Basic Rules for Dynamic Braking Resistors

  • by

The Chemistry of Successful Braking Resistors

Braking resistors have 2 properties, and both are really crucial to the good results of the sleek deceleration of the VFD with no interruption of drive faults:

Resistance in Ohms (Negative Braking Torque)
Wattage (Duty Cycle)

Buying the biggest resistor isn’t the solution. No one wants to buy a boat anchor whenever they purchase a resistor. Thus, why don’t we review what these characteristics are and the way they work.

Let us begin with the Ohm worth for the brake. Consider the V=IR equation? Effectively, if the R (Ohms) element is simply too small for a particular V (voltage), meaning I (current) will boost at that voltage level. If the present is absurdly high, the braking transistor (sometimes known as a chopper) is going to fail because of too much current running through the circuit.

Rockwell AutomationĀ® drives come with an installation manual which is distinct from the programming hand that describes a minimum Ohm value for which 7th insulated gate bipolar transistor (IGBT) or even chopper. You have to remain above the minimum value, or maybe the chopper will fail. The 7th IGBT just switches on once the DC bus voltage of the drive goes up & triggers the resistor into the circuit (for instance, the DC bus voltage at 480VAC input is 650 VDC, the chopper will turn on above 750 DC to stay away from tripping (fault)).
The Math of Successful Braking Resistors

Precisely why is this critical? The lower the opposition, the taller the present in the braking circuit as well as the greater the damaging motor torque. When you would like to stop quickly, that gives extra torque together with the DC injection braking the drive is providing (stop setting on the drive is “ramp”).

Thus, why don’t you pick the lower impedance resistor all of the time? The present flow causes heat, which heat has to dissipate, thus the next aspect of success: wattage.

The higher the inertia (Ā½ times the mass x the radius squared), the trickier the mass is bringing to a stop. The longer to stop, the greater number of heat must be mitigated. The alternative circumstance which emits heat is small diameters that stop and start often. And also this will raise the % braking (duty cycle).

A terrific approach to assess the way to determine % braking is this: grab a timer and go away with the working area. Count the amount of cycles in 2 minutes. Calculate just how long the deceleration time period is for every cycle (cycles x deceleration period)/120 sec x hundred = % braking).

Let us use the example of a conveyor that’s heading a little bit downhill along with gravity is impacting the stopping performance of the drive. The deceleration was enhanced on the drive to avoid tripping, which has reduced production. In case you lessen the deceleration time, the drive faults on bus overvoltage. The bus regulator has to be activated on the drive, and also a powerful braking resistor has to be loaded externally.

The conveyor stops 5 times in a two minute window with 10 seconds duration. What’s the % Braking of the program?

(5 Cycles x ten Sec Decel)/120 Second Window x hundred = 41.6 % DC Braking

ProposalWorks software program from Rockwell Automation is going to give you a number of options of braking resistors. The program is going to select the correct resistor with the right % braking and % motor torque to avoid overheating the resistor. Simply observe the two minute window of the process of yours and also compute the % braking. A winter switch under the shielding cage of the resistor is added as a standard to monitor and stop the drive in case the resistor does overheat. You are able to wire the winter switch to an allow input on the drive to avoid harm and also stop the drive.

Fewer cycles with smaller period is able to deliver a better braking torque, along with repeated stopping means higher power level and also less braking torque. These’re common rules with minimal math that will help you evaluate an ideal choice in the selection of yours.

Set up is easy; mount the dynamic braking resistors to the structure, plus wire the BR- and BR+ terminals of the drive on the 2 resistor terminals under the shielding cage of the resistor. I highly suggested utilizing a DC type wire for example locomotive cable between the drive braking terminals as well as the resistor.