactors (inductors) are used to reduce the di/dt of the peak currents so that in fault situations, the drive OverCurrent trip function has time to react and shutdown the IGBTs before the fast rising current would cause them to fail.

2. Change the impedance characterestic between the drive and motor so as to reduce dv/dt at the motor.
HOWEVER: incorrect selection of the reactor impedance value can cause a resonant peak to occur at the motor, and for this reason, is not always a wise dv/dt solution (ask Toshiba....)

... and now we come to the two non-relevant references gleaned from your use of a search engine.

Question: did you take the time to explore those two references for relevance before posting them ???

The two references you listed are designed to analyze power distribution systems....not VFD output harmonics. They aren't even capable of analyzing VFD harmonic contribution to the input power distribution system as they do not have models for the associated VFDs.... (every mfgr's VFD design is different .. ergo ... a different simulation model is required for each mfgr.)

JB: you will not find a canned program for analyzing VFD load side harmonics.

Not a lot of call for that one. Some PhD types have undertaken such analysis within particular research projects (see IEEE IAS Journals over the last 10 years for relevant papers).


Suggestion: JB ... Read IEEE 519-1992.
When you do, you will discover that it is entitled...
&quot;IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems&quot; ...... and has nothing to do what-so-ever... with harmonics on the load side of VFDs !

 

[HarryDampers]

Some critical info is missing Mishlove . Some good advice has been given already but I would make the following comments:

The THDI that you quote is certainly on the INput of the drive . That figure would have no effect on what is happening on the Motor side anyway .

Measuring the Motor side Harmonics is (IMHO) practically impossible.

What I think you are seeing is the effect of the VSD OUTput harmonics on the outer edges of the Rotor Bar section . It is probable (although you do not tell us) that the Motor is overheating although it is drawing less than FLC.

You have to be very careful here because the overall temperature of the Motor may conceal the fact that the Rotor bars are blue with heat .

Lowering the switching frequency may help but the solution is probably to put Motor chokes at the drives output.

Just be careful though , these chokes need to be rated with the drives output waveform in mind .......dont just buy these from anyone or you will be left with a cooler Motor and stinking hot Motor Chokes!

I have seen this on two other occasions and I would be interested to know whose drive this is on ?

Both times the supply Voltage was over 415 Volts (once 575 and once 690)

 

[jraef]

JOmega, just to add to your list of reasons for adding output reactors...
3) Reduction of EMI / RFI to help meet CE requireents. Danfoss and PDL are both doing this as a standard feature now, but they are only 1-1/2 - 2% reactors. They are very careful to inform the user that they are NOT intended to mitigate output harmonics issues and even recommend additional reactors to help with the first 2 items you mentioned. Unfortunately some suppliers will see the literature that mentions the existance of the load reactors and ASSUME that they will work for the other purposes. That may be the ase with our OP, mishlove.

Mishlove,
More reactance or some long-line filters may be called for. Another resource is MTE Corp. Try this link...

http://www.mtecorp.com/tutload/reactors2.pdf

I have used their products many times with great success.



Quando Omni Flunkus Moritati

 

[mishlove]

Gentlemen, Thanks for all the reponses, most interesting. I will discuss some of them with the company responsible for the total install.
I'll try to reply to as many questions as possible.
- Ambient: 43^C
- VFD Carrier frequency: 5Kz (per cust @ factory default)
- Motor nameplate FLA: 189Amps
- VFD readout % load: 73% (Variable Torque application)
- Per cust. no changes made to the system other than the introduction of the VFD and related equipment.
- New VFD motor supply cable installed, special shield and grounding, approx. 200ft.
- I havge been told that the load filters are a &quot;tuned dv/dt&quot; filter ??. Voltage transients measured on the output side are around 2050V..this exceeds current NEMA limits, but I do not beleive is the reason for the excess heating. Wire used in rewind is inverter / spike resistant wire. Tuned dv/dt filters should be able to reduce these transients..but are not.
- Magnet wire used in the rewind is indeed Class H. I do not know the level of the rest of the insulating materials. (Repair shop is not forthcoming).

..........and now for bad news...the motor, she died.

Which, at the temperatures measured is not unexpected. The mill installed another motor, same everything, also motor manufacturer, but not with VFD wire. They have gone back to DOL and motor is running at 180Amps and the measured surface temperature, worse spot = 60^C. Which is totaly in line, actually for a 200HP, 6Pole in a 447T frame, quite cool.

I am left with 2 thoughts. (1) Yes the VFD is the prime source of the excess heating and (2) its possible that the repair shop did a bad burnout and damaged the lamination which increase the watts loss/lb, coupled with the VFD caused the high heat.

They will be doing some further investigations, with some of yr suggestions, as the mill will not allow the VFD back on line until they have some answers. In the meanwhile any other thoughts would be welcome.

Most sincerely to you all....thanks

 

[jOmega]

Mishlove:

First point:

According to EASA, with a burnout, upt to a 10% loss of torque can be experienced. However, this would be minifested by a higher amp draw at rated speed, assuming that on DOL operation, the pump did not draw more than rated (at 1.0 s.f.) current.

Second point:

Design & construction of the motor; namely, the construction of the rotor. Deep Bar rotors are notorious for producing a lot of heat when operated from inverter power sources. Motor manufacturers have done much in the way of rotor bar geometry design to minimize the heating in the rotor.

Perhaps you would be so kind as to advise the name of the motor manufacturer and the model number as a minimum... and as a maximum, complete motor nameplate information would be appreciated, with which we could contact the motor mfgr. and make some inquiries about the design characteristics of your particular motor.

jO

 

[jsummerfield]

Motor selection should consider a VFD during the initial purchase. This could be a factor with an existing motor.

Refer to IEEE Std 841-2001, IEEE Standard for Petroleum and Chemical Industry—Severe Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage Induction Motors— Up to and Including 370 kW (500 hp)

3.2 Unusual service conditions
e) Operation from solid-state or other types of adjustable frequency or adjustable voltage power supplies for adjustable speed applications

Also, Toshiba furnish an optional long lead filter for their VFDs for distances exceeding about 100 meter.

John