Wound Rotor vs Squirrel Cage 14

[OhioAviator]

Hi All,

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?

Thanks!

 

[DougMSOE]

Having installed the 3 different types of motors in scrap yards WRIM,SCIM and Synch (and a few DC) the shock loading can take a motor from operation RPM to 1/4 of the at RPM in less than 2 seconds! Be advised, what is constant with scrap-- NOTHING! While in the theory world 'white goods' are only 'white goods', you may also have the "engine block" mixed in with them. By far a WRIM is totally the only motor that can handle 4* rated current for 30m seconds while the solid state world is in smoke. Been there and saw that. Reliabiliy is best when you can fix it youeself.

 

[OhioAviator]

Thank you, DougMSOE.
I'm finding myself pretty much in complete agreement with you, especially as time goes on and I gain more and more operating experience with these shredders. You are so right that 'white goods' ain't always 'white goods'. And I doubly agree with your statement that "Reliability is best when you can fix it yourself"! Been there, done that... too many times. I've spent way too many long nights and weekends learning how to fix the supposedly superior 'latest and greatest' electronic gadget, wishing I had a simple molded case circuit breaker and an across-the-line starter, instead.

Gentlemen (and ladies, too) thanks for your input on this thread. I think I have enough info for now. If a new posting comes up I'll check it out but I can't promise I'll respond to further postings.

Again, many thanks!

John R.

 

[Aquarius]

I know you've said you do not want any more emails, so please do not feel under obligation to replay.
I was on holidays and missed the whole discussion
Apologies to jbartos, electricpete,dougmsoe, gsimson if I repeate what you have already said; you guys provided v.good replays.

OhioAviator- you did not state at the beginning what is the application you are looking for, speed control or soft start ?
Anyway, here is my contribution:

Solid State Soft Start Systems reduce the available full voltage start torque when applied to SQ or WR induction motors. Hence applications requiring a maximum starting torque cannot be used with a soft starter. However, where the WR motor has been used to reduce and control the starting torque application of soft start control can be readily adopted.

The characteristic of increasing torque with decreasing current frequently determines the selection of the WR motor when starting current limitations are severe. I.e if a power company regulations limits LRC to 1.5 FLC , WR motor would still produce a LRT of 1.5 FLT.

Each addition of R in the rotor circuit reduces motor speed . Speed reduction is practical only to 50 % of synchronous speed. Beyond that speed becomes unstable because high slip characteristics are produced in the a rotor that operates with a high resistance in its circuit. For this reason application where speed is to be reduced <50% are used only where a constant loads are involved( bridge, trolley, crane)
Normally, a WR motor is designed to operate with small slip and high efficiency at full load. So, when speed is reduced , slip increases and efficiency decreases.

When operating at anything other then max speed the resistance control of WR motor consumes power that would otherwise be used to move the load. This inherent inefficiency of WR design has lead to its obsolescence in variable speed application.
Consequently, applications requiring variable speed 3 ph motors now use a variable frequency drive (VFD) to adjust the speed of a standard SQ motor. The VFD electronically changes the frequency of the AC current supply , thus changing the synchronous speed of the motor. This has a huge benefits in power consumption over the resistance control.

The other problem with application of WR motor in variable speed application is that if the secondary resistance value of each phase become unbalance, the vibratory torque is generated.

WR motor can be applied on either constant speed or adjustable-speed drives. They are particularly suitable for smooth acceleration of loads in application which require high starting torque with low starting current or impact load (elevators, ventilating fans, printing press, pumps, compressors, conveyors, pulverizers, stokers, positive pressure blowers, crushers, shredders .
In summary major reasons for selecting WR motors are:

1. The load can be started at the max torque
2. Large starting torque can be obtained in comparison with low starting current
3. In case when starting frequency is too high for the thermal resistivity of the an ordinary SC motor
4. In case GD2 of the load is too large for the thermal resistivity of an ordinary SC motor
5. In case load requires cushion starting

Today, owning to their complicated construction and hence high maintenance and cost, WR motors are used mainly in applications when relatively high starting torque is required, but in which the starting current may not exceed the FLC much-application 2 above.

