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However, new research shows that even a temperature sensor attached directly to a servomotor's electrical winding won't always protect the motor from overheating.

Reconsider the four-parameter estimation of winding and case heatup at 1× power dissipation.

Here, our 60-mm motor winding and case are heated with 16x power dissipation.

Shown in Fig. 5 is the dynamic heatup of both — only this time, the motor generates 4× peak torque output, corresponding to 16× power dissipation.

Here, the four-parameter model calculates that with 4× peak torque and 16× power dissipation, winding temperature rises rapidly from 25°C ambient to its 130°C rated value in only 25 sec. Meanwhile, the case temperature only rises to 30°C. In effect, for 16× power dissipation, winding heatup is adiabatic with little heat power transferred to the case during the first 25 sec.

In the next 70 sec, the winding approaches 280°C and is now in the process of burning up while the case has barely reached 40°C. In contrast, if one uses the two-parameter model to calculate the entire motor's dynamic temperature rise, the windings are inaccurately estimated at less than the 130°C rated value at 70 seconds.

Sensor limitations

Servomotor manufacturers must decide on what type of temperature sensor to use: thermocouple, thermistor, or temperature switch. Along with the drive, this sensor is supposed to protect the motor from overheating during all possible modes of operation — but not cause nuisance motor shutdowns.

Many commercial drives only interface with temperature switches and not thermistors or thermocouples. In addition, most modern drives use the pulse width modulation (PWM) to produce output voltage and current … and PWM drives are electrically noisy. Here, accurately measuring dynamic winding temperature with a thermocouple is difficult, due to this electrical noise. For these reasons, many servomotors contain either a temperature switch or a thermistor mounted inside the motor.

Location

Where are temperature sensors best installed inside a motor? The four-parameter thermal model shows how motor windings heat fastest, which might suggest that sensors should be mounted on motor windings.

Consider this: Many manufacturers tout UL 1004 and CSA 22.2 adherence for their motors. To meet these standards, electrical insulation must be constructed in compliance with the UL 1446 Insulation System Standard. As described in UL 1446 Section 4 and Table 4.1, the winding's maximum allowable hotspot temperature, occurring at any point and time, is determined by the insulation Class used to construct the winding. Therefore, to comply with UL 1446, a winding's insulation must have a maximum hotspot temperature equal to or greater than the maximum continuous winding temperature.

To provide an over-temperature safety margin during peak torque output, it makes engineering sense to construct motor windings using a higher Class insulation such that the winding's maximum hotspot temperature is never exceeded, or UL 1446 violated.

To keep a servomotor in compliance with UL 1446, shouldn't the temperature sensor also be placed at the point in the motor's electrical winding where its hotspot temperature occurs? In fact, this isn't always possible, especially in smaller 20 to 90-mm-diameter servomotors. As mentioned, many servomotor drives only interface with temperature switches, such as a Thermik SO1.

Given the physical size of a temperature switch and the packing density of these motors' electrical windings, manufacturers often attach the switches to winding end turns. However, the end turn doesn't always correspond to the winding's hotspot location.

In fact, in many 20 to 60-mm diameter-servomotors, manufacturers locate the temperature switch inside the motor but (due to physical size of both the switch and the winding) this “normally closed” switch isn't attached to the winding at all, so the winding's dynamic temperatures are not measured directly.

Other servomotor manufacturers also specify motors with Class B (130°C) or Class F (155°C) insulation but specify 130°C or 155°C as the maximum continuous winding temperature. In addition, they can specify 4:1 or even 5:1 as the peak to continuous torque ratio, though this provides no safety margin between the winding's maximum continuous and its maximum hotspot temperature.

Four-parameter thermal model plots show how a servomotor's electrical winding heats up most quickly. As motor peak torque output increases above the 1× continuous value, the dynamic temperature difference between the winding and case becomes increasingly greater … and above 2× peak torque, the initial winding heatup is adiabatic and the rise in case temperature lags behind.

Therefore, with no safety margin between the winding's maximum continuous and its hotspot temperature actual measurement shows that for greater than 2× peak torque output, it's extremely difficult (if not impossible) for a motor temperature sensor not directly attached to the winding to react quickly enough to prevent (with the drive) exceeding winding maximum hotspot temperature — and violating UL 1446.

Even if a temperature sensor is attached directly to the windings, it may not be at the winding's hotspot location. Here, the temperature gradient common in servomotor windings can prevent the temperature sensor from detecting the dynamic rise in hotspot temperature fast enough to prevent the maximum allowable value from being exceeded — and UL 1446 from being violated.

A full list of references and supporting documents is available from the author at welch022@umn.edu. For more information, call (952) 368-3434 or visit exlar.com.