12
Magtrol Dial Weight Dynamometers
User’s Manual
3.9
EDDY CURRENTS
There is some Eddy current generation within the brake rotor. These magnetically induced currents
cause an increase in brake torque proportional to speed.
The larger the hysteresis brake, the higher the rotor surface velocity. Additionally, as brakes become
larger the rotor cross-sectional area increases. Each of these factors increase Eddy current generation.
The combination results in speed-related torque increase, exhibiting a more pronounced effect on
larger dynamometers.
On the smaller dynamometer sizes, 2% to 4%/1000 rpm is typical.
3.10
TEMPERATURE RISE
Temperature rise has a more complex effect on hysteresis brake load torque and is difficult to quantify.
As the temperature of the brake increases, differential expansions cause dimensional changes that
tend to increase torque. Conversely, electrical resistance in the rotor increases with temperature,
resulting in decreased Eddy current generation and torque, all in a variable frame.
Where current and speed remain fixed, there may be a gradual torque increase over a period of a
few moments. This will generally stabilize at 0.5% (for small dynamometers, up to HD-500-1) to
1.5% of the starting torque value, per 1000 rpm of applied speed.