Magtrol DSP6000 User Manual - Appendix B
28
INERTIAL EFFECT ON MOTOR TEST
DATA
A major advantage of the DSP6000 is its ability to obtain
full motor performance data (free run to locked rotor)
by continuous load application with an absorption
dynamometer. Data acquisition is fast, resulting in
minimal motor I
2
R losses, and loading characteristics
simulate actual end-use applications.
When a motor is accelerating or decelerating, the
measured torque is the sum of the true motor torque ±
the inertial torque, or stored energy, of the system.
Unless inertial torque is excluded, motor performance
will vary in proportion to the rate of acceleration or
deceleration.
This type of error can produce problematic test results.
For example, during rapid deceleration, system inertia
can produce apparent efficiency greater than 1.0. This
error may occur if output power is divided by input
power without extracting the stored energy in the
system.
Since "inertial effect" is only a factor when speed is
changing, and because inertial torque is proportional to
the rate of change, inertial value may be expressed as a
unit of torque per change in RPM in a given period of
time. With the DSP6000, properly adjusted PID values
yield constant change in RPM so that the inertial torque
can be expressed as a constant.
PROCEDURE FOR INERTIA
CORRECTION
1. Determine the torque Correction Factor (CF) as
follows:
•
Adjust the PID loop properly
•
Establish a torque value equal to the inertial
torque.
2. Use the "Program Down" command (PD#) to ramp
to 75% of the free-run speed.
3. Select a data point on the performance curve where
speed will be approximately 78% of the free-run
speed. Let this represent the dynamic speed-torque
value.
Appendix B
Appendix B
Appendix B
Appendix B
Appendix B: Inertia Correction
: Inertia Correction
: Inertia Correction
: Inertia Correction
: Inertia Correction
4. Immediately program your DSP6000 (Nddddd) to
a speed equal to the dynamic speed value. When
the speed stabilizes, use this as the static torque
value.
CF = Dynamic Torque - Static Torque
To correct your data, subtract the CF from each torque
point obtained during the ramp.
Example:
FR
0
TORQUE
DYNAMIC
TORQUE VALUE
SPEED VS TORQUE
CURVE WITH CONSTANT
DECELERATION
.78 OF FR
STATIC TORQUE
VALUE
CF = DIFFERENCE IN TORQUE
S
P
E
E
D
DIFFERENCE IN TORQUE
K
EY
C
ONDITIONS
•
Select appropriate value. The test point
selection of 78% is typical for an induction
motor. Use a value in the linear portion of the
motor curve where there is a substantial torque
change with speed.
•
Acquire data rapidly. Rapid data acquisition
is necessary so that motor heating does not
degrade performance by adding a false
difference between the static and dynamic
torque values.
•
Use a regulated power source. The input line
voltage must be stable for the time necessary
to perform the test. Torque varies by the square
of the change in line voltage.
•
Obtain new CF value for various
deceleration/ acceleration rates. The CF is
only valid for its specific ramp rate. To calculate
other CF rates, use the following equation:
CF
new
= (CF
old
/ramp rate) X new ramp rate