Transformers and flux

[mbrownn]

I am interested to learn more about transformers, from what I understand, as the current drawn from the secondary increases, the flux in the core reduces. Is this correct?

Is it also the case that the flux reduces when when the secondary is not wound over the primary but on a different part of the core, say two halves of a toroid?

[snowgoose]

The physical basis of a transformer is "mutual induction" between two circuits linked together by a common magnetic flux. In its simplest form, a transformer consists of two inductive coil which are electrically separated but magnetically linked through a path of low reluctance. Then if one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core which produces a mutually induced emf in the second coil. If the second coil is closed, a current floes in it and so electrical energy is transferred entirely magnetically from the first coil to the second coil. The amount of induced voltage in the second coil depends upon the transformers voltage transformation ratio.


When a transformer is put "on load", there are iron losses in the core and copper losses in the windings for both the primary and secondary coils. A secondary current is set up determined by the characteristics of the load. This secondary current also generates its own magnetic flux within the secondary coil which is in oppostion to the main primary flux. This opposing secondary flux weakens the primary flux momentarily causing more primary current to flow. However, as this additional primary current is in anti-phase with the secondary current the two cancel each other out. In other words, the magnetic effects of the increase in secondary current are immediately neutralized by the additional primary current which increases exactly at the same instant in time.

The effects of mutual induction are the same for core-type or shell-type transformers. The advantage of toroidal transformers is that the primary and secondary coils are wrapped around a single powdered-iron core improving the magnetic coupling of the two coils and also allowing them to be used at much higher frequencies.

[boguslaw]

The physical basis of a transformer is "mutual induction" between two circuits linked together by a common magnetic flux. In its simplest form, a transformer consists of two inductive coil which are electrically separated but magnetically linked through a path of low reluctance. Then if one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core which produces a mutually induced emf in the second coil. If the second coil is closed, a current floes in it and so electrical energy is transferred entirely magnetically from the first coil to the second coil. The amount of induced voltage in the second coil depends upon the transformers voltage transformation ratio.


When a transformer is put "on load", there are iron losses in the core and copper losses in the windings for both the primary and secondary coils. A secondary current is set up determined by the characteristics of the load. This secondary current also generates its own magnetic flux within the secondary coil which is in oppostion to the main primary flux. This opposing secondary flux weakens the primary flux momentarily causing more primary current to flow. However, as this additional primary current is in anti-phase with the secondary current the two cancel each other out. In other words, the magnetic effects of the increase in secondary current are immediately neutralized by the additional primary current which increases exactly at the same instant in time.

The effects of mutual induction are the same for core-type or shell-type transformers. The advantage of toroidal transformers is that the primary and secondary coils are wrapped around a single powdered-iron core improving the magnetic coupling of the two coils and also allowing them to be used at much higher frequencies.

Thank you, very good explanation. Simple and straightforward. One thing I don't understand. What is forcing primary current to increase to neutralize increase in secondary current ? Does it mean that increasing secondary current due to load (right ?) change core permeability (how? lower? increase?) which on other side cause inflow of more current to the primary from power source(if possible of course) because of lower resistance of primary to the power source frequency ?


Wow, a lot of questions ... and this is only starting point....

[mbrownn]

The physical basis of a transformer is "mutual induction" between two circuits linked together by a common magnetic flux. In its simplest form, a transformer consists of two inductive coil which are electrically separated but magnetically linked through a path of low reluctance. Then if one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core which produces a mutually induced emf in the second coil. If the second coil is closed, a current floes in it and so electrical energy is transferred entirely magnetically from the first coil to the second coil. The amount of induced voltage in the second coil depends upon the transformers voltage transformation ratio.


When a transformer is put "on load", there are iron losses in the core and copper losses in the windings for both the primary and secondary coils. A secondary current is set up determined by the characteristics of the load. This secondary current also generates its own magnetic flux within the secondary coil which is in oppostion to the main primary flux. This opposing secondary flux weakens the primary flux momentarily causing more primary current to flow. However, as this additional primary current is in anti-phase with the secondary current the two cancel each other out. In other words, the magnetic effects of the increase in secondary current are immediately neutralized by the additional primary current which increases exactly at the same instant in time.

The effects of mutual induction are the same for core-type or shell-type transformers. The advantage of toroidal transformers is that the primary and secondary coils are wrapped around a single powdered-iron core improving the magnetic coupling of the two coils and also allowing them to be used at much higher frequencies.

Thankyou, you have just given me the exact information I needed. Great explanation.

Transformer actions are used in some configurations of universal motors to reduce arcing by what is called inductive compensation. Your explanation could be one reason why these motors are so inefficient. This also gives another reason why resistive compensation is the preferred method in universal motors as the transformer action is eliminated.

