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AC Servo drive

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kal.a

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Hello Gentlemen,

I'm curious as to how AC servo motors are driven or be specific how is commutation achieved. With DC motors of its variety PWM is used with MOSFETS/IGBTs. AC motors controlled with VFDs, again use PWM and IGBTs.
Is an AC servo drive designed partly like a VFD with the AC signal being simulated using PWM ?
And what do they mean by an amplifier when referring to servo drives, does it really amplifies the voltage or is it a misnomer for using low voltage to switch higher voltage?


Thanks

Kal
 

MaxHeadRoom78

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Amplifier and drive are synonymous, AC and BLDC servo motors are practically identical, physically, the difference is in the commutation and control, AC is fed by a 3ph sinusoidal power, and commutation is usually by exercising the motor slightly at power on and taking an initial current sample to find the position of the poles via the encoder.
A BLDC motor has what used to be 3 hall effect devices placed at the correct angle for commutation and only 2 winding's are powered at any one time, simulating a DC brushed motor turned inside out.
Now the hall effect devices are replaced by equivalent tracks on the encoder.
The PDF shows the commutation and the relative winding phases when the motor is rotated and generating using a BLDC motor, as an illustration.
Max.
 

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dr pepper

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Yes pretty much, only the position of the servo motor rotor usually has feedback, so the chip in control can tell which coil to drive, sensorless is also possible by measuring the back emf from the motor coils, there will be an amount of error on start up which will vary on the gear ratio of the servo this way.
Another methos would be to use a synchronous motor, then the motor is always 'in time' with the drive signals, like a stepper motor.
 

spec

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Hy Kal,

Nice to see that you are still investigating electronics.:)

Here is a general diagram for a closed loop servo:

2016_08_15_!ss1_ETO_SERVO_LOOP.png
(1) In control theory there are two fundamental types of systems: open loop and closed loop.
(2) An open loop system has no feedback, so in the diagram above there is no sensor but there still may be an amplifier. Examples of open loop systems are vacuum cleaners and a spin driers.
(3) A closed loop system has a sensor which feeds information back to a servo amplifier. Examples of closed loop systems are electric drill speed controllers (speed feedback) and domestic ovens (temperature feedback).
(4) The principle of a closed servo loop is that you give it an input and the closed servo loop does its best to set the sensor to match the input.
(5) The servo amplifier compares the input with the output from the sensor and amplifies the difference to drive the motor. So yes, a servo amplifier is an amplifier which amplifies voltage or current or both depending on the motor. In fact, you can use an opamp or an audio amplifier as a servo amplifier. Note that motor here has the general sense and is a device that converts electrical energy to mechanical energy.
(6) In the above diagram, you may want to position the motor shaft at a particular angle between 0 degrees and 355 degrees. In that case you may fit a sensor to the motor shaft that has an output of 0V for 0 degrees and 5V for 355 degrees. So, for example, if you wanted the shaft to be at 205 degrees, you would command an input voltage of, 205 *5V/355.
(7) On the other hand you may want the motor to spin at a particular speed, as in electric drill controllers. You would then fit a sensor that generated a voltage output proportional to RPM.

The world is full of open loop and closed loop control systems, not just electrical, but the principle is the same. In fact, you have been a servo amplifier yourself in a closed loop control system if you drive an automobile: you control road speed with the accelerator and you monitor road speed (feedback) by looking at the speedometer.:p

spec
 
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MaxHeadRoom78

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I think the question was regards to commutation, not servo loop or feedback;)
Commutated Servo drives can be operated open or closed loop.
Max.
 

spec

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And what do they mean by an amplifier when referring to servo drives, does it really amplifies the voltage or is it a misnomer for using low voltage to switch higher voltage?
 

MaxHeadRoom78

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But an amplifier or drive (same one in some instances) is used whether the motor is used in closed or open loop configuration.
Max.
 

MaxHeadRoom78

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I just wondered why you clouded the issue with Closed Loop Servo?
The question was on commutation, I thought.
Max.
 

spec

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I just wondered why you clouded the issue with Closed Loop Servo?
The question was on commutation, I thought.
Max.
May be Max, but clouding is one thing you cannot accuse me of.:wideyed: My post #4 is specifically structured so that the OP can pick out the parts that he is interested in, like most of my posts.

By the way, you made no mention of dr pepper's earlier post #3 on a similar theme, or the thousands of other posts on ETO that are way off the ball.

spec
 
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MaxHeadRoom78

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Post #3 went toward answering the commutation issue I thought?
For example I use a BLDC motor and A-M-C servo drives, with this drive I can use it in velocity (open loop) feedback where it establishes rpm feedback from the 3 commutation pulses, the is essentially open loop as it is capable of rpm control but not accurate positioning, I use the identical motor, sensors and drive and set the drive for Torque (trans-conductance amp mode) which is useless unless used in conjunction with a PID loop control.
Both modes use the same drive and method of commutation.;)
Max.
 

dr pepper

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There is a machine that I maintain which does both, for a certain product the motor/box acts like a geared drive maintaining a accurate speed ratio to the master but not position, and when in another mode speed and position are maintained at an overall slower rate.
The system uses siemens ac servo's, 2.2kw I think, and the comms protocol to the main plc is drive cliq, it works well.
 
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kal.a

Member
Thanks a lot everyone.


I would like everyone to know that all replies are much appreciated and to give you a little insight on how a newbie looks for help is the following:

- I wanted to know more about servo motors as I encounter them regularly and recently read about AC servo motors.
- I read a bit and watched some youtube and learned a new term: "Commutation". I think I know what it means but I may be wrong.
- I did more reading and downloaded a lot more to read but was impatient to wait till I read the answers to my questions so I came looking help here.
- I asked two questions:



Is an AC servo drive designed partly like a VFD with the AC signal being simulated using PWM ?

Kal
And what do they mean by an amplifier when referring to servo drives, does it really amplify the voltage or is it a misnomer for using low voltage to switch higher voltage?

Kal
The second was answered directly but not the first. I realize that the first question my have been answered indirectly by Max in the following statement :
"AC and BLDC servo motors are practically identical" but until it is answered directly here or in any of the literature I will read, I will not be satisfied.

So we have a servo drive and an AC servo motor. The servo drive takes two voltages (I believe) one control voltage and one motor voltage (which I believe can be 100-200VAC! ). If the AC servo motors are identical to BLDC servo motors then the AC motor voltage is first rectified then an AC waveform is simulated and output though IGBTs or MOSFETs.
 
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dr pepper

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I spose there are different startegies, some smaller drives might not use pwm and just on/off, other larger ones may generate 3 pahses of pwm synchronised with the motor feedback.
You can use a 3 phase bldc motor as a servo, heres a piece of code for the arduino I wrote to position a bldc motor from a hdd, pwm is used to generate the 3 phases, the code generates a random position for the servo then puts it at that position, the 'control' is a little floppy as the device uses voltage not current control.
 
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