Re: How to calculate the braking resistor & wattage value for a 3.7 KW 3 phase invert
There is not any simple formulas ...
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).
The lower the ohmic value, the higher the power, shorter the brake time.
The minimum ohms are set by the manufacturer, and will produce braking power at the peak rating of the drive (or its chopper module)
Higher ohmic values can be used; they will reduce the braking power proportionately, and hence increase the stop times of any given load.
The ohmic value is less than the value recommended by the drive manufacturer – result-in VFD failure (or its Chopper module). The correct ohmic value of the brake resistor should be equal or slightly higher by 10% of the value set by the drive manufacturer.
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, it is suggested 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.
you find some tables here....
6.1 BRAKING UNIT AND BRAKING RESISTOR UNIT APPLICATION LIST
**broken link removed**
http://www.grelectronics.co.in/braking-resistors.pdf
GR Electronics