Calculate magnetomotive force, current, or number of turns from two known values and convert between amps, kiloamps, and amp-turns with unit conversions.
Magnetomotive Force Formula
The following equation is used to calculate the Magnetomotive Force.
\mathrm{MMF} = I \times N- Where MMF is the magnetomotive force (ampere-turns, At)
- I is the current (amps, A)
- N is the number of turns (or, more generally, the number of conductors linking the magnetic path)
To calculate the Magnetomotive Force, multiply the current by the number of turns.
What is a Magnetomotive Force?
Definition:
Magnetomotive force (MMF) is the quantity that drives magnetic flux in a magnetic circuit (analogous to voltage driving electric current). In a coil, MMF is produced by the current through the winding and the number of turns.
For a coil, MMF depends on two variables: the number of turns and the current flowing through the winding (MMF = N·I). Coil geometry affects the magnetic circuit (for example, reluctance and how the field is distributed), which changes the resulting flux and field strength for a given MMF, but it does not change the MMF itself.
How to Calculate Magnetomotive Force?
Example Problem:
The following example outlines the steps and information needed to calculate the Magnetomotive Force.
First, determine the current of the system. In this example, the current is 400 amps.
Next, determine the number of turns in the coil. For this problem, the number of turns is 5.
Finally, calculate the magnetomotive force using the formula above:
MMF = I * N
MMF = 400 * 5
MMF = 2,000 ampere-turns
FAQ
What is the significance of the number of turns in a coil for magnetomotive force?
The number of turns in a coil directly influences the magnetomotive force (MMF). The more turns a coil has, the greater its MMF for a given current. Since MMF is calculated as the product of the current (I) and the number of turns (N), both factors are crucial for determining the driving force that produces magnetic field strength (H) and magnetic flux in a given magnetic circuit.
Can magnetomotive force be negative?
Yes. MMF can be treated as a signed quantity with respect to a chosen reference direction around a magnetic path (per Ampère’s law). If the current direction is reversed (or the reference direction is reversed), the MMF changes sign. In many practical calculations people use the magnitude of MMF (a positive value) and handle direction separately.
How does the geometric construction of a coil affect its magnetomotive force?
For a coil, MMF is set by the ampere-turns (MMF = N·I) and does not depend on the coil’s shape, size, or layering. Geometry and construction do affect the magnetic circuit and field distribution (for example, the path length, core material, air gaps, and reluctance), which changes the resulting magnetic flux and flux density for the same MMF.
