How to Select the Right VFD (Inverter) or AC Motor Drive

The choice of the right AC motor drive
(VFD)
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.


Selection Note for VFD

Why to use Choke for VFD?
When the AC Motor Drive (VFD) is connected directly
to a large-capacity power transformer (600kVA or above) or when a phase lead
capacitor is switched, excess peak currents may occur in the power input circuit
and the converter section may be damaged. To avoid this, use an AC input
reactor (optional) before AC Motor Drive mains input to reduce the Current and
improve the input power efficiency.
When a special motor is used or more than one motor
is driven in parallel with a single AC Motor Drive, select the AC Motor Drive
current ≥1.25x (Sum of the motor rated currents).
The starting and accel./decel. Characteristics of a
motor are limited by the rated current and the overload protection of the AC
Motor Drive. Compared to running the motor D.O.L.(Direct On-Line), a lower
starting torque output with AC Motor Drive can be expected. If higher starting
torque is required (such as for elevators, mixers, tooling machines, etc.) use
an AC Motor Drive of higher capacity or increase the capacities for both the
motor and the AC Motor Drive.
When an error occurs on the drive, a protective
circuit will be activated and the AC Motor Drive output is turned off. Then the
motor will coast to stop. For an emergency stop, anexternal mechanical brake is
needed to quickly stop the motor.
Parameter Settings Note for Variable Frequency
Drive(VFD)
The AC Motor Drive can be driven at an output
frequency up to 400Hz (less for some models) with the digital keypad. Setting
errors may create a dangerous situation. For safety, the use of the 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 accel./decel. time is determined by motor
rated torque, load torque, and load inertia.
If the stall prevention function is activated, the
accel./decel. time is automatically extended to a length that the AC Motor
Drive can handle. If the motor needs to decelerate within a certain time with
high load inertia that can’t be handled by the AC Motor Drive in the required
time, either use an external brake resistor and/or brake unit, depending on the
model, (to shorten deceleration time only) or increase the capacity for both
the motor and the AC Motor Drive.

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