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2 Phase #29883 09/27/03 03:13 AM
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aldav53 Offline OP
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Think this was talked about a while back but how does single phase compare with 2 phase?


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Re: 2 Phase #29884 09/27/03 03:18 AM
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Pinemarten Offline
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I don't think 2 phase exists much anymore.
I think it was 2 phases 180 degrees apart.
Single phase (in NA), is 240V, one phase with a grounded tap at the center of the secondary winding of the transformer.
This provides 120V to ground/neutral.

Re: 2 Phase #29885 09/27/03 03:22 AM
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aldav53 Offline OP
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What was the voltage on the legs.


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Re: 2 Phase #29886 09/27/03 03:51 AM
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Scott35 Online Happy
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This has been coming up alot lately! [Linked Image]

To answer the original poster's questions:

1Ø is nothing at all like 2Ø.

First off, Single Phase is just thst - Single Phase!

Two Phase is a Polyphase System - like 3 Phase is.

2Ø has Two individual Phases, which are 90° offset - like 3Ø has Three individual Phases, which are 120° offset.

There are 2Ø 3 Wire, 4 Wire and 5 Wire systems (more like "were"...). It's an older, antiquated polyphase Power System, which has been made obsolete by 3Ø systems.
It's still in use on Stepper / Servo Motors, along with the control + clock circuitry for them.

The closest comparison to 1Ø would be on Motors.
A Single Phase Split Phase Induction Motor resembles a Two Phase Induction Motor, being that the 1Ø Motor has both a "Run" winding and an "Aux./Start" winding, which in the starting sequence will have a Phase offset between 30° and 75° - maybe coming close to 90°.
Of course, the Two Phase Motor will have no start switch, no start Capacitor, and will have at least one additional Phase Line connecting to the windings. Both windings will always be active, and both will have 90° offset (± may come into effect).

Check out the Technical Reference area for Transformer and Motor Schematics for 2Ø Systems.
They may be accessed from the Menu For Technical Reference Area


Scott35

p.s. direct url is: https://www.electrical-contractor.net/ubb/Forum15/HTML/000064.html

edited some silly cornfusion!

[This message has been edited by Scott35 (edited 09-27-2003).]


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Re: 2 Phase #29887 09/27/03 05:26 PM
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Trumpy Offline
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Pinemarten,
2 Phase is still used, although not as much as it used to be.
3 Phase is used more often because it gives a better balancing to a Supply system.
However, Industrial Welders still use 2 phase.
Over here in NZ, we still have cookers that are connected to 2 Phase + Neutral, for places way out in the country that suffer from Voltage Drop. [Linked Image]

Re: 2 Phase #29888 09/27/03 07:39 PM
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classicsat Offline
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That's technically not 2 phase, but 2/3 3 phase.

Re: 2 Phase #29889 09/27/03 07:45 PM
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aldav53 Offline OP
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What are the voltages with 2 phase? Do they vary?


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Re: 2 Phase #29890 09/30/03 04:44 AM
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Pinemarten Offline
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I assume that the voltages on 2ph were similar to the modern 3ph systems, depending on the equipment being serviced.
7-12.5 kV knocked down to 480, 460, 240, 230 208 etc. I think 600V was developed after 2ph went away.
I once heard that voltages have slowly increased to shrink the size of the wire, and therefore the size of motors and feeders. We have motors at work that have 230/480V wiring diagrams (delta/wye or parallel/series). The amperage is double for the lower voltage.
The first 600V motor I saw was a 10HP and it was about the size of a breadbox.
Does anyone have any pics of 5-20 HP motors in single phase? They must be huge.

Re: 2 Phase #29891 09/30/03 11:12 AM
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winnie Offline
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Actually, all other things being equal, changing the voltage to a motor will not change the size of the motor. It will change the size of the conductors needed to feed the motor, and it will change the requirements for the switchgear needed to control the motor. It will change the termination requirements inside the motor, and it will change the insulation requirements. But once you account for these differences, the motor size will stay the same for different voltages.

When you increase the voltage to a motor, the current required to deliver the power goes down, and thus the required wire size goes down. But since you have the same rotor spinning with the same magnetic flux going through the same winding cross section, the volts/turn stays the same, and thus you needs a greater number of turns. The result is that the higher voltage version of the motor has more turns of thinner wire, with the same 'net slot current' (current per conductor times number of conductors in the slot).

As with building wire, the same insulation system is pretty much used for everything below 600V, so the insulation thickness for a 120V motor and a 600V motor will be essentially the same. At higher voltages, special insulation systems are required; the insulation starts taking up a lot of space at this point and will change the size of the motor. I've seen a white paper describing motors built with the same sort of insulation systems used for underground high voltage cables; the idea is that you run your motor directly at 69KV, and eliminate the need for a step down transformer. The motor itself gets larger, to make room for the extra insulation.

If you really want to make a motor small, what you need is _speed_. The size of a motor pretty much scales with _torque_, not power. Rotational mechanical power is the product of torque and speed (just as electrical power is the product of volts time amps). So you can double the power of a motor by doubling its speed. 400Hz motors are _very_ small for their power rating. The limit here is the mechanical capability of the various parts to deal with the rotational stresses.

-Jon


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