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Revision August 2008, 03VE, SW V2.04 C-1
Appendix C How to Select the Right AC Motor Drive
The choice of the right AC motor drive for the application is very important and has great influence
on its lifetime. If the capacity of AC motor drive is too large, it cannot offer complete protection to the
motor and motor maybe damaged. If the capacity of AC motor drive is too small, it cannot offer the
required performance and the AC motor drive maybe damaged due to overloading.
But by simply selecting the AC motor drive of the same capacity as the motor, user application
requirements cannot be met completely. Therefore, a designer should consider all the conditions,
including load type, load speed, load characteristic, operation method, rated output, rated speed,
power and the change of load capacity. The following table lists the factors you need to consider,
depending on your requirements.
Related Specification
Item
Speed and
torque
characteristics
Time
ratings
Overload
capacity
Starting
torque
Load type
Friction load and weight
load
Liquid (viscous) load
Inertia load
Load with power
transmission
●
●
Load speed and
torque
characteristics
Constant torque
Constant output
Decreasing torque
Decreasing output
● ●
Load
characteristics
Constant load
Shock load
Repetitive load
High starting torque
Low starting torque
● ● ● ●
Continuous operation, Short-time operation
Long-time operation at medium/low speeds
● ●
Maximum output current (instantaneous)
Constant output current (continuous)
●
●
Maximum frequency, Base frequency
●
Power supply transformer capacity or
percentage impedance
Voltage fluctuations and unbalance
Number of phases, single phase protection
Frequency
● ●
Mechanical friction, losses in wiring
● ●
Duty cycle modification
●
C.1 Capacity Formulas