Note – in very large applications 8000 + kW a solid state slip energy recovery (SER) system in conjunction with the conventional slipring resistors is used to perform variable speed control.

 

[jbartos]

Suggestion/question: It is not clear from the original posting whether or not the cost is a factor. Please, would you address that point? Any better solution will probably be much costlier; especially, considering modern power electronics options aligned with the squirrel-cage induction motors and wound rotor induction motor.

 

[OhioAviator]

Hello Aquarius and Mr. Bartos...

Thanks for your postings to this thread.

Aquarius - The primary parameters I'm looking for are current limited starting, kW Demand control, high slip capability to accommodate very high short term shock loads, and reliability with simplicity. Variable speed control really isn't a goal, just high slip capability.

JBartos - Cost isn't the primary factor, but it probably ranks second. Reliability in this particular application (automobile shredders) is paramount. Simplicity is also important, as we typically don't have highly skilled electrical technicians on the payroll.

As I eluded to previously, I think I'm pretty well convinced that the WRIM is the best option, primarily due to the severity of the duty required for our application. Automobile shredders are &quot;crushers&quot; taken to the extreme. As DougMSOE stated above, a typical automobile shredder is shredding light metal (car bodies) together with heavy chunks of metal (engine blocks). The impact shock loads are tremendous.

Again, thanks!

 

[jraef]

OhioAviator,
I am coming into this late due to the holidays as well, but I would like to add my opinion. I am a big proponent of MV solid state controls, but I, like GGOSS, consider each application on its own merits.

I have applied SS starters to SCIM motors in many shredder applications successfully and they work just fine. However they were waste shredders not AUTO shredders. IMHO, in your application the WR motor is probably better for the shock load capability as mentioned by others. Many of the above posts refer to starting torque issues but may not realize that a shredder NEVER starts with the load already in it (the exception being hydraulic powered shredders, but thats a different discussion). You are looking for a reduction in starting torque since the shredder is always unloaded at start. The slip recovery capability of the WR motor can be &quot;adjusted&quot; by altering the resistance on the secondary, a very useful feature for your application. Look at it this way, a WR motor has all of the capabilities of Design A, B, C and D SCIM motors by simply changing the rotor resistance. With a SS starter on a SCIM, once it is at full output the starter has NO control of torque capability. Your motor is on its own with just the torque and slip recovery capabilities inherent to its design.

One other possibility is what was mentioned by rlpuck on 1/5/04, the use of a Design D squirrel cage motor. It has the slip recovery capability equal to the maximum available in the WR. If you go that route, the RVSS starter would be useful to keep the starting torque to a minimum. If you don't already have a motor you may want to consider this. If you already have a WR motor it would not be worth swapping it out.

By the way, don't be fooled by the Benshaw marketing of their &quot;solid state wound rotor&quot; control. It is just a SS starter for the stator, combined with a fixed resistor in the rotor, which is shorted out at full speed. If you are going to go with WR, have a competent control manufacturer build a starter and resistor control package that matches your needs. SS is kind of a waste on WR motors. Also IMHO liquid rheostat WR controllers are good for things like flow control etc., but the maintenance costs and care required to keep them running would make them poor candidates for your application. Search this forum for the term &quot;Liquid Rheostat&quot; and you will see several discussions pertaining to LR maintenance issues. Shredder operators are not known for being mindful of routine maintenance!

Quando Omni Flunkus Moritati

 

[OhioAviator]

Hello jraef,

Thanks for your post. And thanks for confirming my suspicions that at WRIM is probably the best choice for our application. And you're right... shredder operators are DEFINITELY not known for routine maintenance.

BTW... thanks for making me look on the internet for the translation to your latin phrase (curiosity, you know)!

Cheers

 

[jbartos]

Suggestion: Visit

http://www.elec-toolbox.com/motorchar.htm

for:
Squirrel-Cage Induction Motors - The most simple and reliable of all electric motors. Essentially a constant speed machine, which is adaptable for users under all but the most severe starting conditions. Requires little attention as there are no commutator or slip rings, yet operates with good efficiency.

