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breaking resistor duty in VSD


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Hello amjad


I suspect drive manufacturers vary in their approach to this, however from what I know;


Braking Duty (%) = 100 x Braking time/Cycle time


Cycle time is defined as the period of time from initiation of one start signal to initiation of the next start signal. It therefore includes acceleration time, run time, deceleration time and the period of off time before the next start is initiated.


As indicated in my opening sentence however, you may wish to check with your drive supplier in case they adopt a different approach for specifying braking duty.


Hope that helps.




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Hi Amjad, GGOSS is correct as far as I am aware. The ratings are concerned with the resisters ability to reject heat over time. The resister will have a kW rating which is concerned with the amount of energy that can be dispersed by the resistor and the duty cycle is related to the time that it can reject the heat. The resistor will require an amount of time to recover from a braking operation ie cool down. It is important to select the correct duty cycle to prevent premature failure of the resistor. Your drive supplier should be able to recommend the correct type of resistor for your application as it is not enough to just apply a rule of thumb to the application.


An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
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There is also another consideration: the duty cycle of the braking transistor.

I think you'll find that the braking transistor is not rated for continuous duty.

And, the heat that the braking transistor must

dissipate, (which is not uncommonly mounted

on the same heatsink assy. as the inverter bridge transistors IGBTs)... , will raise the temperature of the heatsink assy. and under heavy braking duty, could cause a H.S. O/T fault to occur ...


As for the duty cycle of the braking resistors..... the wattage of the resistors

is the limiting factor combined with ability to cool the resistors i.e., get rid of the

heat dissipated in them .....


Can dynamic braking components be rated for 100% continuous duty ...


Yes.... the resistors must be sized appropriately, as well as the braking

transistor and its associated heatsink...



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I appreciate the help/concerned shown by jomega, Kens & GGOSS.Actually I want to known how to determine the breaking duty if different applications.For a specific VFD I have to select the breaking resistor and the supplier sent me a chart suggesting a 20 ohms 4 KW resistor for 10% duty cycle and 20 Ohms 13 KW resistor for 40% breaking duty I am having difficulty in selecting the resistor power need guidelines in determining breaking duty cycle.
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Hallo amjad,

probably you have an inverter for a 45 kW motor, or also bigger if your resistor has a good overloadability.


During braking time, many "braking units" are "choppers", this means that they don't have a full conduction during time. Some of them cut the voltage for few seconds without choppering, after that they will start to chop the DC voltage.

About resistors, if braking is prolongued in time, you have to select a nominal power very close to calculated regenerative power.

If instead your braking happens rarely, for emergency, you can reduce the resistor size, overloading 4 or 6 or also 8 times, if that resistor could accept an high peak power for a certain time.

If your emergency braking happens rarely but is long (i.e.: a centrifuge), you have to select the full size of power resistor.

To have an idea of maximum power dissipated, knowing the AC line voltage you can multiply by 1.41 and by 1.15 (this is only an indication) and you will find a certain voltage, i.e. 800 VDC.

Using a 20 ohms resistor, you will have (resistor cool) 800 VDC/20 ohms = 40 A = 32 kW of dissipated power. If your resistor is rated 4 kW, you could use it a little bit over 10 % of absolute time, afterthat you have to wait 90 % of time.

Please remember that as soon as the resistor becomes hot, the resistance increases immediately, therefore maximum current decreases and also braking torque. Using an higher power of the same ohmic value you will have a better thermal stability and therefore a more constant braking torque.



Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Hi Amjad, if you give us some further information on the application we may be able to provide some more specific assistance.


An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
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Hello AMJAD,


GGOSS stated how to calculate duty cycle .. and while it is very non-specific / non-detailed (it is how it is defined in the literature on the subject of Duty CYCLE).


I would surmize that it does not give you the information you are seeking; a specific formulation for calculating the duty cycle of your particular application.


Please understand that you have not supplied the readers of this forum any information specific to your application .... no details supplied..... and so, we are unable to be more specific in our replies to you.


I did some digging and came up with the below which I think may be helpful to you.


here is a link to an "Application Solution" published by Reliance Electric ( part of the Rockwell Automation group) .... The paper is entitled, " Brakng and Regenerative Energy with AC Drives" .



This paper Will guide you thru typical calculations.


If you still have questions, then I think we need to move this discussion to the next level and request that you provide us with the details of your application....

such as:


Braking: from what speed to what speed .... RPM just prior to braking ..... and RPM at end of braking

Braking Time: how fast must the speed change (time in seconds)

Braking Events: How much time between the START of one Braking Event and the start of the next

Braking event ... in seconds


Here's an example to consider ...


A motor operating at 1000 rpm is required to decelerate to 500 rpm over 2 seconds.


At the end of the 2 seconds, the motor is accelerated to 1000 rpm and maintains 1000 rpm for 8 seconds,


and then is decelerated to 500 rpm again (the start of the next braking event) .... and the sequece repeats


as described above.


So here's what we know:


Braking: 1000 rpm to 500 rpm

Braking Time: 2 seconds

Braking Events: 10 seconds ( 2 seconds + 8 seconds = 10 seconds Total Time)


From this we determine the duty cycle as follows:


%Duty Cycle = (Braking Time / Total time) x 100


%Duty Cycle = (2 seconds / 10 seconds) x 100


%Duty Cycle = 0.2 x 100 = 20%



Now it may be that your application has braking times and time between events that are not all the same .... in which case, you would need to calcuate the AVERAGE duty cycle.


The paper by Reliance Electric shows you how to make this calculation.


Hope this helps,





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Hello amjad,


mariomaggi & jOmega have provided some excellent information for you.


This is just a quick note to advise if the link provided by jOmega does not work for you, try deleting the asterix following .pdf




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Thanks mariomaggi and jomega for providing value data and guidelines.For information my application is long travel drive of over head travelling crane with following specs.

We have two long travel motors of 15 KW each at both ends 415 volts 50 hz 4 pole.

Deceleration time is 2 sec from full speed to zero.

Acceleration time 3 second from zero speed to full.

Dead weight of crane is 40 tonnes and swl is 25 tonnes.

crane duty is M8.





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for safety reasons, on cranes it is not recommended to save few money for a bigger power resistor, full power. Please select the 13 kW type.


Are you absolutely sure about the suggested value of 20 ohms? It seems too low for that size, but you don't indicated the inverter type and therefore I cannot check.




Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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