Braking Resistors in Variable Frequency Drives

Braking Resistors are used in Variable Frequency Drives to dissipate the energy released by the motor into the power system.  The Braking Resistors perform the duty of absorbing the power from the rotor when the VFD reduces the speed to zero and preventing rotor heating.  They also prevent the rotor from exceeding the synchronous speed set by the Variable frequency drive.

A variable Frequency drive consists of three main components - the rectfier which converts the AC supply into DC, the DC busbars and the inverter which converts the DC supply into a variable AC supply.  The VFD varies the speed of the motor by varying the frequency of the AC supply applied at the motor terminals. 

When the motor is required to be stopped suddenly, the Variable frequency drive reduces the supply frequency to 0 HZ.  In this condition, the rotor is rotating at speed higher than the synchronous speed.  This causes the motor to behave like a generator and send power in the reverse direction, into the DC bus bars.  During this time, the voltage across the DC busbars can rise to very highlevels.  The braking resistors absorb power in this situation and prevent the voltage from rising beyond limit and damaging the Drive.     

The value of the resistances determines the rate of fall of the motor speed (braking). 

Special provision is made for cooling the resistors which can generate a huge amount of heat when in operation. 

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