Agitator motors

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gadfly56

gadfly56

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GoldDigger said:
I think that the appropriate units for viscosity are pound-seconds per foot squared.
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I looked around, and Haji has the right form for viscosity which is M/(T*L) for M = mass, T = time, L = length. The usual form for Re is (Rho * D * v)/mu (sorry, Alt key not working for Greek letters). I think he left out a conversion factor.
 
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GoldDigger

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If you measure density in pounds per cubic foot instead of mass the other conversion between Mass over (Time x Length) works out right as equivalent to Force x Seconds over Feet squared.
The common misuse of pounds (unit of force) instead of poundals (unit of mass) causes the English system to be a lot harder to keep straight that the SI (MKS) or the Gaussian (cgs) system. If only I could get used to pressure in Pascals I would be happy using SI.
I also have a problem with using propellor "diameter" in feet squared!
In any case a 20% change in speed or in diameter will not change the Reynolds number enough to change flow regimes.
 
T

topgone

Senior Member
ptonsparky said:
The idea of these pits and stirring machines is to keep the solids in suspension until it is run through a separator and compression cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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AFAIK, agitator rule of thumb considers the amount of slurry inside a specific tank and the power required for that volume. If you can, please tell us how big a slurry pit you are dealing with, we can compare with usual practice.

IIRC, for an intensive agitation, the recommended input power per 1000 gallons of slurry is around 10 HP. LINK, page 292
 
ptonsparky

ptonsparky

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topgone said:
AFAIK, agitator rule of thumb considers the amount of slurry inside a specific tank and the power required for that volume. If you can, please tell us how big a slurry pit you are dealing with, we can compare with usual practice.

IIRC, for an intensive agitation, the recommended input power per 1000 gallons of slurry is around 10 HP. LINK, page 292
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Roughly 30 by 32 foot with a working depth that will vary between 2.5 to 5 feet. 17.9 to 35.9 thousand gallons, I think. Two 15 hp agitators in each tank with what are now 10 hp props. Why not 10 hp motors? IDK. Not operational yet.
 
ptonsparky

ptonsparky

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My memory is as long as my uh, little finger so bear with me. The way I set the overloads for a motor when adding PFC, is I read the current of the motor prior to and then with the correction included. Then divide the corrected value by the prior value to determine the reduction in current. 11.4/16=.7125. .7125 * 19 = 13.54 which would be the new value for my motor overloads. 19 being the nameplate FLA. I do not remember how I came up with that solution. Is it close?
 
H

Haji

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The most accurate way for finding the O/L setting for any motor involves using one of the fine small power analyzer meters . Simply measure the fully loaded motor current drawn through the protective overload device before and after capacitor installation, and reduce the overload ampere rating by the amount of overload current reduction caused by the capacitor installation (taking vectorial addition of real and imaginary components). The final ampere value of the overload selected should not exceed the fully loaded motor current measured through the overload multiplied by the appropriate percentage from 430.32
 
ptonsparky

ptonsparky

Tom
Location
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EC - retired
Haji said:
The most accurate way for finding the O/L setting for any motor involves using one of the fine small power analyzer meters . Simply measure the fully loaded motor current drawn through the protective overload device before and after capacitor installation, and reduce the overload ampere rating by the amount of overload current reduction caused by the capacitor installation (taking vectorial addition of real and imaginary components). The final ampere value of the overload selected should not exceed the fully loaded motor current measured through the overload multiplied by the appropriate percentage from 430.32
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These are all the same Motors with the same PFC. Seems like a wide variation to me, but I don't know the depth of each unit nor the angle of the dangle.

Motor 1. 15.9 amps reduced to 13.9 by capacitor. Your Method would be to reduce Heater size from 19 to 17. amps
Motor 2. 17.9, 14.3, 15.4
Motor 3. 17.1, 13.1, 15
Motor 4. 16.6, 12.2, 14.6

FWIW, the Tech set them all at 17. (He didn't want to come back in over the weekend) We were lucky to differentiate between full numbers on those small scale dials anyway.

Close enough.
 
T

topgone

Senior Member
ptonsparky said:
These are all the same Motors with the same PFC. Seems like a wide variation to me, but I don't know the depth of each unit nor the angle of the dangle.

Motor 1. 15.9 amps reduced to 13.9 by capacitor. Your Method would be to reduce Heater size from 19 to 17. amps
Motor 2. 17.9, 14.3, 15.4
Motor 3. 17.1, 13.1, 15
Motor 4. 16.6, 12.2, 14.6

FWIW, the Tech set them all at 17. (He didn't want to come back in over the weekend) We were lucky to differentiate between full numbers on those small scale dials anyway.

Close enough.
Click to expand...

Just some points to remember here.
  • You set the overload based on the motor nameplate amps because thats the level of current the motor designers use when sizing the winding conductors.
  • Similarly, TOL selection is done such that the lowest dial adjust will be around 15% lower than the TOL trip setting. Doing this will allow the lowering of TOL tripsetting especially when motor operating history tells you your setting is a bit too high (motor burnouts occur often).
  • The placement of the PFC relative to the TOL determines whether the current passing thru the TOL is the "reduced" current due to PFC. If you connected the PFC unit just after the starter/contactor terminals (compared to connecting the PFC right after the TOL terminals, in parallel with the motor lines), the TOL setting should be solely based on the motor rated amps.
Hope that helps.
 
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