Touching Desired - So DC Motors Stay Cool

A DC motor that is operated near the nominal torque can get very hot. In continuous operation, the winding reaches temperatures of up to 155 ° C, which results in a housing temperature in the range of 120 ° C. In surgical handheld devices, DC motors must not become so hot. What shall we do? If you neglect the friction, there are two main sources of loss that heat up the engine: electricity heat losses and iron losses.

When DC motors get too hot

The electricity heat losses are required with the proportional to the electricity

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The considerations up to this point are based on continuous operation, in which the maximum temperatures are only reached after around ten minutes. In handheld devices, however, you are usually dealing with intermittent operation, which can take up to 30 minutes and longer. This means: A continuous operation analysis must also be used here, however with the effective value (RMS) of the load current (quadratic averaging over the entire load cycle). The mean heating then corresponds to continuous operation with the RMS load torque.

DC motor heats up even without load

The iron losses are linked to the speed. The eddy current losses increase quadratically with the speed and warm the motor when turning - even without load. In handheld devices, this can be a problem with grinders and cutters that operate at tens of thousands of revolutions per minute.

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Such high-revving motors require a special design to keep eddy current losses small. They are typically designed with a small number of magnetic poles, an ironless winding and ultra-thin iron sheets with deep hysteresis in the inference. The ECX Speed ​​program from Maxon combines these special properties. The brushless DC motors with their long design and diameters between 16 mm and 22 mm fit perfectly in handheld devices that are operated at high speeds of several tens of thousands of revolutions per minute.

PWM control and inductance

However, it turns out that engine warming is not just a question of torque, speed and design. It also depends on the design of the PWM control and the setting of the control parameters. Recently a customer complained to Maxon Motor about his hot engine (80 ° C and more) even when idling. A more detailed analysis showed that the control and the supply voltage had a significant influence.

Ironless windings have a very low inductance, which results in a small electrical time constant. Accordingly, the current reacts very quickly to changes in voltage; that's good for dynamic engine behavior. However, if the motor is controlled with a pulse width modulated (PWM) output stage (which most controllers do), the motor current follows the rapid voltage changes - which can lead to a large current ripple. While the PWM voltage and the current ripple have no influence on the mechanical behavior of the motor - the motor essentially “sees” the mean value of current and voltage - the current peaks of the ripple heat up the motor. Similarly, stiff control loops lead to strong and fast current reactions with corresponding heating.

How a DC motor works:

Tips to keep the current ripple small

Countermeasures to keep the current ripple small are:

• Reduce the supply voltage of the PWM output stage in cases where this is possible due to the speed requirements of the application.
• Increase the PWM frequency to give the current ripple less time to train.
• Install an additional inductor (motor choke) in series with the motor connections. This increases the electrical time constant and dampens the current response. This last measure is not very attractive. Because it increases costs and requires extra space.
• Select control parameters that are as soft as possible.

The Maxon controllers take into account the deep inductance of the Maxon DC motors. They operate at high PWM frequencies from 50 kHz to 100 kHz and are equipped with enough additional inductance for most motors and situations.

The book "Practical Manual Drive Design" helps in the selection of the essential components of electrical drive systems: motor, gearbox, actuator, mains supply and their additional components. The calculation is also dealt with intensively.

The customer's temperature problem was quickly solved: it was sufficient to replace his oversized control with an Escon controller from Maxon. The Escon solution has less, but enough power. It works with a higher PWM frequency than the existing controller and contains a larger built-in motor choke. That alone would have done a lot. But the temperature could be reduced even further: To do this, the supply voltage was brought down to near the absolutely necessary minimum.

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The world of DC motors

High demands on DC motors for surgical power tools

When surgeons perform an operation in the operating room, they use battery-operated tools. These have to be precise and reliable and have to survive over a thousand sterilization cycles. This also places high demands on the micromotors. The hand-held surgical devices are visually very similar to machines from craftsmen, but they have to meet much higher requirements in terms of precision, heat generation and vibration. Reliability is also an important point. The same applies to the DC motors that drive the power tools. Brushless DC drives are particularly suitable because they are characterized by a long service life and high speeds.

"The operating conditions for motors in surgical hand tools are brutal," says Anthony Mayr, Senior Project Leader at Maxon Motor. The drives have to withstand very strong vibrations. There are also high temperatures in overload conditions (either torque or speed) as well as contact with moisture and alkaline solutions due to the strict sterilization and cleaning requirements. Mayr explains: "Maxon's DC motors and gearboxes work under all these conditions." The extensive developments and tests in Maxon's own laboratory also contributed to this.

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* Dr. sc. Nat. Urs Kafader is training manager at maxon motor ag.