Keeping the motors cool in medical hand tools

Electrical heat losses

When DC motors deliver torque they require current. This current causes the motor winding to heat up. The more torque the motor produces, the more current is needed and the higher the winding temperature.  In continuous operation, the motor winding can reach temperatures up to 155°C, which results in a housing temperature around 120°C. These sorts of temperatures are much higher than the human skin can tolerate, around 50°C is about the limit. The electrical heat losses increase with the square of the current.

However, if a motor is running at only half its nominal current, then temperatures are more moderate (usually under 50 °C) and suitable for contact with human skin. For motor selection, this usually means a need to oversize it. However, the hand tool must be easy to control and hold, so the smaller and lighter the motor, the better the device feels when in use. So merely oversizing the motor is not necessarily the best option.

These considerations are based on continuous operation, where the maximum temperature is only reached after 10 minutes or so. Hand-held devices, however, are usually worked in intermittent operation, with the on-time measured in anything from a few seconds to a few minutes. This helps with the selection of smaller motors, but one still must consider the continuous operation point. The continuous operation point is based on the effective load current (RMS, the root mean square) over the entire load cycle. The mean heat build-up is equivalent to that caused by continuous operation with the RMS load torque.

Iron losses 

A motor can become too hot to touch, even at no load, i.e. the current draw and electrical heat losses are very low. The heat comes from the iron losses (Eddy current losses). Eddy current losses increase with the square of the motor speed, irrespective of load/current. For hand-held devices, this can become a problem with grinding and milling tools that run at speeds of several tens of thousands of revolutions per minute (rpm). High-speed motors are specifically designed to minimise eddy current losses. They are typically built with fewer magnetic poles, ironless windings, and ultra-thin iron plates with a low hysteresis in the magnetic return. As an example our own maxon ECX SPEED programme combines these special characteristics. With their long design and diameters from 16mm to 22mm, these brushless DC motors for hand-held devices operate at speeds significantly upward of 10,000 rpm.

PWM control and motor inductance

Heat generation in a motor depends on more than just torque, speed and construction. It also depends on the design of the pulse-width-modulated (PWM) controller and the setting of the control parameters. Ironless windings have a very low inductance, which results in a low electrical time constant. As a result, the current responds rapidly to changes in the voltage, which is desirable if dynamic behaviour is a design goal. However, if such a motor is controlled with a PWM output stage, which is what most controllers have, then the motor current follows these fast voltage changes.

This can cause a large current ripple. While PWM voltage and current ripples do not affect the mechanical behaviour of a motor, the motor basically "sees" only the mean current and voltage. Peak currents in the ripple cause the motor to heat up. Similarly, rigid control loop settings cause strong, rapid current responses with a corresponding heat build-up. 

Possible countermeasures to minimise current ripples are: 

Reduce the supply voltage of the PWM power stage as far as speed requirements and the application type permit.

Increase the PWM frequency to give current ripples less time to develop.

Install an additional inductance (motor choke) in series with the motor connectors. This increases the electrical time constant and attenuates the current response. This last option is not very attractive, as it increases cost and requires extra installation space.

Select soft control parameters. Our own controllers account for the low inductance of our DC motors. They operate at high PWM frequencies between 50 and 100 kilohertz and have sufficient additional inductance for most motors and situations. 

It may be possible to lower the temperature even further by decreasing the supply voltage until it was close to the required minimum.

Case study

A UK based medical hand tool manufacturer was having problems with excessive heat build-up when the motor was running at high speed. They contacted us, and ultimately, the solution came in 2 parts:

1.     We correctly tuned the motor using our latest motor driver. All our motor drivers have an autotune function that runs through a series of moves and calculates the optimum tuned parameters.

2.     We reduced the PWM voltage, so it was just high enough to reach the maximum required speeds. This is calculated by using both the speed constant (kV rpm/v) and the speed/torque gradient (rpm/mNm).

When using motors in high-speed medical hand tools, it is always going to be a challenge delivering the desired power requirements, without the generation of excessive heat. Here, we specialise in the development and production of high-speed motors and gearboxes for such applications. By following a few simple guidelines, and by using motors with the highest power density, it's possible to meet all the design requirements.

Our motors perform in high-precision devices such as active implants, insulin pumps, surgical robots, power tools, respirators, ventilators and prostheses. Drive components for medical technology applications must meet demanding requirements. Precision, sterilisability, smooth running and long service life, as well as low heat build-up in DC and EC drives are essential. In close partnership with our customers, we strive to develop the suitable drive systems based on a modular approach, a standard solution or the creation of a fully customised solution tailored to specific needs. maxon is ISO 13485 certified. Discover more about motors for medical devices here.