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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.
Load type
Load speed and
torque
characteristics
Load
characteristics
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
Revision July 2008, EG03, SW V1.06
Item
Friction load and weight
load
Liquid (viscous) load
Inertia load
Load with power
transmission
Constant torque
Constant output
Decreasing torque
Decreasing output
Constant load
Shock load
Repetitive load
High starting torque
Low starting torque
Related Specification
Speed and
Time
torque
ratings
characteristics
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Overload
Starting
capacity
torque
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C-1
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