Wound-Rotor (Slip Ring) Induction motor - Used for constant speed-service requiring a heavier starting torque than is obtainable with squirrel cage type. Because of its lower starting current, this type is frequently used instead of the squirrel-cage type in larger sizes. These motors are also used for varying-speed-service. Speed varies with this load, so that they should not be used where constant speed at each adjustment is required, as for machine tools.
Comment marked ///\\Reviewing the above postings and agreeing with the above link, the squirrel-cage induction motor will be the best solution. The flywheel for smoother ride-throughs should not be ruled out.

 

[DougMSOE]

With all due respect jbartos.
Squirrel cage motors have been used in the auto shredder applications, BUT the tons/KWH is less for a SCIM than for a WRIM, all things the same.
Unless you have actually been to and worked in an auto/industrial scrap yard where the cars/busses, etc are sherdded from their present form to 'half-dollar size' in less than 10 seconds the true appreciation of the WRIM will never be appreciated. I have worked in the heavy power industry for over 30+ years and watching a shredder do its job is amazing to say the least. A 20,000 HP motor driving a BFP, ID or FD fan is no comparison to what a 7,000HP motor goes through in 10 min of its job in a scrap yard.
Yes, a SCIM is good but will NEVER come close to taking the shock loadings that a WRIM does. Further, the WRIM can be made to operate very closly to the SCIM in this application.
PS I have installed about 10 shredder motors and have worked on over 50 of then.
DougMSOE

 

[SRC99]

The metal type resistive controller for WRIM is more reliable than liquid rheostat starters. The controller consists of only three durable parts: stainless steel resistors, vacuum contactors and PLC. It’s reliable and maintenance free. PLC controls contactor close/open to switch resistor in/out to control the torque and speed. The controller allows the motor to deliver up to its breakdown torque to the driven load during starting or running.The controller is pre-programmed to meet your specific load characteristic

 

[Marke]

Metal type resistors tend to have a positive temperature constant causing the value of the resistors to increase with heat. Liquid type resisters have a negative temperature coefficient resulting in reducing resistance with heat.
A wound rotor starter requires reducing resistance as the motor accelerates so there is an advantage in using the liquid type resistor.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: The Squirrel-Cage Induction Motor (SCIM) has some room for customization based on its intended application. SCIMs with NEMA Design Letter D can handle very demanding applications, e.g. openings of the rusty valve, punch presses, cranes, hoists, press brakes, shears, oil-well pumps, centrifugals, etc.
Reference:
Donald D. Fink, H. Wayne Beaty &quot;Standard Handbook for Electrical Engineers,&quot; 13th Edition, McGraw-Hill, Inc., 1993,
Section: Characteristics of Polyphase Induction Motors on page 20-33 objectively compares various motors characteristics.
Visit

http://www.ecmweb.com/ar/electric_keep_speed_motor/

http://turbolab.tamu.edu/pubs/Pump20/p20pg095.PDF

http://www.dac-3d.com/Knucklebuster/PDFFiles/E410-S02.pdf

etc. for more info

 

[DougMSOE]

The resistor grid system has been used in several WRIM scrap yards and they do work reasonably well. The problem is that they become very hot and with all of the dust and fluff from the car seats you will have a fire. Been there and have installed the liquid rheostats that do not have the problem.
Further, the square footage req'd for the liquid Vs the grid system is much less about 60%. And with the grid system you need large contactors to control the speed torque curve such that the max torque can be placed at the resultant RPM from the load on the shredder.
Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job.
I do not care for any book that you might be able to cite that a SCIM is better than a WRIM in this application. There is NO COMPARISON in this application. A NEMA D does not even get close to what a WRIM can do in this application.

 

[jbartos]

Question to the previous posting: Please, would you be more specific in engineering and design terms in your statement:
&quot;&quot;Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job.&quot;&quot; specifically, if the WRIM flexibility is clarified?

