Keep cool and reap the rewards

Linear motors offer designers a range of attractive benefits – but they are hot stuff in more than one way. We look at how you can calculate your cooling requirements and get the best out of this technology.

Machine designers are increasingly incorporating linear motors into packaging machines, electronic components assembly equipment and machine tools. They eliminate backlash, friction and machine jitter and give you non-contact motion, high speed operation, resolution and accuracy which are limited only by the position transducer and servo amplifier. Typically the system is more expensive than a rotary motor with a rotary to linear conversion mechanism but the performance advantages are considerable.

The major performance limiting factor of a linear motor is thermal stress. A properly cooled linear motor can produce 50% more force than a linear motor with no cooling.

Brushless linear motors and rotary motors are electrically identical. The linear motor’s forcer coil (primary) receives electrical current and travels over a permanent magnet secondary section. But in a linear motor the current flows through a smaller area than in a rotary motor building heat up faster. So many manufacturers recommend that you choose a motor with a heat sensor in the forcer coil.

Manufacturers’ motor data should give maximum coil temp of the motor. You will also need to know the ambient temperature of the operating conditions and the potential thermal stress in application. From this data you can establish whether the motor needs to be cooled and select the most appropriate cooling method: conduction, radiation or convection.

The cooling method can be determined by either building a prototype system and taking temperature measurements or by solving heat transfer calculations. A few commercial packages for heat transfer analysis are available.

The most effective cooling method is convection with forced water it is also the most expensive.

OTHER MOTOR COMPANIES

Linear motors are frameless brushless motors and require a bearing system and a mechanical superstructure. Except in extreme cases neither mechanical bearings or non-mechanical bearings add significant amounts of heat to the motor.

The most common feedback device on a linear motor is the linear encoder, offering accuracy down to sub-micron levels. For even higher resolution, laser interferometry can be used. Heat can degrade either feedback system, affecting the sensor and the system resolution. Linear scales expand when heated and the interferometer light wave length changes with temperature. So you must move the linear scale from the heat source or, if the motor is using forced air cooling use this system to cool the scale too.

Extracted from an article by Robert Novotnak, Robert Sobek & Stephen A Botos of Aerotech.