Temperature sensor NTC

Introduction

An NTC - Thermistor (Negative Temperature Coefficient Thermistor) is a type of resistor that decreases the resistance when temperature rises.

The SMD (Surface Mounted Device) version of an NTC can be used to measure the temperature of a maxon motor.

The SMD element has the advantage that it can be integrated directly on the motor PCB.

The disadvantage is that there is no direct contact to the winding (compared to the PTC in the EC 60). This leads to an increased thermal time constant (longer delay) and a higher thermal resistance (lower temperature measured). The SMD NTC on a PCB without direct contact to the winding is therefore a solution to control continuous operation and requires testing in the customer application. For rotating operation 1 (one) sensor is sufficient. For operation at standstill (positioning) 2 (two) sensors are required to control the temperature of all windings.

maxon motors with integrated NTC's

• EC frameless (catalog standard)
• ECX SPEED 16, 19, 20 (configurable option)

Practical information about NTC

The NTC resistance is not a linear function of the temperature as one can see in the two plots below (which refers to an NTC specified R₂₅ = 10 kΩ and beta 25..85 = 3490K). Even by restricting the temperature range (say from 0 to 50 °C), still the function can hardly be approximated by a line. As explained before, in an NTC the resistance decreases as the temperature increases.

NTCs have two major parameters: the nominal resistance R₂₅, which is their resistance at the standard temperature of 25 °C (T25 = 25 °C = 298.15 K) and their constant beta, which somehow represents the "temperature coefficient". The beta value is optimized for a certain operating range, e.g. 25°C..85°C.

With these parameters the temperature can be calculated as follows:

With: 

• beta:  Constant (temperature coefficient) optimized for the operating range of the motor (catalog value)
• R₂₅:  Nominal resistance at standard temperature R₂₅ (catalog value)
• T₂₅: Standard temperature of 25 °C (298.15 K)

Or use this spread sheet to see how resistance changes over temperature:
NTC Calc Sheet 20150706.xlsx

NTCs are widely used temperature sensors because of their low cost and their availability in many sizes and shapes. With modern micro-controllers, it is easy to program the above explained equation and get direct readings in °C (or any other temperature unit) without the need of complex analog linearizing circuits.

There are a downsides that should be mentioned:

• the working temperature range is limited to about --50 to +150 °C. This depends, of course, on the particular model of NTC. Since the majority of NTCs use silicon, these limits cannot be exceeded.
• because of the logarithmic change of resistance, the wider the temperature range accepted by the circuit, the lower the precision.

NTCs are usually not factory calibrated: the actual R25 and Beta can vary from one NTC to another and some sort of circuit adjustment is always required to do absolute temperature readings.

A last note: When using an NTC as a temperature sensor, one should be careful in not running to much current though the NTC, since the current will heat the NTC and introduce a measurement error. For this reason, high resistance NTCs (10 kΩ or more) are better for thermometers.

Two NTC's on EC frameless motors

On EC frameless motors two NTC's in parallel are used to get an indication of the winding temperature:

In this configuration NTC1 and NTC2 are placed on the PCB under two different motor windings.

At continuous rotation operation both NTC have the same temperature. The resistance measured between NTC IN and NTC OUT is:

Thus with the formula T(R) =... from above you can calculate the temperature for this type of Operation.

At standstill (positioning tasks), the total resistance goes in the direction of the lower (more critical) individual resistance. Thus, only half of the measured resistance should be used to determine the temperature.