decreasing stopping time of induction motor

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harsh

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aurangabad,maharasthra,India
Dear members,
I am working in a optical fiber manufacturing plant and am engineer in the maintenance team.We wanted to reduce the stopping time of the motor installed in the take up assembly of the plant.Specification of the motor and drive are as follows:-
Motor specification:-
rated power=4 kw
rated speed=1500 rpm
Braking employed for stopping motor is dynamic braking.During braking energy is first stored in capacitance bank and then excess energy is dissipated in the regenerative resistors.It takes around 5 s for the motor to stop by above method.
Is it possible that while stopping motor we completely disconnect the drive from the motor and connect it to some other external circuit which helps in a faster braking.As different braking methods are more effective at different speeds we wanted to use the combination of two methods,the methods would be at higher speeds capacitance self excitation braking and after bringing motor to a lower speed through this method switching to dc injection braking.Drive and motor employed are of omron and yaskawa. Minimum regenerative resistance value is 28 ohm.
 
Is it possible that while stopping motor we completely disconnect the drive from the motor and connect it to some other external circuit which helps in a faster braking.As different braking methods are more effective at different speeds we wanted to use the combination of two methods,the methods would be at higher speeds capacitance self excitation braking and after bringing motor to a lower speed through this method switching to dc injection braking.
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Accomplishing braking involves the transfer of energy, the kinetic energy as represented by the spinning load. Dynamic Braking involves converting (transmuting) that kinetic energy into electrical energy, then transmuting the electrical energy into heat energy. To do this, the VFD maintains magnetic excitation on the motor windings, then lowers the stator frequency relative to the rotor speed, so that the rotor is always faster and therefore acting as a generator. The VFD then monitors the DC bus voltage so that the motor stays in regeneration mode constantly, which raises the DC bus voltage. At some predetermined level, the DB circuit triggers a 7th transistor, the "braking chopper", to pump that excess DC bus energy off into a resistor bank. The resistance value of the DB resistor has to do with the current rating of the Braking Transistor, so the options are not really there to change that without risking damage to the transistor. However the WATT rating of the resistor is something that you can alter, because that has to do with the resistor's ability to dissipate that energy as heat without causing damage to the resistor itself.

So in that effort, it is essentially pointless to "disconnect the drive from the driven load and connect it to some other external circuit", because that "other external circuit" would have to look EXACTLY like the drive! Dynamic braking however has one fatal flaw, the "law of diminishing returns". The faster the load is spinning, the more potential braking energy you have to stop it. But as it accomplishes this, it is at the same time REDUCING the potential braking energy. So DB is very good at removing MUCH of the energy from the circuit and putting it external to the motor as heat, but it has a difficult time finishing the job.

DC Injection Braking works differently, by creating a stationary magnetic field in the motor stator, which makes the rotor try to line up with it and come to a stop. The fatal flaw here is that the first law of thermodynamics is still in place, stating that energy cannot be created or destroyed, only transferred. ALL of that kinetic energy then is trapped INSIDE of the motor, as opposed to being moved away into external resistors. So using DC Injection Braking has a much more severely limited duty cycle as far as the motor is concerned.

In your Yaskawa / Omron VFD, you should be able to engage both technologies. Use the Dynamic Braking to bring the motor speed down as quickly as possible, then have the DCIB set up to be triggered to come on when there is very little kinetic energy left in the motor, usually set at around 10% speed.

Just so you understand however, no form of electronic braking can make the motor stop any faster than it can start at full voltage (DOL). So if you can, observe how fast the motor is able to start DOL, then that becomes your hard limit as to how fast you can brake it electrically. If it takes 5 seconds for it to accelerate the load, you are not going to get it to stop any faster, unless you use a mechanical friction brake.
 
Best way to break an induction motor is to apply a DC current across its windings. If its high enough breaking is quick.


But, the post below me makes a good point, heat wise you will have a limitation.
 
Perhaps the mbrook's method works on a synchronous motor in which 'permanent' magnetic poles on rotor available?
 
Sahib said:
Perhaps the mbrook's method works on a synchronous motor in which 'permanent' magnetic poles on rotor available?
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Of course it would work on synchronous motors too, but why would one bother when they could just short the motor leads together and let it stop fastest? DC injection is not generally used on synchronous motors for this reason, hence, it IS for induction motors.
 
your best bet from what you are describing IMO is to add some kind of friction type brake as another poster mentioned.

