explanation of 3 phase motor wires l1, l2, l3?
This question is regarding my home brew RPC project.
How do I know which wires into my 3 phase motor are l1, l2, and l3?
I have the idler motor wired for low voltage. The diagram for the nine wires to run low voltage shows #6,5,and 4 all connected.
* 9,3 together
* 8,2 together
* 7,1 together
Is 7,1 actually l1?
L wires are line wires bringing the power to the device.
If a RPC, there will be but two L wires since you power it from a single phase source.
1 & 7 are winding wires and connect to L1
2 & 8 are winding wires and connect to L2 in a three phase system
3 & 9 are winding wires and connect to L3, or possibly L2 when powering a RPC
There is only L1 and L2 to feed a RPC
You will ALSO have L wires LEAVING the RPC to feed what ever you want to have fed three phase to. These can also be L1, L2 and L3, and are connected to the related winding wires as noted above in a 220/230/240 volt system.
L2 of these LEAVING or OUTPUT wires are often also connected to run caps, which are in turn connected to possibly both L1 and L2 of the RPC FEED line wires
"I have the idler motor wired for low voltage. The diagram for the nine wires to run low voltage shows #6,5,and 4 all connected.
"* 9,3 together
"* 8,2 together
"* 7,1 together
"Is 7,1 actually l1?"
The phases are labeled A, B and C, rotating in a clockwise direction [ * ] , from lowest-numbered to highest-numbered, from the outside to the inside.
Therefore, the start of the A winding is T1, the start of the B winding is T2 and the start of the C winding is T3, while the end of the A winding is T4, the end of the B winding is T5 and the end of the C winding is T6.
However, on a two voltage motor, the start of the second A winding is T7, the start of the second B winding is T8 and the start of the second C winding is T9, while the end of the second A winding is T10, the end of the second B winding is T11 and the end of the second C winding is T12.
But, on a 9-wire motor, what would be T10, T11 and T12 are internally connected and insulated from the others, and are not available outside the motor's housing.
So, for low voltage, (T1 and T7), (T2 and T8) and (T3 and T9) would be paired as shown and independently brought to the control terminals labeled, respectively, L1, L2 and L3, while T4, T5 and T6 would be connected and insulated from the others.
There is usually a bar under the T6 and T9 terminals, and these appear as: 6 and 9, respectively.
T1 to L1 to A, T2 to L2 to B and T3 to L3 to C is the usual connection.
To reverse the load motor, reverse any two of the connections to L1, L2 or L3.
[ * ] A few utilities use A, C and B phase rotation, which is counter-clockwise, therefore the actual rotation should be confirmed when the machine is installed.
Thanks for the responses. Very kind on my first post here. The first time I tried registering for practical machinist the administrator denied my registration. Not sure why, but glad I'm on here now.
Sounds like my question should have started at my circuit breaker box where I have 220V(or 208V, I haven't measured it). How do I know which wires are L1 and L2 there? There is a red wire and a black wire that feed the metal bars that the 30amp 2 pole breaker snaps onto. I did mark the wires out of the box all the way to the other end of my garage where they go to the RPC setup I'm building.
Those are the two hots that feed each two pole breaker, one of which will be used (maybe?) to feed the RPC. I.E., you can call Red L1 and Black L2
There is a red wire and a black wire
"How do I know which wires are L1 and L2 there?"
Perhaps a better question is: "How do I know which wires are A and B there?".
By custom, three-phase power is labeled A, B and C.
Also by custom, and now by code, a three-phase panelboard is labeled A, B and C, left-to-right or top-to-bottom. This places B, which can be the so-called "high-/wild-leg" in the middle. A high or wild leg should not be used as part of a single-phase source.
And, in an RPC situation, the manufactured phase is B, too. The manufactured leg also should not be used as a single-phase source.
So, the two single-phase lines will eventually become A and C, and the correspondence is then L1 (single-phase) to A (three-phase) and L2 (single-phase) to C (three-phase).
In some shops, there are two panelboards, one for single-phase loads and another for three-phase loads.
It then makes more sense to equate L1 (single-phase) to L1/A (three-phase) and L2 (single-phase) to L3/C (three-phase), with L2/B being the manufactured phase.
The manufactured phase should be "identified", and the required identifier is orange wire or another non-white and non-green wire to which has been affixed orange tape wherever this wire is seen, but certainly at each end.
So, it makes sense to assign black to L1 (single-phase) and to L1/A (three-phase), orange (or black with orange tape) to L2/B (three-phase) and red (or black with red tape) to L2 (single-phase) and to L3/C (three-phase).
The wiring works. I can pull start the 7.5hp idler motor with a cord and then flip on the knife switch with 30 amp fuses.
Now my next challenge is getting a push button start to work since pulling the rope isn't too much fun. When I apply single phase with momentary push button to l3/C I expect the motor to start rotating but it doesn't. I have qty 2 208 MFD capacitors wired in series that are inline with the l3/C wire. This is how I understand the circuit should be. Once the motor is spinning I will manually flip on the knife switch. At least that is the plan.
So here's what I have wired up. Are capacitors directional? I wired them in series. Hopefully the picture of the schematic shows up when I post this.
Here is the link. I think the direct attach didn't work for the image
They have to be in parallel.
"Are capacitors directional? I wired them in series."
When wired in parallel, you get the effective capacitance as the simple algebraic sum of the two capacitors, as
Ceff = Ca + Cb
When wired in series, you have to compute the effective capacitance by applying the "series capacitor law", as
Ceff = 1 / ( ( 1 / Ca ) + ( 1 / Cb ) ) )
I found out they have to be parrallel today too. I also heard the voltage rating should be higher if wiring in parralell. It doesn't matter now, I found a different capacitor. On attempting my push button start I started blowing the 30 amp fuses in the knife switch safety on/off box and it snowballed from there.
