Long-term experience driving standard induction motors with VSDs


Thread Starter

Peter Talas

I am looking for input from anyone out there who has had long-term operating experience using VSDs to drive non-inverter duty rated large HP, 460 - 600 V rated, three-phase, wound-rotor induction
motors with shorted rotor windings.

Best regards,

P. Talas
What is the principle reason for VSD application to wound-rotor motor.

Is it age of existing equip? or Soft-Start? or Variable speed?

Phil Corso, PE
Trip-A-Larm Corp
(Deerfield Bch, FL)
The main reasons were obsolecence of the vintage 1960 wound rotor speed control and dynamic braking systems, the excessive maintenance required to keep the old control systems operational and the customer's desire to reduce start-up stresses on the equivalently old driven equipment. The wound rotor motors were left in place due to outage time limitations with the intent to later replace them with inverter duty rated ones. The old motors have been functioning flawlessly since the VSDs were installed last September (i.e: no overheating, no shaft currents and no apparent insulation degredation). I am leaning towards leaving the wound rotor motors installed "as-is" but, as a precaution, I am seeking input from others who have long-term operational experience with similar applications (bad or good).

Pete Talas, PE

C. Ward Yelverton

Peter - it was common practice for a while in my field to replace manual wound-rotor speed controllers (manual drum switch control of secondary resistor bank) with adjustable frequency controllers and leave the existing
motors in place. The existing motors were very rugged, had a very low failure rate, and were interchangeable with the motor used in other
installations that were not being rehabilitated at the same time.

Shorting the rotor brushes together in the motor's rotor junction box approximated a Design-B performance curve and worked quite well with the AF controllers. The brush resistance was important in producing this curve. If the motor is disassembled and the rings shorted together in a misguided effort to eliminate the "failure prone" brushes, the curve is crunched up close to a Design-A and the performance just wasn't acceptable for our application.

But as I mentioned in another reply to someone in this subject thread, the problem with this arrangement was that the ability to adequately handle overhauling overloads just wasn't delivered by the AF controllers, even with
greatly oversized "dynamic braking" resistors; and now we've returned to the 70yr old adjustable voltage control which performs superbly in this
application. (Admittedly, SCR's are very nice replacements for mercury ignitrons).

C. Ward Yelverton

Phil - haven't fount the original message yet (I'm a bit behind), but use of wound-rotor motors with adjustable voltage drives for variable speed control of draw bridges has been pretty standard for years. It's the most reliable drive when it comes to handling the huge overhauling load under wind and snow/ice conditions in the lousy electrical environment (picture a open drawbridge as a really big lightning rod). With single motors, Design-D will work, but when you start paralleling motors the wound-rotor is required
for matching curves ... and in most cases, the wound-rotor lets you match the curve to the load (again, torque limits and overhauling loads) which
might not even be possible with a conventional Design-D and would almost certainly require a much larger motor (keep in mind we're talking about 150%-200% slip, rotor currents get kinda high and the winding does warm up).
Thanks for your input Ward. The following is intended as additional information on the application. The VSD systems are made up of coupled rectifier/inverters modules. One is connected to the motor load the other to the supply transformer. The input side rectifier/inverter is used to regenerate into the supply transformer during dynamic braking and load lowering. The motor loads are transmissions which lift and lower navigational gates. Two motors are utilized in a torque sharing
arrangement per gate. The VSDs are also used to hold the gates in the stopped position so that the static brakes are only applied after the
gates stop moving or in emergencies.

With respect to your comments, we shorted the rotor windings on the load side of the slip-rings and intend to leave this arrangement "as-is" and
to maintain the slip-rings/brushes as required. The VSDs utilize junction transistor packs and operate at an output frequency of 3kHz.

Best regards,

P. Talas, P. Eng.

Amr Elaguizy

The bottom line is cost as you are already are aware of. The cost could be broken down to short term and long term, you must figure that out and presented to the management in terms as return of investment. Short term cost will be the purchasing
and replacement of the motors. Long term cost is as follows:
a. The new motors are energy efficient ( > 95%) versus the old motors which if it has been rewound two or three times in the old days(by burning the varnish in an open fire), could be as low as 50%.( Motor repair shops have software programs to test and figure the efficiency of an existing motor, or give you a rough figure of
the motors efficiency number based on the year, manufacturer and size of motor). For your own information I was able to justify replacing the old U frame motors versus T Frame motors in less than one year, only based on the energy saving in the late 1980's.
b. Historically how many times the motors have been rewound & taken out of service and the expectation of future failures, with the associated cost of repair, loss of production and labor cost of removing and repairing. My experience is the old form wound motors were over designed (plenty of copper), thus they are able to handle overloading and over temperature ( from slow speed) with no problem. Thus if you are responsible for the maintenance you might be happy
with the status quo (why fix it if it ain't broken?). kind of cover your "a**." Of course this is an unethical way of thinking? but good intentions are not appreciated when a new equipment are installed and it fails at the beginning of a new project, even if it was not your error.
Last point how did you establish "no apparent insulation degradation"? Did you do an insulation test before and after?
Good luck
Amr Elaguizy
Thanks for your note Amr. As far as I know only two of the four motors were rewound over the last 30 years. We hi-pot tested the insulation of all the motors before the changeover and we are
retesting them in the near future. I used the words "no apparent winding insulation degredation" since the "after utilization" hi-pot
test results are not yet available.


P. Talas.
Replying to Pete Talas' comments:

I certainly laud you in updating the starting methodology. Regarding motor condition, you should be able to determine insulation condition by trending megger tests. On the other hand the motors are 40 years (+) old, and are probably close to their PLE.

On a positive note, the VSD substitution has certainly worked, based on your comments.

However, I'm curious about your solution. Did you short-circuit the rotor winding? At the slip rings on the rotor? Elsewhere?


Phil Corso,PE
Trip-A-Larm Corp
(Deerfield Bch, FL)

I worked on a railroad bridge project (Salmon Bay Bridge in Seattle for Burlington Northern) and we used vector drives with design B 100 HP Motors. We took a novel approach and summed the encoder
feedback registers from both motors in a motion controller and used this feedback for both motor position loops. This created an electronic differential that worked great. The civil engineers for the job felt that we extended the working life of the bridge by decades simply because we could now control the accel/decel so precisely. We data logged 120% current reversals when raising the bride due to wind gusts. The drives were modified to allow line loss when
lowering the bridge. The regen bridge kept the logic alive all the way to the seat and then the drives capacitors kept things going for another 10 seconds beyond that. It was a pretty cool design!

Ken Brown
Applied Motion Systems, Inc.