DC Motor Speed Control

chip7238

Member
Join Date
Oct 2004
Location
Knoxville, TN
Posts
13
I've got an interesting problem that has been put on my plate.

I need to replace an old DC motor speed controller on a saw with a controller that can be integrated with an AD 205 PLC.

Here's the story - we have some ID saws on site (like the ones used to cut silicon boules) that are getting older. We recently converted one to run on an AD 205 PLC with touchscreen interface as a testbed for converting other saws. This arrangement has been working great, but one of the old parts we kept, the DC speed controller (for the blade speed) is going bad. Unfortunately, it doesn't look like we will be able to get a direct replacement for the part. It is now an opportunity to upgrade the functionality of the HMI a little more by integrating the speed controller with the PLC.

The motor being controlled is about 2HP 15A 200V DC.

I have some experience with PLCs, but this is my first foray into motor controls.

Let me know if there is more info needed. Any help would be greatly appreciated!

Thanks!
Chip
 
That shouldnt be a problem but you didnt state how you controlled the speed...is it more or less manual?

With the DL205 you could use an analog card that would work just like a potentiometer, which any controller can use...technically you could make the HMI show a pot and turn the dial or something similar to represent the speed.

Grainger sells a variety of DC controllers that could possibly be used.

To give a better idea of what I am saying. The HMI is setup for speed control, lets say based on RPM, full speed is 100 rpm. The analog output is setup to send a 0-10volt signal to the speed controller, 0v would be no speed and 10v would be 100 rpm...ie 1v = 10 rpm etc etc. On the display you may enter 50rpm and the analog card would output 5v. This is an example, the exact configuration would depend on the application.

It may also be possible to configure the speed to be automatic depending on the application.

Some DC controllers have other options for controlling via a PLC. www.grainger.com has a variety of dc controllers.

http://www.reliance.com/prodserv/standriv/dc/ offers a variety of industrial drives, depending on budget and the necessary control options they will have something that is applicable. With them you can use communication, devicenet or ?

Hope this helps.
 
I've used the KB Electronics drives with good success in the past. But be a little careful when interfacing them to a PLC. One of the ways they make them so inexpensive is the power section is not always isolated from the control section. So what you think is +/- 15 VDC is actually riding on top of 220 VAC. I blew up a couple of analog output channels before I realized that one. But then again I didn't read the manual first. KB sells isolators for just such a case, as is shown in their manual.

By all means use drives like this. Just be a little heads up.

Keith
 
Thanks for the info and links everyone! I'll do a little more research on the sites suggested.

Currently, the speed is controlled manually (with a pot) and monitored with a separate tachometer.

I think using the PLC to control the speed while also using it to monitor actual speed would be ideal since the saw runs automatically and any substantial drop (or increase) in blade rotation speed can have a bad influence on the cut.

Thanks!
Chip
 
sounds like FUN

I think using the PLC to control the speed while also using it to monitor actual speed would be ideal since the saws run automatically
Now you can put a ct with 4 to 20 out on your
dc drive at let your speed be a function of the current. Equals top speed depending on the current of the saw for what ever your cutting.
(Did I say that right)? :mad:

:site:
 
I use KB drives all the time to do just this very thing. I use the KBIC-240D (I like this one since it auto switches between 90 and 180VDC based on the input power (110/220)). You will also need the SI-5 signal isolator. Both combined are around $100-$110. I would then use the F2-02DA-2 to control the board (0-10VDC). One nice thing about the SI-5 is that it supplies you with terminals to connect to instead of spade connectors.

It all works great!
 
On a Series DC machine, Speed varies directly with voltage, inversely with current (flux). This is why an unloaded series DC machine will tend to overspeed (speed is X divided by epsilon [small number approaching zero]).

On a shunt DC machine, speed varies directly with voltage, and inversely with field Flux. The formulas are actually the same, but with a shunt machine, the field is typically separately supplied and constant, so speed becomes the controlling variable with voltage.

That said, the suggestion for the CT for speed is not correct. It is more correct to use this for torque. Speed times torque is proportional to horsepower (watts). Volts being the analog for speed, and current being the analog for torque, you get I * E for power... which can easily be seen to be Ohm's Power Law.

Use volts for speed if you have a fairly constant load OR a separately excited field.
 
Before you buy anything, it would be wise to discover exactly how the present DC drive is controlled. In other words, what kind of signal is it receiving to vary the speed, determine Run/Stop, determine Forward/Reverse maybe, and any other control signals that the drive receives as input.

Once you have identified these, you can better choose a new replacement drive.

Most DC drives these days will give you an output which represents motor speed. Typical would be 0-10vdc or 4-20ma. This might eliminate the existing encoder from your system.
 
Not to stir the pot but...

Originally posted by DonsDaMan:

Speed times torque is proportional to horsepower (watts).
Emphasis added.

Peter, he didn't say this is equal to horsepower, but that it is proportional to horsepower. Since the other number in this equation is a constant, what DonsDaMan said is indeed true.

Keith
 
Kamenges,

I hope you're right or else I'm going to have to give up my day job.

Peter,

I know I over-simplified, but if you work it out in units, it does come out. This one is easier with metric units:

Velocity = m / sec
Torque = kg * m / sec^2

Multiplied: kg * m^2 / sec^3

Watt: kg * m^2 / sec^3; units correspond

http://www.essex1.com/people/speer/units.html

In standard:

Velocity: ft / sec
Torque: lbf (a force: mass times acceleration: lb * ft / sec^2)

Horsepower: ft * lbf / sec (ft^2*lb/sec^3)
 
Well, after all this talk of torque, power, and voltage, I think I'm going to do away with the current method of reading the motor RPM (armature voltage) altogether and use a prox sensor on the shaft that is attached to the saw blade.

Here is my reasoning (let me know if I'm off base):

Reason 1: I need to set the saw up to be able to truely run unattended. Currently, it can run automatically, but not unattended. If the RPM of the blade slows, for instance, the controls (PLC) have no way of knowing this and continue to drive the blade down through the cut without enough rotational speed. This is a good way to ruin the material and the blade.

Reason 2: The saw blade is belt driven. If the belt breaks (which is not often, but still a consideration) then I have a probelm similar to reason 1, but worse. The PLC needs to know what is going on with the ACTUAL blade speed.

Reason 3: Reliability of RPM reading as a standard to use on other machines. For instance, a different size saw may read armature voltage RPM slightly different than a smaller saw (different motors and belt/pulley configurations and ratios). I would prefer that since different processes require different blade speeds, that after all upgrades are complete, 1000 RPMs on one saw is 1000 RPMs on any other (at least very close) to keep confusion to a minimum. All conversions for different saw blade speeds would be calculated in PLC so the operator can see actual RPM of spindle AND actual blade speed.

Any more thoughts?

Thanks!

Chip
 

Similar Topics

Does anyone have any experience with working on piezoelectric motor control? I want to use off the shelf components to hopefully change the...
Replies
5
Views
715
So it looks like i am stuck with an old reliance DC drive on this project instead of the powerflex dc w/ 22-COMM-E. That being said i am trying to...
Replies
0
Views
1,184
powerflex755,,,, I set ptp as per page 128 of this document, but the motor does not work. I try to control using Absolute Encoder. Please let me...
Replies
0
Views
1,538
Hi all, Got an application where the customer is looking to use a servo motor, purely because of the very wide speed range required for different...
Replies
12
Views
3,657
Quick question on my first from scratch program needing to control the speed of a motor via controlled input to a motor drive; is it more...
Replies
10
Views
2,118
Back
Top Bottom