48
Magtrol Hysteresis Dynamometers
Chapter 4 – Testing
OPERA
TION
4.1.4
f
ixTureS
anD
C
ouPlingS
When mounting the test motor, please consider the following:
• Construct precise fixtures that provide proper shaft alignment.
• Secure the test motor in the fixture to prevent torsional movement and bolt the fixture to the
dynamometer base plate.
• Give consideration to the interaction of materials between the motor and test fixture. For
example, a (magnetic) steel plate placed against the exposed lamination of an open frame
motor can significantly influence performance. Some thin shell PM (permanent magnet)
motors may be similarly affected.
• The dynamometer base plate material is an aluminum tool plate that is easily drilled and
tapped. The use of helix thread inserts is a good idea if you are going to interchange fixtures
often.
Note:
For an additional charge, Magtrol can perform base plate
modifications.
The following diagram illustrates examples of possible shaft misalignment.
PARALLEL OFFSET
ANGULAR
Figure 4–22 Examples of Possible Shaft Misalignment
The use of high quality double flexure couplings is recommended to help correct any misalignment
problems. This type of coupling—two flexing elements separated by a solid link—inherently tolerates
greater parallel offset. If you would like more specific coupling recommendations, contact Magtrol
Technical Assistance. For precise misalignment tolerances, consult your coupling manufacturer.
4.1.5
w
inDage
Windage is proportional to the square of speed and magnifies rapidly above rated speed. The air
friction is tangential to the surface and impinges upon the stationary field assembly. This acts as
viscous drag and becomes part of the motor load and torque reading. However, there is a small
amount of air dissipated as pumping loss. Since this appears as a load on the motor, not measured
by the dynamometer, it becomes a source of error.
Windage effects on accuracy tests have been conducted on all Magtrol dynamometers. The percentage of
torque loss due to windage ranges between 0.025% and 0.20% of full scale at maximum rated rpm.
Windage is proportional to the square of speed and magnifies rapidly above rated speed. Conversely,
at one half of the rated speed, the effect becomes immeasurably small.