Appendix C How to Select the Right AC Motor Drive
VFD-EL-W
C-4
C.2 General Precautions
Selecting an AC Motor Drive
When connecting the AC motor drive directly to a large-capacity power transformer (600 kVA or higher), or
when switching a phase lead capacitor, excess peak currents may occur in the power input circuit and may
damage the converter section. To avoid this, use an AC input reactor (optional) before the AC motor drive
mains input to reduce the current and improve the input power efficiency.
When using a special motor or when driving more than one motor in parallel with a single AC motor drive,
select the AC motor drive current to be ≥ 1.25x (sum of the motor rated currents).
The starting acceleration and deceleration characteristics of a motor are limited by the AC motor drive
rated current and the overload protection. Compared to running the motor D.O.L. (Direct On-Line), you can
expect a lower starting torque output with the AC motor drive. If a higher starting torque is required (such
as for elevators, mixers, tooling machines, etc.), use a higher capacity AC motor drive or increase the
capacities of both the motor and the AC motor drive.
When a fault occurs on the drive, a protective circuit is activated and the AC motor drive output is turned off.
The motor coasts to stop. For an emergency stop, use an external mechanical brake to quickly stop the
motor.
Setting Parameters
You can set the AC motor drive to an output frequency up to 400 Hz (less for some models) with the digital
keypad. Setting errors may create a dangerous situation. For safety, setting an upper limit frequency
function is strongly recommended.
High DC brake operating voltages and long operation time (at low frequencies) may cause overheating of
the motor. In that case, forced external motor cooling is recommended.
Motor acceleration and deceleration time is determined by motor rated torque, load torque, and load inertia.
If you activate the stall prevention function, the acceleration and deceleration time is automatically
extended to a length that the AC motor drive can handle. If the motor must decelerate within a certain time
with a higher load inertia than the AC motor drive can handle in the required time, either use an external
brake resistor and/or a brake unit (depending on the model) to shorten deceleration time only, or increase
the capacity of both the motor and the AC motor drive.
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