compumotor drive burning out

We have 7 machines for the same process and they've been around quite a few years with no major problems until the past couple of weeks. For head positioning they are using compumotor pds-15 drives.

Every machine seems to be eating these drives for lunch. They will get to the point where the drive has to be slowed down to the point thats beyond aceptable then a shift or two later the drive dies. None of the machines are the same age so life expectantcy isnt the issue.
There haven't been any mods to the head weight and the motors seem ok.

Any ideas?

A suggestion would be to check your line voltage. Make sure it is not running too high or low.

Rick I checked the line volt and its running about 112vac. But I don't have access to a scope so I can't check for spikes. The drives get very hot (can't hold drive very long if at all in bare hand). Is this normal ?

If the Drives are hot this is not normal. Check the drives with a amp probe. Read the nameplate on the drives to see the running amps on them.If drawing to much undo the drives and read amps again.

Does your positioning routine require that the motor hold a position?

Do you have a lot of DC-Hold current applied?

Is the Hold-Current required?

What is the method of position control?

If you are using a PID type thing, your control might be too stiff.

We need a hell of a lot more details!

Does the motor heat up when it is holding position?

Do you have access to a display that tells you the holding current?

I've done several (dozen?) compumotors in the past. They are pretty damned good! If we could see your parameter list, I think we would have a much better shot at workin' this out!

What is the age spread of the machines?

Your comments seem to indicate that something changed in the last few months. Where is the 115V coming from? Anyone make a program change? Some difference in the product?

When you say that they die- just kaputt? Do the steppers' positioning degrade?

Have you talked to the machine builder? Maybe they changed something when the last machine was installed.

Frank
undo the drives and read amps again
I'm not sure what you mean by this?

Terry
These drives are nothing but slaves- no programming in them. There are three pots to adjust slow/fast/accel and 8 switches.
1-selftest-off
2-standby current reduction-80%
3-int/ext speed control-external
4,5-step resolution-4000
6,7,8-peak current setting-5 amp
The plc does the positioning,and to the best of my knowledge there have been no programming mods done (my crappy laptop with ai will connect using the oldest program available- thats approx 2 1/2 years old-without telling me the progs don't match)
Yes the drives have to hold a position but I don't know the holding current applied to them (drive does have a standby reduction of 50or80% according to manual ). It seems that when the operators go on break the drives actually cool down so I don't think the holding current is the problem.

The process has the stepper motors moving at least 50% of the time.


Rick
The 115v comes from a machine transformer but i have no taps that are appropriate to adjust voltage up a bit, now the 480 side reads low also but I don't have the option of moving a tap. As far as the positioning degrading as the drive ages I believe it is.
What happens is we need to keep slowing the heads down because the drives can't seem to stop at the required position so they end up bouncing around at almost desired position until they achieve the desired position. Slowing the motors down corrects the problem for a few days until the same thing happens- so we slow them down some more-and so on and so on.



We are using a double prox(about 2 inches apart) reading a half circle on the shaft. The 2 proxes run counters to count shaft revs and that is the feedback for position.
When the counter equals the preset 1 its in desired pos 1 and the same for 2. I was think of using geq and leq instead of equ to allow the position to vary a bit and stop the positiong problem. But the oldest machine is approx 7 years old and I feel I'm just masking another problem .

Hope this clarifies things a bit for all.
Thanks
Drewcrew6

Does the program try to hold the motors at an "exact" position?

You might need to allow a "range". That is, instead of "Go to and Hold at Position = 2500", you might try "Go to and Hold at 2490 < Position < 2510 (2500 +/-10)".

Of course, whether or not you can use the +/-10 depends on your particular application. You might only be able to get away with +/-2. But then, you might be able to get away with +/-100!

Another concern is INERTIA.
That's game where you have to keep the demand on the motor/drive within their limits.

As the driven mass gets larger or the acceleration (or deceleration) demand gets greater, the more power the motor/drive needs to meet the demand. Of course, if this demand is outside the limits of the motor/drive, then something is gonna die!

Another concern is FRICTION.
I don't know what your layout is, but, you should be able to power-down the system, disengage any braking, and then turn the motor shaft by hand. If it doesn't turn freely then you might have a bad gear-box or someone might have adjusted the travel-guides a little too tightly, or...

The plc gets the motor to an exact position. Allowing a window is what I meant by stating geq and leq (greater than or equal to , and less than or equal to). But the machines have been running this way so long I feel there may be a problem that I am just masking. I'm not saying I'm not gonna try it but I don't want to mask a problem when theres a resolution besides masking. I feel (along with some others at work) the machines will run better with that modification. But I would like to correct the existing problem before making that modification.

Basically the down position has to be more tolerant than the up. And the up is where most of my problem is. When the head is down its resting on top of a product with a little bit of pressure down to acquire somewhat of a seal. When the head is up it just has to let the product pass underneath, so a larger window is acceptable.

