Hi all,

I would like to clarify a theoretical topic regarding Magneto Motive Force in a magnetic circuit with a solenoid.

If, for example, a coil has 2000 turns around a ferrite core and a current in it of 0.5A then we have a value for f = NI = 1000 Ampere-Turns (AT).

If we now have two of these coils in parallel, while the combined inductance will be like resistors in parallel and give a combined reduced value of L/2, what happens to the MMF measured in AT? Is that also reduced to AT/2 and so will be 500?

Thanks

Justin

 

Do the two inductors in parallel divide the 0.5A of current?

 

Hi all,

I would like to clarify a theoretical topic regarding Magneto Motive Force in a magnetic circuit with a solenoid.

If, for example, a coil has 2000 turns around a ferrite core and a current in it of 0.5A then we have a value for f = NI = 1000 Ampere-Turns (AT).

If we now have two of these coils in parallel, while the combined inductance will be like resistors in parallel and give a combined reduced value of L/2, what happens to the MMF measured in AT? Is that also reduced to AT/2 and so will be 500?

Thanks

Justin

Are these two inductor wound on the same core, or on different cores?
If they are on the same core, then it is no different to winding one inductor with wire twice as thick.

 

The inductance is not that meaningful if there is a constant DC current through the coil.
V/L=dI/dt, so dI/dt is zero for a constant DC current, so that the voltage across the inductance will also be zero. The coil resistance will dominate, so the voltage across the coil will be IR, where R is the DC resistance.

 

I’m assuming the total current will rise to 1.0A as there will be 0.5A in each coil branch (as per resistors in parallel). The two coils are wound on separate cores and so are not co-axial but discrete units wired in parallel and sitting about a coil diameter apart.

 

Would ac current change the NI calculation at all? So does inductance have a role to play in the calculation of MMF between the two setups?

 

I’m assuming the total current will rise to 1.0A as there will be 0.5A in each coil branch (as per resistors in parallel). The two coils are wound on separate cores and so are not co-axial but discrete units wired in parallel and sitting about a coil diameter apart.

There are only two ways that the current would remain steady at 0.5A (or 1A)
1. It is driven from a constant current source
2. The current is being limited by the resistance of the inductor.
If it is driven by a voltage source, then the current would rise continuously at a steady rate.

 

Would ac current change the NI calculation at all? So does inductance have a role to play in the calculation of MMF between the two setups?

Inductance in an AC circuit would limit the current.
Magnetomotive force would remain proportional to the current.

 

Let’s assume there is a constant current source. My query is trying to see what happens to the turns aspect of NI when coils are in parallel. Do they behave numerically like resistors in parallel or can’t it be modelled like that?

The current has gone up from 0.5 to 1A as there are now two branches so does one now say 2000 turns x 0.5A in each branch (1000A-T) therefore a total of 2000A-T in total, or is that not how the total works out?

 

Let’s assume there is a constant current source. My query is trying to see what happens to the turns aspect of NI when coils are in parallel. Do they behave numerically like resistors in parallel or can’t it be modelled like that?

The current has gone up from 0.5 to 1A as there are now two branches so does one now say 2000 turns x 0.5A in each branch (1000A-T) therefore a total of 2000A-T in total, or is that not how the total works out?

They behave like resistors in parallel, because they ARE resistors in parallel. With a constant current source, the inductance is irrelevant.
The current will be shared between them depending on ONLY their resistance.

 

Yes, I agree, so the NI for two identical coils in parallel will be double that of a single coil with a value of 2000 in this case.

If AC was used instead then that would invoke their inductances, but that doesn't appear to enter into the calculation for the NI of a coil.

 

Yes, I agree, so the NI for two identical coils in parallel will be double that of a single coil with a value of 2000 in this case.

If they were both connected in parallel to a single constant current source then the magnetomotive force would be the same as for one coil, because each would take half the current.
If they were both connected in series to the same constant current source then the magnetomotive force would be twice as much as a single coil, because there would be twice as many turns.

 

Ok then I mean a constant voltage source so total current doubled.

 

Ok then I mean a constant voltage source so total current doubled.

What is limiting the current then?

 

A power supply

 

A power supply

A power supply is not a perfect voltage source.
If it is driving a perfect inductance then the current would increase indefinitely if it were a perfect voltage source.
EITHER: the current is set by the resistance of the coil.
OR: the current is set by the power supply's current limit, in which case it is being driven by a constant current source.

 

A power supply

Wrong, a constant voltage source cannot control the current. If the power supply is liniting the current, it is in constant current mode. Otherwise it would be violating Ohm’s law.