Changing AC to DC motor

ettore

Member
Join Date
May 2005
Location
Canada
Posts
10
Hello,

I haven't posted in a while, my history is that I am a noob.

Ok, we have an existing machine with an indexing table being run off an AC motor with no feedback. It rotates until a limit switch is set and this gives the motor the 'start' and 'stop' commands.

Well, as luck has it, it's overshooting every so often. Since we have no feedback, there is basically no way to verify it's overshooting situation.

SO, here's my problem. My boss wants to change the motor to a DC motor so we can easily vary it's speed using an existing variable motor speed controller that we had laying around ( :( ). On a positive note, it is a decent module (Camco Vari-Pak) that takes AC in and outputs a friendly DC out including some acceleration considerations.

So, where are we now? We have an AC motor (3 phase, 1HP) being controlled by a single logic output telling a contactor to turn on (Telemecanique LC1D09BD). This is funneled thru it's respective fuses as well as an overload protection module (Telemacanique GV2P06) to tell the logic if the motor has overloaded (I am assuming here). Part of the system also includes a Telemecanique LADN11 module that I have NO idea what it does.

So, basically, we need this thing to stop overshooting. I am quite the noob, seriously. I don't want to be screwing with logic just yet as I have never programmed ANYTHING and this machine is online right now. I understand logic well enough and I know how to read the drawings (assuming they're right) to see how to add logic to where I need it if necessary (I can requisition an electrical guy to do the changes for me if need be).

Adding feedback would be great but I don't think it's a good idea due to my lack of experience. Eventually, sure; today, I don't think so ESPECIALLY since I don't have the programming software nor do I have the existing logic (lucky me). Changing to a DC motor I don't think will fix any problems with our current setup but, again, I am requesting divine intervention if possible.

So, he wants a plug and play fix for our problem. Ideally, in his eyes, we take out the old motor, remove the contactor, feed AC into our motor speed controller, and slow down this new motor on the shop floor until it works (it has a big knob on top). It seems like a band-aid to me but I don't know the magnitude of how bad this problem is. This appears to be of a similar problem to pstephens 'the source weighs a lot' problem that may have caused overshoot. However, since our motor has no speed-up / slow down feature, it's relying on luck.

Any reply is appreciated, this is the first 'job' I have had so far.
 
Last edited:
Reducing the speed will not fix the error, but it should reduce it proportionally. Does the thing have a brake? If it is as originally designed, it must have some kind of brake. Maybe you just need to adjust pad clearances. Is it chain/belt driven? Could the drive mechanism be loose? Worn keyway? Eliminate all mechanical possibilities first.

If you go through all this trouble changing motors and slowing everything down, how is that gonna affect the machine cycle rate?$? Also, it would be much, much cheaper and easier to install a variable speed AC drive if you absolutely have to slow it down.
 
Ok, we have an existing machine with an indexing table being run off an AC motor with no feedback. It rotates until a limit switch is set and this gives the motor the 'start' and 'stop' commands.

Every indexing table I've built/programmed (about 50 and if they are round we call them a 'dial') was controlled by a DC motor and a variable speed controller.

Every one also had a dwell, meaning that the drive motor moved the table to the next position and kept running, the table didn't move (the dwell time) , until the limit switch (use a prx, not mech. one) was activated by the lobe on the index/dwell cam.

I presume that you too have a dwell/index cam that the switch is activated by.

If you are having random problems, first check the switch to see if it's not being activated soon enough on the dwell cam. This is the first check and most of the time the problem.

Even if you do change to a DC motor, if the switch is flakey, same problem. If mech. switch, change to Prx.

Check the dwell cam. May be loose on the shaft.

If you slow down the index much, the plc should have been programmed to detect this with a timer, incase the index got stuck somehow.

As a side note, the PLC should also have been programmed to stop operations if not on the dwell cam and not all 'heads back'
 
As I was writing that question, they came to me with more info.


APPARENTLY, the table was designed to use this motor controller I have + a DC motor but for SOME reason they wanted an AC option. As a result, this AC motor is REAL fast. They're saying almost fast enough that the item we are installing almost comes out. So, we are going BACK to the original intended design including the motor controller that came with the table.

I will get back with more information but it looks quite simple now that I understand the controller better.
 

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