Resistors in Motion

Braking is a fundamental part of any transportation vehicle. Whether it’s cars and trains or farming and construction equipment, manufacturers need to know their products have consistent and reliable braking systems. And an important part of a functional braking system is a quality power resistor. 

Fundamentally, resistors come in a variety of forms and styles, and they are used in all kinds of electrical circuits. Depending on the application, they are used to control the flow of current, limit current, protect circuits, adjust signal levels, divide voltages, bias active elements, and terminate transmission lines, amongst many other. In addition, resistors can dissipate power (wattage) to enable designs like motor starting, load testing, motor braking, generator testing and maintenance, and battery testing. 

Both AC and DC motors are found in applications ranging from automobiles and transportation to industrial, including farming and construction. That means any moving device powered by a motor of any kind must need a decelerating system to control its speed, and then bring it to a stop.  

Power resistors play a vital role in such deceleration systems by dissipating power and slowing down the mechanical system. This process is called dynamic braking, and the resistors used are called dynamic braking resistors or simply brake resistors.  

To decelerate an electric motor, kinetic energy acquired during motion is transformed back into electrical energy. This electrical energy is dissipated using a power resistor. Ohmite understands the need for braking applications and offers a variety of resistors for many motors used in mobility applications ranging from fast moving trains, automobiles, and motorbikes to slow moving farm equipment, forklifts, earth movers and more.  

Ohmite has served the needs of automotive, locomotive, and industrial applications for nearly 100 years. We have designed and developed rugged resistors that are capable of working under harsh conditions, including high temperatures, corrosive materials, moisture, dirt and dust, while also withstanding vibrations, static charges, chemicals like pesticides and fertilizers, and more.  

Ohmite resistors also include metal plating to prevent oxidation and comply with lead-free, Restriction of Hazardous Substances Directive (RoHS). They also come in modular and scalable formats, enabling the users to find the proper braking resistor required to fit their needs when it comes to motor deceleration. Ohmite’s team of engineers are always available to consult customers to discuss solutions to braking problems.  

Ohmite’s many resistor families cover a wide range of ohmic values and power ratings to meet the needs of many braking applications. For example, the company’s SGM series is constructed using stamped grid technology in a modular format, comprising several resistive plates, fitted on steel rods, and separated by ceramic rings. This means the SGM series offers size and weight advantages over regular wirewound grid assemblies, and they come in NEMA 3R rated enclosures manufactured from galvanized steel for harsh indoor and outdoor applications. 

Similarly, the 14984 series of high-current round edgewound resistors deliver a variety of ohm and power ratings common to transit use. All high-current/braking resistors for dynamic braking applications are listed on the company website.  

Dynamic braking can be further divided into rheostatic and regenerative. In rheostatic braking, the energy is dissipated as heat from the resistor. While in regenerative braking the electric power is fed back into the system. The last option generally has a higher cost. Brake resistors offer several advantages over friction braking systems as listed below. 

Advantages of dynamic braking resistors over friction braking: 

• Lower wear of components.
• Control motor voltage within safe levels. 
• Faster braking of AC and DC motors. 
• Less service required and higher reliability. 

Likewise, resistors are also used for starting DC motors. At the time when motor starts, back emf is zero and therefore, motor draws a very heavy current, limited only by armature resistance which is very low. Consequently, to limit this current, a resistor has to be added to the armature circuit.  

Ohmite brings decades of experience in offering power resistors designed to maximize watt dissipation for starting DC motors. Some of these include the WLRH series, 270-280 series, PFE-PFR series and SGM series. The WLRH series provides tough, versatile resistors for motor starting and speed control. Offering 45 models ranging from 0.25 to 250 Ohms to best fit users’ specific requirements, these resistors feature nickel-copper, nickel-chrome or stainless-steel wire wound in a double helix around the center core, depending on size and model. All have ceramic cores with a groove to grip the coiled wire. The 90° flat bus style terminals are corrosion resistant stainless steel. 

Ohmite is a leading power resistor manufacturer and braking solutions provider. The company offers decades of knowledge and experience, as well as access to resources to provide the correct resistor for any automotive, locomotive, and other industrial breaking applications. Contact Ohmite today.