Wound Rotor vs Squirrel Cage - Eng-Tips

Hello OhioAviator

A wound rotor motor, with an appropriate secondary reistance starter is able to produce a high starting torque from zero speed through to full speed. This will result in a higher acceleration rate than you will achieve with a squirrel cage motor. The starting current will be lower and the motor will be able to start in loaded situations where a standard cage motor will not.
The negatives, are that both the motor and the starter will require a lot more maintenance than a standard cage motor, and the purchased price is higher.
Best regards,

Mark Empson
Thank you jbartos,

I'm not sure about the MTBF being lower for a soft starter; they seem to be getting more and more reliable these days. But I do not have experience with medium voltage starters in the HP+ range, either.

Back to wound rotor motors...
Are liquid rheostat starters still the current, most reliable technology? If not, are there other types of wound rotor starters out there that are more reliable and less maintenance? If so, what manufacturers?

Again, thanks all for your help!

--John Ruble Suggestion to the original posting marked ///\\I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?
///The mechanical load profile torque-speed needs to be known to be able to match the motor torque-speed characteristics. Assuming that high starting torque is required, then the squirrel-cage induction motor Nema Design D may be required. This could be the better solution than the wound-rotor induction motor since the motor may be DOL started, if the power distribution allows it, and it will be simpler to maintain than the wound-rotor induction motor. Normally, starting and operating conditions of this size of motors are simulated by using software, e.g. EMTP.\\
Hello OhioAviator

Although you can certainly reduce the starting current by using a soft starter, you will not get the same starting effectiveness (Start torque/start current) using this method.
If you begin with a standard cage motor, particularly at this size, your starting torque will be low compared to that achievable with a wound rotor motor. The initial start torque may be in the region of 100 - 120% at 600 - 800% start current as opposed to to say 250% current for 200% torque with the would rotor.
When we apply a soft starter to a standard cage motor, we reduce the start current and also reduce the start torque by the current reduction squared.
For equal start current, the wound rotor motor and secondary resistance starter will produce many times the start torque of the soft starter and cage motor.

If you do not require a high start torque, then the soft starter and cage motor are definitely a very viable option. The reliability of correctly engineered soft start applications is very high. Some installations that I have been involved in are still operating correctly and without problems after twenty years.

I suspect, that for this application, you will require a high start torque, hence the suggestion of the wound rotor machine. This is based on my experience, but would depend on your actual requirements and parameters.

Best regards,

Mark Empson
I have personally been to installations that used soft starters, and they were very sorry that they had them.
Like the previous posts, a wound rotor induction motor (WRIM) will have faster acceleration and an adjustable speed torque curve, which a squirrel cage motor (SCIM) does not have.
One important point that many SCIM manufactures do not understand is the KWH demand billing. What this means is that as the motor is crushing cars, there is a demand limit that you do not want to go over otherwise you will be paying substantially higher energy bills to compensate for that (for example) 5 second period that you went over that value.
Yes, you will get full load torque at standstill with about 70% of your full load (rated) starting current, provided that the liquid rheostat is operating correctly AND the brine solution is correct.

PS The best way to determine if the brine solution is of the correct concentration is to use a hydrometer (density test of the solution). Please take note that the concentration needs to be temperature compensated. Auto shredder motors see far more than shock loading, the main area which I did not see covered is the extended period of time where the load on the motor reduces the speed of the driven equipment. This is where the SCIM fail, they do not work well when punished at less than full speed.
I have seen many shredder motors and they are far beyond the capabilities of any other motor. The service factor is typically 2. Schorch make one of the best motors I have seen for this application. They are of course the wound rotor type. P&H may have a shredder motor also.
On SCIM motors the rotor will usally break up as a result of the rotor running slower than design. Thank you, all, and especially to DougMSOE and to Mendit. Both of you obviously have specific knowledge of the scrap metal processing industry (far more than I have, that's for sure!) and car shredders in particular. I originally come from the mining industry and while that industry has large crushers, I've never seen mining crushers (and motors) that take the punishment that car shredders do.

