Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 108)

that the harmonic content of the output current from the frequency converters may possibly
change the characteristic of the protective devices and the devices will also be subject to
additional thermal loading. See Section
2.4.3.5 .
It should also be noted that self-ventilating motors are not suitable for continuous operation at
low speeds. For such applications, external ventilation should be provided. In order to assure
motor protection even at low speeds, temperature sensors, for example thermistors (PTC), must
be inserted in the motor windings.
LVSAM-WP001A-EN-P - April 2009 3-32