7 constant power calibration – Rockwell Automation SA500 AC Power Modules User Manual
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4-14
SA500 Power Modules
Resolver Restrictions
The PMI cannot discriminate between X1 and X2 resolvers. It only detects electrical
rotations. One mechanical rotation is equivalent to one electrical rotation for an X1
resolver and two electrical rotations for an X2 resolver. The practical limit of electrical
speed that the module can detect is dependent both upon the resolver selected and
upon the resolution selected during drive parameter configuration. See Appendix A.
4.6.2 Analog Input
Use 18-22 AWG twisted pair shielded cable to connect the analog device to the
terminal block. Connect the shield to the SHIELD terminal, as shown in figure 4.4.
Figure 2.5 in chapter 2 shows the analog input circuit. Note that the input impedance
has a finite value of approximately 1.3 megohms. This must be taken into account
when connecting to a source with a high output impedance. Take steps to reduce
noise and the possibility of ground loops. In the case of grounded sources, note the
common mode voltage limit. Avoid connecting a remote ground into the Power
Module. Use the differential connections to reduce noise. Keep input cable lengths as
short as possible, and ground the shield at the source’s earth ground.
4.7
Constant Power Calibration
Typically, an induction motor operating at or less than its rated speed will be capable of
generating its rated torque output. When a motor is operating at more than its rated
speed, its ability to generate torque decreases as the speed increases. To
compensate for this reduction of torque-producing capability, the magnetizing current
can be reduced as a function of motor speed or an external RPM signal. Reducing the
magnetizing current (Iz) enables the motor’s voltage to remain constant at speeds up
to four times base speed. This allows operation in the motor’s constant power region.
See figure 4.5. The maximum speed attainable is dependent upon the motor and the
application. Motors typically can only be operated in the 1.5:1 to 2:1 speed ranges.