Application Guide

63-706211
Motor
Tt
Insulation Life
Vibration Ventilation
Rated
Temperature
Rise
Load
(Duty Cycle)
Ambient
Altitude
Voltage
Frequency
Waveform
Fig. 9. Causes for insulation failure.
Three of the elements in Fig. 9 can be or are affected substantially by application of a VFD. Therefore, they have a bearing on
the temperature of the motor and thus its life expectancy:
Vibration
Although a modern VFD produces good quality current waveforms, there is a small amount of additional vibration produced at
the motor. Thus, there is potential for small reduction in the motor life expectancy. However, motors are often installed on
inadequate frames or machinery that has a tendency to vibrate. Consequently, the life expectancy of the winding is affected far
more by the installation than the VFD.
Ventilation
The installation of a VFD should have no affect on motor ventilation, as this is a purely mechanical function. However, a VFD
tends to cause motors to run slightly warmer than they would if driven from a commercial power supply: typically 5 degrees
Fahrenheit. Normally this is well within the motor design limits and there are no adverse effects from the small increase in
temperature.
Rated Temperature Rise
Typically, motors are designed to produce an 176°F temperature rise at full load with nameplate conditions, in an ambient
temperature of 104°F. As additional temperature rise caused by VFD application is only 5°F and experience shows that most
motors are selected with 80% of design capacity and rarely run with an ambient temperature of 104°F. Again there is little
effect on the life expectancy of the motor, when driven by a VFD.
Load (Duty Cycle)
Generally small motors from fractional Hp up to maybe 5 or 10 Hp can withstand many starts per hour, typically 100-200,
without overheating. There is an increase in the motor temperature over that reached in a motor left running constantly. This
increase in operating temperature is due mainly to:
— The surge current at start up, which may be as high as 5 or 7 times full load current.
— Periods when the motor is stopped, where the motor cooling fan is not running, therefore the body temperature will
increase to compensate.
As increase in temperature reduces motor life, frequently starting even small motors can substantially reduce the life
expectancy. As the motors increase in capacity above 10 Hp, the frequency of starts per hour should be reduced to 3 or 4
starts per hour for motors of 125 Hp to 250 Hp.