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Chapter 4 – Testing
4.1.8
C
ogging
This cross-section shows (by one tooth) the magnetic relationship of the hysteresis brake elements.
If the dynamometer shaft is at rest with torque applied, and if the torque control is then reduced to
zero, a magnetic salient pole will be temporarily imposed on the rotor of the brake.
If the shaft is then rotated slowly, the magnetic poles on the rotor will attempt to align with the adjacent
case-pole tooth form. This is often referred to as "cogging". The action is sinusoidal—first it tries to
resist rotation and then, as the rotor passes through the tooth form, it subsequently supports rotation.
At a few hundred rpm, these forces integrate resulting in an effective torque of nearly zero.
To avoid magnetic cogging, before the shaft comes to rest, reduce the torque control to zero.
To remove cogging, once established, reapply current on the dynamometer. Then, decrease the
current to zero while simultaneously rotating the dynamometer shaft.
4.1.9
e
C
DDy
urrenTS
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.
With the HD-800 through 815 models, the Eddy-current torque component adds approximately
10%/1000 rpm to a static, fixed current, torque value. On the smaller dynamometer sizes, 2% to
4%/1000 rpm is typical.
Rotor Poles
Case
Pole
N
S
S
Figure 4–23 Hysteresis Brake Cross-Section
50
Magtrol Hysteresis Dynamometers
Rotor
N
Assembly
N
S
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