Can DC drives be run without a tacho input?
Can DC drives be run without a tacho input?
(OP)
Don't the DC drives need a tacho input for speed & current control?
The client is running the motor without tacho input and with 'armature control' and is facing the problem of drive tripping on overcurrent.
The client is running the motor without tacho input and with 'armature control' and is facing the problem of drive tripping on overcurrent.
Muthu
www.edison.co.in





RE: Can DC drives be run without a tacho input?
RE: Can DC drives be run without a tacho input?
RE: Can DC drives be run without a tacho input?
RE: Can DC drives be run without a tacho input?
dis - Thanks. The client changed the motors of different makes (both have same armature voltage, current, speed & KW but different field voltage and current) and claims previous motor had no such issues for the same load. Could it be the drive tuning issues?
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
The speed controller is, when you run without tach or encoder, the same thing as the EMF controller and you need an isolated EMF signal fed back to the EMF controller. That signal is usually galvanically isolated (use an isolation amplifier) or derived from the armature voltage by using a differential amplifier plus a load (armature current) adjustment to compensate for the load-dependent speed drop.
It is a trivial thing to set such a simple drive up, and if you ran the machine in your test bench with good results, it is the customer and/or his consultant that needs to do the actual adjustments. They should know how to. Or get help from outside.
My question is: Why was the motor changed in the first place? Was it because the drive failed? If that is the case, the drive should be fixed instead of the motor. A simple resistive load bank (use incandescent lamps) can be used to test the drive. And then - when drive is fixed - connect and tune (if necessary) the motor.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can DC drives be run without a tacho input?
Yes, the motor was loaded up to 350 A (rated 2000 A) in our shop and black commutation was seen. Vibrations in all directions were less than 2 mm/sec peak.
The client changed out the motor since they wanted to send the running motor for a due overhaul. Talking with their engineers over phone, I realized their understanding of how dc machines work was not that good. I told them what you said - call the drive supplier to fix it since it's a drive issue and since they don't have the expertise. And to fix the damned tacho as the OEM intended. Haven't heard from them since. Fingers crossed.
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
If this is a new motor you must do the autotuning......standstill and rotating, which will automatically include the PID adjustment you are talking about. You cannot use the previous values..and you may need to de-tune the PID a bit, as it will be tuned only to suit the drive and not so much the application or the mechanics of the system....found this with Siemens 6RA24/70 anyway.
You can run DC without encoder, but needs to be tuned correctly...overcurrent as you are indicating is usually a sign of needing an autotune.
RE: Can DC drives be run without a tacho input?
1. Tacho
2. Encoder
3. Armature Voltage (taken from the drive itself)
You can use each depending on the speed accuracy you need, been Nº3 the less accurate and Nº2 the most
RE: Can DC drives be run without a tacho input?
1. Current controller. This has to be done with the motor stopped (sometimes it is need to mechanically stop the motor from spinning during tunning due to residual magnetism or series field). This tunning gets the armature resistance and armature inductance values
2. Speed Controller. This has to be done with no load and with the machine free to spin. This tunning gets the P and I gains for speed control
RE: Can DC drives be run without a tacho input?
RE: Can DC drives be run without a tacho input?
dis - yes, the new motor is being fed right field current and voltage.
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
ABB started it, if I remember right, and Siemens and others followed. The idea is that equipment shall be serviceable (there shall be knowledge and spare pars available) so that the customer/user of equipment doesn't need to be lucky to find support. It is now getting ridiculous, with equipment being declared obsolete after ten years or so.
But, for critical equipment, where a still-stand costs a lot more than the equipment as such, it may be a good idea to upgrade every ten years or so. The MTBF thinking doesn't seem to be valid any more.
I have seen so many examples of "obsolete" equipment in the new sense of the word that I am beginning to accept this thinking. We old-timers are getting fewer and the new guys are a lot better at more complex things, like data networks and graphics. And more sophisticated tuning is available in all drives nowadays. So, get new equipment that the new guys can handle. There are still enough old equipment left, obsolete in the real sense or not, to create plenty of jobs and challenges for us dinosaurs.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can DC drives be run without a tacho input?
