Servo Motors versus Induction Motors

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Motors are the class of electromechanical devices which convert the electrical energy into mechanical energy. In some applications, pure torque is needed to drive a mechanism, and in some applications, the position and the rotational speed of the mechanism have to be controlled. Induction motor delivers pure uncontrolled torque, while servo motor driver deliver controlled torque, where speed and the position of the shaft (rotor) can be adjusted.

More about Induction Motors

The Induction Motor is perhaps the most common type of electric motor in the world. It does not have a commutator or brushes. In general the less moving parts and more simple any device is, the better the longevity. This type of motor is powerful and efficient. It is used in newer diesel trains, industrial applications, pumps, compressors, fans, dishwashers, and countless other things.

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. The slip, which is a measure to determine the motor’s torque, gives an indication of the motor efficiency. 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.

More about Servo Motors

A servo motor is a dc, ac, or brushless dc motor combined with a position sensing device(e.g. a digital decoder). In this section, our discussion will be focused on the three-wire DC servo motors that are often used for controlling surfaces on model airplanes. A three-wire DC servo motor incorporates a DC motor, a geartrain, limit stops beyond which the shaft cannot turn, a potentiometer for position feedback, and an integrated circuit for position control.Of the three wires protruding from the motor casig, one is for power, one is for ground, and one is a control input where a pulse-width signals to what position the motor should servo. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. As the coded signal changes, the angular position of the shaft changes.

Servos are extremely useful in robotics. The motors are small and are extremely powerful for thier size. A standard servo such as the Futaba S-148 has 42 oz/inches of torque, which is pretty strong for its size. It also draws power proportional to the mechanical load. A lightly loaded servo, therefore, doesn’t consume much energy. The guts of a servo motor are shown in the picture below. You can see the control circuitry, the motor, a set of gears, and the case. You can also see the 3 wires that connect to the outside world. One is for power (+5volts), ground, and the white wire is the control wire.

Servo motors can deliver high torque and the position and the speed of the motor can be controlled. Therefore, servomotors are extensively used in robotics and control systems related applications.

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).


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