I think I'm convinced now that a WRIM really is the better choice for our applications in this industry.

One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters. Is this because you can continuously vary the resistance of a liquid rheostat while in operation? And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit? Does rotor resistance help limit KW Demand?

Again, thank you everyone!

--John R. Suggestion to the previous posting marked ///\\One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters.
///The cost and reasonable reliability including simplicity may be the main reasons.\\ Is this because you can continuously vary the resistance of a liquid rheostat while in operation?
///Yes, in comparison with wound resistors; no in comparison with soft starters.\\ And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit?
///The resistors require less maintenance, can have better location, can be remotely controlled, etc.\\ Does rotor resistance help limit KW Demand?
///In which context or comparison?\\///Please, notice that the above postings did not differentiate among squirrel-cage induction motor Nema Design letters. Nema design letter D has noticeably different characteristics from other Nema design letters. The Nema design letter D motor has substantially higher starting torque than other ones.
Another aspect that has not been addressed yet would be adding a flywheel. However, this would impact the operation of the crusher since it would have to start empty to minimize the starting torque unless the flywheel is disengageable. \\ Hello OhioAviator,

Although I fully agree with and support much of what has been said here, I believe you need to consider each application on a case-by-case basis, particularly if you are aiming achieve best performance with the lowest possible capital outlay.

Wound Rotor Motors are capable of producing high torque when controlled via a ‘properly engineered’ liquid resistance starter. Their torque/speed characteristics are also well suited to applications that present transient over-load conditions and this can be further enhanced in shredding applications through the addition of a flywheel to the mechanical system. On the down side they do require more maintenance than a Squirrel Cage Motors and this needs to be factored into the decision making process.

Medium (and High) Voltage Squirrel Cage Motors are notorious for their poor start performance, often exhibiting levels of Locked Rotor Torque less than rated or Full Load Torque. As has been advised by Marke the addition of a soft starter will act to reduce starting torque further and therefore it is likely problems will be experienced here. This might also explain comments by DougMSOE! On a more positive note some motor manufacturers will design and manufacture Medium Voltage Cage Motors to your specifications, thus allowing you (possibly) to utilize Full Voltage Starting techniques.

As suggested by TheDOG, a Cage Motor controlled via a Variable Speed Drive will provide the best overall performance......dollars permitting of-course!

If your clients are anything like mine (unlimited dollars are not available to them), an analysis of motor and load curves should assist to determine best starting method for the intended application.

Regards,
GGOSS
"Thank You" to TheDog, GGoss, and GSimon for your replies to this thread. While I agree that a SCIM/VFD combination might produce starting torques comparable to a WRIM, the up-front capital costs tend to be prohibitively higher. (Remember, our world is dominated by bean counters who, by and large, look only to the end of the current fiscal quarter.)

What I don't have a good handle on, though, is how it is possible to continuously manage (control) KW Demand with a SCIM/VFD combination like I can a WRIM/Liquid Rheostat. Is it possible?

Thanks,
J Ruble Ohio Aviator.

You should consider both, starting and full speed operation of the motor.

My opinion is that WRIM with a proper matched resistance will provide the higher starting torque with the lower KVA inrush.

At full speed operation, your load application could develop sudden high peak Torque overloads. A flywheel could provide the extra required torque combined with some resistance in the rotor circuit to allow the rotor to slip under a shock load and then gradually accelerate at operating speed again.

The kilowatt demand is due to load plus losses and you have to provide it since it is pure energy ( the flywheel, if you have one, will provide momentary kinetic energy but then the motor will return that energy while accelerating back to full speed ) A variable frequency driver will develop a Volt/Hertz electric input to the motor but the voltage drops following the frequency to avoid magnetic saturation, then your torque could be constant but not larger than under 60 Hz operation.

A Olalde.