A motor design to defy lenz law ?

[Proud Teslian]

1. In the given figure, the two rotors (purple, green) are electrically isolated but mechanically fit to rotate along same axis

2. The motor action on the left side rotor will move the armature set, this will produce currents in right side rotor that will pass via a variable load.

3. do you see the magnetic feilds of both rotors cancelling each other especially when the no. of turns are large and the two rotors are close enough.

4. Will this cancel lenz effect on generator side or both sides?

[John Cuthber]

"A motor design to defy lenz law" will fail.

[Proud Teslian]

it is difficult to move rotor of generator (green) because the magnetic field of induced current is opposing the rotation (lenz law), but if that magnatic feild is cancelled by another adjecent rotor, then the question is will the generator rotor move freely in the field (the diagram shows only half a turn in each rotor, so please imagine more turns)

[studiot]

I wondered if you envisaged more turns when you first posted and I can see the machine you are describing as a combined motor and generator with alternate turns connected to the motor and the generator.

 

This would work, but it is not in contravention of Lenz law, which I think you misunderstand.

 

Both the motor windings and the generator windings are subject to a generated emf, known as back emf.

However the back emf serves different purposes in the two sets of windings.

 

In the motor section the back emf serves to limit (reduce) the 'applied voltage' and thus the drive current. It does indeed opposes the driving emf directly.

 

In the generator section there is no driving emf or applied voltage. There is only mechanical driving of the shaft by the motor section.

The Faraday voltage generated obeys Lenz law and provides the emf to power the external load lamp.

 

This explanation from 'Nightingale' may help.

 

 

Exactly what happens will depend upon the relative circuit values; the driving battery voltage, the circuit resistances in the motor and dynamo circuits, the lamp load resistance.

 

However there are devices known as rotary converters which can convert (battery) DC to other values of DC or AC supply.

These are less common today with modern electronic alternatives, but are still used for very high powers.

Rotary converters work on essentially the same principles as we are discussing.

Edited April 30, 2016 by studiot

[swansont]

it is difficult to move rotor of generator (green) because the magnetic field of induced current is opposing the rotation (lenz law), but if that magnatic feild is cancelled by another adjecent rotor, then the question is will the generator rotor move freely in the field (the diagram shows only half a turn in each rotor, so please imagine more turns)

 

 

That's not an example of defying Lenz's law, any more than going up a set of stairs defies gravity.

[John Cuthber]

"A motor design to defy lenz law" will fail.

Perhaps i should expand slightly on that.

Since Lenz's law follows from Newton's laws of motion and the conservation of energy, you are not going to break it.

Anything designed to defy it will fail.

[Proud Teslian]

I wondered if you envisaged more turns when you first posted and I can see the machine you are describing as a combined motor and generator with alternate turns connected to the motor and the generator.

 

This would work, but it is not in contravention of Lenz law, which I think you misunderstand.

 

Both the motor windings and the generator windings are subject to a generated emf, known as back emf.

However the back emf serves different purposes in the two sets of windings.

 

In the motor section the back emf serves to limit (reduce) the 'applied voltage' and thus the drive current. It does indeed opposes the driving emf directly.

 

In the generator section there is no driving emf or applied voltage. There is only mechanical driving of the shaft by the motor section.

The Faraday voltage generated obeys Lenz law and provides the emf to power the external load lamp.

 

This explanation from 'Nightingale' may help.

 

Induc2.jpg

 

Exactly what happens will depend upon the relative circuit values; the driving battery voltage, the circuit resistances in the motor and dynamo circuits, the lamp load resistance.

 

However there are devices known as rotary converters which can convert (battery) DC to other values of DC or AC supply.

These are less common today with modern electronic alternatives, but are still used for very high powers.

Rotary converters work on essentially the same principles as we are discussing.

 

Nice Explanation @Genius i recheck my idea about where i was wrong

[Enthalpy]

I have nothing against the possibility of building a DC transformer this way, especially if the ripples are addressed.

 

I have much against its usefulness whatever the power. Three decades ago, power electronics was already the best choice at Itaipú to convert 12GW between three-phase, the DC transport line, and three-phase back.

 

Lenz' law applies here too but with caution... It doesn't differ much from a DC generator by the way. That is, the induced current opposes directly the cause when the self-inductance limits the current in the induced circuit. When the resistance limits the current, this current is in phase with the inducing voltage instead of lagging 90°. In intermediate cases, the phase between the inductor field and the induced current is somewhere in between and the effect of the induced current adds vectorially.

 

At a rotor where the mechanical angle evolves over time, a phase lag in a current or voltage that offsets them in time also changes their mechanical angle as seen by an observer on the rotor. As a consequence, the induced current has a variable angle with the inducing field, and so does the field it creates.

 

Take a DC generator for instance, with just two horizontal poles and a vertical axis as on your drawing. The inductor field creates the maximum voltage (or better EMF) in the loop parallel to the drawing plane at a given time.

• If the loop is essentially inductive, the current changes most at that time, and is maximum when it doesn't change: when the loop is perpendicular to the drawing plane. The induced current creates a field that fully opposes the inducing field.
• But if the loop is essentially resistive, the induced current is maximum when the EMF is maximum: when the loop is parallel to the drawing plane. The induced current creates a field perpendicular to the inducing field, which doesn't oppose directly. The effect is much smaller.

In a big DC machine (already at MW), the induced current (or more generally the rotor current as it's the same story in a motor) can be stronger than the inductor current and would create a strong field. This is avoided by a special winding at the stator that is mechanically offset by 90° from the inductor winding and through which the rotor current is passed too. Even at smaller power, this is important to reduce the voltage among the loops switched by the commutator and let the commutator live longer.

 

The same effect happens in three-phase machines, but no special winding is added there because the three or more windings suffice to create the field in any direction and with any phase lag - including to compensate the field created by the induced current. One consequence is that the mechanical angles between the fields, voltages and currents change at the stator and rotor - nothing to see at the physical components. Though, it still does need more current and lost power, and it also implies that the lagging consumed current is the main limit of a three-phase generator.

 

It also applies to squirrel-cage motors. The rotor must be resistive so you get a torque. If the rotor gets too inductive, for instance because the induced frequency rises with the slip speed, then the induced current still increases with the EMF but it creates a field parallel to the inductor field and less torque. Starting rheostats, double rotor windings, and now electronics, are all meant to address this weakness of a smaller torque at low speed.

 

One funny case appeared as I developed the contactless chipcard (RFID, NFC). I put a resonant LC circuit on the card and noticed that the induction in the reader's field increased much when I brought the card close. It puzzled me as I had expect the consequence to reduce the cause. My explanation is that the induced current creates a field at nearly 90° phase lag with the inductor field, hence capable of exceeding it without compensating it.

 

Back to your example: you've drawn a resistive load, so the rotor current creates a field that combines at 90° with the inductor (stator) field, so it doesn't oppose directly, nor does it change the EMF much, neither at the primary nor at the secondary winding.

[Proud Teslian]

That is one deep review @Primate thanks for your Explanation