Delta VFD-EL-W Series, User Manual, Page: 195 / 197

Appendix C How to Select the Right AC Motor Drive

VFD-EL-W
C-5
C.3 How to Choose a Suitable Motor
Standard Motors
When using the AC motor drive to operate a standard three-phase induction motor, follow these
precautions.

The energy loss is greater than that for an inverter-duty motor.

Avoid running the motor at low speed for a long time. Under this condition, the motor temperature may rise
above the motor rating due to limited airflow produced by the motor’s fan. Consider adding external forced
motor cooling.

When the standard motor operates at low speed for a long time, the output load must be decreased.

The load tolerance of a standard motor is according to the following diagram.
3 6 20 60
100
82
70
60
50
0
60%
40%
25%
to
rq
u
e
(%
)
continuous
Frequency (Hz)
Load duty-cycle

If 100% of continuous torque is required at low speed, it may be necessary to use a special inverter-duty
motor.

Motor dynamic balance and rotor endurance should be considered once the operating speed exceeds the
rated speed (60Hz) for a standard motor.

Motor torque characteristics vary when driving the motor with an AC motor drive instead of a commercial
power supply. Check the load torque characteristics of the machine connected to the motor.

Because of the high carrier frequency PWM control of the VFD series, pay attention to the following motor
vibration problems:

Resonant mechanical vibration: use anti-vibration dampers to mount equipment that runs at varying speed.

Motor imbalance: special care is required for operation at 60 Hz and higher frequencies.

The motor fan is very noisy when the motor speed exceeds 60 Hz or above.
Special Motors

Pole-changing (Dahlander) motor:
The rated current differs from that of a standard motor. Check before operation and carefully choose the
capacity of the AC motor drive. When changing the number of poles, stop the motor first. If over-current
occurs during operation or the regenerative voltage is too high, let the motor free run to stop (coast).

Submersible motor:
The rated current is higher than that of a standard motor. Check before operation and carefully choose the
capacity of the AC motor drive. A long motor cable between the AC motor drive and the motor reduces the
available motor torque.

Explosion-proof (Ex) motor:
Must be installed in a safe place and the wiring should comply with the (Ex) requirements. Delta AC Motor
Drives are not suitable for (Ex) areas that require special precautions.

Gear reduction motor:
The lubricating method of the reduction gearbox and the speed range for continuous operation are different
and depend on the motor brand. Carefully consider the lubricating method when operating for a long time
at low speed and for high-speed operation.

Synchronous motor:
The rated current and the starting current are higher than those of standard motors. Check before