Engineering Guide CDA3000
3-75
3 Selection of inverter module
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Summary: 40% higher acceleration torque
In a drive design for 70 Hz the motor is run at a speed higher by the factor
1.4. As a result the maximum power output delivered by the motor is
achieved as low as a frequency of 50 Hz and remains constant beyond
that level up to 70 Hz. Above 50 Hz the torque falls proportional to the
inverter output frequency. The higher rotation speed of the motor shaft is
compensated by s transmission ratio increased by a factor 1.4. As a result
of the speed adjustment the available torque increases by 40% between 0
and 50 Hz and 0 and 68 rpm. This is equivalent to 40% more acceleration
torque with no increase in cost.
40% more overload reserve and break-away torque
Proportional to the acceleration torque, a 40% higher maximum torque is
of course also achieved (see characteristics 5 and 6 in Figure 3.34) and
thus also a 40% higher break-away torque.
60% larger speed manipulating range
The motor speed increased by a factor of 1.4 produces an approx. 60%
larger speed manipulating range on the gear output shaft. Referred to to
the application set out in Figure 3.32, Figure 3.33 and Figure 3.34, the 70
Hz design even means that no external cooler is needed, and so the
space take-up is reduced.
Or a reduction in motor power by one type step
A drive design with field weakening (70 Hz design) can, however, also be
designed to usually produce a reduction in motor power by one type step.
A reduced motor power saves space and money.
It should, however, be noted that the choice of maximum speed has a
major influence on the required acceleration torque and thus on the
acceleration time. In practice, at desired acceleration times below 400 ms
no reduction in the motor power or inverter output by one type step is usu-
ally attained.
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