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

Josh Pozner | November 6th, 2017

When looking to spec a motor for your application, do you select an induction motor or a servo motor? What are the advantages and disadvantages of each? Let’s break down the benefits of both to see which motor technology is best suited for your application.

The two main factors to consider when choosing between servo motors and induction motors are of course performance and cost. We can evaluate the performance of a motor by looking at its torque density, inertia, dynamic performance, and how easy it can be controlled by a VFD. For your motor application, you will want to get the best performance at a the lowest possible cost. Now let’s delve deeper into the performance of servo and induction motors.

Offset gearing with servo motor
A KEB servo motor with integral gearing. Servos tend to have a long narrow body.

 

Servo Motor Advantages

Permanent magnet motors, or servo motors, have been used in applications in the machine tool industry due to their easy motion control for both rotary and linear motion, as well as their high maximum speeds and short acceleration times. The torque density is also a highlighting feature of the servo motor solution. Servo motors can produce between 40 and 60% higher torque capacity than the equivalent-sized induction motor. The motor compactness is particularly advantageous for machines where weight and footprint size are critical.

permanent magnets in servo
Permanent Magnets provide increased power density

The rotor size of the servo motor is also smaller in diameter than that of its equivalent induction motor counterpart, leading to smaller inertia. This makes servo motors particularly attractive for dynamic motion control profiles.

Servo motors will provide full torque at zero speed. This is not the case with a line started induction motor. Applications that require full load at zero speed like a dynamometer or winder will benefit from this operating characteristic.

Advancements in VFD technology have allowed for the increase in usage of servo motors, because now manufacturers can offer VFDs with the capability to do complex current calculations and rotor pole identification in real time. They can also offer solutions encoderless configurations, such as the KEB SCL solution.

 

 

Most servo motors are rated IP55 or IP65 as standard. This is a higher IP rating than traditional ODP or TEFC motors that are going to be IP44 or IP54.

fanless servo motor
Servos like this one do not typically have a fan. One less point of ingress means the IP rating is typically higher.

Servo Motor Disadvantages

As far as costs go, servo motors have traditionally been more expensive due to the permanent magnet material cost. But the cost gap between servo and induction motors has been shrinking for years. It used to be that a servo system was mabye twice the cost of an induction motor system. That difference has now shrunk to maybe 10-40%.

In some applications the servo motor advantages become disadvantages. Machines like a crusher might benefit from a higher inertia motor that can ride through torque impulses. Also, applications that do not require speed or position feedback can be more easily solved with a line start induction motor. There are some line fed PM servos out there but a majority will require the use of a servo drive or amplifier.

Servos can be optionally equipped with brakes but they are mainly for holding or e-stops. Having said that the size of the brakes offered with servos might be limited. Also, brake options like hand releases or microswitches might not be readily available. Another important point is that some Servo motors use Permanent Magnet brakes. While PM brakes are power-off, they are not considered “failsafe” and might not be the best option for safety-related applications. It is possible to equip a servo with a spring-set brake but its a point worth considering.


Induction Motor Advantages

Three phase squirrel cage induction motors have historically represented the premium low-cost choice for simple, single speed applications, such as material handling conveyors, rotary turntables, fans and other simple systems. Although the cost difference of a servo motor in comparison to a similarly sized induction motor is not nearly what it used to be 10-20 years ago, induction motors still have their place in certain applications, and a servo motor is not always the right replacement for an induction motor if you can justify the added cost.

For example, if you need to maintain a constant speed at a large load, say for applications greater than 30 or 40HP, the induction motor would likely be the better choice. This is due to the high costs of producing the large number of permanent magnets needed for such a large motor, coupled with the fact that quick accelerations, high dynamic speeds, and accurate positioning aren’t requirements for the application.

Also, although vector driven induction motors do require feedback controls, or encoders, to compete in performance to servo motor control, advancements in drive technology now allow us to run the motor in closed loop vector mode without the need of an encoder. KEB calls this its ASCL drive technology.

induction vector motor encoder
Vector motor control still offers positioning and precise speed control capabilities


Induction Motor Disadvantages

In general, Induction motors will be larger and have more inertia than Servo motors. So they will not be the best option for highly dynamic applications although they can do positioning when equipped with the correct feedback device.

Induction motors will be slightly less efficient than servo motors, particularly at smaller sizes. However, with recent legislation, the required efficiency levels have been increased and the gap between Servos and Induction motors has been reduced.


Conclusion

Both induction motor systems and servo systems can be the right fit for your application depending on your performance and pricing needs. Induction motors can offer lower cost, reliable, and rugged solutions for single-axis applications with simple motion profiles. Servo motors can offer high dynamic performance and torque density, but at a premium cost.

Here at KEB, we are glad that we can offer both induction motor solutions ranging from .25 to 50HP and servo motor solutions ranging from .25 to 20HP. We can also offer both the induction and servo solution with integral gearing.

We have helical inline, right angle helical bevel, right angle helical worm, parallel shaft, and planetary gearing available. If you don’t know what solution is best for your application, we are more than happy to help you spec the correct motor.

Please contact a KEB representative near you if you have any application questions or need more information.

 

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