[snowgoose]

Universal motors are so called because they operate on either DC or single phase AC supplies. They are basically series connected DC motors, so when used on AC supplies, the AC current reverses direction in both the field and armature at the same time leaving the direction of rotation and torque unchanged. Also the high reactance of the DC armature and series winding reduces the AC currents makes universal mnotors inefficient when used on AC supplies. This high reactance can be reduced, increasing AC current and torque by using a compensation winding on the stator which is counter wound to the field winding so that the magnetic fluxes cancel each other out.

Inductive compensation is achieved by shorting out the compensating winding so that it behaves like a shorted transformer secondary coil with the motors armature winding acting as the primary coil effectively reducing the reactive armature currents due to transformer action. Inductive compensation also has the effect of increasing the overall reactive power consumed by the motor, increasing efficiency. In other words, an uncompensated universal motor is inefficient losing power due to its high AC reactance.

Typically universal motors are used in portable tools as they are small, cheap and lightweight operating at very high rpm. The disadvantage of these motors is that due to their high-speed the life of the carbon brushes and commutator is short resulting in high audible noise and electrical noise as well as large eddy currents being induced into the motor laminations overheating the motor if used for long periods of time.

[mbrownn]

Universal motors are so called because they operate on either DC or single phase AC supplies. They are basically series connected DC motors, so when used on AC supplies, the AC current reverses direction in both the field and armature at the same time leaving the direction of rotation and torque unchanged. Also the high reactance of the DC armature and series winding reduces the AC currents makes universal mnotors inefficient when used on AC supplies. This high reactance can be reduced, increasing AC current and torque by using a compensation winding on the stator which is counter wound to the field winding so that the magnetic fluxes cancel each other out.

Inductive compensation is achieved by shorting out the compensating winding so that it behaves like a shorted transformer secondary coil with the motors armature winding acting as the primary coil effectively reducing the reactive armature currents due to transformer action. Inductive compensation also has the effect of increasing the overall reactive power consumed by the motor, increasing efficiency. In other words, an uncompensated universal motor is inefficient losing power due to its high AC reactance.

Typically universal motors are used in portable tools as they are small, cheap and lightweight operating at very high rpm. The disadvantage of these motors is that due to their high-speed the life of the carbon brushes and commutator is short resulting in high audible noise and electrical noise as well as large eddy currents being induced into the motor laminations overheating the motor if used for long periods of time.

Thanks again, you do understand your motors.

Yes the high resistance is a huge problem, to get round this I have been running high power (2kw at 220v) motors on low voltages. The motors run well on 12v but obviously there is not much power. The high reactance can be useful in recovering the inductive kickback for us overunity seekers but the iron losses are huge. Using pulsed DC instead of AC does help but it is still not enough.

I have started looking at transformer actions in these motors to see if it can be used to help the situation as we get this action with little additional iron loss to the motor running. There are patents that mention using this method but the geometry of how its done is not mentioned.

It makes sense to me that the current in the compensation winding be used as an output to power a load or be fed back into the motor by some means, hence me wanting to know more about transformers.

[mbrownn]

The motor efficiency relies on high flux in the rotor and stator, if the flux is reduced by this transformer action it will not help us but as you say the motor is more efficient with compensation so I don't understand this unless drawing power from the compensation winding just causes more current to flow through the other windings, this has no effect on efficiency and only increases power.

[snowgoose]

The purpose of the field winding is to produce the maximum magnetic flux with the minimum number of coil turns. The main purpose of the compensating winding is to decrease the armature self inductance, in other words to increase the effective armature reluctance, and therefore its ratio to the fields reluctance as a high ratio results in a higher power factor inproving efficiency.

Higher efficiency is advantageous in that the energy consumption of the motor (input power) is as low as possible, while still satisfying the needs of the motor load (output power). The efficiency of a universal motor is therefore the ratio of output power to input power (out/in).

The reactive current of the motor remains practically constant at all loads, but the power factor of a universal motor (or most motors for that matter) is very low at no-load or on light load conditions. Then motors should not be oversized (since they will then be lightly loaded). To improve the power factor of a lightly loaded machine, requires a non-inductive shunt across the field such as a resistance or capacitance, The down side of this is that power is wasted.

[mbrownn]

The purpose of the field winding is to produce the maximum magnetic flux with the minimum number of coil turns. The main purpose of the compensating winding is to decrease the armature self inductance, in other words to increase the effective armature reluctance, and therefore its ratio to the fields reluctance as a high ratio results in a higher power factor inproving efficiency.

Not sure I understand this, maybe its just the terminology. The part about the turns makes sense to me but the reluctance part I am not sure on. Got any links where i can read about it?

If compensation reduces self inductance then it allows more current to flow and gives more torque as well as more current in the inductive compensation coil, this I see as positive. If it is the opposing magnetic field caused by the compensation coil that is the reluctance then this is negative. Reluctance being like resistance but in the magnetic circuit. Is this how it works?

