Soft Starting Slip Ring Motors

[marke]

I recently came across an article written by an equipment supplier expounding the virtues of a "torque" soft start on a slip ring motor with the slip rings shorted out. The application was a large band saw and the comment was along the lines that the Torque start was able to start the machine where tradditional soft starters would not be able to.

 

I find this hard to follow as to start the machine, the motor must produce torque and the ability of the motor to produce torque is a function of the rotor impedance and the stator current applied. This is a motor characteristic issue rather than a starter characteristic in my book.

Any comments??

 

Best regards,

[jraef]

My first comment is that it smells of bravado that has not actually been properly investigated. It may have worked, but from my investigation of the "true torque" ramping scheme, they do not (cannot) directly limit current in that process. So although it may have worked to start the motor, they may have turned a blind eye to the fact that current was going sky high to do it!

 

The other possibility, although less likely, is that we are sitting back pontificating on why it will not work, and they went out and tried it, finding that it did. I once did that on a centrifuge starter system where all the pundits at the time said it couldn't be done. When we did it, and it started better with a soft starter than it did with Y-Delta, they scratched their heads and said "what do you know...".

 

Is that article available on line anywhere?

[GGOSS]

Beats me. Maybe they used high resistance wire to connect stator to rotor or just left all the connections loose.

 

On a more serious note, I would certainly like to know more so please share with us whatever you find.

[Guest]

You can download the article at http://www.drivedynamics.co.nz/news.html

[marke]

This article describes the use, but not the improved mechanism.

 

By my understanding, there are two reasons that we use some resitance in the rotor circuit when using a soft starter and these are "motor" related reasons rather than "starter" related.

 

• To improve the starting characteristics of the motor.

• to dissipate the full speed kinetic energy of the driven load.

  A slip ring motor, with it's rings shorted, exhibits a very high locked rotor current (typically 900% - 1400%) and a very low locked rotor torque(60 - 120%). If we reduce voltage start this, we will produce a very low start torque for a moderate start current. i.e. if the motor has a locked rotor current of 1200% and a locked rotor torque of 100%, then at a start current of 400% FLC, the start torque will be 11% of FLT which will slowly wind up an inertial load provided that there is no real load torque requirement during acceleration. If we add some resistance to the rotor circuit, we can easily achieve a start torque of greater than 50% for the same current which would accelerate an inertial load in one fifth of the time. - much better for the stator and for the supply.

   

  The wound rotor, is not designed to dissipate or absorb power. When a machine is started, the energy dissipated in the rotor circuit is equal to the full speed kinetic energy of the driven load. With no resistance in the rotor circuit, that means the total energy is dissipated in the rotor winding and this will cause a very high temperature rise in the rotor winding. The addition of resistors to the rotor circuit dramatically reduces the temperature rise of the rotor winding.

   

  Best regards,

[Guest]

We have recently installed, on site, two of the soft starters referred to in previous e-mails in this thread. Both were used to start existing slip ring motors with no external resistance connected to the rotor. I am please to advise that both applications were extremely succesful.

 

The first application was a 45kW vacuum pump which was started with a current limit of 270% motor FLC. The second application was a 132kW bandsaw which started with a current limit of 375%. Both loads started very smoothly and accelerated in what I would expect to be a normal time frame for the load types. This seems to defy the theory as expressed by Marke.

 

I am not certain what "magic" the soft starter uses to accomplish this but can definitely confirm that it works. I am now a convert!!!

[jraef]

Slip energy dissipation is not something to be trifled with IMHO. Perhaps one of the issues at hand is that not all WRIM designs are the same. Some rotors are in fact designed to dissipate more load energy safely within their limits, and as such would in fact be candidates for this operation. Many, if not most, however are not. The ones that are tend to be labeled as "Mill Duty" and/or "Crane Duty" motors, used origially for hoists, mining skips, overland conveyors etc. I did a lot of work in retrofitting old hoist control systems a few years ago, and found that it is not safe to assume anything about the motor design if you don't have the documentation on it. We would apply soft starters to different motors on identical hoists, one would work fine with a shorted rotor, the other (motor) would burn out in less than a year. We determined that the safest method was to always have some sort of resistor in the circuit during acceleration so that the rotor slip energy has somewhere to go OUTSIDE of the rotor at low speeds, then short it out at full speed.

 

When investigating the problem back then, I got ahold of a 30 year old white paper from Square-D (pre-Schneider obviously) on this subject that explained in wonderful detail what was happening. Unfortunately I failed to retain that when I left that employer. Now I doubt it is available.

