Drives and Control Solutions

Motors, Control Solutions, Power Transmission and Advanced Motion Technology                                                                 

BrakingResistor1August 4, 2020

By Mike Keefe

When designing a motor control system, it is not always clear if a braking resistor is required and, if it is, how to proceed in selecting a braking resistor. This post is intended to simplify that process so it is clear when and how to select a braking resistor for your application. 

Why are braking resistors necessary?

Braking resistors are introduced into a motor control system in order to prevent hardware damage and/or nuisance faults in a VFD. They are required because in certain operations, the motor controlled by the VFD is acting as a generator and power is flowing back towards the VFD, rather than towards the motor. A motor will act as a generator whenever there is an overhauling load (e.g. maintaining a steady speed as gravitational forces try to accelerate an elevator as it moves down) or the drive is being used to decelerate the motor. This causes the drive’s DC bus voltage to rise and will lead to over voltage faults in the drive if the generated energy is not dissipated.


There are a few basic ways to deal with the energy generated by a motor. First, the drive itself will have the capacitance to absorb some amount of this energy for a small amount of time. This is typically the case when overhauling loads are not present and a fast deceleration is not required. If there are portions of the duty cycle where the energy generated is too great for the drive alone, then a braking resistor can be introduced. The braking resistor will dissipate the excess energy by converting it to heat across a resistor element. Finally, if the regenerated energy from the motor is continuous or the duty is high, then it may be more beneficial to use a regen unit rather than a braking resistor. This will still protect the VFD from hardware damage and nuisance faults, but allows the user to capture and reuse the electrical energy rather than dissipating it as heat.

What should be considered when selecting a braking resistor?

Once it has been decided a braking resistor is needed for the application, there are two main factors when selecting the resistor: the resistance value and the power dissipation capacity of the resistor.

Minimum Resistance Value

BrakingResistor3VFDs that use a brake resistor will also have a “chopper circuit” or brake transistor. When the DC bus voltage gets too high, the brake transistor shunts current from the DC bus across the brake resistor. This brake transistor circuitry has current limitations and the VFD manufacturer will typically list a maximum current value and duty cycle.

Since V=IR, if the voltage is constant then a smaller resistance will lead to a larger current. Thus if the max voltage is known to be the KEB over voltage level of 840VDC, it is possible to calculate the minimum resistance that would keep the current value below the braking transistor’s max current rating. While the minimum resistance value doesn’t affect the operation of the resistor or its ability to dissipate power, it is crucial to ensure it works properly with the VFD.


Power Dissipation Capacity

The second consideration when selecting a braking resistor is its ability to dissipate power. KEB braking resistors are listed with the amount of power they can safely dissipate if used continuously (PD) as well as three values for intermittent duty. Each of the numbers in P6, P25, and P40 refer to the cumulative number of seconds the resistor is used over the course of two minutes. For example, the KEB 10BR100-1683 resistor could safely dissipate up to 2200W for one stretch of six seconds over the course of two minutes or instead could do two cycles of three seconds each over the course of two minutes. BrakingResistor5

Now that it is known what resistance values will safely work with the VFD and the power dissipation capabilities of the various resistors, it must be considered how much energy will be generated back towards the drive that will need to be dissipated. This will ensure the selected braking resistor has enough capacity to safely dissipate the energy generated from the motor. The first way to do this is through calculation.  It is possible to calculate the power generated from the motor if the mass moment of inertia of the motor and load, motor torque, speed change, and time of the deceleration are all known. More information on performing these calculations can be found in the braking resistor manual. However, in real world applications it can be difficult to know and/or calculate the mass moments of inertia, particularly of the load. Because of that, it is commonly necessary to determine the proper power size of the braking resistor through a testing method. BrakingResistor6

The general rule is that the larger the load and the faster the deceleration, the more power that will need to be dissipated.  However, by utilizing the scope function in the Combivis 6 software, it is possible to record the drive’s DC bus voltage throughout the operation to get a more accurate picture of the braking resistor usage. With the scope it is possible to monitor if a resistor with greater power dissipation is required or if instead the resistor is sufficiently sized. In the latter scenario, it may be possible to adjust the operation to improve performance, such as making the deceleration faster. 


Braking Resistor Installation

The final consideration when selecting a braking resistor is to ensure that it is installed properly.  If a braking resistor is not installed according to UL standards, the circuit can fail in a manner that is a fire hazard. More information on safe connection of a braking resistor can be found here.

