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interruptedz

difference between servo motor and ac squirel cage induction motor

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im a lil confused about the difference between servo motor and ac squirell cage induction motor. they both have encoders.and i i saw applications in positioning that both uses servo and ac motor in a same application. whats the difference in the theoretical sense and application wise.

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To my humble technicians understanding I have always classified motors as one of the following types. 1. 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 DC Motor was the DC 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.

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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. http://en.wikipedia.org/wiki/Servomechanism Now in practice the technical definition doesn't allways wash. 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. And technically, one of these precision motors, used without any feedback or without a closed loop control, is not a servo, no matter how special the motor is.

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thats not quite accurate sir, as far as i know, servomotor usually has permanent magnet (i have never encounter a servomotor without permanent magnet)and ac motor doesnt have any.dc motor has magnet(if i remember it right) but usually servomotor works in ac provided by an amplifier.thats why usually to check wheter a servomotor is good or not is to short two power leads(this is not powered) and rotate the motor(by hand) if it has resistance(i.e. stiff to rotate) then the servomotor is said to be ok.you cant do that in induction(ac squirell cage) motor. the ac squirell cage motor we talking about here is the one you usually start with softstarters or wye-delta starters or inverters.

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Someone better educate GE because they are using an AC squirrel cage motor with an encoder on their 300 series of VFD to achieve Servo function. I used them and they are every bit as good for the applications I used them in as DC servo.

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interruptedz, Alaric is technically correct, a servo has closed loop control and even an AC induction motor gain have closed loop control. However, I usually call AC induction servo motors by the technology used to control them like flux vector drives and motors. I normally use servo motor to describe DC or brushless DC motors but more often now I and must calling the motors by their technology too which is BLDC for a brushless DC motor. NOW GET THIS. EVEN A BLDC MOTOR IS NOT A SERVO WITHOUT CLOSED LOOP CONTROL. IT IS JUST A BLDC MOTOR, GET IT?

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The AC induction motor uses a transformer effect so that when voltage is supplied to the stator (motor base)coils, a corresponding magnetic field is generated (induced) in the rotor (motor shaft). The interaction of the two magnetic fields creates a force between the rotor and the stator. In basic terms, the rotor field tries to align itself with the rotating (AC sinusoid) magnetic field of the stator. Because of losses and the load attached to the motor shaft, the rotor will not keep up with the stator field when left at a steady frequency. The lag and losses are referred to as "slip". The typical DC Brushless motor uses the magnetic field of a permanent magnet in the rotor to interact with the stator magnetic field. The stator magnetic field is varied by varying the voltage to different sets of magnetic poles inside the stator. As several people above mentioned, servo control has very little to do with the motor being used. It is a control technique of using feedback and error to provide precise motion profiles. Any device that creates motion can be "servo" driven. Because DC brushless motors are the typically the fastest reacting rotary motor and the magnetic field of the permanent magnet is always present (no delays), it was easiest to develop precise control methods for these types of motors. Because they were typically used for servo applications, over time people began referring to DC Brushless motors as servomotors. Over time, techniques have been developed for compensating for the "slip" of the induction motor by varying the frequency (and waveform) of the supply voltage. Because the induction motor is easy to manufacture and large permanent magnets are very hard to come by, the use of induction motors for servo control solutions is very attractive where high speed change of direction and acceleration is not necessary. The advanced control techniques in use today allow these simple, inexpensive motors to do jobs that required DC control in the past. There. Clear as mud. Ian

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interesting question. I think the lines have blurred in recent years. How would you guys classify regular AC induction motors that are connected to a VFD that features sensorless positioning? Is the motor "regular" or servo depending on what is driving it?

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This is purely semantics, and people refer to things in terms that are not technically correct all the time, but servo refers to the control scheme not the components used in it. And "sensorless vector" drives are not truly sensorless. The sensors are just located inside the drive and not in the field with external connections. In the context of a servo control system, whatever actuator is used is the servomotor. It could be induction, DC Brushless, hydraulic, pneumatic, whatever.

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Interestigly, some of the first servo motors were steam motors. In the latter part of the 19th century as ships became much much larger it was impossible to turn the rudder from the wheel. The rudders of the larger ships had to be manned by a crew of several dozen men using block and tackle rigs to move the rudder in response to commands sent from the wheel house. This method obviously left a lot to be desired. Then a steam powered servo motor was developed to move the rudder - a servo in every right because it responded to command input from a wheel located in the wheel house with feed back and gain to move the rudder precisely to the commanded position. IMO, some of those early steam and pneumatic servos were some of the cleverest devices ever invented. Not necessarly complex, but very very clever.

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thanks pal, i also thought about it that way at first because im a "theory" guy, i know the term servo control means something about feedback and master slave thing.but the industry blurred it so much to make me confuse sometimes Well, newly graduated engineers here in philippines usually overwhelmed with math and theory but lacks applications.so here goes this site for us to learn.but if you have questions regarding control engineering and the maths, im glad to answer. just dont ask about screws and bolts check this site, it controls both "servo motor" and ac induction motor http://www.ssddrives.com/usa/Products/AC890/AC890.php credits to the poster from another site Edited by interruptedz

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Shame on Parker for that piece of advertising. I can certainly see where someone could get confused by that ad. Apparently the marketing department ignored or misunderstood the technical people (at least I hope so) to make it look pretty. If they would have left out the induction motor versus servo motor titles they would have been okay - velocity/torque control versus positioning control. It's pretty bad when the company selling the technology can't even describe it correctly.

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Virtually any motor that can be controlled can be made into a servo motor - we have fitted hydraulic motors with encoders and closed a servo loop on them with good success and even hydro turbines in generating plants can be turned into servo motors and this approach now allows exact positioning of the generator rotor such that optimum synchronization with the line is achieved prior to closing the breaker by controlling rotor position rather than looking at the generated sine wave. (note that this requires servo positioning of both turbine blades and wicket gates which are controlled with low pressure hydraulic pressure from the same water source that is driving the turbine) . . . reviewing the designs from the 20's and 30's and tearing those antique governors out and replacing them with a DSP based controller gave me mixed emotions. The real question should be - what motor technology is best suited to the application - assemble the speed torque characteristics of the various motor technologies - look at the requirements of the application, select the motor based on that criteria - add an encoder and a "current, flow, pressure" amplifier with suitable bandwidth and control it with a servo loop and you have a servo motor. We have developed a huge database of induction and servo motors from a dozen or so manufacturers - all are characterized by torque / inertia specifications as well as synchronous speeds, field weakening characteristics, etc. It often surprises folks when they see a $1000 Baldor induction motor with encoder perform as well as a $12,000 Indramat Servo motor in an application simply because the application torque, speed, stability requirements are better served by the cheaper induction motor. Edited by motion guru

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Hii Guys I place a photograph eye on top of my living ceiling fan to count the fan blades, regenerate it to motor speed feedback, and tied that back to the fan speed controller, would my fan motor count as a servo motor? Technically, yes. Its not what we tend to typically regarding once talking about servo motors, however its a minimum of nearly as good as some servo setup in a very budget aware faculty laboratory. Thanks Xol brin

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