breaking resistor duty in VSD
Posted 05 July 2006 - 04:42 AM
Posted 05 July 2006 - 06:17 AM
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.
Posted 05 July 2006 - 08:48 PM
Posted 05 July 2006 - 11:17 PM
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...
Posted 06 July 2006 - 04:55 AM
Posted 06 July 2006 - 08:45 PM
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.
Posted 06 July 2006 - 09:54 PM
Posted 06 July 2006 - 11:00 PM
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....
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,
Posted 06 July 2006 - 11:20 PM
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
Posted 07 July 2006 - 09:26 AM
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.
Posted 07 July 2006 - 09:33 AM
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.
Posted 07 July 2006 - 08:18 PM
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