How would you take Foot Per Minute and work that back to needed frequency for a drive

Rob S.

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Sep 2008
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Good Afternoon ,

I'm upgrading a machine , and all the conveyor speeds are in Foot Per Minute. That is what the operator will select on their screen. How would I take the Foot Per Minute of a conveyor let's say 46.12 fpm , coming from a 6.130 " head pulley , being driven by a 28 RPM Gearmotor and come up with a Frequency Command for a Frequency Drive ?

I can come up with the RPM , but coming up with the Frequency is what I'm strugging with .

Thanks so much,
 
I like to write the ratios and units out longhand to be sure I'm getting them right.

46 feet/ minute x 12 inches/ft = 552 inches/minute

A 6.130 inch diameter head pulley has a circumference of (pi * 6.130) = 19.258 inches.

So the head pulley will have to spin at 552 inches/minute divided by 19.258 inches/revolution = 28.663 revolutions per minute.

You referred to a "28 RPM gearmotor". I suspect that means that it runs at 28 RPM when the motor is running at 60 Hz.

So your ratio is essentially 46 FPM : 60 Hz.

Take the operator's entry in Feet/Minute, multiply by (60/46 =) 1.304, and that will get you the command frequency for the VFD.

(please, folks, check my geometry and my calculations !)
 
Last edited:
Good Afternoon ,

I'm upgrading a machine , and all the conveyor speeds are in Foot Per Minute. That is what the operator will select on their screen. How would I take the Foot Per Minute of a conveyor let's say 46.12 fpm , coming from a 6.130 " head pulley , being driven by a 28 RPM Gearmotor and come up with a Frequency Command for a Frequency Drive ?

I can come up with the RPM , but coming up with the Frequency is what I'm strugging with .

Thanks so much,
You need to know the base motor speed or the gear ratio of that gear motor for that. For example if the base motor speed is 1725 RPM at 60Hz to get the shaft speed of 28 RPM, that’s a gear ratio of 61.6:1, so once you know that you use it to calculate what Hz you need to get a particular RPM.
 
Thanks for your wisdom , gentlemen.

So you are saying that each foot per minute would be 1.304 HZ ?

"Take the operator's entry in Feet/Minute, multiply by (60/46 =) 1.304, and that will get you the command frequency for the VFD."

Thanks for your help.
 
After you do all that- then put a hand tach on it and get the real speed and adjust your scale factor
:)


Same here. When setting up new equipment, we set everything up initially just to run at 100% speed, then measure the actual speed, and scale accordingly. All of my HMI screens have a calibrations page to enter the actual measured values, so everything is always scaled into EU in the PLC.
 
I agree with all of the above.

Also, if I simplify the calculation in a PLC by using a simple scale factor, I like to put the formula in a rung comment so future readers (usually me) will know where I came up with that number.
 

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