Transformer

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This has got to be filed under rule #75

1. Try thinking of the most elementary transformer with just one primary turn and one secondary turn.
2. The turns can be side by side (along the axis) or one around the other, corresponding to the two fundamental ways of arranging transfo windings.
3. Imagine the a section through the coils on a cutting plane through (and parallel to) the coil axis. You will have two sets of adjacent conductors, one set above and one set below the axis seen end-on to the wire.
4. Imagine a short length of the wire, and think of it being straight, not slightly curved.
5. The problem has now been reduced to the forces between straight parallel conductors, which is a fundamental fact you can test with some wire and torch cells, and which is covered by basic magnetic theory.
6. Now you just have to assign the current directions and consider whether you have attraction or repulsion between the adjacent conductors, and what that is doing to the coils.
7. Do not forget to consider the parts of the coil below the axis, as well as above and both the side by side and one around the other force directions.
8. In practical coils with many turns, you will have forces between the turns of the same coil (if the coil is multi-layer not solenoid, there will be radially close as well as axially close turns) and between the two coils.

Transformer is a static device which transfers voltage from one circuit to another circuit without changing thefrequency (changes the voltage level).

Transformer is not rotating because for rotation, rotating magnetic field is required to be setup, which is produced only when the current is passed through the windings which are displaced physically by 120 degrees. In transformer, there is no physical displacement of 120degrees hence transformer cannot rotate as rotating machines.

For more clarification, better you screw your mind.

Vinu_Answers Sure Answers

120 degrees? Vinu Answers forgets that single phase induction motors work happily with windings displaced 90 degrees or even with only one winding and a shaded pole.

Actually, most transformers rotate once per day.

Is the rotation reversed in the southern hemisphere?

In electical point of view the secondary of 3 phase transformer is not short circuted.So it won't rotate.

Hithuanand,

Slipring induction motor with resistance in circuit still rotates at reduced speed! Force is exerted on current carrying conductor in magnetic field, unless conductor is parallel to magnetic field lines. I have a memory that induction motor theory says that only the resistive part of the rotor current creates rotational torque - and a short circuit has no resistance! All the bars in the rotor contribute to its magnetic field and I have a feeling that if they are totally inductive with no resistance, their net field is perpendicular to that of the stator, which gives no net torque.

Maximum torque of IM in running condition is Tmax=(3.E2^2)/(2.pi.Ns.2.X2) So maximum torque is independent of rotor resistance. I think 3 phase transformers are still built on copper windings rather than superconductors.

ref. Hithuanand, post #21

I find my ancient brain was thinking of article 6-4 page 295 of my paperback copy of Fitzgerald and Kingsley's "Electric Machinery" 2nd Edition 1961 [McGraw - Hill] formula 6-90, of which I attach part copy :-

I2 and r2 are rotor current, resistance and s is slip. This formula indicates r2 = 0 means torque T = 0. I agree r2 = 0 is not realistic, but it has a major effect on starting torque, which was what I was thinking about (s=1), because an induction motor is most like a transformer when the rotor is not moving.

1. Does your formula have an X suffix 2 in it?
2. What does it correspond with? Rotor Reactance?
3. And how is it voltage E and speed Ns come into a torque formula, but not current or power?
4. Can the motor size have no bearing on torque?

Regards,

67model

Just to give a counter example, consider a wound rotor motor. The "secondary" of a wound rotor motor is not short circuited, especially during startup, yet it rotates.

If secondary of the 3 phace transformer is short circuited, can it rotate?

Secondary of the transformer is fixed mechanically and at short circuit condition also it cannot ratate.

Motors are designed to get torque to the rotor and mechanically it is free to rotate.

Rotation is motion or relative displacement . In a motor the rotating magnetic field generates motion since the rotor is free to rotate and the stater is fixed . Where as in Transformer the motion is possible only when one of the two the windings ( primary and secondary is movable on the same axis (like a motor ) . In trans former since both windings are on the same core there cannot have any displacement or rotation but only energy transfer or induced voltage.

No displacement, V.I.Abraham? Try this, two windings on transfo core -

The load currents in primary PRI and secondary SEC produce fluxes in opposite directions, which cancel each other (leaving the primary magnetising current flux, this current usually being much less than the full load current). So N and S poles of the winding electromagnets are as shown. So PRI and SEC windings repel each other with peak force each half cycle (and zero current, zero force, twice a cycle). They vibrate back and forth a distance decided by the elasticity of their restraining blocks, causing 100 Hz [and harmonics] acoustic noise and insulation stress.

But regarding the original post, I can see no rotating force.

In transformer alternating field is produced while in Induction Machines rotating field is produced.

There is no distributed winding in transformer so there is no rotating magnetic field While in induction machines winding is distributed in space so magnetic field in space is also distributed i.e

B=BcosX

Where X is in radians and when resultant mmf of three balanced phase winding is determined. It comes out to be constant for all instants So Taking knee point as the operating point Magnetic flux can be found out from MMF.

This is called rotating magnetic field which is responsible for flux cutting so a torque is developed.

Please Correct me CR4 if you find me wrong

with regards

Because induction motors have bearings while transformers do not.

Re: the operating principle of Transformer & Induction motor are same

The operating principle of a transformer and an induction motor are the same only to the extent that they both use magnetic induction. As others have stated, in a transformer, the coils are not intentionally arranged to give a rotating magnetic field, which is an additional operating principle / requirement for an induction motor.

So, transformers and induction motors share one operating principle, but not all required operating principles.

In a 3-phase transformer, there is a rotation of magnetic fields; the primary drives the secondary with a phase lag depending on vector group.

As has been noted by RHKramer in post #11, transformer and induction motor share use of the property of magnetic induction. But transformers do not need the principle of electromagnetic forces, which is essential to the motor. So the induction motor has two operating principles, not the one "principle" implied by the original post.

A large diesel engine driving a generator doesn't go anywhere, while the same thing in a truck goes all over the place. Each is meant to do a specific, but different, job, though operating on the same principle.

In transformer winding and core are not free to rotate but fixed.In induction motor secondary is mounted on core fixed with anti friction bearings meant to rotate.You can compare the linear motor too in which secondary or rotor is free to move.

So what ?

Let Tr. be not rotated, at least phase sequence is rotating , then why you want Tr. to rotate ?

It rotates very very slowly, you may have to watch it for hours.

Why is the transformer humming...it forgot the words.