Is a SERVO motor specific or any motor control can be SERVO?

[AB2005]

Hi;

Normally if someone talks about SERVO, its refer to such a motor control system in which a motor, having permanent magnet on rooter, or a DC motor is connected with a drive with a feedback encoder or resolver. But on the other hand, a normal Asynchronous motor with feedback encoder/resolver, driving just a belt is also called servo motor.

Do someone put some light to clear my concept?

 

[goghie]

Generally speaking, servo motor is any motor with closed loop capabilities.
I mostly use either AC servo motors with permanent magnets that are controlled with normal servo drive, but often I use classical AC motors with encoder that are controlled with VFD with advanced vector capabilities.

 

[LadderLogic]

In my mind, a servo is any motor with the associated control system that can be commanded to move to a certain position and stay at that position infinitely long, provided the torque/force limitations are not exceeded.

Of course a servo motor can move at some velocity or maintain some torque as well. But the ability to maintain position to me is the most important distinction.

 

[hagos]

Cheers to all

But isn't the servo motor controls both velocity & position?

Does normal AC asynchoronous motor have the capability to do on the fly position control?

In my opinion servo motor is the motor that have the capability to control both, Position & velocity not the one that controls only velocity

Regards

 

[kamenges]

By the classical definition, a servomotor is any motor that has feedback used in a closed loop system that uses that feedback to regulate some variable the motor has control of. That "some variable" could be position, velocity, speed, torque, force, or any of a number of other physical values.

The problem is that a segment of the industry has taken control of a term and infer that it means something it did not originally mean. In the current industry vocabulary, most will think of a servomotor as a low inertia permanent magnet motor with integral high resolution feedback. That is a very small subset of the motors that could be servomotors. But the motion industry would have you believe that only that type of motor can be a servomotor.

Keith

 

[Peter Nachtwey]

Ditto what Keith said.
Some "servo motors" are MUCH better than others.
You need to check the specifications.
Most are fine for driving a conveyor or continuos speed applications. The ones that can do intricate motion like flying shears or cutoffs that require high accelerations are more expensive. The ability to hold a load at position is good to know too

 

[hagos]

Cheers to all,

Can a normal asyncrous AC motor do a position shift (on the fly position correction) while it is running?of course a servo motor can.

Isn't this the difference?

Best regards

 

[joseph_e2]

Cheers to all,

Can a normal asyncrous AC motor do a position shift (on the fly position correction) while it is running?of course a servo motor can.

Isn't this the difference?

Best regards

I'm by NO means an expert, but I would expect this to be more a function of the drive/controller than the motor itself.

 

[hagos]

Joseph_e2

You are right,
But its both the motor & the controller

For this reason I mentioned that Servo controller has capabilities that a normal AC motor & a VFD doesnt have.

A servo controller has the ability to lock its position(using the hall transitors), also it has the ability to shift its position while running.

Servo controller contains two loops , velocity loop & position loop.

All of the above a normal AC driven by an VFD cant achieve.

Best regards

 

[goghie]

Joseph_e2


All of the above a normal AC driven by an VFD cant achieve.

Best regards

There are VFDs that can achieve it, but AC motors have to have encoder, and additional ventilator.
Mitsubishi FR-A800 does it without problem. Limitation is in response comparing with standard servo drives/motors.

 

[kamenges]

Originally posted by goghie:

Limitation is in response comparing with standard servo drives/motors.

This is really the big distinguishing factor between AC induction motors and permanent magnet servomotors used in servo positioning applications. This is especially true of NEMA or IEC spec AC motors.

Permanent magnet servomotors are usually very long relative to their diameter. this has the effect of increasing their torque to inertia ratio which increases responsiveness. NEMA and IEC spec motors usually have shorter, larger diameter rotors that decrease torque to inertia ratio due to the increased rotor inertia.

AC motors also tend to go into magnetic saturation at lower torque levels than permanent magnet servomotors do. It isn't unusual to see a PM servo with a peak torque 3-4 times its continuous torque where an induction AC motor can typically only get to about twice continuous torque. This allows for higher short-term acceleration and better responsiveness with PM servos.

There is a specialty class of induction servomotors out there. They are usually higher horsepower than most PM servomotors (15HP and above, for example) and are constructed with that same long, lean high torque to inertia ratio design. I have seen these on rotary knives and other similar applications and they work very well. As in all cases you just need to make sure the physics work out before you commit.

Keith

 

[I_Automation]

Use a VFD and PLC with analog output and high speed counter.


I had a customer scrapping out an old welding cell with a 'normal' welding robot and a tiny robot under it - barely 14" tall.


6 axis' of 240V motors with encoders and 24V brakes.


Put a SLC504 rack with 6 small Lenze VFD's. The PLC outputs enable & reverse along with analog speed and turns on the brake relay for each motor and the encoders are wired to HSCE cards for position.


This won't run any production job, but makes a great toy - I mean promotional display.