
Soft Starting Slip Ring Motors
#1
Posted 19 April 2005 - 09:32 AM
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,
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#2
Posted 20 April 2005 - 03:16 AM
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?
#3
Posted 20 April 2005 - 04:24 AM
On a more serious note, I would certainly like to know more so please share with us whatever you find.
#4
Guest__*
Posted 04 May 2005 - 05:12 AM
#5
Posted 07 May 2005 - 05:57 AM
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,
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#6
Guest__*
Posted 25 July 2005 - 07:13 AM
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!!!
#7
Posted 04 August 2005 - 05:48 PM
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.z...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.
#8
Posted 05 August 2005 - 04:28 AM
#9
Posted 05 August 2005 - 05:00 AM
#10
Posted 07 August 2005 - 09:36 AM
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

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
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#11
Guest__*
Posted 11 August 2005 - 10:22 AM
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
#12
Posted 11 August 2005 - 05:57 PM
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.
#13
Posted 15 August 2005 - 10:20 PM
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
#14
Posted 16 August 2005 - 05:06 AM
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,
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#15
Posted 16 August 2005 - 05:09 AM
#16
Posted 16 August 2005 - 06:19 AM
Bob
#17
Guest__*
Posted 16 August 2005 - 04:00 PM
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.
#18
Posted 16 August 2005 - 06:44 PM
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,
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#19
Guest__*
Posted 16 August 2005 - 09:03 PM
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.
#20
Posted 17 August 2005 - 06:47 PM
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.
As to the article, I have another problem with it.
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