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Thermistor Motor Protection Relays
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Thermistor Motor Protection Relays
Operating principle and fields of application for thermistor motor protection relays
The CM range of thermistor motor protection relays are used to control motors equipped with PTC temperature sensors. The PTC temperature sensors are incorporated in the motor windings to measure the motor heating. This enables direct control and evaluation of the following operating conditions:
- heavy duty starting
- increased switching frequency
- single-phase operation
- high ambient temperature
- insufficient cooling
- break operation
- unbalance
The relay is independent of the rated motor current, the insulation class and the method of starting. The PTC sensors are connected in series to the terminals Ta and Tb (or Ta and Tbx without short-circuit detection). The number of possible PTC sensors per measuring circuit is limited by the sum of the individual PTC sensor resistances: RG = R1 + R2 + RN 1.5 k.
Under normal operating conditions the resistance is below the response threshold. If only one of the PTC resistors heats up excessively, the output relay de-energizes. If the autoreset function is configured, the output relay energizes automatically after cooling down. Devices with manual (push button on front-side) or remote reset configuration have to be controlled via the control input by the required signal.
Further applications include the temperature monitoring of equipment with PTC sensors integrated, such as:
- machine rolling bearings
- hot-air ventilators
- oil
- air
- heating installations, etc.
 Download the Electronic Products and Relays catalog in *.pdf format
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