# Y or Delta configuration?

Discussion in 'Electrical Engineering' started by Hiking, Mar 9, 2005.

1. ### HikingGuest

Hello all,

We have a motor which can be hooked up either in Y configuration, or
Delta configuration. Which configuration will give more start torque,
and then the most operational torque, and why in each case?

Thanks.

2. ### SQLitGuest

Looking on the name plate of the motor has not explained the difference?

Or is this a home work question?

3. ### Guest

I would say delta since it might be slightly more efficient (240 vs
208v) but the difference would be miniscule. What do you have in the
building? That is what you will use.

4. ### EddGuest

Is this a wye start delta run single voltage motor or a dual
voltage motor with a voltage ratio of 3^.5?

-=-=-
.... Is it ignorance or apathy? I don't know and don't care.

5. ### CLTGuest

The 2 configurations will give you the same power, couse the delta
config is for lower voltage, let say 220 or 110 volt 3 phase and the
Y config is for higer voltage, 380 or 220 volt 3 phase. The are
motors that use 660 volt for high (Y) and 380 for low (delta).

6. ### HikingGuest

Never mind guys, I wasn't so much concerned with which configuration to
connect for its own sake, the boys here will do trial-and-error and will
wire it from there. The guys here don't care why. For myself, I was
only interested in the "why", in understanding the "why"s of this situation.

I remember from school many, many years ago that torque was a result of
the phase differential between the outer winding (what do you call this
one again?) and the induced EMF in the armature, the greater the
difference, the more torque you get. As the armature gains speed and
nears synchronization with the rotating EMF in the outer core, the
torque is reduced conversely. I understand the EMF in the outer core is
not really "rotating", but the effect is of a rotating field... and if I
understand this correctly, by increasing the frequency of the outer-core
field, one could increase the operational speed of the armature,
correct? I suspect, however, that this higher frequency would also
result in a reduced starting torque capability for the same motor?

But this is in a Squirrel Cage, single-phase induction motor, I have no
idea what the situation is with a 3-phase motor... I expect it's the
same operational principle, only difference being that there are three
outer-core windings.

What I was really hoping to understand is how these dynamics are
different with these two different wiring configuration.

If anyone can explain what happens in this motor/system under these two
varying configurations, and has the time and desire to explain it, I am
all ears and appreciative.

Can you guess I drove my elect teacher nuts? I have to hand it to him,
however, he was extremely knowledgeable about the internal, theoretical
method of operation of elect motors... good 'ol Mr. O'Brien. Though he
seemed to think it was a waste of time for me to understand elect theory
to this level, he did humour me and I did understand a lot of it and it
still is a big help... the problem being that as soon as I learned it, I
ended up going into another field of work and never used/applied, nor
thought of electricity since, for over 12 years, so, unfortunately,
forgot a lot, and am just trying to refresh my memory.

If you're in a teaching mood, I'd love to learn/understand this subject
a whole lot better. Thanks.

7. ### BudGuest

My recollection is that Y start delt run is normally for high HP motors.
The windings are the same but you connect them differently. You
start in Y configuration, which draws less current (you don't have full
line voltage across the windings), then switch to delta to run. Ths
requires a Y-delta starter or connecting multiple starters to do the
same. A Y-delta starter probably has the contactors mechanically
interlocked, which is better.
Y-delta is to reduce the start current on large HP motors - so you don't
dim the lights on your block - also may be used for high inertia
mechanical loads that take a long time to start.
3 phase squirel cage motors operate the single phase except you don't
need a start winding because the 'rotation' of the phases gives a
direction for the rotor to turn.

Your description sounds good. The current in each phase produces what
appears to be a rotating magnetic field in the stator. This produces a
current in the cast aluminum bars in the rotor. The induced current
produces a magnetic field that pulls the rotor to catch up with the
stator's rotating magnetic field. The greater the difference between the
stator field rotation and the rotor rotation, the greater the rotor
current and thus torque. The rotor can' rotate as fast as the stator's
magnetic field (synchronous speed) because there is no torque at that
speed. The difference between the speeds is the "slip". Hope this isn't
overkill.

Bud--

8. ### HikingGuest

Thanks Bud,

that seems to confirm that I have the right idea about how the motor
works, or is made to work (rotate), but what I'm trying to visualize is
the different winding configs (Y as opposed to Delta), and how each
works (trying to understand this in the same way as I understand the
1-phase Squirrel Cage induction motor).

Now 'THAT' may be overkill, but if someone wants to tackle it, I'll take
the beating.

9. ### Don KellyGuest

--

---------------
The rotating field of a 3 phase motor is a true rotating field. That of a
single phase motor has two rotationg components acting in opposite
directions. The 3 phase machine doesn't need special tricks to get started
and has smoother torque than a single phase machine as well as a better
size(weight) to power ratio.
The rotor only sees the rotating field. The torque is dependent on speed as
you indicate and the output power depends on the product of torque and
speed. Consider a 3 phase 208V 1 HP 1760 rpm motor. supplied at 208V and
connected in delta. The torque at rated speed will be the same as that of a
208V, 1HP 1760rpm motor connected in Y. The line current and voltage will be
the same in both cases but the coil voltage will be higher by a factor of
root(3) in the delta than in the Y. The coil current will be lower in the
delta by the same factor. Total power input will be the same at a given
torque and speed. A delta winding will have more turns per coil by the same
factor so the ampere turns producing the field will be the same in both
machines. The rotor doesn't see the difference. A dual voltage machine is
designed with multiple coils per phase which can be reconnected to either Y
or delta without exceeding coil voltage and current ratings.

There is no rotating stator emf by the way- there is a rotating field which
induces an emf in a shorted rotor which in turn produces a rotating field in
synchronism with the stator field but not in phase. (The rotor field moves
at slip speed with respect to the rotor and the rotor is moving at
synchronous speed -slip speed with respect to the stator so the rotor field
is synchronous but lagging behind the stator field).

I think I have some notes around which are in Microsoft word format and
possibly in HTML format. Equations might get a bit messed up- will check.

Don Kelly