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Required: Hotspot safety margin

As mentioned, the two-parameter model isn't accurate enough to calculate dynamic winding temperature when more than 1× maximum continuous current value is supplied to the motor; windings heat up much faster than calculated by the two-parameter model.

In fact, even the four-parameter model isn't perfect. Although it differentiates a separate dynamic winding temperature, the entire winding is assumed to have one uniform temperature value — which isn't realistic. That said, four-parameter accuracy is sufficient in most cases, including for hotspot safety margin calculations.

Few servomotor manufacturers publish the four-parameter thermal model values for brush and BLDC motors. Therefore, it's reasonable to assume that most perform motor sizing and dynamic winding temperature calculations using the two-parameter thermal model. Because manufacturers generally publish only one value for both the motor's winding to ambient thermal resistance plus its thermal time constant, motor users tend to use this two-parameter model in making dynamic temperature calculations — unless they measure the more reliable four-parameter values themselves. This is rather easy.

As shown in Fig. 1, sizing and selecting the optimum motor for an application begins by defining the dynamic motion profile, along with the total ambient condition in which the motor will operate. Next, using the two-parameter thermal model in combination with the time averaged power dissipation technique, a candidate motor's RMS operation point is determined and entered onto its continuous torque-speed curve as shown in Fig. 2. If this RMS operation point lies outside the boundary of the continuous torque-speed curve, then this particular motor-drive combination will overheat and cannot be used unless the motion profile is modified (or the ambient condition is changed.)

Conversely, if the RMS operation point lies within the boundary of the motor's continuous torque-speed curve, then the motor may be suitable, but one more investigation is needed.

Figs. 3 and 4 show the four-parameter model: Even though the “time averaged” power dissipation technique and two-parameter model indicates that the winding's maximum continuous temperature shouldn't be exceeded, the four-parameter model shows (and actual measurement proves) that during times of peak torque output, maximum continuous winding temperature can be exceeded.

Therefore, for the best performance and UL 1446 protection, the insulation Class of a servomotor's electrical winding must allow a hotspot temperature that is greater than maximum continuous winding temperature … and the greater this hotspot temperature safety margin, the better the protection.

Some servomotors have a 130° C maximum continuous winding temperature and Class H insulation, for a 180° C maximum allowable hotspot temperature — and a 180° - 130° C = 50° C hotspot temperature safety margin.

These servomotors are also specified with a 2:1 peak-to-continuous torque ratio; combined with their 50° C hotspot temperature safety margin, this provides these motors with the highest possible thermal protection during times of peak torque output.

In comparison, many BLDC servomotors have a hotspot temperature safety margin that's 15° C or less (many have zero margin) and harsh peak-to-continuous torque ratios between 3:1 and 5:1.

References and supporting documents are available from the author at welch022@umn.edu. For more information, call (952) 368-3434 or visit exlar.com.

Topics covered

• Winding temperature and torque
• Models and their accuracy
• Temperature variations
• Safety margins