Dynamic Braking in Variable frequency Drives

Dynamic braking is the method of stopping the motor by dissipation of heat in the resistor. When a motor is driven by VFD, the motor acts as a generator when the actual speed of the motor is more than the VFD frequency. The generated energy dissipates in the resistor( Dynamic braking resistor) and it leads to braking of the motor.

What is braking?

When the motor is required to stop faster than the normal speed of the motor is known as the braking of the motor. During deceleration of the motor, the actual speed of the motor can be more than the drive set point if the load inertia is more.

The opposite torque against the normal running torque is applied to bring the motor speed down as per the set point of the inverter frequency.

dynamic braking circuit diagram

Braking is required in applications like centrifugal fans, rotary kilns because of high inertia loads. The braking is also required in machine tools, winches, and cranes applications. The motor can be operated in four quadrants, Forward and reverse driving and forward & reverse braking.

four quadrant operation of induction motor

Why Braking is Required?

Because of high inertia, the load connected to the motor shaft does not allow the motor to decrease the speed at the same rate as inverter frequency. In this situation, the motor speed is more than the synchronous speed of the motor and it will work as a generator. The synchronous speed of the motor is;

s= (Ns-N)/Ns

s- slip, Ns- Synchronous speed, N- actual Rotor speed,

If the actual speed of the motor is more than the synchronous speed the slip of the motor is negative and the motor will act as a generator and, as a result, the motor starts feeding energy to the source side. The energy can be either dissipated in resistance or can be fed to the source for braking the motor.

Methods of Braking Induction Motor Fed from VF Drive

There are three methods that can be used for breaking the motor.

1.  DC injection in the stator of the motor
2.  Dynamic braking
3.  Regenerative braking

In this article, let us first discuss dynamic braking that is mostly used in variable frequency drives.  

Dynamic Braking in Variable Frequency Drive

First, let us understand the working principle of the VF drive. The drive has three major sections, the converter, DC capacitor, and inverter. The three-phase supply is rectified in the converter section using the p-n junction diodes or the IGBTs. The converter section can be of 6 pulses or 12 pulse configuration. If the drive is of large rating, 12 pulse rectifications should be preferred so that lower-order harmonics 5thand 7th get eliminated. The block diagram of the VF drive is as given below.

block diagram of variable frequency drive

The input three-phase supply is fed to the diode bridge rectifier that converts AC voltage into DC voltage. The magnitude of the DC voltage at the capacitor terminal is 1.35 * Vrms, where Vrms is the root mean square (RMS) value of the input supply.  

The DC voltage is fed to the inverter circuit. The inverter circuit has six IGBTs (Insulated Gate Bipolar Transistor). The IGBTs are switched on and off through the PWM(pulse width modulator) controller.

In pulse width modulation, the carrier frequency (Range 1KHz-15 KHz) is modulated with the sinusoidal frequency (Range 0.1 -50 Hz) to get the base drive pulses. In the PWM inverter, both the voltage and frequency are increased/decreased at the same rate to maintain the constant flux in the motor so that constant torque delivery can be ensured.

Working of Dynamic Braking in VFD

When the motor is driving a high inertia load and if the speed of the motor is reduced by decreasing the frequency, the speed of the driving equipment does not lower down according to the set point of the inverter.

In this condition, the actual speed of the motor is more than the synchronous speed, and the motor acts as a generator and it starts feeding energy into the capacitor bank through IGBTs of the inverter. As a result of this, the voltage across the capacitor bank increases. In normal conditions, for 440 Volt drive, the voltage across the capacitor is 635 volts. However, if the motor goes in generating mode the voltage across the capacitor bank increases.  

The additional energy that gets stored in the capacitor bank during generating mode operation needs to be dissipated for lowering the speed of the motor.

The block diagram of the dynamic braking in VFD is as given below. 

Dynamic Braking in Variable frequency Drives

When the voltage increases above its rated voltage the drive sense it as an over-voltage and the controller switch on the dynamic braking transistor and the additional energy gets dissipated in the braking resistor. The capacitor voltage returns to normal voltage level after dissipating the additional energy in the braking resistor.  

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