vfd and ac servo drives

seee · Nov 24, 2011

hello dear friends.

as you know in some of vfd and ac servo drives there is a feature with name of "auto tune".i am useing a teco ac servo drive and a servo motor to control speed of spindle.after connecting the motor to the coupling and set some parameters then active the parameter"auto tune" and then give setpoint and servo on the motor rotates without any problem but when i pull out the motor from the coupling(the motor without load)and apply power and setpoint to drive the drive display "over current alarm".

what is the problem?

should the "auto tunning " be done with load or without load
on the motor?

best regards.

iraiam · Nov 24, 2011

I have only seen "auto tuning" on AC servo drives, After the drive was auto tuned under normal load conditions, If something changed beyond whatever tolerance was allowed, the drive would fault. The other question I ask is, will the drive fault once again after you start cutting/drilling with a tool in the spindle? If that is indeed the purpose of the spindle.

It seems better to me to "auto tune" the drive under the same conditions in which it will operate.

seee · Nov 25, 2011

hello.
thanks for your reply.
most of vfds also have "auto tune" capability. for example vfd from
danfuss,lg and etc.
yes when i connect the motor to the coupling(load on the motor)it work good but without load it has problem with alarm overcurrent.
when should i use "auto tune"?

best regards.

OkiePC · Nov 25, 2011

Consult with the drive manual or manufacturers recommendations.

Some drives can adequately "static tune" a motor and run quite well afterward. Other drives need a full autotune where the drive runs the motor and measures its response.

In most cases, the static tune can be performed with the load disconnected from the motor.

With autotuning, some drives allow this to be done without the load (motor only) and others (most?) prefer that you perform the tuning with the load to measure the inertia of the whole system.

If your load is unusual the auto tuning results may not be satisfactory at first. You may need to make adjustments manually to the gains to get best performance.

Bottom line, read the book, follow the vfd recommendations carefully. If it does not work to your satisfaction, call the manufacturer, and ask their advice after describing your system to them.

Peter Nachtwey · Nov 25, 2011

I don't see how an auto tune can do an accurate job if the load is disconnected. A lot depends on the load that the motor sees relative to its own inertia. Even if one does the inertia matching with a gear box the load will appear to be equal to the load of the motor and therefore the gains can be off by a factor of two by a factor of two. See page 2/30 to see the gain calculations formulas.
http://www.deltamotion.com/peter/Mathcad/Mathcad%20-%20DC%20Motor%20Velocity.pdf
J is the inertia. If J changes by a factor of two it will have an effect on the gains.
The modern trend is to ignore the inertia match and the load inertia may be significantly greater than the inertia of the motor on which case the gains will be way off.

A drive doesn't have a magical ability to estimate the load inertia unless it is connected.

I know my example is a for a DC motor the general principles apply and the effect of inertia will be the same.

OkiePC · Nov 25, 2011

Thanks, Peter, your expertise in the nitty gritty details of what goes on under the hood will surely be valuable here:

Peter Nachtwey said:
I don't see how an auto tune can do an accurate job if the load is disconnected.
---
The modern trend is to ignore the inertia match and the load inertia may be significantly greater than the inertia of the motor on which case the gains will be way off.

A drive doesn't have a magical ability to estimate the load inertia unless it is connected.

My experience is mostly with A/B products, and I have seen this "trend" you mention over the years working with their older servo products (which always asked for a coupled load) and the newer stuff which has gotten away from that.

Excerpt for Powerflex70:

PF70 User Manual said:
“Ready” (0) = Parameter returns to this setting following a “Static Tune” or “Rotate
Tune.” It also permits manually setting [IR Voltage Drop] and [Flux Current Ref].

“Static Tune” (1) = A temporary command that initiates a non-rotational motor
stator resistance test for the best possible automatic setting of [IR Voltage Drop].
A start command is required following initiation of this setting. The parameter
returns to “Ready” (0) following the test, at which time another start transition is
required to operate the drive in normal mode. Used when motor cannot be
rotated.

“Rotate Tune” (2) = A temporary command that initiates a “Static Tune” followed
by a rotational test for the best possible automatic setting of [Flux Current Ref]. A
start command is required following initiation of this setting. The parameter
returns to “Ready” (0) following the test, at which time another start transition is
required to operate the drive in normal mode. Important: Used when motor is
uncoupled from the load. Results may not be valid if a load is coupled to the motor
during this procedure.

DickDV · Nov 25, 2011

At least in AC drives, their are two different processes that are sometimes referred to as "tuning". The first is a bit of a misnomer since it is to be done with the motor uncoupled and the drive is not tuning anything but is simply building a motor model to use for reference when running "sensorless vector" Some manufacturers call this motor model build or motor ID run or similar and those are probably better names.

The second tunes the response of a closed speed loop. This can only occur if there is an encoder or tach on the motor shaft (flux vector control) or a calculated and therefore estimated speed feedback signal derived from the previously built motor model in the drives memory (sensorless vector control). Since the power train and load on the motor are a part (often the dominant part) of the speed loop, it certainly should be included in any attempt to tune the speed loop.

vfd and ac servo drives

seee

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iraiam

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seee

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OkiePC

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Peter Nachtwey

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OkiePC

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DickDV

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