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Wye connected system

gjoo

Member
In a wye connected system, it states in my book that if you get the polarity backwards on one of the windings then you reverse the direction of phase rotation. Can someone please give explain.
 
Take a pencil and paper and draw three equal length vectors, label them R,S,T at 0, 120, 240 degrees respectively, and joined at the center.
What you drew is the classical representation of Wye connected three phase system.

Now, invert R, which essentially means adding 180 degrees.
Now draw R, S, T at 180, 120, 240 degrees.
What did you get?
I will allow you now to calculate the vector value of the R to S and R to T. Does this resemble a balanced three phase voltage?
It reverses the phrase sequence
 
The term BLDC comes from Brushless DC, as it simulates a Brushed DC motor turned inside-out IOW, two commutation conductors

Not in any serious technical description I've ever seen. The "inside out brushed motor" is a just a simplified explanation, not a literal requirement.

(Also, DC brushed motors can have more that two brushes - four or eight sets are not unusual).

Wikipedia:
A brushless DC electric motor (BLDC), also known as an electronically commutated motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows.

Science Direct:

Brushless Dc Motor​

The brushless dc motor used in the section is excited by a three-phase sinusoidal supply and driven from a six-step inverter, which converts a constant voltage to a three-phase voltage with a frequency corresponding instantaneously to the rotor speed.

Texas Instruments also consider field oriented control to be one of the systems for BLDC drivers:


Some companies may use different terminologies to try and make their gear stand out from others, but the key criteria is the "Electronically commutated from a DC supply" part, whether it's full three phase or a minimal system as in many PC fan motors.
 
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Not in any serious technical description I've ever seen. The "inside out brushed motor" is a just a simplified explanation, not a literal requirement.
All I can go by is experience and I have used the two types, which are interchangeable, BTW, just the amplifier/drive and the commutation is different.
The bottom line is, that any BLDC drive I have used, utilizes two windings at any one time.
Also it has been recorded that slightly higher torque is experienced with BLDC commutation rather than 3ph AC.
It is useful to have terms which differentiate between motor technology, rather than lump all under one heading which does not indicate the commutation method required.
 
The book is technically correct in saying that a Y connected transformer will have the opposite phase rotation if one winding is connected in reverse. However, two of the phase to phase voltages will be wrong, and the phase angles will be wrong between the different phases, so it really isn't a helpful comment in the book. It's not a situation that would ever be of any use in a practical installation. I think that the book is talking about how to spot a reverse connection on a transformer with no load, before it is connected to a load, so that the connections can be corrected before damage is done.

In a two phase system it's easier to understand. You normally have two windings with opposite phases. If they are like normal American supplies, there will be two phases at 120 V each, and 240 V between the two live wires. If one phase is reversed, then both phases will still be 120 V, but there will be 0 V between the live wires. 120 V loads will work fine, but 240 V loads won't function at all, and the neutral wire may be overloaded.

On a Y connected three phase system correctly wired, with 120 V phase - neutral, there is 208 V between any two phases. If the three phases are labelled A, B and C, with neutral called N, and A - N is taken as 0° and the order of phase rotation is A, B, C, then B - N is 120 ° and C - N is 240 °.

If phase C is reversed, then C - N becomes 60° so now the rotation is A, C, B but the angles are not evenly distributed, There is 60 ° between A and C, 60 ° between C and B, but 240 ° between B and A,

If you look at the phase - phase angles in a delta - connected load powered by the Y connected transformer when wired correctly, A - B is 330° , B - C is 90° and C - A is 210° . Each phase is 120 ° from each of the other phases. All three phase - phase voltages will be 208 V

If phase C is reversed, it all gets messed up.
A - B is still 208 V at 330°, but B - C becomes 120 V at 180° and C - A is 120 V at 120°. So now two of the phase - phase voltages are only 120 V not 208 V, and there angles are no longer even. The angle spacing is 150 , 60 , 150 instead of the usual 120, 120, 120.

A motor that is expecting three phase - phase voltages of 208 V will rotate in the wrong direction if connected to that, but it will not work at all well.
 
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