12 hours later and my RPC has a total makeover. Ditched the knife switch completely. Got a huge contactor and 40 amps fuses at ABC electric. Now I have a on/off toggle switch. I also picked up a 500 MFD capacitor for starting. It starts up real fast. You can't start and stop it frequently (less than a minute or two) or it will misfire on start up and blow a fuse. I suspect the capacitor has to cool down a little. Says it's rated from 200V and 250V peak. So life's great there (as far as I know anyhow).
On to the run capacitors...
Without any run capacitors I get 120V to ground from both A and B. I get 191V to ground from the wild leg (C).
The formula I read for starting point said 15 MFD/hp (7.5x15 would be 90). Well I ended getting some 50 MFD's. Bridging both A to C and B to C the suckers hum real loud. The voltage difference got worse too (208V from C to ground).
My references to wire names here matches the posted diagram, however I have taken note and labeled the manufactured/wild leg as B on my actual physical set up.
when measuring voltage for 3 phase a,b,c to ground is irrelevant.
you need to measure voltages between leads eg: voltage a-b,b-c,a-c to get a proper value.
also when using capacitors the value a-b would work for your formula but in my case was actually higher with my 7.5hp idler but the ratio is a general rule60/40 value between a-b vs b-c.
eg: my final results
I have a 7.5 hp converter wired to 240v.
after ballancing I have:
B= wild leg
> A-B 255v
> B-C 255v
> A-C 238v (line voltage)
> Fitch Williams rule of thumb is line*1.08= generated legs (unloaded) so:
> 238*1.08=257v (I think I got close)
> cappacitors used:
> AB 130 uf (not counting start caps of 380 uf)
> BC 62 uf
> AC 42 uf (power factor)
my amps were: input amps on both single phase lines were 2.65A
L3- 3.70A (I tried hard to ballance this closer but this was the final result)
amps were measured before the transformer on the 240v side but on the output of the idler.
I used 440v run capacitors and 330v start capacitors.
these measurements were taken with a 2hp mill under light load,there is also a load from my 6kva autotransformer which upgrades my voltages to work with my 550v mill.
after connecting the L1(A) and L3(C) leg to 240v input,600 v output and the L2(B) leg to 240v input and 480 v output my volatges are 575v between each leg. right or wrong thats how I have things working right now.
as far as wiring in the capacitors goes you do need them to be a higher voltage value than what you intend to produce.
if you wire 2 or more equal voltage capacitors in parrallel you add thier capacitance values but do not effect thier voltage capacity.
if you wire 2 or more equal voltage capacitors in series you cut thier capacitance value by 1/2 but double thier voltage carring ability.
it may or may not help but here is the diagram I followed to wire my converter with a starting circuit which worked great:
if you use the potential relay I will have to find the box to give the model and number I used at a later date after I retrieve the box under my bench.
hope this helps you out
Last edited by racer55; 12-31-2009 at 05:10 AM.
as a side note the potential relay I used was a Mars 68
pickup voltage: minimum-325 maximum 345
drop-out voltage: maximum-135
which seems to be a very good fit for me as is.
another side note is that I used a new WEG 7.5hp tefc motor for my idler so depending on the era and model of idler you use results may be different.
This has been a very interesting thread for me.
As I understand it, the L1 and L2 lines from the home panel become L1 and L3 on the output side of the RPC, with the output L2 being generated.
It also recommends (or requires?) that L1, L2 and L3 (output side of RPC) be wired to A (1-7), B (2-8) and C (3-9) respectively on the idler motor and the end application motor(s).
In addition, to reverse the rotation of an end-application motor, any two of the input leads to the end-application motor are to be reversed.
The question I have is: Is there any consequence for NOT wiring in the manner identified above?
I have a 7.5 HP rotary converter that I have used to power multiple motors (usually just one at a time).
In the past, I was not aware that there is a specific recommendation (requirement?) to hook the input leads to a specfic motor lead (L1 ->A, etc). Sometimes the motor, when connected to the RPC output wires (without regard for which wires were L1, L2 and L3), would either not start or be under powered. I would then switch the connections around till the motor would run as I expected it should. I also had one time when the motor did not start, the RPC chattered.
Were these failure symptoms a consequence of not connecting leads as recommended in this thread?
Can someone provide a further explanation of why it makes a difference?
I have very little knowledge of electrical code but my understanding is that the generated leg needs to be identified at each end of the circuit by something orange and connected as B,L2 in every case so that it is not confused and it is also code as stated above.
On some applications where switches/contactors or single phase connections are required the best results are achieved by connecting to line power not the generated leg.
eg: if you need to supply single phase 120v power to a switch or pump or light ect. then it should be connected to A,L1 or C,L3(line direct from the pannel) because these connections to neutral will give the proper voltage.For a 240v setup these lines to neutral will give you 120v.The generated leg B,L2 to neutral will give a totaly different voltage of about 191v as posted above and make things work badly or not at all.
Also when you have magnetic starters or the like that use 240v coils then the proper connection is with A,L1-C,L3 for the coils,you might have had the generated leg connected here which could have affected the the contactor coils and a no start condition. the generated leg is not a consistant power source.
this is a rough explanation and any errors or omissions are welcomed to be pointed out.
Any stories on getting RPC's approved with a state/local electrical inspection? I'm afraid to ask but curious...
A friend of a friend has small business and could not pass electrical inspection with presence of RPC in his shop (as far as I know it was RPC, not static converter).