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DC Injection Braking in VFD

In this article, we will discuss the types of DC injection braking in vfd. There are many applications that require braking for process control.

When a motor functions in the motoring zone, it delivers torque to load for driving it. When we stop the motor keep on running at slower speed because of the kinetic energy of the motor. If we dissipate the kinetic energy in the other form of energy, the motor can be stopped immediately.

Thus, the dissipation of the energy after stopping of motor cause motor to stop. This process is called braking of motor. The amount of braking torque developed depend on the rate of dissipation or re-utilization of the energy.

There are many braking techniques used for an induction motor. In this post, we will discuss the braking technique used for the motor driven by the VFD.

VFD Braking Techniques

The five most commonly used braking methods in VFD are as follows.

  1. Coast to stop,
  2. Ramp to stop,
  3. Regenerative braking and
  4. Dynamic braking.
  5. DC Injection braking

Coast to Stop

Coast to stop through VFD means disconnecting the motor from the VFD output voltage and frequency.

This method reduces the VFD frequency to zero in no time. Even after reduction of frequency to zero, the motor comes at standstill after some time because of the stored kinetic energy in the motor.

In coast to stop method, the drive immediately reduce the frequency and voltage to zero after giving stop command to drive. However, the actual motor stopping time depends on the load connected to the motor.

Ramp to Stop

In a ramp to stop method, the VFD gradually reduce the voltage and frequency in a ramp-down manner. The ramp-down slope can be set according to the system requirement.

If the load inertia is more, then VFD reduces the motor speed slowly to avoid regeneration in the motor. If the load inertia is small, the VFD reduces the motor speed fast.

Regenerative Braking

When motor actual speed is more than the VFD frequency, the motor acts as a generator. The motor converts its kinetic energy into the electrical energy.

Where this regenerated energy go?

The regenerated energy pumps into the DC bus capacitor bank. It increases the capacitor voltage & the drive trips with over-voltage.

How can this regenerated energy be re-utilized?

However, this way the motor braking is not effective. To solve this problem, the regenerated energy needs to feed in the system again. It is possible to feed this energy, if the VFD has active front end rectifier unit.

The active front end rectifier functions as an inverter, and converts the DC bus voltage in the incoming supply. Thus, it is possible to pump back the energy in the mains.

Dynamic Braking

The Dynamic braking is also applies in the case of regeneration braking.There are two methods of the braking in the case of regeneartion.

  • Regenerative braking
  • Dynamic braking

In the regenerative braking, the generated energy of the motor is back feed in the system for braking. Whereas, in the dynamic braking, the generated energy dissipates in the dynamic braking resistor. In the case of regeneration the dc bus voltage increases.

We set the VFD parameter( basically DC bus over-voltage) to switch on the IGBT mounted in parallel to the DC capacitor to take dynamic braking resistor in the circuit. Thus, the generated energy dissipates in the resistor and the brake applies to the motor.

DC Injection Braking in VFD

Ideally, the motor must stop after a stop command given to VFD. However, practically motor keeps running and comes at standstill after some time. This happens as per the Newton’s law of inertia. The motor keeps running unless some external force is applied against the motor torque. In normal case it is air friction. But, in this case the motor stops slowly.

After stop command to VFD, motor keeps running. And in this case, when frequency of VFD is zero the VFD injects DC voltage in the stator winding. The VFD can inject the voltage in one or more winding according to the braking torque requirement. The motor comes at standstill after injecting DC in its winding. Therfore, this method of motor braking is called DC Injection braking.

Now, let us understand how the DC Injection brake the motor.

Working of DC Injection braking in VFD

When we inject the DC in the stator winding when the motor is in rotation( Under supply off condition). The stator winding generates a magnetic field. As the rotor is rotating in a magnetic field, the voltage induces in the rotor winding.

The rotor winding of induction motor is short circuit at the end rings. Therefore, the induced voltage cause current to flow in the winding.

The winding has certain resistance and heat dissipation (I2R) take places in the rotor winding. The heat dissipation is nothing but the kinetic energy of the motor. Thus, the brake applies to the motor.

The braking torque produced in the motor depends on the winding resistance and the current flowing through it. The resistance of the stator winding is fixed for a particular motor. Thus the braking torque is proportional to the square of the stator current. The stator current depends on the magnitude of the DC voltage injected.

Thus, we can control the braking torque of the motor in the following ways.

  • By increasing stator resistance- This can be done by taking more ststor winding in the circuit
  • By increasing DC voltage – This parameter we can set in VFD.

The huge braking torque is possible to produce through DC Injection braking. However, all the heat dissipates in the rotor winding at the time motor is slow rotating. Therefore, it affects the insulation life of the winding.