
ECO REFERENCE MANUAL
8-6
SC
MOTOR LIMITATIONS
The motor speed is determined mainly by the
applied frequency. The motor slows down a little
as the load increases and the slip increases. If
the load is too great the motor will exceed the
maximum torque and stall or ‘pull out’.
The standard induction motor is cooled by a built
in fan connected directly to the shaft that runs at
motor speed. This is designed to cool the motor
at full load and name plate rated speed. If a
motor runs at a lower frequency and at full torque
- that is high current not normally an operating
condition of pumps and fans - motor cooling may
be inadequate. Motor manufacturers generally
give the necessary de-rating information, but a
typical de-rating curve would limit output torque at
zero frequency rising to full output torque
capability at 50% of nameplate rated speed (see
diagram). Ensure that these limitations are not
exceeded for long term operation.
Use the i
2
t function to help protect the motor (
See P074 in parameter descriptions ) or consider
using a motor with built in thermal protection such
as a thermistor (PTC).
Pull out torque
Torque
Speed
Normal operating
point
Slip
Variable frequency
operation