Balancing Loads on a three Phase System

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Smart $ said:
No need for transformers, just connect the two motors either line-to-line or line-to neutral but not the same line-to-line or line-to neutral. Balancing depends on the 1Ø motors having different HP ratings and thus FLA's. Then there's the issue of power factor and whether it plays any role in balancing.

For an overly simplified answer, assign HP of 5, 10, and 15 to the connected 1Ø motors, connect the other two LN to the same lines as the already-connected 5 and 10 conversely to the existing 5 and 10. Single phase motors will have 15 HP connected to each line. The 3Ø motor is irrelevant to the proposal.
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An important condition in the statement of the problem is "to keep the load balanced". That implies that it is already balanced with the three single-phase motors already in place.
That takes your simplified answer out of consideration. <sigh>

I will not say any more, other than to confirm that if you can freely choose the size of the additional motors and you can change the existing wiring it can be done.
 
GoldDigger said:
An important condition in the statement of the problem is "to keep the load balanced". That implies that it is already balanced with the three single-phase motors already in place.
That takes your simplified answer out of consideration. <sigh>

I will not say any more, other than to confirm that if you can freely choose the size of the additional motors and you can change the existing wiring it can be done.
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Emphasis. :p
 
Smart $ said:
No need for transformers, just connect the two motors either line-to-line or line-to neutral but not the same line-to-line or line-to neutral. Balancing depends on the 1Ø motors having different HP ratings and thus FLA's. Then there's the issue of power factor and whether it plays any role in balancing.

For an overly simplified answer, assign HP of 5, 10, and 15 to the connected 1Ø motors, connect the other two LN to the same lines as the already-connected 5 and 10 conversely to the existing 5 and 10. Single phase motors will have 15 HP connected to each line. The 3Ø motor is irrelevant to the proposal.
Click to expand...

Then throw in a reality check - see how many single phase motors you can find over 1-1/2 hp that have the leads made available to operate them at 120 volts. You may find a few rated 2 or 3 hp but go over that and they are nearly non existent.
 
The answer is no.

Because regardless of the Horsepower you don't know what the load on those motors will be.


JAP>
 
Thanks, guys for your views, suggestions and opinions, hopefully my next class is next Wednesday 02 March, my instructor would reveal the connections and I will let the forum know. But I will please welcome any view or suggestion before the next class since I get credit for the right answer on my research work. Again thanks guys.
 
mnkworkstation said:
Thanks, guys for your views, suggestions and opinions, hopefully my next class is next Wednesday 02 March, my instructor would reveal the connections and I will let the forum know. But I will please welcome any view or suggestion before the next class since I get credit for the right answer on my research work. Again thanks guys.
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So from the suggestions you have been given, what is your solution and how did you arrive at it?
 
jap said:
The answer is no.

Because regardless of the Horsepower you don't know what the load on those motors will be.


JAP>
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OP said:

My instructor also added that, the system could be any voltage, HP as well as amperage etc. He only wants the two single phase motors (regardless of HP and connection type: Phase neutral or phase phase) be added to keep the load balance.
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So I take that as meaning we get to play around with the numbers and connections as long we have the required amount of motors.
 
If the two motors can be any voltage, HP, but single phase, then hook extra #1 as a 10HP motor A-B and extra #2 as a 5hp C-N? Not seeing how they can be hooked up just by connecting lines in the drawing w/o some more information.
 
JFletcher said:
If the two motors can be any voltage, HP, but single phase, then hook extra #1 as a 10HP motor A-B and extra #2 as a 5hp C-N? Not seeing how they can be hooked up just by connecting lines in the drawing w/o some more information.
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By tuning the motor HP and design voltage appropriately, what you suggest should work. Motor #1 would have to be designed for the L-L voltage while motor #2 would have to be designed for the L-N voltage. And then, as others have pointed out, the mechanical load applied to the two motors would have to be matched too. (Twice the mechanical load on #1 as on #2 assuming equal efficiency.)
 
GoldDigger said:
By tuning the motor HP and design voltage appropriately, what you suggest should work. Motor #1 would have to be designed for the L-L voltage while motor #2 would have to be designed for the L-N voltage. And then, as others have pointed out, the mechanical load applied to the two motors would have to be matched too. (Twice the mechanical load on #1 as on #2 assuming equal efficiency.)
Click to expand...

:thumbsup: in a much more intelligent fashion than I put it I might add. :)


JAP>
 
JFletcher said:
If the two motors can be any voltage, HP, but single phase, then hook extra #1 as a 10HP motor A-B and extra #2 as a 5hp C-N? Not seeing how they can be hooked up just by connecting lines in the drawing w/o some more information.
Click to expand...

Dang
should have posted before reading up to page 4 lol

This
motor 1 line a - line b 10 fla
motor 2 line c - neutral 10 fla

since the ckt ahead is balanced and we added 10 A to a, b, c and n should remain balanced
 
Ingenieur said:
Dang
should have posted before reading up to page 4 lol

This
motor 1 line a - line b 10 fla
motor 2 line c - neutral 10 fla

since the ckt ahead is balanced and we added 10 A to a, b, c and n should remain balanced
Click to expand...
There is one small problem with this that the instructor is likely to ignore:
The current in a and b (and c) initially will be at a uniform phase angle offset dependent on the motor design and load.
But the current in a and b from the motor #1 L-L connection will be leading on one and lagging on the other, resulting in an unbalanced vector sum current when comparing a, b, and c afterward.

Maybe the OP can get extra credit for knowing that. :)

The resistive components should be close enough for government work.
 
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