Single phasing

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electrics

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hi, what happens if one phase of an induction motor is gone?
i know it is a very cliche question but i want to learn more else,
1 . single phasing means one phase is gone or one phase is on which one?
if one phase is gone its name is a bit weird for me.(it sounds as if one phase is on )
2- İf the two phase is on so will they each draw 1,5 times normal phase current if not why?
3- İF the two phase is not enough to drive the load, so how a decline of speed and stall situation can be proposed? what is the logic of this..?
4- iF the load increases with one phase gone, thermal overload relays is said to protect motors from single phasing, why are they used for this_? any switch wont work in an overload situation ???
best regards...
 
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electrics

Senior Member
I have had no time for reading it, pls till i read it tell me if one phase is gone, the other two one creates a revolving or one direction or whatever oriented magnetic field ? the two will have opposite phase angles to each other so what can be said?
 
mivey

mivey

Senior Member
Single-phasing is the loss of one phase. Single-phasing can cause currents in the remaining lines to increase to 173% (square root of 3) times normal. It may be as high as 200% because of power factor changes. It may approach LRA for high inertia loads.

The overloads may not protect the motor if they are sized for full load and the motor is lightly loaded. If 173% of the lightly loaded motor does not cross the threshold, circulating currents can still damage the motor.

Thermal sensors are used in addition to current sensors because sometimes the rate of temperature rise may be greater than the rate of current rise.
 
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winnie

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Electric motor research
http://forums.mikeholt.com/showthread.php?t=123185
electrics said:
hi, what happens if one phase of an induction motor is gone?
i know it is a very cliche question but i want to learn more else,
1 . single phasing means one phase is gone or one phase is on which one?
if one phase is gone its name is a bit weird for me.(it sounds as if one phase is on )
Click to expand...
The answer is _both_. If you have a three supply conductors supplying 3 phase power, and one conductor fails open, then you are left with only 2 conductors supplying power. 2 conductors can only supply _single_ phase power. 3 phase minus 1 phase = 1 phase ;)

electrics said:
it tell me if one phase is gone, the other two one creates a revolving or one direction or whatever oriented magnetic field ?
Click to expand...

When a motor is 'single phased', the connected supply current creates an alternating but not rotating magnetic field. This alternating magnetic field will interact with the already spinning rotor to provide some torque, in much the same way that a 'capacitor start' motor will work. Motor performance is significantly degraded, and the motor will be unable to start.

The answers to your other questions depend upon the specific circumstances, so read the document that bob linked. If you don't have time to read it, how can you expect other people to have time to write?

-Jon
 
E

electrics

Senior Member
winnie, it is a big pleasure for me to read what u wrote, thank u, these answers have satisfied me..thanks mivey to u too..
 
mivey

mivey

Senior Member
It appears some moderator removed my post but I'm not sure why (maybe they did not like the joke), so here is the meat of what I said in another post that gives more detail on why we say "single-phase" when we lose one of three phases (part of the OP's original question):

When you take a voltage reading between two points, you only have one waveform. Even though the two points may have a voltage that is different when referenced to a third point, the result is a voltage waveform that is the difference in voltage between the other two waveforms.

One waveform is one phase. The number of different waveforms helps determine how many phases you have.

The canonical rule for the number of phases is that the # of phases equals the number of equal magnitude and equal frequency waveforms that have a different phase angle and where waveforms of opposite polarity (180? phase angle difference) are counted as separate phases. Waveforms are taken from any two points as long as the set of points is only used once.

If I have other waveforms with different magnitudes and frequencies, I have a combination of systems. If n waveforms in a system are evenly dispersed through 360 degrees, I have a classic poly-phase system with n phases. If they are not evenly dispersed, I still have a system of n phases, but they do not fit the classic definition of a poly-phase system.

A multi-phase system can be used to supply other systems of a lower order. (Like a 3-phase supplying a 1-phase). Transformers can be used in certain cases to derive systems with different orders.

I'm sure some get touchy because the historical use of the terms but their historical use for certain cases does not subsequently invalidate their use elsewhere.
 
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