In addition to our traditional resistors, KEB is increasingly selling intrinsically safe brake resistors which fail much like a fuse protecting the system in the case of short circuit failure.

The installation environment is also important. Hazardous locations and installations with fibers (textile, sawdust) that are flammable will need special consideration.

Interested in learning more about how KEB drives and braking resistors can be utilized in your application?  Contact an Applications Engineer at KEB America today!


Editor's Pick: Featured Article

Condition Monitoring ABBBy Blake Marchand

Artur Rdzanek, ABB’s Mechanical Power Transmission Global Product Manager for Sensored Products, notes there are three core tenants to ABB AbilityTM sensors and remote monitoring systems: safety, extending system life, and significantly, or even completely, eliminating downtime.

From a safety perspective, the advantage is being able to gauge component health without having to be continuously around rotating equipment. The ability to monitor equipment remotely is particularly important when it comes to equipment that is difficult or dangerous to access.  Usability is at the core of sensors. 

Read More




PB 26 Allied logo 400The fourth industrial revolution, or Industry 4.0, builds off the computerization and digitization of previous shifts and employs artificial intelligence, IoT, cloud computing and remote dashboards that measure and monitor parameters and minute data points related to industrial operations.

Industry 4.0 offers endless possibilities as more production lines are designed and configured with quality sensors. Those who utilize the right sensors, in the right place, for the right amount of time should experience improved system maintenance.



Motor Feature

Motor and Drive Systems 2021The 2021 Program Unveiled

Motor & Drive Systems 2021 is focused on the latest technical advancements impacting the design, integration and efficiency of motor, drive systems and motion control for automation, robotics, manufacturing and industrial, as well as utilities and automotive applications. Attendees will have full access to the simultaneous conference, Magnetics 2021.

The 18th annual Motor & Drive Systems 2021 virtual program has been unveiled. Motor & Drive Systems 2021 will provide the best forum to learn from leading motor and drive system manufacturers...

Product News

  • Prev
Hubbell Control Solutions announced the release of the newly improved FX Room Controller, ...
Unscheduled stops and faults in the production process can be avoided using our new ...
Provides protection for instruments or electronics requiring routine attention or detail.  ...
New from Weidmüller: the BR/SUxxxUC Series offers a complete line of UL489 CSA-C22.2 No. 5 and ...
CARLO GAVAZZI is pleased to announce the launch of our new Capacitive Proximity Sensors with ...
It's now easy to display historical process variable data in a machine's HMI application.   ...
With the new RJ Industrial MultiFeature series, HARTING can better meet the wishes and ...
Since 1960 ODE has been one of the leaders in designing and manufacturing a complete line of ...
With the faster adoption of IoT, many industries, including pharmaceutical and healthcare, have ...
AutomationDirect now offers native CAD files downloadable directly from their Web store to assist ...

New Product

FX Room ControllerHubbell Control Solutions announced the release of the newly improved FX Room Controller, a vital component for room optimization and control in conjunction with the use of NX Distributed Intelligence™ System Devices.

The FX Room Controller is the first of its kind in the industry with a built-in time clock for scheduling capability of room controls without the need for additional network components or a central time server. The new FX Room Controller is part of the NX Room Controller family and it fundamentally changes the architectural role of the controller and the ease of installation for electrical contractors and specifiers.


Read More


Latest News

  • Prev
The 18th annual Motor & Drive Systems 2021 virtual program has been unveiled.
Yaskawa, a manufacturer of motion control, robotics, and variable speed drives and Phoenix Contact, ...
PC-based control and TwinCAT provide a foundation for advanced Industrie 4.0 and Internet of Things ...
Join ABB for an in-depth look at how variable frequency drives assist in aeration applications for ...
Leading Canadian forest product company, Chantiers Chibougamau, has selected ABB to help overhaul ...
ABB Motion Canada alongside our HVAC Partners nationwide has introduced a special program for our ...
Brock Solutions is pleased to announce the recent award by the Region of Peel of the Clarkson and ...
This webinar (Friday April 17, 2020 at 11:00am - 12:00pm EDT) will review simple and easily ...
ABB is proud to announce it has been selected as one of Canada’s Best Employers by Forbes Magazine ...
Kerrwil Publications Great Place to Work. Certified December 2019 - December 2020

538 Elizabeth Street, Midland,Ontario, Canada L4R2A3 +1 705 527 7666
©2020 All rights reserved

Use of this Site constitutes acceptance of our Privacy Policy (effective 1.1.2016)
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of Kerrwil