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.
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
Note: these are both induction motors.
RE: Using one motors rpm reading to control another motors rpm
TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com
RE: Using one motors rpm reading to control another motors rpm
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.
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RE: Using one motors rpm reading to control another motors rpm
Keith Cress
kcress - http://www.flaminsystems.com
RE: Using one motors rpm reading to control another motors rpm
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
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
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
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
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
RE: Using one motors rpm reading to control another motors rpm
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
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.
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RE: Using one motors rpm reading to control another motors rpm
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
Bill
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