When and why to use Dynamic Braking Resistor in VFD

 

Dynamic breaking resistor is used where motor connected to VFD which is generating regenerative energy due to high inertia or in case where deceleration time is so less that VFD cannot control motor to stop. Due to high inertia motor acts as a generator and starts producing energy. This energy is feed back to VFD through free wheel diodes connected across IGBT and it tends to increase DC bus voltage of VFD.

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When the DC bus voltage increase beyond maximum DC bus capacity Controller of VFD turn on Braking transistor/ Braking Chopper/ Braking Unit for some time. When braking chopper if turn on DC bus voltage is dissipated through braking resistor in form of heat energy.

When DC bus voltage of VFD comes in limit, VFD is able to control Speed of motor. Even when application like has no of start stops and less deceleration time braking resistor comes in picture.

Crane, elevator, Hoist, agitator, vertical conveyors, winders, etc applications needs braking resistor

Some of the VFD’s (Mostly smaller ratings) comes with Braking chopper inbuilt. But for higher rating VFD’S you need to purchase braking chopper/Braking resistor/dynamic braking unit seperatly.

 

Selecting braking Resistor:

Resistance (Ohms):

Resistance of braking resistor should be equal to value specified by manufacturer if VFD. Kindly contact Manufacturer or refer Manual provided by manufacturer. You can also contact us if you need it. Remember you should not take a braking resistor of lesser value than recommended by manufacturer else braking resistor may get damage or even drive can get faulty. If resistance is over value braking might be slower or drive might get trip on overvoltage.

Wattage selection:

Generally Manufacturers will provide you wattage Value considering around 10-15% of braking duty cycle.

For example Application like Elevator Duty cycle is around 15%

So for 3.7kw considering 15% duty cycle braking resistor value will be 555 Watt

So generally you will be advice to take 600watt Braking resistor.

In some of the application such as hoist with more height and without counter weight will have higher duty cycle and you may require to increase wattage of braking resistor

Types of Braking resistor

 

wire-wound-braking-resistor-solims aluminium-braking-resistor-solims

There are many types of braking resistors available in market but for VFD purpose mostly two types of resistor.

  1. Aluminum house braking resistor
  2. Wire wound resistor.

 

If you need any help related dynamic braking or braking resistor please feel free to contact us or you can email us to info@solims.com.

 

We will be happy to help you

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