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When it comes to the law of thermodynamics, we know that energy cannot simply go away; it needs to be transferred or transformed into something else.

In industrial motor applications, this law presents itself in two common situations:

  • Excess energy that is generated whenever an induction motor continues to spin after one stops sending electrical energy to the motor (called regenerated current)
  • When the deceleration time for a VFD is too short to dissipate the stored energy in the VFD.

This excess energy can wreak havoc to the VFD if not properly configured.

Motor Overhauling Condition: Any situation in which the motor’s rotational velocity has exceeded the synchronous velocity designated by the connecting VFD thus creating excess energy.  

Generally we see Dynamic Braking Systems used in applications such as: centrifuges, pumps, fans, certain conveyor belts, rapid/ continuous braking, and applications that require rapid slow down and reversing.  

Imagine it like this, the motor you’re working with is moving 10,000lbs of gravel along a conveyor and you need to stop the system. Even if you were to cut the power, the motor still has a large deal of momentum and will continue to turn; making the motor a generator of current that is now being sent back to the VFD.  Whether you’re dealing with a conveyor belt or milling machine with quick accelerations and decelerations, the excess energy generated by the motor or already stored in the VFD needs to be addressed.

How do we go about dissipating this excess energy? The AC drive/VFD connected to the motor is designed to accept some of this energy generated by the motor in the DC Bus of the VFD.

DC Bus: A discharge medium installed in the VFD which stores the incoming voltage in charging capacitors. If the maximum voltage capacity is met per the DC Bus specifications, the VFD will go into error mode and/or shut down.

In order to reduce potential damage, the DC Bus has a maximum capacity which will trigger an overcurrent error if the energy has nowhere to go. This is where the Dynamic Braking Systems come into play.

On drives that have the dynamic brake option already installed, an additional IGBT transistor is used to remove extra current coming back into the drive from the motor.

What are the components for Dynamic Braking Systems?  These systems will either include a factory installed brake transistor (a.k.a. Brake Chopper) or a supplementary Dynamic Brake Unit (external to the VFD). 

This transistor or DBU controls the DC link voltage by connecting the brake resistor (separate device) across the link when the voltage reaches a predetermined level, thus dissipating the excessive energy in the brake resistor as heat.

Brake Chopper:  An electronic switch that is used to interrupt one signal under the control of another.

We also want to keep environmental factors in mind when selecting your Dynamic Brake Unit. With the surface temperature of these resistors reaching up to 90°C (194°F) it is important to consider proper ventilation and safety regulations.

Manufacturer’s charts for the necessary components have been known to vary…awesome, right?

We’ll try to make it a little easier; more often than not they will be described at:

 

  • 100% braking torque at a 20% duty cycle ≈ 12 seconds of max. braking time
  • “100% braking torque at a 50% duty cycle ≈30 seconds of max. braking time
  • Other manufactures may even use 10% duty cycle.

In many cases the VFD might already have the transistor (brake chopper) built in, at least up to certain specifications based on HP and model. Once we get to higher HP levels, the item is removed and an independent brake module or multiple modules will be required along with the resistor or resistor(s). 

Best approach is to follow the VFD manufacturer guidelines and tables to achieve your application goals.

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