With power off I have no signifcant resistance to rotation (the rotation is a screw shaft that the head rides up and down on).

Thanks
Drewcrew6

Has business been slow? Have the heads been in the up position a lot? Are they counterbalanced? I am not sure about steppers, but with servos there is a problem with holding a vertical, unbalanced load for a long time. All the current is going through one winding (phase). The motors/drives need to be upsized to handle this. The usual solution to counterbalance with air.

Business was a little slow (no ot) but is picking up now. As far as the heads balanced they are not. But I was told stepper motors have no problem holding a load still. The tool weight is fairly low (about a total of 25 lbs) and with the rotational drive I would guess the drive sees very little torque when still judging by how it is to turn by hand. I will check what motor is installed and get the specs on that.
The only time that the drive has to hold the load still for extended periods is when the line breaks down or when its break time. Inbetween shifts and weekends when unit is not used, the power is shutdown to drives and the heads drop to bottom of travel.


Drewcrew6

So...

You have a stepper motor that drives a screw which moves a "head" between two positions "TOP" and "BOTTOM".

Position feed-back is developed through counting pulses.

When the counter equals the preset 1 its in desired pos 1 and the same for 2.

Here's what I think the program should look like... sorta...

First... I should expect that the "HEAD" goes through a "Homing" Routine at Start-Up.

"HOME" should be a position between the "TOP" Position and the Physical Limit of the Upward travel.

Assuming that the Home Position is at a point higher than TOP (AND below the Physical Stop!), and that you have something (Limit Switch, Photo-Eye...) to indicate "AT HOME", the motor turns in the appropriate direction to bring the Head to Home.

I've designed a few of these Homing Routines where the subject (using a 4000 Count/Rev Encoder) moves to HOME at high speed. When the "AT HOME" Sensor goes OFF (Fail-Safe), the drive stops, using the built-in, or programmed, Decel Rate.

When it has stopped, I reverse the direction and move off of the "AT HOME" Sensor at SLOW speed. When the "AT HOME" Sensor goes ON, the drive stops, using the built-in (or, programmed) Decel Rate.

Then, depending on how precise I need to be, I use a slow/slower/slowest speed to approach the "AT HOME" Sensor.

As soon as the "AT HOME" Sensor goes OFF again, the encoder (counter) is Zero'd Out - This is "HOME". The motor is still moving but stopping at the built-in (or, programed) Decel Rate.

       HOME = 0

        TOP = W

   NEAR TOP = X

NEAR BOTTOM = Y

     BOTTOM = Z

 GRAB LIMIT = Z + ?

Now, knowing where the Head is, the program and drive send the motor, at high speed, to the "NEAR TOP" Position - based on counts.

Then the program and drive send the motor to the TOP Position at SLOW Speed. When the count indicates "AT TOP", the drive and motor stops.

Now, assuming a cycle consists of the following operations...

• "Go to Bottom"
• "Grab Part"
• "Go to Top"
• "Go to Drop"
• "Drop Part"
• "Go to Pick-Up"

IF "Ready for Next Part" and "Part Ready"... (Begin cycle)

• Set "Do Cycle"

(Use Set, Latch, or a latching rung, whatever...)

While "Do Cycle"... (this spans the entire section of code for the cycle)

IF "Do Cycle and NOT "In Cycle"

• Set "In Cycle"
• Set "Go to Bottom"

IF "Go to Bottom" and NEAR TOP, Drive-1 down at LOW SPEED.
IF "Go to Bottom" and NOT NEAR TOP and NOT NEAR BOTTOM, Drive-1 down at HIGH SPEED.
IF "Go to Bottom" and NEAR BOTTOM and NOT BOTTOM, Drive-1 down at LOW SPEED.
IF "Go to Bottom" and BOTTOM, stop Drive-1.
IF "Go to Bottom" and BOTTOM and Drive-1 is Stopped,

• Reset "Go to BOTTOM"
• Set "Grab Part" (Turn ON Vacuum?)

While NOT "GOT PART"("GOT PART" = Vacuum Switch is ON)...
While NOT GRAB LIMIT...
IF "Grab Part", Drive-1 down LOW SPEED.

IF "Grab Part" and "GOT PART",

• Reset "Grab Part"
• Set "Go to Top"

IF "Go to Top" and NEAR BOTTOM, Drive-1 up at LOW SPEED.
IF "Go to Top" and NOT NEAR BOTTOM and NOT NEAR TOP, Drive-1 up at HIGH SPEED.
IF "Go to Top" and NEAR TOP and NOT TOP, Drive-1 up at LOW SPEED.
IF "Go to Top" and TOP, stop Drive-1.
IF "Go to Top" and TOP and Drive-1 is Stopped,

• Reset "Go to TOP"
• Set "Go to Drop"

IF "Go to Drop" and NEAR PICK-UP, Drive-2 Counter-Clock-wise at LOW SPEED.
IF "Go to Drop" and NOT NEAR PICK-UP and NOT NEAR DROP, Drive-2 Counter-Clock-wise at HIGH SPEED.
IF "Go to Drop" and NEAR DROP and NOT DROP, Drive-2 Counter-Clock-wise at LOW SPEED.
IF "Go to Drop" and DROP, stop Drive-2.
IF "Go to Drop" and DROP and Drive-2 is Stopped,

• Reset "Go to Drop"
• Set "Drop Part" (Turn Vacuum OFF)

IF "Drop Part" and NOT GOT PART (Vacuum Switch is OFF)...

