×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Using one motors rpm reading to control another motors rpm

Using one motors rpm reading to control another motors rpm

Using one motors rpm reading to control another motors rpm

(OP)
Below is the sytem I am trying to put together... however if you have a better simpler idea please chime in.

I want to take an rpm sensor with a 4-20ma output and connect that into a 4-20ma input on a VFD so that as the rpm's of one motor fluctuate so will the other.

Can you connect them directly or do you need something between them to buffer the signal? Ideally I would like both motors to operate at their design speed, however I know that is not practical with varying loads on the sytem.

RE: Using one motors rpm reading to control another motors rpm

(OP)
I put a longer write up on here and then my computer froze so I put the short version up. I have already selected both motors and was suggested to use a toshiba VFD for the motor... so I can provide more details if needed. As a start I just wanted to see if I was even on track. I assume I will end up needing some sort of PLC to control the whole system.

Note: these are both induction motors.

RE: Using one motors rpm reading to control another motors rpm

SEW Eurodrive's controllers will do this, IIRC.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

RE: Using one motors rpm reading to control another motors rpm

This is something that is done all the time, in the VFD world it's called "slaving": one motor is the "master" and the second is the "slave / follower". You have the basics correct, but the implementation will not be as simplistic. There are a lot of "gotchas" hidden in there. Error handling, slew rates, accel and decel issues, signal drift, hunting, those are all potential problems you may need to think about. You also have not provided anywhere near enough information for a definitive answer. From the tone of your question I would hazard to guess you have little experience in VFD applications, so my suggestion would be to elicit the help of someone who does, or anticipate a lot of learning time (and the associated management patience if you work for someone else).

By the way, I assume you have a VFD on both motors, because if not, how are the RPMs varying on the master? If they are varying because of load, you might be better served by using a torque sensor on the master shaft or motor and having the slave match the torque, not the speed. But again, we don't know what your final need is here.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Using one motors rpm reading to control another motors rpm

Or perhaps run both the motors under constant speed so they don't vary at all.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Using one motors rpm reading to control another motors rpm

Agree with jraef.  If both motors are powered by VFD's, send the same speed reference to both drives and set them up as precise speed regulators.

If no VFD's, then you need to tell us more about how you are controlling speed.

RE: Using one motors rpm reading to control another motors rpm

(OP)
Ahhh my ignorance of all things electrical shines through! Yes I am a mechanical engineer that has very limited experience dealing with the electrical side of design, hence why I am here asking questions.

I will layout the whole scenario so you can better understand the situation.

"master motor" (installed on site currently operating) - 20hp, 1800rpm, 3-phase 460v power source, Licoln induction motor. This motor drives (v-belt driven) a rotary gear pump. The pump is used to move hot liquid asphalt. No VFD installed currently.

"Slave motor" (proposed system,not purchased) - 3hp inverter duty, 1750 RPM (nominal), 3/60/230/460 volt. V-belt drives a rotary gear pump. This pump is the injection pump that will supply the asphalt addative.

VFD (proposed item, not purchased) - Toshiba model VFS11-4022PL, 3hp, 3/60/460 volt, 5.5A.

This is the initial phase of design... so I am just starting to ask all the question I need to. I have been told that the rpm's on the master motor vary due to the varying load, however how much they vary and how often I am not sure (I will find out). Also I need to determine how precise I need to be (I will find out). I will update when I get more info.

End goal of this system: Run the asphalt pump at 1800rpm so it injects XY gallons of aspahlt into the system. Then run the injection pump at 1600 rpm so it injects XY*.06 gallons. Once we determine the correct ratio the setup will not change and we are only running one batch process. So if I can get the system to accurately run at pre-set rpm's without fluctuation I should be good. I can then just use the VFD to dial in the injection pump to the proper rate.

Question back to you guys:

How could I get the motors to just run a constant speed, even with varying loads?

Thanks for all the help, like you mention this is way over my head and is better suited for someone who is already familiar with the products. I plan on purchasing the mentioned items and then having the electrical company that will install the components work out the fine details to make it run like we intend (I have already spoken once with there design engineer). However I would like to have as good of an understanding of the system as I can so that I can be helpfull.... and not look like a moron.

-Jake

RE: Using one motors rpm reading to control another motors rpm

This type of proportioning system is very common in industry. There are a number of ways to implement it but the best solution depends on the details like accuracy requirements and sensitivity to process fluctuations.