 

I now use this paper to explain the phenomenon however, even though it is not quite as detailed.

http://www.ckit.co.za/Secure/Conveyor/Trou...tart_Mechs.html

By the way, it is a good reference for all kinds of heavy duty starting applications since overland conveyors are some of the toughest one can imagine. It is understandibly biased towards fluid coupling solutions, but I think their treatment of electonic and electro-mech starting methods is fair. I suggest making a copy of it before it dissapears from the web as so many good papers do.

[Gigger]

I have followed this post with interest with being involved in the industy,I to have tried to start many applications involving slip ring motors with a 60/40 success rate.On heavy duty I have always found that most times a small kick start time has to be inserted to get the load away.Regarding the 132kw that the people in New Zealand have said they started at 3.75FLC on a head rig I find hard to beleive,in fact I phoned the site they supplied the starter to only to be told due to lack of torque they have removed the motor and replaced it with a conventional motor.For this to work properly the last bank of resistance surely must be left in.

[jraef]

Now THAT is really interesting news! I can't say that I'm surprised, but it is especially interesting in that this company is using them as a reference, but failed to check up on the application! Poor marketing if you ask me. Thanks Gigger, and welcome to the forum.

[marke]

There certainly are differences between different slipring motors, and any application rules are not absolute.

If the slip ring motor was an ideal slip ring, then it would have a very low rotor impedance and there would be no way of starting it with the rings shorted.

In reality, there is a finite rotor impedance and the motor will generate some torque, however in most cases this is insufficient to start machines.

 

It would be interesting to know what the rated full load speed of these slip ring motors is. I believe that the full load slip with a shorted rotor could be a good indicator as to the starting torque under these conditions. The higher the slip, the greater the chance that the motor could develop enough torque to start a machine with the rotor shorted.

 

The other issue of course, is where does the slip energy go during start? With external resistors, we keep the bulk of the slip energy out of the rotor. With the rotor shorted, all the energy is dissipated in the rotor winding. This winding has a standard varnish insulation and is far more sensitive to the effects of heating than the standard cage rotor.

 

In any starting situation, it is important to ensure that a) the motor can develop enough torque to start the machine, and the rotor can absorb the slip energy.

 

I am concerned that in most situations, the torque available under shorted rotor conditions is not sufficient to start machines, and more particularly, that the slip energy will cause damage to the rotor winding.

 

Best regards

[Guest]

Dont ytou people out there realise that you already have a "Soft Starter" controlling your motor?

So much the better if it is a liquid type because the torque (& current) can easily be adjusted for specific applications.

If you have a moving electrode starter, you have a truly vesatile starter which can be used for starting under torque or current control, provides controlled deceleration, stressless reversing, emergency braking, and, to boot variable speed potential - allthis with much lower starting currents than an electronic soft starter can achieve.

I rest my case.

Fintan1

[jraef]

Originally posted by Anonymous

Dont ytou people out there realise that you already have a \"Soft Starter\" controlling your motor?

So much the better if it is a liquid type because the torque (& current) can easily be adjusted for specific applications ....balance snipped for brevity...

I rest my case.

Fintan1

 

Hello Fintan1,

Yes, most of us do, but there are many reasons why users might want to replace existing WR controllers with solid state devices. Liquid Rheostat systems are impractical in a lot of industrial applications, and maintenance of them is problematic now for companies that do not want to outlay the expense of having qualified and well trained maintenance staff on hand. It is a sad truth, but a truth nonetheless. A lot of older electro-mechanical controllers were tap changing switches that brought sections of the resistor grid on-line in steps, and those tap changers are now difficult to maintain or find replacement parts for. So when users investigate the cost of rebuilding them with contactor control, they discover that they can usually replace the entire starter with a solid state version for the same price or even less. What invariably happens though is that they want to take it too far, and eliminate the resistor all together.

 

That is what we are trying to educate them on. Marke's original post was about another "newbie" player in the field that thought they had discovered some super secret way of cheating the physics, and as it turned out, they didn't.

 

Thanks for posting though. It's always good to add experienced voices to discussions. Please consider registering so we don't see you as "Anonymous" any longer.

[rotormtr]

I have been watching this thread with interest and thought it timely to add some comments to correct all of your thinking. Let me start by saying that for a forum that is to contributed by "industry experts" I am disappointed to see that many of you have jumped to conclusions based on little to no fact.