Can this reluctance be compensated for by adding permanent magnets to increase flux? I think it can.

Thanks again for your assistance

[snowgoose]

Magnetic reluctance is the equivalent magnetic term generally used to describe the resistance of an air-gap between pole faces. Reluctance is related to inductance through permeability and reducing reluctance also increases inductance and vice versa.

Since the ratio of armature reluctance to field reluctance improves power-factor of the machine, the higher an armature reluctance and as low a field reluctance as possible the better, in other words as good a magnetic field circuit and as poor magnetic armature circuit as possible which is achieved by inductive compensation of the motor. The original query.

Don't thing fixing permanent magnets to the side of the motor will make much difference but try it anyhow.

[mbrownn]

Ahhh reluctance and inductance are another trade off, especially when there is a core.

Its just another compromise I have to work out hehe

The use of magnets on the external part of a motor does work to some extent as shown by the stargate motor, I have also had some success introducing a parallel path on an unmodified universal motor but for best results using this method you have to match the power in the coils to the magnets.

You have given me enough info to point me in the right direction in my research although as expected I am faced with another trade off

[yipyipdog]

Mutual Inductance

How a transformer works can be a little complicated and designing one is very complicated. The first thing to understand is that we have to start with the primary creating a small amount of flux in the core by running a small amount of current through the primary. This is called the magnetizing or exciting current and is part of the losses in the transformer. This exciting flux will be greatest at no load and will decrease slightly as the transformer is loaded do to other losses.

The mechanism that lets a transformer operate is the concept of mutual inductance.

Putting two stand-alone inductors in series is like putting two resistors in series and their self inductance's just add together. In this case their magnetic fields and flux flows are internal to each one and do not have any influence on the other.

In a situation where the two inductors share a common core and therefore a common flux path, their fluxes each have an influence on the other. This case is no different than the way each turn of a single inductor has an influence on the other turns. Take a 10 turn coil for example and lets look at the formula for self inductance. ind = uN2*A/l - this says that the self inductance equals the permeability of the core times the number of turns squared times the area of the core divided by the length of the core. This is different depending on core geometry, but this is fine for what I am trying to illustrate here. Now lets logically divide the coil into two 5 turn sections without physically altering it and analyze the self inductance from this point of view. We are not going to calculate the actual self inductance, just their relationship to one another and therefore the characteristics of the core are not important here so we can just get rid of them and are left with (ind = n2).

This means that if we double the number of turns, then the inductance will increase 4 times. Lets say that the either one of the 5 turn coils have an inductance of 10mH. So if we consider them as one 10 turn coil, we have doubled the number of turns and therefore quadrupled the inductance from 10mH to 40mH.

This is the same result we would get if we calculated the self inductance of the two 5 turn coils using the formula : (L1 + L2 + 2M) where L1 and L2 are the inductance's of each coil and M is the mutual inductance given by : (square root of L1 * L2). Mutual inductance in this example is therefore the square root of 10mH * 10mH, which is still 10mH. So we have:

10mH + 10MH + (2)10mH = 40mH

We can see from this example where the mutual fluxes are going in the same direction that they can increase the self inductance of the whole by as much as 4 times. If the fluxes are going in the opposite directions from each other as in the case of a transformer, then the flux put out by a loaded secondary will cause the flux created by the self inductance of the primary to decrease. A decrease in flux in the primary means that we have also reduced its inductance. By reducing its inductance, we have decreased its resistance to current flow and the current increases in the primary. Since the current increases, so does its flux. This occurs simultaneously as the secondary is loaded and results in core having the same flux in it as we started with. In reality the resulting flux will be slightly less than this because of losses.

[rosehillworks]

@All
I am enjoying reading this thread it is useful information Thanks

[lotec]

question 4 yipyipdog or snowgoose or anyone

I remember reading about transformers somewhere and they said the reason why the primary current increases when the secondary is loaded or shorted is because the primary or any coil for that matter creates flux when a current goes through it. This flux inducts a current or potential that tries to travel back down the wire in the opposite direction causing a head on collision with the primary current or a bucking effect or impedance. The presence of a secondary gives this current a path to travel through so that it no longer impedes the primary current. That sort of makes sense to me. Then they went on to say when the secondary is fully loaded or shorted the current is nearly as much as that of the primary. Therefore the flux of the secondary is nearly as much as that of the primary, which nearly cancels the flux out completely leaving a net flux closer to zero. Other people hold that that flux remains nearly at the original level. I dont know what to believe, but I am interested in both points of view and how they come to their conclusions. A fascinated novice.
Regards wil

[boguslaw]

I'm interested in other aspect. They stated that energy from primary is coupled and transfered via magnetic field to the secondary. If energy is transfered to secondary then why when I disconnect primary I get flyback spike from primary ?