For the magnetic variety, a sensor reads north-to-south-pole changes along the circumference of a magnetized wheel,
again resulting in an electronic pulse.
Most VFDs operate with an encoder input of 1024-PPRs
(pulses per revolution) and provide superior motor performance. 1024-PPR is a good balance between encoder reliability and smooth speed control, and is ample resolution to
control the motor smoothly, even at low speeds.
Making the Right Choice
Choosing an encoder for speed control applications typically
involves several criteria: resolution (PPR), mounting method,
electrical specification, and environmental sealing. Once resolution is determined (usually dictated by VFD requirements),
mounting is typically the next selection criteria.
For motor control, common sense suggests the selection of a motor-mount encoder. A variety of motor-mount
encoders exist, and the most popular are hollow-shaft units.
The hollow-shaft encoder rides directly on the motor shaft (or
stub-shaft), secured by an anti-rotation arm known as a tether.
This eliminates the need for brackets and couplings that are
prone to failure, and simplifies installation and service.
When selecting this type of encoder, electrical isolation
from the shaft is an important feature. Electrical current that
Typical encoders output a pair of signals that can be increased
in resolution by counting both the rising and falling edges of the
waveforms.
can build up on the rotor wants to find its way to ground, and
without an isolated encoder bore it can pass through the encoder’s bearings – not a desirable situation with optical encoders.
This current can ultimately damage the bearings and cause
premature failure. In addition, an isolated bore helps prevent
unwanted heat conducted from the shaft to the encoder.
Electrical requirements, such as voltage and current specifications, and output signal type are the next major pieces of
the puzzle. The type of cable, cable length, and drive requirements all play a role, but typically differential line driver outputs are the preferred choice because of their ability to support
a wide voltage range and provide a clean, reliable signal over
long cable runs and reject noise.
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