Rockwell Automation 8510 AC Spindle Drive System Programming Manual - Manual (Page 39)

Drive Tuning
Chapter 3
3-36
“Per Unit” System Description The units used to express gain are arbitrary. When tuning analog drives by
adjusting potentiometers, units are completely unknown and gain is only an
arbitrary scale on the pot. Thus, it is possible to tune drives without even
knowing the gain units.
With digital drives, parameter values must relate directly to mathematical
or computer models of the system that are implemented in the drive.
Therefore, the units must follow some defined format.
The 8510 uses the Per Unit system. This system is employed for the
following reasons:
– The Per Unit system is independent of any local system of units. Units
do not change among industries or locality.
– The Per Unit system yields gain values which are very similar over a
wide range of motor/drive sizes. Once you become familiar with typical
values for P, I, and Droop gains, nearly the same values can be expected
for a very wide variety of motor/drive sizes.
– The Per Unit system is becoming the worldwide drives industry
standard.
Per Unit System Defined
Unit Velocity, V, is defined as the motor base speed
Unit Torque, T, is defined as the motor peak torque
Unit Inertia, J, is defined as the time to accelerate to Per Unit speed at Per Unit torque
Unit Time, t, is defined as 1 Second
Example 1:
Unit motor speed is defined as base speed. The base speed of a certain
motor is 1500 rpm. How many units of speed is the motor running at when
it is rotating at 6000 rpm?
6000 rpm
1500 rpm
= 4 Note that the Per Unit speed does not have a unit
Example 2:
The peak torque of the same motor is 200 N-m (147.5 lb.-ft.). What is the
Per Unit torque when operating at 50 N-m (36.9)?
50 N-m (36.9 lb.-ft.)
200 N-m (147.5 lb.-
ft.)
= 0.25
Note that the answer is the same with torque
expressed in lb.-ft. or N-m.