What are the advantages and disadvantages of each? 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. 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. The motor compactness is particularly advantageous for machines where weight and footprint size are critical. The rotor size of a servo motor is typically 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 where fast cycle times are desired.
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 servo drive technology have allowed for the increase in usage of servo motors, because now manufacturers can offer drives with the capability to do complex current calculations and rotor pole identification in real time.
KEB even offers the ability to position a servo without any feedback. This current creates a magnetic field in the rotor and the interaction between the stator field and the induced field drives the motor. Induction motors are made to operate with both single and poly-phase currents; latter for heavy duty machines that require a large torque.
The speed of the induction motors can be controlled using either number of magnetic poles in the stator pole or regulating the frequency of the input power source. Since the short-circuited rotor windings have small resistance, a small slip induces a large current in the rotor and produces large torque. Yet the rotational speed of the rotor is slower than the input power source frequency or the rate of rotation of the stator field.
Induction motors do not have any feedback loops for control of the motor. Technically, a servo motor is any motor that has feedback and closed loop control, and it is only a part of a servo mechanism in which the negative feedback is used to control the performance of the motor. But, commonly used industrial servo motors are normal AC induction motors with added features such as Low inertia rotor, high torque brake, and inbuilt encoder for speed and position feedback. This, of course, is based on similar electronic controls being used with the latest technology and approximately the same cost , leaving the cost of motors the differentiating issue.
Thanks to Frank Bartos, Control Engineering contributing editor, and to the four authors below, for the information above. The articles below were in the cover story in a shorter format. Links to the longer online versions follow. Permanent magnet motors outperform induction motors in many applications. Permanent magnet synchronous motors offer significant advantages on high-energy-consuming and high-dynamic applications, compared to induction motors.
See table, photo gallery. Permanent magnet servomotors versus induction motor systems. Motor selection: When deciding on the use of permanent magnet PM servomotors or induction motor systems, consider dynamic performance, torque density, inertia matching, and control schemes. Servos versus induction motors: Look at performance, costs. Choosing between servos and induction motors depends on the level of performance required by the application and costs. This theme may be out of date.
Run the support tool in the AdminCP to restore the default theme. Sign in to follow this Followers 0. Share this post Link to post Share on other sites. To my humble technicians understanding I have always classified motors as one of the following types. Stepper Motor 2. Induction Motor 3. DC Motor 4. Servo Motor The Stepper required a special controller and moved in dicrete steps at all times even if the were very small and very fast recurring.
The Stepper offered the greated accuracy of position, The Induction Motor was the AC powered continuously running motor. USed most often for belts, screws and other conveyance devies. The Servo Motor was either an AC or DC motor with an encoder attached so that position control and or speed control could be achieved.
As their technology improved Servo's were often used in applications previously reserved to Steppers to save money. VFD drives in the latest generations from the major manufacutrers have added servo like functionality. Technically, a servo motor is any motor that has feedback and closed loop control. If the motor has an encoder and closed loop control, its a servo motor.
So technically, attaching an encoder or resolver or other feed back device to a general purpose three phase squirrel cage motor and closing the control loop makes it a servo motor. Its a matter of semantics, but usually when people think of a servo motor they are meaning a motor that is made to much more exacting tolerances and is designed to have a lower rotor inertia yet also be able to withstand larger currents than a similar power general purpose motor.
As has been mentioned, it can be a squirrel cage induction motor or a DC permanent magenet motor.
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