Braking Resistor for VFD

[rockman7892]

I am researching some calculations and specifications for adding a braking resistor to a Siemens MM440 70hp VFD.

I saw an equation for calculating the maximum breaking power of the load in which you must first calculate the braking touque given by:

Mbrake_appl = [(Jmotor + Jload)* Nmax] /(9.55 + tbrake_appl)

Where J = moment of inertia
Nmax = max motor speed
tbrake_appl = time braking is applied

My question is in regards to the moments of inertia used for both the motor and load. Are these the same moment of inertias used for starting these loads as well?

I notice on some motor datasheets the motor moment of inertia is given in the form wk^2 (obviously units may vary) Is this wk^2 or starting moment of inertia the same as what is to be used in the above equation? If not how do you determine the moment of inertias to be used in the above equations?

 

[burnt2x]

Refer to the motor data sheet sheet for Jd² (or Wk²).

 

[DickDV]

You might have some special reason for doing the calculations you mention but, usually, selecting braking resistors is much easier.

You start with the drive and select the brake chopper that is rated to the drive. This "chopper" is the voltage sensing system which detects high DC bus voltage and diverts some of the bus energy to the braking resistor. This chopper will have a maximum current capacity which may be specified directly or indirectly by specifying a minimum acceptable resistance value.

Even if you don't need all of the drive hp as braking, I would suggest that you buy a resistor with that minimum resistance value.

Finally, you need to determine what wattage the resistor must be. A resistor will have a continuous wattage rating but, for a few seconds on a cold resistor, you can push it to 10x the continuous value. So, duty cycle is an important element in the wattage selection. Certainly, the maximum possible wattage would be the drive hp converted to watts. This would only apply to continuous braking as in an unwind tensioner drive. If the required braking hp is less than the drive rating, the resistor wattage comes down accordingly. Also, as the duty cycle becomes smaller, the wattage comes down further. Usually, a good bit of "dead reckoning" and "seat of the pants" judgement comes into play here with the wattage deliberately oversized if there is any doubt. This is desireable and easy to accept because resistors are generally comparatively cheap.

There, you have it without any formulas. I suppose the critics can pick it apart but, for most ordinary applications, it works just fine.

 

[rockman7892]

DickDV

Thanks for the response. You are correct, I do not need to do any calcuations for selecting resistor, the drive manufacturer specs a certain type of resistor for the hp of the drive as you mentioned.

I was just reading a tech paper from the manufacturer showing how the braking power was calculated and how these calculations led to these particular resistor sizes. I was just trying to understand the theory behind these calcualtions and how these sizes were achieved, and the one question I had was regarding the inertia as described above. The paper also talked about making sure the resistor had an average or continuous power greater than that of the average braking power of the load.

On a side note how do you correctly size the cable going from the drive to the brake? I was thinking to simply take the max bus votlage which in my case if 820Vdc and divide this by the resistor impedence (8.2ohm) to arrive at the max current for this resistor which in this case is 100A. Then select a cable rated for 100A ampacity. Do I need to add 125% to this figure? Does this sound correct?

 

[DickDV]

I would calculate the maximum current as you have done and then use the code rules for wire ampacity to pick the wire guage. While I am not an expert on code, I do believe that you can use the wire ampacities directly from the chart. There are no inrushes or surges that would cause the 125% rule to apply, as far as I can see.

 

[rockman7892]

I know that not all loads require a braking resistor, but when applying a VFD to a particular application what is a rule of thumb for determining weather or not a braking resisotr should be included.

For example, I know that for a flat overland conveyor I will not need a braking resistor, however for a shaker I most likely will. Should you simply just step back and look at each application to see if there is a chance of the load overdriving the motor?

 

[LionelHutz]

Yup, if the load is over driving the motor then a resistor is likely required.

If the load is constantly over driving the motor then a drive with regeneration capabilities is highly recommended.

 

[waross]

Cut the power on a running load and observe the stopping time. If you want it to stop faster, use a resistor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Marke]

The change in kinetic energy of the load during deceleration is dissipated in the brake resistor.
If you decelerate the load from full speed to zero speed, the energy dissipated in the resistor is equal to the kinetic energy of the driven load and motor minus the load and rotational losses.
High inertia give high kinetic energy.
The deceleration time determines the rate of energy flow (KW).

Best regards,

Mark Empson
L M Photonics Ltd

 

[ozmosis]

rockman7892
with respect to the specific VFD you are talking about, this document should be useful:

http://cache.automation.siemens.com...neering_braking_chopper_operation_V1_2_en.pdf