Does the drive has to be tuned for any of the above parameters? For example, does GD2 matter at all?
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
RE: Can DC drives be run without a tacho input?
It looks as if the resistance of the armature plus compensating plus inter-pole resistance of the new motor is about 15% less than the old motor.
If the drive is using the applied voltage, compensated by the current to derive the speed, that change in resistance may be causing a derived speed error which in turn is causing the over current. If you can find the compensation circuit and add a small amount of resistance this may prove the concept. The added resistance does not have to be exact, it just has to show a reduction in the overcurrent. If this is successful you can then look for the appropriate adjustment.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can DC drives be run without a tacho input?
Bill - I have a very basic understanding on how drives work. So, not about to advise the client on how to proceed. :)
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
My work with DC drives was back in the day of MG sets, relays, resistor networks and Amplidynes. My question to Gunnar was based on a partly remembered understanding of how the drive looked at the motor parameters to determine speed and to limit overcurrent. Although the drives today bear almost no relationship to the drives of yesterday, The motor basics and the relationship between the currents and voltage drops across the commutating poles, compound poles and the armature remain.
My post was more a question to Gunnar to help my understanding, than a suggestion to you. Sorry for the misunderstanding. My bad.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can DC drives be run without a tacho input?
Speed is not given. Do these voltages and current give a speed that is similar in both motors? If not, one may have to look at that.
Otherwise, drives always come with a current limit that normally cannot be overridden much and it is not common to have an instantaneous overcurrent trip if the drive is OK. Which it seems to be. How is the current limit set?
I am not to tell you, Muthu, about interpoles and compensation windings, but if they have the wrong polarity (happened in a large steel mill motor only a few weeks ago) you will have bad arcing and also overcurrent when reversing. I know, I shouldn't even mention it. But I did, sorry.
Speed controller tuning should not be considered yet. This is either the gating unit or the current controller/current measurement that don't behave as expected.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can DC drives be run without a tacho input?
Maybe overcurrent apear when start after a long pause and motor cool down to 25-30°C. At 25°C armature resistance is much lower than at 75°C and current go to 2600A.
RE: Can DC drives be run without a tacho input?
Drives have current controllers and very effective current limiting. So, the armature resistance has nothing with overcurrent tripping to do.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can DC drives be run without a tacho input?
From your reaction I must assume that this is no longer a common practice.
Thank you for your patience Gunnar.
Yours Bill
PS Have a virtual drink in the Pub, on my tab. I am sure that that is still allowed.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can DC drives be run without a tacho input?
I asked the client for the voltage/speed/current data for both the motors and still didn't get them before they swapped out the motor.
At what point the motor parameters (and which ones) demand a drive tune-up? 10%? 20%?
I also find it odd that the heavier rotor has a lower GD2.
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
Muthu, there is no magic limit where new tuning is needed. So, as long as the armature current is steady and not reacting too slow, it shouldn't be necessary to do any adjustments. For a rubber (mixing) mill like that, speed varies as load changes over the cycle, but speed precision or constancy is not very important. Not like a paper mill where you want to regulate down to around 100 PPM.
I think that a proper commissioning where Gating Unit, Current Measurement, Current Controller and, not to forget, auxiliary power shall be checked. In cases like this, it is very easy to forget that +/-15 V (or 12) and +24 V needs to be constant and smooth and if they are not, you will have problems of all sorts.
Also, if it is a Four Quadrant drive, there may be a problem with the torque (current) sensing comparators. One cannot get to grips with these drives without the necessary knowledge and experience. There must be an old-timer laying around in your part of the World. Dust him off and bring him on site. That will help.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can DC drives be run without a tacho input?
Muthu
www.edison.co.in
RE: Can DC drives be run without a tacho input?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Can DC drives be run without a tacho input?
I suposed is a sensorless driver that use emf controller only, w/o current sensor in current loop.
RE: Can DC drives be run without a tacho input?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.