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Encoder is tailored to the needs of AC motors

28 July, 2007

Most incremental encoders fitted to asynchronous AC motors are general-purpose devices and may not be ideal for this application, says the German encoder specialist Sick l Stegmann. The company has therefore developed a high-resolution hollow-shaft encoder to cope with the idiosyncrasies of AC motor mounting.

Sick motor encoder

So, for example, the VFS60 encoder (shown above) has a larger bearing spacing to reduce runout, an isolated shaft to minimise stray currents, and a robust metal encoder disc.

All incremental encoders have two shaft bearings, usually close together. But this makes it difficult to produce an encoder without run-out, which will appears as ripple on the speed feedback. The new encoderís bearings therefore are widely separated to minimise run-out.

In most incremental encoders, the bearings are not electrically isolated, allowing stray currents in the motor shaft to pass through them to earth. Over time, this causes the bearings to become pitted and can lead to failure as encoderís disc starts to make contact with the pick-up electronics. To prevent this happening, the new encoder has an isolated shaft.

Conventional encoders usually incorporate either accurate, but fragile, glass code disks, or tougher, but less accurate, plastic discs with restricted operating temperatures. The new encoder uses a metal disk, said to produce as good a signal quality as a glass encoder but with the robustness of a plastic disc and an operating temperature range of -20 to 100įC.

The 60mm-diameter, IP65-rated encoder offers a choice of metric (10-15mm) or imperial hollow-shaft mounting options, and has a resolution of up to 10,000 lines (equivalent to 40,000 pulses/rev after multiplication).

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