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Introduction to Servo Motors for Industrial Applications

Mike Keefe | January 5th, 2021

As servo motors become more commonplace, it can be confusing to identify what they are exactly and when they should be selected.  This post will explain the unique features of a servo motor and how they benefit certain applications.

 

How is a Servo Motor Different than an AC Induction Motor?

AC induction motors and, in particular, three-phase squirrel cage motors have been an industry staple for decades.  They offer a rugged, low-cost design that is perfect for single-speed applications.  However these motors are not a universal fit, which is where servo motors, also known as AC synchronous or permanent magnet motors, come in.

Related Article: Servo or Induction Motor – Which is right for your application? 

 

A KEB induction motor with the stator (left) and rotor (right) beside each other for comparison.

 

Synchronous motors differ from induction motors in the construction of the rotor. The rotor is the rotating portion of the motor that contains the output shaft. While an induction motor has conductive bars that form a cylinder that look like a squirrel cage (hence the name), synchronous motors have rare earth permanent magnets in the rotor core. This difference in construction leads to many benefits for the servo motor.

 

What are the Benefits of Using a Servo Motor?

When selecting a motor, it is important to consider what is required for the specific application. Servo motors offer many benefits that may make them advantageous to an induction motor.

 

More Powerful and Compact

One of the most obvious benefits of using a synchronous motor is that they have a much higher torque density relative to induction motors. A servo motor of a similar physical size relative to an induction motor will typically produce 40-60% more torque. This means that to reach the required torque, speed, or power required by the application requires a servo motor that is smaller and more lightweight than its induction counterparts. This makes a PM motor ideal for applications with space and/or weight constraints.

 

Low Inertia for High Dynamic Response

Because a servo motor is more compact, it inherently has a lower inertia to comparable induction motors.  With its low inertia, the synchronous motor can accelerate and decelerate to/from its rated speed much faster.  It also allows for much more precise starting and stopping from full speed.  This makes a synchronous motor ideal for highly dynamic or motion control applications.

 

Full Torque at Zero Speed

Another important benefit of the PM motor is that it allows for full torque at zero speed continuously.  This is a stark difference to most induction motors where the low speed torque and stability are limited.  VFD adjustments (e.g. Voltage Boost) can be made for low speed operation but this creates additional motor heating and the performance is limited.   Thus if a holding torque at zero speed is necessary, or if the application requires running at low speeds, a servo motor (with feedback) would be necessary.

 

Higher IP Rated Protection

In addition to its benefits for motor control, servo motors often typically come with benefits in their housing designs.  KEB’s synchronous motors do not require a cooling fan which allow it to reach IP65 protection.  Induction motors on the other hand are commonly IP44 or IP54.  So if the motor is going in a harsh environment, a servo motor may be beneficial to prevent premature failure.

Increasingly Economical

Finally, since the servo motors offer so many advantages over the induction motor, you may be asking why anyone would ever select an induction motor.  The historical answer has been that servo motors have been significantly more expensive than induction motors.  And while it is true that servo motors are still more expensive, the price gap has been shrinking.  Synchronous motors of similar power specifications to an induction motor can now be found for as little as 10-20% more expensive.  In the past, the servo motor could be 2x or more the cost of the induction motor.  As servo motors become more commonplace, this price gap should also continue to shrink.

One thing to note is that induction motors can be fed off of the line (fixed 60Hz).  Servo motors will typically require a servo drive or amplifier to run them.  However, there are a number of other advantages that drives introduce including accurate motor shaft performance, increased protection parameters, and lower starting currents.

Related Article: 4 Factors to Consider When Sizing Servo Motors

KEB Servo Motors: An Application Advantage

KEB’s line of servo motors and servo drives allow the user to take full advantage of the benefits offered by servo motors.  Want to learn more about how a servo motor could benefit your application?  Contact an Application Engineer at KEB America today!

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