 

Firstly, the application of the soft starter on this slip ring motor was 100% sucessful. Yes the motor was changed because of lack of torque, but this was NOT starting torque. Rather, the motor was undersized for the application and was slowing as the saw was cutting. If you read the original article you will note that it was an American NEMA motor (460V, 60Hz) that was installed. The site engineers then geared the motor up to try and gain the 20% loss in speed suffered when installing it on 50Hz. This gear change combined with the voltage variance resulted in insufficient torque being produced by the motor during the log cutting phase. IT STARTED PERFECTLY EVERY TIME without excessive rotor temperature rise!

 

While I agree with most of you about the issues with slip energy etc and the care that is needed with applying a soft starter on a wound rotor motor, you are failing to accept that there is a product now available on the market that will sucessfully start a slip ring motor with the rings shorted.

 

Gigger, my guess is that you didn't speak with a technical person when making your site enquiry otherwise you would have realised the above information

 

Jraef, maybe you should take your foot out of your mouth and retract your second to last post. I suggest you base comments on fact, after all this is meant to be an engineering forum. May I quote you from an earlier comment of yours "The other possibility, although less likely, is that we are sitting back pontificating on why it will not work, and they went out and tried it, finding that it did."

 

Lastly, the application of this new soft starter technology is not haphazrd, it is engineered. I happen to have spoken to the R&D engineer who developed this starter and know that he spend many hours working on a control algorithm that permitted the sucessful starting of slip ring motors. The application of this model soft starter on slip ring motors has not been a one off. In fact I can confirm that a number of traditional soft starters have been replaced - models of which some of you sell or have sold in the past.

 

Rotormtr

[marke]

Hello rotormtr

 

Welcome to the forum.

 

for a forum that is to contributed by \"industry experts\" I am disappointed to see that many of you have jumped to conclusions based on little to no fact.

Firstly, do not feel that you are being disbelieved, rather we are looking for reasons for this to be a step forwards.

You are correct that we have little or not fact other than classical theory and a lot of experience.

 

There certainly are situations where it is possible to start a slip ring motor with the rotor circuit shorted, but it is risky to expect this in every case. Additionaly, as outlined before, there are rotor heating considerations to bear in mind, so it is not just a case of starting the motor, but being able to start the motor in a fashion that it lasts for an extended period of time is also important.

 

If you use a reduced voltage technique to start the motor, you will have a high slip energy and this must be got rid of from the rotor circuit. If you vary the frequency, you can operate in a low slip mode and develop more torque at a lower current and a low slip energy.

The only way that I can see of achieving this is to use a for mo cycloconvertor technology, but with six SCRs, this is somewhat limited. If you used 18 SCRs, you can get good results.

 

 

IT STARTED PERFECTLY EVERY TIME without excessive rotor temperature rise!

That is great. At the end of the day, if it works and everyone is happy, there is no problem, however you must forgive some of us for asking for more information, as we have all seen many unsubstantiated claims made. The energy saving topic is always a good one for snake oil. If you search back through one of the threads you will find one poster who claimed that the iron loss of induction motors mysteriously doubled under light load conditions and that al the theory etc was incorrect. I think that reality did strike home in the end.

 

I for one am happy to share my knowledge and experience, but I do so with an open mind and would be keen to see how the slip energy is avoided. note the slip energy is not a sloft starter issue, it is a fact of physics. The accelerating torque field in the stator is at line frequency. The shaft power is a function of the rotor speed and torque. The slip power is a function of the stator torque and the difference between the stator torque speed and the rotor speed.

 

Tell us more! then we can not "jump to conclusions based on little to no fact".

 

If you refer to my first post, I was asking for comment rather than disputing the claim. I would like to see some substance rather than a simple statement of "it works".

 

Best regards,

[Gigger]

I am still not convinced that this magic soft starter can produce the torque required for this slipring application,yes Rotormtr I did talk to someone on site with Technical knowledge regarding this.In your message above you go on about the facts that this magic starter can do this,but where are the facts,perhaps the R & D guy you talk about will do a few notes on the theory of this starter.This would clear the problem that I have up fully and hopefully change my way of thinking.

[bob]

Basically a vsd may fulfill this funtion easily.

 

Bob

[Guest]

As far as I am concerned...there isn't any magic bullet that is different than another. There isn't very much that can be done different from one soft starter to another. That would put everyone in the same boat on this post.

I have to agree and am also curious to know what technologically advancement was made to enable this soft starter to start the motor over and above any other on the market.

Lets face it....SCR's can only be phased on and off a certain way. No magic here.

