Motor Cycle time

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
I have a 7.5 HP motor that will be cycling. A grain auger. Starts and stops loaded with product. Run time about 35 seconds, Off about 15.
The small detail I'm missing is the current while running. SEWAG at the moment is 3/4 load. 8-9 amps. An hour or two every couple days.

Too often?
 
NEMA MG-10 chart shows a 7-1/2 HP 4 pole motor (assuming here) is supposed to be good for the LESSER of 13.9 Starts/Hour, or the product of S/H x Wk2 = 39, whichever is lower, with an off time of 44 seconds between cycles. So without knowing the load inertia (Wk2), the limit is going to be 13.9 S/H. That's roughly once every 5 minutes including the rest time, or roughly 8x what the motor is designed to handle, so you are going to be exceeding the motor design limits and affecting the motor life. What that boils down to in terms of motor life is difficult to quantify, but you can just explain to the user that the motor is essentially a "consumable" here.
 
Jraef said:
NEMA MG-10 chart shows a 7-1/2 HP 4 pole motor (assuming here) is supposed to be good for the LESSER of 13.9 Starts/Hour, or the product of S/H x Wk2 = 39, whichever is lower, with an off time of 44 seconds between cycles. So without knowing the load inertia (Wk2), the limit is going to be 13.9 S/H. That's roughly once every 5 minutes including the rest time, or roughly 8x what the motor is designed to handle, so you are going to be exceeding the motor design limits and affecting the motor life. What that boils down to in terms of motor life is difficult to quantify, but you can just explain to the user that the motor is essentially a "consumable" here.
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Thank you. My guess was it would be toast before long
I have an older scavenged VFD I’ll offer for use until millwrights and owner determine a solution. Start it full speed, then back off and maybe we can get it to run continuously.
 
ptonsparky said:
I have a 7.5 HP motor that will be cycling. A grain auger. Starts and stops loaded with product. Run time about 35 seconds, Off about 15.
Click to expand...
Are there sensors that indicate when the product is fully loaded and also when it's getting low, such that they determine when the motor stops and starts? Or is there a timer that's cycling it?

If sensors are controlling it, then you could have the sensors switch a speed reference voltage to the VFD between two different levels instead of turning the motor on and off. The higher speed would be somewhat above the average speed that's needed by the system, and the lower speed would be somewhat below it. That way the motor wouldn't have to stop, but instead would cycle between the two speeds.
 
Would it not be easier for them to close the chute to save wear on the motor?


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synchro said:
Are there sensors that indicate when the product is fully loaded and also when it's getting low, such that they determine when the motor stops and starts? Or is there a timer that's cycling it?

If sensors are controlling it, then you could have the sensors switch a speed reference voltage to the VFD between two different levels instead of turning the motor on and off. The higher speed would be somewhat above the average speed that's needed by the system, and the lower speed would be somewhat below it. That way the motor wouldn't have to stop, but instead would cycle between the two speeds.
Click to expand...


brantmacga said:
Would it not be easier for them to close the chute to save wear on the motor? There used to be a gate to limit the grain flow, but for whatever reason the millwrights removed it.

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This is related to my 'LOAD RPM' thread.
The current of one motor gives me a clue as to how close the Mill is to plugging. We have modified the control to shut off the Feed motor at a certain point and keep it off until the current goes down again. This saves the mill from locking up, but is causing the excessive cycling.

In the mean time we have several hundred head of cattle that need to be fed every day until the owners and millwrights decide who pays for what and how to fix the problem.

A drive to load ratio change of sheaves would help. That has been done once already.
 
brantmacga said:
Would it not be easier for them to close the chute to save wear on the motor?


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That don't work if you have direct feed, if it is running product needs somewhere to go. Some sort of surge tank and gravity feed would work though if you need to close a gate frequently. Needing to stop flow frequently might mean it is small batches at a time instead of a continuous flow process?
 
kwired said:
That don't work if you have direct feed, if it is running product needs somewhere to go. Some sort of surge tank and gravity feed would work though if you need to close a gate frequently. Needing to stop flow frequently might mean it is small batches at a time instead of a continuous flow process?
Click to expand...
Gravity feed from a holding bin so a gate would solve the problem, if it was still there.
 
ptonsparky said:
This is related to my 'LOAD RPM' thread.
The current of one motor gives me a clue as to how close the Mill is to plugging. We have modified the control to shut off the Feed motor at a certain point and keep it off until the current goes down again. This saves the mill from locking up, but is causing the excessive cycling.

In the mean time we have several hundred head of cattle that need to be fed every day until the owners and millwrights decide who pays for what and how to fix the problem.

A drive to load ratio change of sheaves would help. That has been done once already.
Click to expand...
Posted about same time as my last post. That seems to pretty much be continuous flow process you maybe need to vary speed of supply auger. Could probably still automatically vary speed based on current drawn by the mill. This could get a little more complex with say a roller mill if you happen to adjust the clearance of rollers very often which likely changes the load on the mill.
 
ptonsparky said:
Gravity feed from a holding bin so a gate would solve the problem, if it was still there.
Click to expand...
Sounds like millwrights screwed up. Who installs a feed mill (hammermill or roller mill) without some method to control input flow? Even basic manual speed control of the supply auger is somewhat effective. Change of moisture level of supply corn is all that is needed to need to tweak the speed a little.
 
ptonsparky said:
Gravity feed from a holding bin so a gate would solve the problem, if it was still there.
Click to expand...

I think I follow.... it’s broken, and of course it can’t be fixed until the grain bin is empty?

We had one break in the closed position last year and had to vacuum 15k bushels out. Not fun.


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