Motor Encoder Units
Encoders for modern AC motors
Being able to accurately monitor the status of rotating components is critical in many applications. Modern AC motors require encoders to continuously monitor speed, position and direction of rotation. These encoders, which are typically external to the motor, are exposed to environmental conditions, which in some cases can have a significant impact on service life.
Integrated units from SKF
SKF motor encoders (fig), however, are mechatronic units that combine sensor and bearing technology. Available for shaft diameters ranging from 15 to 45 mm, these encoder units are mounted directly inside the motor and require only 6 mm of extra space in the axial direction. The sensor body, impulse ring and bearing are mechanically attached to each other, forming a compact, integrated, ready-to-mount unit that is shielded from external influences. SKF motor encoder units, which can be installed on the drive or driven side of asynchronous motors, provide a medium resolution signal between 32 and 80 digital pulses per revolution. They can accurately detect speeds from zero to 20 000 r/min.
Encoder units for harsh environments
SKF also has a series of motor encoder units designed for harsh operating conditions. For more information, contact the SKF application engineering service.
SKF Explorer class deep groove ball bearings
Motor encoder units use SKF Explorer deep groove ball bearings. These bearings are suitable for high speeds and are robust in operation. They can accommodate not only relatively heavy radial loads, but also axial loads in both directions and are typically used to locate the shaft axially. Furthermore they are noted for their high manufacturing accuracy, low noise level and low frictional moment. Efficient seals combined with the grease fill provide a relubrication-free solution for the service life of the bearing.
Active sensor design
The SKF motor encoder unit uses a compact and robust active sensor that produces an incremental encoder signal. The sensor is accurate down to zero r/min. Its main components include an impulse ring, a sensor body with sensors and a connecting cable.
A composite, magnetized impulse ring is attached to the bearing inner ring. Depending on the bearing size, it is divided into a number of north and south poles. The number of pulses per revolution ranges between 32 and 80.
The sensor body has two cells, enabling the direction of rotation to be determined. The two sensors are offset to each other in the sensor body. In a small integrated circuit they contain not only the Hall cell as an active element but also the electronics for signal amplification and conversion. The analogue sinusoidal signal generated by the Hall cell is amplified and converted into a square-wave signal by a Schmitt trigger (fig). The leading signal determines the direction of rotation.
Furthermore two sensors double the number of pulses. For example, a 6206 deep groove ball bearing will normally generate 64 pulses per revolution. However, with two sensors, the same bearing will generate 128 pulses per revolution. When counting the rising and falling edges of the pulses, a maximum accuracy of 256 pulses per revolution can be obtained, which corresponds to a resolution of 1,4 angular degrees.
The sensor requires an external voltage supply. The signal output is fed via an open collector circuit. Speeds down to zero r/min can be recorded.
SKF motor encoder units (fig) consist of
- an SKF Explorer deep groove ball bearing with a RS1 contact seal and a snap ring groove in the outside surface of the outer ring (a)
- a magnetized impulse ring (b)
- a sensor body (c)
- a connecting cable (d).
On the side opposite to the seal, the impulse ring and sensor body form an effective labyrinth seal.
The impulse ring is a composite, magnetized ring. The number of north and south poles (between 32 and 80) depends on the bearing size. The impulse ring is attached to the inner ring.
The sensor body protecting two embedded Hall cells is attached to the outer ring - a patented SKF solution. The multi-wire cable extends in the radial direction, connecting the motor encoder unit and the signal processing electronics and has a standard length of approximately 500 mm. To account for the different interface requirements, SKF can provide customized solutions with the connector and cable length that best fits the application. For additional information, contact the SKF application engineering service.
SKF motor encoder units are based on SKF Explorer deep groove ball bearings in the 62 series. The boundary dimensions comply with ISO 15:1998 except for the width. The unit is 6 mm wider than the ISO standard due to the integrated sensor.
The bearings used for SKF motor encoder units are manufactured to P5 tolerances (d = 25 mm) or P6 tolerances (d = 30 mm) as standard and conform to ISO 492:2002.
Bearing internal clearance
SKF motor encoder units have C3 radial internal clearance as specified for deep groove ball bearings in ISO 5753:1991.
SKF motor encoder units are designed to suit the limiting speed of the appropriate sealed bearing. If motor encoder units are to be operated at higher speeds than indicated in the product table, contact the SKF application engineering service.
SKF motor encoder units have a temperature range from –40 to +120 °C, as extensive long-term testing has proved. For temperatures constantly exceeding +120 °C and up to +150 °C, contact the SKF application engineering service.
Electrical interface data
Active sensor operation requires a regulated supply voltage from 5 to 24 V. Signal output is affected via an open collector (fig). Resistors inserted between the conductor (connected to the voltage supply) and the conductors for the output signals, limit the output current to 20 mA (table). The signal output features are shown in the table.
SKF motor encoder units can be used in systems functioning under the most arduous electro-magnetic environment such as described in the European Standard EN 50082-2.
Selection of motor encoder unit size
To determine the size of an SKF motor encoder unit for a particular application, use the same factors and calculation methods normally used for a standard deep groove ball bearing without a sensor (see section "Selection of bearing size").
Application of motor encoder units
Two bearings are normally required to support a shaft - a locating and a non-locating bearing. Since an SKF motor encoder unit is typically used as a locating bearing, the opposite shaft end may be supported by a non-locating bearing. If heavy axial loads act on the SKF motor encoder unit in both directions, it should be mounted in such a way that the heavier axial load acts on the bearing outer ring side-face opposite the sensor unit.
In accordance with general recommendations for a non-locating bearing, the inner ring should have a tight fit on the shaft and the outer ring a loose fit in the housing bore. The sensor connection cable emerging radially from the encoder unit determines the position of the outer ring relative to the housing. A sufficiently dimensioned cable duct must be provided in the housing or housing cover (fig). The cable protruding from the sensor body should be protected against excessive rotation by a radial notch in the housing, having a circumferential width of 9 to 15 mm.
The inner ring that has a tight fit, is normally located axially on both sides of the bearing, e.g. by a shaft shoulder, a distance sleeve or a snap ring. The outer ring side-face opposite the sensor should abut a housing shoulder. The axial location on the other side of the outer ring depends on the bearing size.
For encoder units with a bore diameter = 25 mm, the outer ring side-face with the sensor can be axially located either with
- a snap ring if the unit is only subjected to light loads or not loaded (fig).
- an end cover bolted to the housing and via a snap ring in the groove of the outer ring if heavier axial loads occur.
For larger encoder units the outer ring side-face with the sensor can be axially located either with
- a thin-walled slotted distance sleeve, that abuts the encoder unit on one side and a snap ring on the other (fig) or
- an end cover bolted to the housing.
The abutment dimensions are listed in the product table.
For more information, contact the SKF application engineering service.
During mounting, SKF motor encoder units must be handled with great care in order to avoid damage to the sensor unit and the connection cable. On request SKF can provide customized assistance to optimize the mounting and connection process. Contact the SKF application engineering service for additional information.
Lubrication and maintenance
SKF motor encoder units are sealed, "plug and play" bearing units that are filled with a polyurea grease. The grease was carefully formulated so that the bearing would not require relubrication under normal operating conditions (between –40 and +120 °C.) The grease fill is adapted to the bearing size.