Selecting a Drive

International English

7

6

5

4

1ph 750W

3ph 750W

3

2

1

0

Fundamental

3rd

5th

7th

9th

th

th

th

11

13

15

 

Figure 3-3 Typical Harmonic currents in 230 V single and three-phase supplies, (750 W Inverter).

In order to calculate the harmonics in a particular supply system it is essential that the supply impedance is known. This is usually stated in terms of fault current levels, transformer size and installed impedance such as line inductors etc.

Computer programs are available to calculate the current and voltage harmonic levels, dependent on the load, type and number of inverters in the system. In general, industrial supplies do not require this level of assessment.

Where supplies have very low impedance (such as below 1%) an input inductor is recommended in any case to limit peak currents in the drive.

3.3Motor limitations

For more information concerning calculation of Power requirements, Torque, and Moment of Inertia, see Appendix B.

The motor speed is determined mainly by the applied frequency. The motor slows down a little as the load increases and the slip increases. If the load is too great the motor will exceed the maximum torque and stall or ‘pull out’. Most motors and inverters will operate at 150% load for a short time, (60 seconds for instance).

The motor is usually cooled by a built in fan that runs at motor speed. This is designed to cool the motor at full load and base speed. If a motor runs at a lower frequency and full torque - that is high current - cooling may be inadequate. Motor manufacturers will give the necessary de-rating information, but a typical derating curve would limit output torque to 75% at zero frequency rising to full capability at 50% of base speed. See Figure 3-4. Ensure that these limitations are not exceeded for long term operation.

Consider using the I2t function to help protect the motor ( see section 5.7.1) or consider using a motor with built in protection such as a PTC.

12

MICROMASTER Applications Handbook