Protecting a motor by monitoring winding temperature

Motor current alone does not explain every overheating condition. Inadequate cooling or a high ambient temperature can cause the windings to overheat. A control unit connected to PTC thermistors embedded in the motor detects excessive winding temperature and signals the stopping circuit. The official IEC standard for this subject is IEC 60947-8:2021.

How the PTC sensor and control unit work together

The resistance of a switching-type PTC used for motor protection rises sharply around its nominal response temperature. The evaluation unit monitors resistance in the sensor circuit and changes its output state when the relevant threshold is exceeded. In a suitably designed circuit, this output can release a contactor or activate a documented stopping input on a drive.

The sensor, evaluation unit and stopping device therefore form a protection chain. Although PTC and NTC sensors may appear in the same product group, they cannot be interchanged arbitrarily. A PTC protection input is also different from a Pt100 input or a general-purpose analog temperature input. Verify the sensor type and input compatibility before making the connection.

Which motor conditions can it help detect?

Sensors embedded in electric motor windings monitor a different location from a thermostat measuring the surrounding air. Monitoring winding temperature can be useful, for example, when a fan motor has obstructed ventilation passages or an application has insufficient cooling. Protection depends on sensor placement, response temperature, the motor's thermal behavior and the stopping chain.

The purpose is not necessarily to display every temperature change in degrees. A typical thermistor protection relay distinguishes an acceptable operating condition from an overtemperature condition. If continuous temperature logging or accurate measurements at several locations are required, select the sensors and measuring system specifically for that task.

Relationship to overload and short-circuit protection

PTC-based temperature protection can complement current-based thermal overload protection. It does not replace short-circuit protection provided by a suitable fuse or circuit-breaker. Overload, short circuit and winding temperature represent different protection needs in a motor circuit, and their coordination must be considered together.

The evaluation relay's output should not automatically be treated as suitable for interrupting the motor's main power current. In many applications, it acts on the control circuit of a contactor. Check coil voltage, output switching capability and the inductive nature of the load against the product data.

Connection and settings with a variable-speed drive

Some drives have an input that directly evaluates PTC sensors; others require an external module. The physical presence of an input is not enough: select the correct sensor type, enable monitoring and configure the fault response. Assessment of a drive under the IEC 61800 series and correct implementation of the motor sensor circuit are separate matters.

Follow the manufacturer's limits for sensor quantity, total cold resistance, cable length and connection arrangement. Resistance or cable limits published for one product must not be applied universally. Sensor wiring also requires different consideration from power wiring; follow the relevant manual for routing, shielding and connections.

Sensor wiring faults, reset and restarting

In addition to overtemperature, the sensor circuit may develop an open circuit or a short circuit. Check how the selected model detects and indicates these faults, and how they affect its output. Diagnostic features available on one product must not be assumed available on another.

Manual, automatic and remote reset options are also model-dependent. A relay returning to its normal state after the motor cools does not establish that automatic machine restarting is appropriate. Resetting a fault and issuing a new start command require separate consideration. The control design must address the possibility of motion restarting during reset.

Product selection and certification checks

Review supply voltage, PTC compatibility, output type, reset behavior and environmental limits together. Information expressed through IEC 60721 environmental classes should be read alongside the equipment's temperature, humidity and other operating limitations. Hazardous-area applications also require the relevant Ex approvals and compliance with their conditions; a general-purpose PTC input alone does not establish suitability.

For UL standards and certification, check the manufacturer, exact model, file number, product category and applicable conditions of use. UL Recognized Component status does not approve the complete machine or panel. For cUL and CSA documentation, check the countries and standards covered. Installation requirements under the relevant NEC framework need separate consideration.

What should be verified during commissioning?

Use the manufacturer's specified checking procedure to verify normal operation, the overtemperature response, any supported sensor-line fault response and reset behavior. Checking the relay's status indicator is only part of the task: verify that the stopping command reaches the intended device and produces the expected response. Permanently bypassing the protection by bridging terminals does not resolve the fault's cause.

If repeated trips occur during service, investigate motor cooling, mechanical load, sensor wiring and connections together. Replacing a control device with one having a higher threshold, or selecting automatic reset, is not a substitute for investigating the underlying cause.

Related standards and product groups

The UL 60947 overview provides routes to UL 60947-1 general rules, UL 60947-4-1 contactors and motor starters, UL 60947-4-2 semiconductor motor controllers and starters, UL 60947-4-3 AC controllers for non-motor loads, UL 60947-5-1 control circuit devices and UL 60947-7-1 terminal blocks. These subjects help distinguish the PTC evaluation unit from other components in the circuit.

For the sensing element, see PTC sensors; for current-based protection, overload relays; for motor control, variable-speed drives; for automation integration, PLCs and accessories; and for a suitable supply, power supplies.