 

[DougMSOE]

jbartos, To your question, I hope that this is what you are asking.
&quot;Please, would you be more specific in engineering and design terms in your statement:

I would ask that you might take a look as to what the difference is between a NEMA design A,B,C,D,and F of a 3ph SCIM. What actually is the difference?
Given all other parameters the same the one major difference is the metallurgy of the rotor bars, and they are not Cu, but a Cu alloy!!! The resistivity of the bars AND the shorting rings are what make the difference.
Now thak a look at the equivalent circuit for an induction motor, NOT the equivalent circuit cited to simplify the circuit such that the well known circle diagram is made. You will notice that the maximun power transfer theorm applies, not just by load but also by RPM!
This is why the WRIM does this job better than any other motor or for that matter a diesel engine, which has also been used.
I the WRIM the secondary resistance may be changed at will to a range of resistances 20 times that of a SCIM even mor if required.

 

[aolalde]

The WRIM could work with variable external resistance values connected to the rotor circuit.

That feature makes it feasible to be adjusted to almost any desired Torque-Line current- slip characteristic.

Put a high external resistance and the inrush current will drop close or even under full load current and at the same time the resultant Torque becomes more current effective.

Adjusting the external resistance properly,you will get any desired “Design Performance” from a NEMA design A to a D and much more.

Adjust the external rotor resistance when the motor is running with load and the speed will be reduced with an increase in the rotor slip.

An ISCM has a fix rotor construction and after it is constructed it will have a fixed rotor resistance and performance.

The heat generated in the rotor winding is dissipated into the rotor of an ISCM. For a WRIM most of the rotor heat is dissipated in the external resistors. A NEMA design A (ISCM) has high efficiency at full load and low slip but very high inrush current and only 150% LRT. A NEMA design D (ISCM) has 275% LRT and 400% inrush but generates too much heat and low efficiency under load.

That makes a (WRIM) Wound Rotor Induction Motor performance very superior if compared to an (ISCM) Induction Short Circuited-rotor Motor (squirrel cage).

Costs of Maintenance and Initial investment are certainly much lower for an ISCM, that is why it is the first selection as far as it could be tailored properly to the load.

 

[DougMSOE]

You've got it aolalde!!!
The only addition to your post is that the starting current of the WRIM with a liquid rheostat is usually set at 70 to 75 % of the full load running current (this is where the maximum torque is usually developed). Typically the liquid rheostat uses a solution of soda ash and temperature (more on the soda ash) to adjuse the Istart via the secondary resistance.
Further, the WRIN is made top operate with the resistance in the rotor circuit.
Have a Great Day!!!
Doug

 

[gsimson]

WRIM with resistor grid / Liquid resistor is a &quot;Load dependent&quot; drive. The torque developed at various speeds is dependendent on load hooked. SCIM with VFD is load independent and developes PULL OUT TORQUE at ALL speeds.

The current requirement is based on the load and the power &quot;wasted&quot; in the resistor. With SCIM /VFD there is no wastage of power . The motor draws power only to the extent required. &quot;Solid state going into smoke&quot; is only hypothetical and certainly not required for real applications.

 

[DougMSOE]

Since the secondary resistance can be increased or decreased as required to match, as you put it, the PULL OUT TORQUE, the load WRIM is load dependent. Certainly, as you describe the SCIM with a VFD is, within the limits of the machine, is load independent. And yoe the heat developed by the resistor bank is some what wasted unless like a few scrap yards have done is to provide heating and cooling as required. Un fortunately I HAVE SEEN FIRST HAND secondary energy pump back systems going into smoke as well as the front end drives. What is required to understand in this application of WRIM in scrap yards is,
1.)easy to fix with only simple parts and simple equipment.
2.)TOUGH BEYOND BELIEF i.e. shock, vibration are standard. Again PLCs have had boards shaken out of their cages.
3.)First cost a big factor
4.)Easy to understand

Ultimate reliability is achieved when you can fix what is broken with a Simpson 260, a screw driver and an adjustable wrench.

 

[jraef]

Don't leave out the duct tape!

&quot;Venditori de oleum-vipera non vigere excordis populi&quot;