[marke]

Hello anonymous

 

Lets face it....SCR's can only be phased on and off a certain way. No magic here.

Not quite true, there are ways that you can control the SCRs, still using phaseing techniques, to create sub harmonics of the supply with sufficient magnitude, that the motor will actually run at reduced speed. There are several soft starters that offer a creep or jog speed.

Unfortunately, this is generally only at certain subharmonics and so there are still high slip requirements.

Hopefully, rotormtr will share some pearls of wisdom that will give us the confidence to accept the technology.

 

Best regards,

[Guest]

That comment was specifically mentioned towards the thread in question....i.e: that there are no magically different way to phase an SCR to "soft start" a motor.

Definately was not discussing creeping, jogging or reversing.

But you are right....we need rotormtr's expert description of this technology. I really am curious on what is done different here. It intrigues me.

[jraef]

rotormtr,

I retract nothing, but fair enough, I was a little 'hasty" in assuming that the installation in question was a failure. Keep in mind that this is a forum, not an on-site gathering, so we can only go by what we see in messages. A claim was made, a counter claim was made, and the supporting information in the original claim gave nothing to help out and in fact looked familiarly suspicious, so I believed the counter claim. Marke is better at being diplomatic than I am, I'm more of the proverbial "bull in the china shop" when it comes to jumping on marketing attempts at claiming heretofore undiscovered industrial "secrets". It comes from my constant battles with the "energy saver" peddlers out there who keep rehashing the hackneyed Nola Circuit chestnut as if nobody has ever thought of that before.

 

I am disappointed to see that many of you have jumped to conclusions based on little to no fact.

I read the information at the website and because it offered no details, just claimed results (which were refuted), I jumped to the same conclusion I always do when someone offers "testimonials" in lieu of details. If I believed all testimonials without details, I would have invested all my money with a Nigerian ex-patriot, bought countless weight loss products and tried to improve my sex life 50 fold with various pills that make me attractive to the opposite sex. Had that site given even a brief description of what type of technology is being used, I may have leaned more towards the possibility of success.

 

As to the article, I have another problem with it.

In the past it would be necessary to either replace all the starting gear, install a VSD or replace the motor with a squirrel cage motor to allow for a soft starter.

The problem I have with this is that a very typical and subtle marketing trick is employed here. Allow the reader to believe your premise by leaving out other pertinent facts. The pertinent fact in this case was the most obvious choice, used by EVERYONE ELSE in the soft starter industry for years. Use the existing WRIM and a new soft starter, but put one resistor in the circuit during start, then short it for full speed running. Not a big deal, every manufacturer who knows what they are doing has been doing this for 25+ years!. So essentially this “ new solution” is for a problem that actually only existed in the mind of the R&D engineer, and/or when considered in a vacuum devoid of other viable and quite ubiquitous solutions!

[jraef]

No response. How typical.

[marke]

I think that we can reasonable assume that it is using a form of cycloconverter algorithm to create a series of subharmonics and rely on these subharmonics to develop the toprque at very low speeds. This type of operation has certainly been around for a while and there are inherent potential issues with subharmonic currents on the supply plus the losses in the motor. Because the driving supply is still at line frequency, the cycloconvertor dois not eliminate the fundemental torque field and so slip losses are still very evident.

 

For those that are unsure as to what the cycloconverter principle is, it is a means of controlling the SCRs so that effectively a lower frequency is built up by a series of pulses of the fundemental frequency.

 

Under normal operation, we turn on all SCRs for a portion of each cycle.

If we now turn on the SCRs following a particular pattern, we get a sub harmonic of the supply. for example:

cycle 1

phase a + SCR ON; phase b - SCR ON; phase c - CSR ON

 

cycle 2

phase a -SCR ON; phase b + SCR ON; phase c - SCR ON

 

cycle 3

phase a - SCR ON; phase b - SCR ON; phase c + SCR ON

 

cycle 4

phase a + SCR ON; phase b - SCR ON; phase c - CSR ON

 

cycle 5

phase a -SCR ON; phase b + SCR ON; phase c - SCR ON

 

cycle 6

phase a - SCR ON; phase b - SCR ON; phase c + SCR ON

 

This simplistic representation creates a rotating magnetic field at one third of line frequency. There will also be a strong rotating field at 50Hz.

 

The current on the supply will be a series of pulses, not a continuous current flow.

 

If 18 SCRs are employed instead of 6, there is a much smoother current flow and more frequency options are possible.

 

This technology is available in a number of soft starters as a low speed jog mode.

 

Best regards,