Servos versus induction motors: Look at performance, costs
The decision to use servos versus induction motors ultimately depends on the level of performance required by the application and costs. The principal strengths of induction motors are that they are simple, low-cost, and represent a very mature technology. Induction motors are also comparatively affordable, straightforward in terms of on/off control, simple to wire, and offer a wide variety of product selection with many vendors able to deliver. In terms of drawbacks, these motors offer limited position control and are typically larger in size.
Higher performance, smaller size
Servos, on the other hand, are more dynamic motors that include a feedback device, such as an encoder or resolver, to control speed and position accuracy. The main strengths of servomotors include much higher performance, the ability to deliver higher speeds, smaller size, and a wide variety of supplementary components. Of course, servos are slightly higher in cost due to the more advanced technology in play. High speeds and torque performance can be limited occasionally by servo drive update time.
Typical squirrel-cage induction motors represent a low-cost choice for velocity control for applications, such as constant speed conveyors, sorters, or similar transmission systems that have reasonable constant loading. Because induction motor torque is generated by percentage of slip, they tend to have a limited flat torque region based on speed when compared to servos.
Common three-phase induction motor applications include machine tools, cranes, pumps, fans, robot applications, and others. In such applications, a synchronous servomotor could be “overkill” relative to the costs involved. However, both solutions clearly have their place. Depending on the application, servos may still be required based on other performance criteria, such as repetitive robust indexing with repeatable positioning and/or higher velocity accuracy. Other instances of where servo positioning systems are necessary include applications that require a range of supply voltages (such as 115 V ac to 480 V ac).
Accuracy justifies cost
As requirements for accurate positioning, higher speeds, and robust indexing moves with limited dwell time become more critical, it’s easier to justify the additional cost for servos to achieve more accurate positioning control. Typical servomotor applications include higher performance machinery for packaging, metal forming, CNC, woodworking, robotics, and more.
Advanced servomotors available today feature a low rotor moment of inertia and a very high overload capacity. Salient pole-wound technology gives rise to a high copper space factor, which helps attain high continuous torques. Very small end turns result in a small overall length. Fully potted stator provides for a thermally ideal binding of the winding to the motor housing. Potting also mechanically protects winding wires against vibrations.
One-cable technology is another advanced feature where power and process data are transmitted in one standard motor cable instead of two cables. Encoder data, rotor position, multi-turn information, and the status of the thermal conditions in the motor are transmitted reliably and free from interference via a digital interface. The benefits here include significant cost savings, since plug connectors and cables are eliminated at the motor and the controller ends.
– Bob Swalley is motors and drives specialist, Beckhoff Automation. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, firstname.lastname@example.org.