I will point out that the faster you decelerate the more force is applied to the parts of the system. They may object to the additional force by breaking (as opposed to braking).
 
Harsh, you did not say if the 4kw motor is synchronous or not; that can determine what your options are; tell us which you have.

You did not state if 240v or 460 either; in either case, if you have a 28 ohm resistor today, then you can pull out 240^2/28 or 460^2/28 or 10kw or 20kw of power from your 4kw motor, so it seems you should be able to stop it quickly now. What is the limiter when you try? A fault? What fault? Overvoltage error? You should tell us exactly what prevents you from stopping faster so we don't guess wrong.

If you have a higher ohm resistor on your system, that may be limiting factor too.

As Jraef said, max accel capability will tell you max decel capability too; worth confirming you have headroom to stop faster.

As already replied, using both stopping methods available in your drive is the proper solution, not disconnecting the motor and trying something else.

There also are 3rd party devices to add to your drive dc bus to help also: For instance,

1) increase the capacitance: http://www.wiki-kollmorgen.eu/wiki/tiki-index.php?page=KCM+Capacitor+Module
2) external additional DB module: https://i-store.hitachi.us/ISDWebSt...ccessories and Options&cat=Inverters#top_logo
3) external line regen unit: https://i-store.hitachi.us/ISDWebSt...ccessories and Options&cat=Inverters#top_logo
 
Respected Members,
Your explanation was very good and informative,also i realized some of my mistakes.Some of the members advised that our omron/Yaskawa drive should be able to apply both dynamic braking and DC injection braking,but i am not able to apply it.I am posting the complete specifications of the motor and the drive :-
Yaskawa drive:-
SGDH 50DEY12
Motor:-
4400 W,28.4 N-m, 1500 rpm ,16.5 A ,400 V
Type:-SGMGH-44DCA6f-OY
I am also posting the link of the motor and drive manual and the page number of the pages of the manual that you will need:-
page:-1-9,2-6,2-31,3-23,4-14(contains the internal circuit diagram of the drive),5-67,5-71,6-33

https://www.google.co.in/url?sa=t&rc...70138588,d.c2E

 
Read page 8-25 and associated pages. Everything you need to know for the drive you have.

Note at bottom of page is what others have described.

The caveats on the pages mean that the igbts or mosfets in the drive get driven at or near ratings during dyn braking, shortening their lifetime.

Part of your engineering trade is what are the advantages to faster breaking vs. replacing drive every few years vs. the plant process impacts.

Overall, well beyond a web page discussion; probably would involve proprietary information from your company also.
 
junkhound said:
Read page 8-25 and associated pages. Everything you need to know for the drive you have.

Note at bottom of page is what others have described.

The caveats on the pages mean that the igbts or mosfets in the drive get driven at or near ratings during dyn braking, shortening their lifetime.

Part of your engineering trade is what are the advantages to faster breaking vs. replacing drive every few years vs. the plant process impacts.

Overall, well beyond a web page discussion; probably would involve proprietary information from your company also.
Click to expand...
I agree. Your thread title says "induction motor", the link is for a servo drive. You would only use a servo in a motion control application, so that is a completely different set of application considerations, far far too complex for the kind of simplistic things a forum like this can address.

By virtue of the fact that you not only had to ask this question but have demonstrated a lack of understanding what you even have, I suggest that you seek out specific, knowledgeable help by a local professional who understands motion applications.
 
Just back to this thread....

Harsh, you realize your motor, rated 16 amp, only can do 650vdc/28ohm=23amp of decel DB... your drive will likely be good for 16*2=32amp.... so your limiting your stopping time to less than its accel capability...

if your drive REALLY limits you to this low decel DB of 28 ohms, then you can do NOTHING more with it to stop faster. Buy a larger drive that allows a lower DB resistor ohms.

OR, if your PLC can really control it properly, add an output contactor and logic to the control. This type motor will stop a matching inertia load in 3-4 Revolutions if you short its 3 leads together thru the proper current limit resistor. A GOOD local motion control engineer will be able to show you how to do this with proper interlocking to disable the drive first.
 
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