• Reset "Drop Part"
• Set "Go to Pick-Up"

IF "Go to Pick-Up" and NEAR DROP, Drive-2 Clock-wise at LOW SPEED.
IF "Go to Pick-Up" and NOT NEAR DROP and NOT NEAR PICK-UP, Drive-2 Clock-wise at HIGH SPEED.
IF "Go to Pick-Up" and NEAR PICK-UP and NOT PICK-UP, Drive-2 Clock-wise at LOW SPEED.
IF "Go to Pick-Up" and PICK-UP, stop Drive-2.
IF "Go to Pick-Up" and PICK-UP and Drive-2 is Stopped,

• Reset "Go to Pick-Up"
• Reset "In Cycle"
• Set "Ready for Next Part"

The point of that whole thing is to illustrate a way to reduce the loading on your drives. As the Head gets NEAR to a target, have the Drive go to a slower speed.

You can define the "NEAR Position" and "AT Position" values in your code.

That listing is written in "C"-code fashion, but it translates very easily into ladder.

Just one note- if your home switch is a limit or other mechanical switch, it is a good idea if you can to put the home position outside the normal travel. That way you aren't actuating it every stroke.

drewcrew6- I am at a little bit of a loss here. Your drive is getting hot, that means too much current. I asked about business in that maybe the heads are staying up more than they used to. 25 lbs can be a hefty load for small motors. I don't know how big yours is. Pitch of the screw is important, too.

I still think that we need to find what changed when your drives started burning out.

You say that the problem spans all seven of the drives, and that it began a few weeks ago. Is this summer any hotter than the last few summers so that the ambient temperature is higher than in the past? Has anything been installed near the drives cabinet that might impede airflow? Have the filters in the cabinet been cleaned/changed? Has anyone blown the accumulated dust off the drive's cooling fins lately?

You also say that the drives cool down during breaks. Has the line speed of the process been increased so that the drives have to go through a cycle more often than they used to? The drive may have a peak current rating anywhere from 1.5 to 2 times its continuous (nameplate) rating. Quite often OEMs take this into consideration when sizing a motor/drive to the application. They let the drive run close to its peak rating during the most severe part of the cycle, knowing that for the balance of the cycle it will run below the nameplate rating, or that there will be a period of no load to allow things to cool down. You may want to calculate the RMS current to make sure it falls within the nameplate rating of the drive.

Consider adding temperature sensors to the drives so you can disable them if they get too hot.

Terry

On the homing sequence there an eye at top of travel and the drive runs on slow up till that eye sees the head then stops and holds (just shy of physical up limit). The homing sequence only happens once a shift assuming no problems with machine. From there it goes down to the programmed up point by counts from the two eyes reading a half circle plate on shaft (poor mans encoder). Resolution is
1 count=1 rev. When the product enters and settles on stop (time based) then head starts in slow for x-amt of counts then goes into high until it reaches x-counts before desired position,then goes to low. Low carrys head to desired position then stops.
After machine does the process. The head starts slow up for x-amount of counts then goes to high until x-amount of counts before desired up then goes into low and puts head into desired position. Machine then gates product through machine and new product enters.

Now when i state desired position the plc will bounce between up and down slow until the half circle plate stays on these two eyes, But it only bounces when the drive is on its way out. It seems that the drive is losing its holding power and can't stop the motor accurately from low speed. And I think this is where the drives are destroying themselves but i'm not sure what's causing the drive to start to weaken in the first place.

Rick
The home limit is outside the normal running range of the process. It is at almost mechanical up limit. I figured thats what you were asking when you asked about if business was slow. When I go in tonight I will get the specs on screw pitch and motor specs.


Steve
Well we have Six machines but each one has 2 drives. I know I said seven but one line has a brand new machine with hydr positioning on heads. I know of 6 drives in the last few weeks and suspect a couple more from things I've heard. The machines were moved to an addition to the plant 2 years ago. The addition has newer style swithgear on the highside and the entire lines are now feed from busduct (1 duct for 2 lines).
As far as the line speed being faster very possible. When the lines were moved some new equip went in down line which is the slowest part of the line. So the drives are running longer before the line backs up and doesnt have as long of a break till the line clears.
There haven't been any other changes to the the machines.


Something I hadn't accounted for!!! I will see if can set something up to monitor the drive temps or maybe i'll set up a fan and force a little more cool are through them.


Thanks everyone
Drewcrew6