Siemens A1 User Manual
Selecting a Drive
International English
12
MICROMASTER Applications Handbook
Figure 3-3
Typical Harmonic currents in 230 V single and three-phase supplies,
(750 W Inverter).
(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.
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.
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.
recommended in any case to limit peak currents in the drive.
3.3 Motor
limitations
For more information concerning calculation of Power requirements, Torque, and
Moment of Inertia, see Appendix B.
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).
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.
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 I
2
t function to help protect the motor ( see section 5.7.1) or
consider using a motor with built in protection such as a PTC.
0
1
2
3
4
5
6
7
Fundam
ent
al
3r
d
5t
h
7t
h
9t
h
11
th
13
th
15
th
1ph 750W
3ph 750W
3ph 750W