How do Hysteresis Brakes Work?

Hysteresis Brakes produce torque without friction plates or magnetic particles. Torque is instead produced magnetically across an air gap. Because of this, life is extremely long, even at high speeds. This makes these types of brakes suitable for slipping applications. These brakes can provide extremely smooth slipping torque across a wide range of speeds, even down to zero RPM.

These brakes rotate freely with no electrical excitation. When electrical excitation, the shaft becomes coupled to the case. When the load torque is less than the output torque, the shaft won’t rotate continuously. When the load torque is increased, the brake will slip smoothly at the torque level set by the coil input current.

What Applications Are Best for Hysteresis Brakes?

Due to their precise and repeatable torque, hysteresis brakes are well-suited for applications that require very precise tension control like web rewinding. During web rewinding, these brakes operate without chatter (or slip-stick). Chatter is usually caused when rotation is too slow for the brake to operate appropriately.

Applications that require high speeds with low torque such as induction motor and mechanism testing or high-speed web tensioning benefit from the function of hysteresis brakes.  This is because hysteresis brakes can provide proper heat dissipation while also offering smooth and repeatable slipping torque. Other types of variable torque brakes will not be able to provide accurate levels of torque for this type of application.

Another common application for these brakes is continuous (24-hour) slipping. This is because hysteresis brakes produce torque without physical contact, resulting in their ability to reliably run at full torque rating and full heat dissipation limits, for almost unlimited time.

Applications that require very low RPM benefit from the ability of these types of brakes to provide smooth slipping below 15 RPM.

What are the Considerations for using Hysteresis brakes? What applications are hysteresis brakes not well-suited for?

Cogging is a potential issue with these types of brakes. Cogging is caused when the input current is brought to zero while the shaft is stopped, there will be magnetized sections in the cup due to residual magnetism. These magnetic sections try to align with the poles, just like a steel bar will try to align between the two ends of a horseshoe magnet.

Cogging can occur when brakes are de-energized to allow the motion of a shaft to start again. This reduction in input current would initiate a hysteresis brake to cog. If the input current can be held constant, the brake will not cog.

Hysteresis brakes are not well suited for applications that require quick acceleration and deceleration. Magnetic particle brakes have faster response time and significantly lower inertia, which allows for shorter stops and starts compared to hysteresis brakes. Typical applications that require quick acceleration and deceleration include tension control of wire in a coil winding machine, and machines with rapid indexing movement of a web.

Hysteresis brakes are also not well suited for applications in which precise holding or positioning is required. A magnetic particle brake is the better choice for precise holding or positioning as this would also cause a hysteresis brake to cog. A magnetic particle brake will not have any shaft movement unless the applied torque exceeds the braking torque set by the input current. A hysteresis brake can hold a shaft in position, but the amount of holding torque is lower than the sliding torque. Braking torque will ramp up as the shaft is forced to turn in either direction, reaching the sliding torque amount in about 10°. The shaft would spring back about 10° when the external torque is removed. For non-precision holding, these limitations would probably not be a problem.