How to mitigate heating in a handheld surgical tool

Here are two main sources of power losses, and hence heating, when it comes to handheld surgical tool motors.

Urs Kafader, Maxon Motor

Motors operated at the rated torque limit can get very hot. In continuous operation, the winding can reach 155 °C, resulting in a housing temperature of some 120 °C. No surgeon would like to operate with a hand-held tool not even at half of that temperature. Neglecting friction, there are two main sources of power losses, and hence heating, in motors.

Source 1: Joule power and iron losses

The Joule power losses are linked to the current, i.e. the required torque load. As it is well known, these losses increase with the square of the current. High currents close to the nominal value will result in temperatures unbearable for humans to touch; running the motor at currents of about half the nominal current results in moderate temperatures (typically below 50 °C) that matches the sensitive human skin.

For motor selection, this essentially means you should go for an oversized motor! Be aware, however, that we consider here continuous operation where the maximum temperatures will only be reached after some 10 minutes. In hand-held tools, one usually has intermittent operation that can expand to 30 minutes and more. The heating is according to RMS average load including dwell times.

The iron losses are related to speed. Eddy current losses increase with the square of speed, heating up the motors simply when rotating – even in a no-load condition. In hand-held tools, this can be a problem for grinders and drills that operate at several ten-thousand rpm. Such high-speed motors need special design precautions to limit eddy current heating. Typically, they are made with a low number of magnetic poles, a slotless winding, and ultra-thin back iron foils made of special low hysteresis materials.

Source 2: PWM driver and inductance

It turns out that heating is not only a question of torque, speed and design but also of the driver. Some users have experienced high temperatures (80°C and more) of a motor even when driven at no-load conditions. In those cases, often the driver and the supply voltage have a major effect.
Slotless windings have a very low inductance resulting in a very low electrical time constant. The current will react very fast; that’s good for dynamic behavior. However, when driven with pulse-width modulated (PWM) power stage (as most controllers do) the motor current is able to follow these rapid voltage changes giving rise to a considerable current ripple. Be aware that the PWM voltage and the current ripple have no effect on the mechanical response of the motor. The motor reacts according to the average current and voltage values.

The current ripple peaks, however, heat up the motor. Counter-measures for minimizing the current ripple are:

• Reducing the supply voltage of the PWM driver if possible by the speed requirements of the application.
• Increasing the PWM frequency to allow less time for the current ripple to build.
• Adding an extra inductance – a motor choke – in series to the motor lines in order to increase the electrical time constant and to dampen the current reaction.

Maxon controllers take the low inductance of Maxon motors into consideration. They work at high PWM frequencies of 50 to 100 kHz and are equipped with sufficient additional inductance for most motors.

The heating problem might be easily resolved by replacing the old over-dimensioned controller with a controller made for lower power with a larger built-in inductance that operates at a higher PWM frequency. The largest effect on temperature, however, is gained by reducing the supply voltage close to the minimum value needed.

Urs Kafader is head of training at Maxon Motor.