Sizing A Motor; Information Required About The Application; Motor Maximum Speed; Torque Required Between Zero And Base Speed - Sevcon Gen4 Applications Reference Manual

Sizing a motor

Information required about the application

To select an appropriate induction motor for an application find or estimate the following
information:

Minimum battery voltage

Maximum motor speed required

Peak torque required at base speed

Peak torque required at maximum motor speed

Continuous (average) motor power output required to perform the work cycle

Peak motor power output required and duration
Include inertia and friction contributed by the motor, as well as any gearing in the drive chain,
when calculating torque and load requirements. If replacing a DC motor with an AC motor in an
existing application, the DC motor torque vs. speed curve is a good starting point to determine
the required ratings.

Motor maximum speed

Determine the maximum motor speed using the required vehicle or pump maximum speeds and
the ratio of any gear box or chain between the motor and the load. Most motor manufacturer rate
induction motors at synchronous speed which is 1,500 and 1,800 rpm for a 4-pole motor when
operated from 50 Hz and 60 Hz line frequencies respectively.
The maximum speed an induction motor can be used at is determined by the limit of the
mechanical speed, typically 4,000 to 6,000 rpm, and the reduction in useful torque at higher
speeds. Increasing losses in the iron of the motor at higher speeds may further limit the
maximum speed. Always check the maximum speed with the motor manufacturer. Check also
any limitations imposed by the maximum frequency of the encoder input signal (see 'Motor
speed sensor (encoder)' on page 5-11).

Torque required between zero and base speed

Calculate the torque required by the application. Use figures for the work that needs to be done
against friction and gravity, plus those required to accelerate the load inertia and momentum. Up
to rated speed the peak torque that can be supplied when using a correctly specified Gen4 is
equal to the breakdown torque. Select a motor with a breakdown torque rating greater than the
peak torque required.
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