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Simple, non-contacting motor speed measurement

25 June 2013

Motor speed is usually measured by means of an internal sensor bearing or an optical encoder with fixed pulse counts, whereby a separate sensor or code disc is required for each pulse count. These are complex and rather inflexible techniques, a fact that prompted Contelec to take a fresh look at motor speed measurement.

When sensor bearings are used, the electronic unit is frequently not rugged enough and so is susceptible to external causes of malfunction. In addition, the time involved in servicing internal sensors and consequent loss of availability in the event of a sensor malfunction brings considerable cost implications.

The use of optical encoders and encoder modules requires precision installation, which in turn demands precision bearings and extremely accurate alignment of the code disc to the optical module.

However, temperature fluctuations which are common in motor operation – high temperatures while running, cooling while stationary – cause material expansion in encoder components which is far in excess of the installed tolerances for optical encoders, bringing the risk of failure.

Their use is further restricted by their vulnerability to dust, with the concomitant risk of failure.

Just as critical is the fact that customers are restricted to the pulse counts provided by individual manufacturers’ products. Then, if production of the sensors ceases and they are no longer supported, users are faced with the high cost and not inconsiderable time involved in finding a suitable replacement.

An alternative approach
To tackle this problem, Contelec has developed a novel sensor concept that measures the speed of the main shaft without making any any physical contact. The system comprises a magnetic actuator affixed to the shaft and a slim, ruggedly housed (IP68) sensor unit attached to the reverse of the motor or to the fan housing.

All that is required in order to attach the magnetic actuator is to tap a hole in the main shaft. Once attached, the sensor unit (designed to have a negligible influence on the air flow) needs only to be aligned.

To simplify installation, Contelec supplies pre-configured sensors; a broad range of pulse counts may be configured without having to change the magnetic actuator in each case. While simple in concept, these flexible motor speed measurement systems nonetheless have some impressive features.

A 12-bit resolution is possible, and pulse counts for incremental output (pulses per channel and rotation) are 1 to 128, 256, 512 and 1024. The incremental high speed operation is greater than 10,000rpm at maximum resolution. A choice of incremental or SSI interfaces are offered, as are TTL, RS422, HTL or OC output stages.

Customer benefits are numerous; apart from the non-contacting nature of the technology, the system is easy to install in existing motor driven equipment (without having to worry about the axial and radial offsets of optical encoders) and just as easy to upgrade using a basic upgrade kit supplied by Contelec.

The system is available in the UK through Variohm Eurosensor.

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