User's Manual

PMAC User Manual
98 Setting Up PMAC Commutation
Determination of the proper magnetization current is best done with a simple experimental technique.
The technique relies on the linear relationship between motor speed and back EMF, in which the
magnetization current provides the constant of proportionality (K
E
, the velocity, or back-EMF constant, is
directly proportional to the magnetization current).
Consider the no-load speed of the motor — equivalent to the line frequency, since there is zero slip at
zero load. For a 4-pole motor at 60 Hz, the no-load speed is 1800 RPM. At this speed, with no load, the
voltage waveforms from back EMF should be just at the saturation point (for example 380V RMS).
Give the motor a starting magnetization current e.g., 5 to 10% of full current (Ix77=1638 to 3276). Set
the slip gain (Ix78) according to the above equations. Measure the back EMF of the motor at a known
fraction of the no-load speed when it is putting out zero torque (use the O0 open-loop command). Either
drive the motor from another motor at the known speed (preferred) or take the motor above this speed
with a small open-loop torque (e.g. O5), then command O0 and take the measurements as the motor
coasts through the desired speed.
If the measured voltage is too low, the magnetization current is too low. If the voltage is too high, the
mag current is too high. The difference should be proportional: if the voltage is only 75% of what it
should be, the mag current is only 75% of what it should be. Adjust Ix77 according to the measurements,
and try again. The calculations should be almost correct on the second pass.
Experimental Setting of Induction Motor Parameters
If it is not practical to use the induction motor nameplate values to set the induction motor parameters, or
if it is desired to check whether the parameters derived from the nameplate values are proper, the
following experimental method can be used. This method should be used on an unloaded motor because
it utilizes open-loop commands that create a large number of revolutions that are not well controlled. The
settings derived from the tests on an unloaded motor are valid even with the load because the induction
motor parameters are load independent, functions only of the electrical properties of the motor.
Select an arbitrary value for Ix77 magnetization current. If there is a calculated number from the
nameplate values, use it. If starting without any previously calculated value, 3000 (about 1/10 of
maximum) is a good starting value.
Select an arbitrary value for Ix78 slip gain. If there is a calculated number from the nameplate values, use
it. If starting without any previously calculated value, 4000 is a good starting value
Gather actual position data from the motor while accelerating the motor with an open loop command.
Use the PMAC Executive program to select gathering of the actual motor position. To perform
the actual
gathering, use the following sequence of on-line commands:
Note:
The values obtained in this test are dependent on the motor’s rotor temperature,
because the resistance of the rotor, and hence its L/R time constant, change with
temperature. It is best to optimize the settings with the rotor hot. In this case, the
motor will run less efficiently with the rotor cold, which will require more current,
heating up the motor towards the proper settings. Run the motor for several
minutes at a significant current level (e.g. O30) before trying to do final
optimization.
GAT O10 ; Start data gathering, open-loop 10% command
ENDG ; Stop data gathering
O0 ; Open loop 0% to stop motor
Do not issue the ENDG command until the motor has stopped accelerating. Upload the data to the PC by
pressing the F10 key.