The simplest solution is to drive the pumps from one motor with appropriate gearing if only one ratio is required. Your approach with two motor is sometimes called electronic gearing. A important part of your system is how will you verify that the actual flow ratios are correct initially and stay that way both short and long term. Gears pumps have internal slippage and flow can change if the back pressure on the pump changes.   

RE: Using one motors rpm reading to control another motors rpm

Are you sure that even if the main motor slows a bit the flow is not going to change so greatly that you need 'gearing'?

I just picture asphalt additives as not needing to be allll that accurate.   Induction motors don't change speed much with loading.  Then you have reduction belt gearing from the main motor which reduces that minimal speed change with load further yet.  Just seems to me to be overkill before the need is proven.

Otherwise you would need an encoder added to the main motor or better yet the pump shaft to avoid the belt slippage. Hopefully this can be done somewhere that asphalt isn't dribbling all over it.

You would then need to use that encoder signal to control the injection pump.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Using one motors rpm reading to control another motors rpm

That "master motor" you describe as 1800rpm is probably nameplated around 1760rpm.  That means that, at no load, the motor shaft will turn very close to 1800rpm and at full load it will turn at 1760rpm.  The speed varies between those two points linearly as the load changes from no load to full load.

The total variation is 40rpm which is 40/1800 = .022 or 2.2%.  Your load will probably never be less than one-half so the actual variation in speed will be only 1.1%.

The first question, clearly, is whether this amount of mixing error is significant or not.

RE: Using one motors rpm reading to control another motors rpm

(OP)
Thank you for the tips... I will continue to investigate and understand better what variation we have in our rpm's.

RE: Using one motors rpm reading to control another motors rpm

I wonder if a different approach might be more direct. Obviously you are observing varying loads on your existing pump or you would not ask the question. I am guessing that the flow of asphalt through the pump might be affected by other factors such as temperature and mixture. Additional factors such as the tank level before the pump and the flow restriction behind the pump may cause the flow to vary. Also, if you are producing more than one grade of asphalt, the difference in the product may also result in different flow. In any case, I am sure that you would want the asphalt pump to run as fast as possible to maximize production and that you would want the additive pump to run as fast as necessary in order to keep the 0.06 ratio.

First, the asphalt pump should run across the line. It is unlikely that you could increase the speed of the pump using a VFD without overloading the motor and you do not want the pump to run any slower than necessary. There is no gain with a VFD.

Next, you should measure the flow through the asphalt pump directly with some sort of flow sensor. At this point you can go two different ways.

You could use the asphalt pump flow sensor as the input to the additive pump drive. You would then control the additive pump rpm proportional to asphalt pump flow. This would be fairly accurate assuming that the additive was of consistent properties and that the flow would not vary much relative to pump rpm.

If you need very good accuracy, the next step would be to install flow sensors on both pumps. You would then control the additive pump flow proportional to the asphalt pump flow. This woud be very accurate but it may require a PLC, I am not sure.

Perhaps someone else from the forum could answer that question or offer a better suggestion.  

RE: Using one motors rpm reading to control another motors rpm

I've done this type of application. The additive, a polyester-like compound I presume, is proportionately mixed into the asphalt to make it stick better or set up faster. The mix ratio is not super critical, it's mixed in at 0.1 to 0.2% of the asphalt, so +-2% of that amount is probably no big deal.

If your asphalt pump is varying it's speed without a VFD being installed, you probably have an undersized pump. Gear pumps have no "give" to them, they are positive displacement and any detectable speed variation is a sign of overloading.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Using one motors rpm reading to control another motors rpm

(OP)
rhatcher - you are spot on with what the industry standard is. Flow meters are how this is typically handled. However they are expensive and have there own set of issues. So we are just looking at alternative designs before we just go spend a large lump sum of cash.

jraef - I have wondered the same, I spoke with the plant manager yesterday and he said that based on the temp. of the asphalt we can see huge variations (I am working with him now to quantify what "huge" is). We only offer one product currently so that is not an issue.  

RE: Using one motors rpm reading to control another motors rpm

A poor man's fix may be to measure the temperature of the asphalt and infer the flow rate from the temperature. The accuracy of this setup will be affected by the accuracy of temperature/flow information. Talk to someone in the industry and see if you can get within your target range with this approach.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources