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difference between servo motor and 3-phase synchronous motor

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ring0

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Servo motor question

I have very little experience with motors yet, so the question may seem stupid, but...

what is the difference between a servo motor and an ordinary 3-phase synchronous motor? I mean, can I control the latter using an appropriate frequency transformer and encoder as if were servo? And what about asynchronous motors?
 

sunderwood

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Servo motor question

Hi,

I think what makes a motor a servo is the way you control it (in a closed loop fashion). Therefore, three phase synchronous and asynchronous motors can be used as servo motors. The methods used to control these two types of motors are different, because of the fact that in synchronous machines the field is either constant (magnets) or controlled independently.
 

    ring0

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greenforest

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Re: Servo motor question

ring0 said:
I have very little experience with motors yet, so the question may seem stupid, but...

what is the difference between a servo motor and an ordinary 3-phase synchronous motor? I mean, can I control the latter using an appropriate frequency transformer and encoder as if were servo? And what about asynchronous motors?

Servo system is closed loop control system, so servo motors must have encoder or resolver, tacho or other measurement devices.
3-phase synchronous motors have measurement devices, they are servo motors.
3-phase synchronous motors do not have measurement devices, they aren't servo motors
 

ring0

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Re: difference between servo motor and 3-phase synchronous m

Thank you sunderwood, I was just thinking the same thing, but I needed someone to confirm it.
 

Mr.Cool

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Re: difference between servo motor and 3-phase synchronous m

huh?

synchronous motors are close loop also. your current must be inphase with the back-EMF of the motor or it doesn't work! true you don't have to control the position, regulate the position if you don't want to - it is optional on synchronous motor. but you definately must be aware of the position and at least route that signal to the rotating vector of your controller. use it directly, or regulate it, in both scenario there is a need for position feedback (as a minimum).

i think the difference is only slight... the stepper motor is a position based controller. that means it rotates a certain distance and then stops at a certain position. so emphasis (acduracy, repeatability etc) is placed on position control. these motors usually have way more than three phases, but so what.. you just replicate the same hardware X number of times and account for this in your control algorithm.

a synchronous motor is emphasis on producing torque under continous rotation motion.

the power electronics is the same, perhaps only the number of phases is different. the control algorithm has same topology too. for example, it is totally reasonable to have position control loop on a SYNCHRONOUS motor too.. depends on the application if it requires that level of precision of speed control. yes, there is a control loop for speed too.. but you may need even more regulation, so you add a position loop. now what is the difference betewen a high precision synchronous motor and stepper motor?

Mr.Cool
 

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