Thermal Sensory Management
by Alexis Montagnat-Rentier
Several technologies exist to measure the temperature in a motor. In many standard Ultra EC™ motors, Portescap provides a Resistance Temperature Detector (also known as RTD), specifically a Negative Temperature Coefficient (NTC) for which the resistance decreases when the temperature increases.
Temperature sensor imbedded in the motor
For several motors, an imbedded thermal sensor – NTC10k – is available. It can be a SMD component assembled on the PCB (22ECS or 22ECT), or a glass encapsulated element directly glued on the coil (30ECT or 35ECS). The operation is similar for both components, but they have different characteristics, the relationship resistance vs. temperature is different even if the nominal resistance is similar for the 2 sensors.
Thermal limitation and its measurement
In a motor, the thermal limitation is related to the winding. The main reason to control the motor temperature is to ensure the winding will not exceed its max temperature, which would cause damage to the motor. For a high peak torque, the winding can reach an elevated temperature in a very short time, much faster than the sensor can display. Therefore, a delay in the response time is expected. For this reason, the measurement is more recommended for steady state temperature. Calibration in the application is recommended to link the measured temperature vs. the winding temperature.
Since the thermistor is a resistive element, current excitation is required. There are many possibilities to integrate an NTC into an electronic circuit to measure the temperature. The following schematics and explanation are primarily derived from an application note (AN685) released by Microship Technology Inc. Please refer to this document for a deeper analysis.
The most common and simple approach is to use the thermistor in series with a standard resistor and a voltage source (Figure A). This is also called the constant voltage drive. The relationship between the output voltage (Vout) and the temperature of the thermistor can be calculated by the following formula Vout = Vref x RNTC / (RNTC + R). In this configuration, the series thermistor system responds to temperature in a linear manner over a limited temperature range. The resistance should be equal to magnitude of the thermistor at the mid-point of the temperature range. This voltage change can be treated as temperature information by a controller.
Another option is to use the NTC resistor in parallel with a reference resistor (Figure B), which creates a composite resistor element. In this case as well, the resistance should be equal to magnitude of the thermistor at the midpoint of the temperature range.
A third circuit consists in a Wheatstone bridge with an NTC thermistor used as one bridge leg (Figure C). With the bridge being balanced, any change in temperature will cause a resistance change in the thermistor and a current will flow through the amperemeter.
Depending of the driver or controller used, it’s also possible to proceed to linearization using software with an empirical third order polynomial or a look up table.
Motor wiring diagram
The typical motor pin out is:
|Green||4 to 24V DC|
|Black or White||NTC|
The black/white or white/white cables are the two extremities of the thermal sensor. For all RTD’s, as for the resistances, both wires can be crossed in the connection without any effect.
Connecting the thermal sensor is not mandatory to run the motor, it depends whether the controller has the capability to accept the temperature sensor output.
Portescap sensor specifications
Two different sensors are used in standard motors:
|Reference||Sensor 3||Assembly||R25||B25/85||Temp. range|
|ERTJ1VG103FA||22ECS / 22ECT||SMD on PCB||10kOhm ±1%||3435 K ±1%||-40°C to 125°C|
|TT4-G10KC3||30ECT / 35ECS||Glass encapsulated glued on coil on head||10kOhm ±1%||3977 K ±1%||-40°C to 155°C|
Following are the relationship curves for the two thermal sensors along with the conversion tables: