Topic 3 MAGNETIC CIRCUITS, MOTOR AND GENERATOR ACTION Magnetic Flux SI unit, Webers (Wb) ϕ Flows from North to South Pole 1
Magnetic Flux Density Measure of Flux/Area SI units, Wb/m 2 = Tesla, B Think of a Magnetic Circuit like an Electric Circuit. Finding the Flux Density allows the designer to properly size the magnetic conductor just like sizing the wire in an electric circuit. = Magnetomotive Force: The relationship between the Number of Turns of a Wire and the Current on it. Flux is increased in two ways: By increasing the current or by increasing the number of turns in the coil. 2
Magnetic Field Intensity: Flux is continuous, it forms a closed loop A longer path requires more MMF to push the Flux A Core is used to focus the Flux into the area it is needed. Measured in Amp-turns/meter (A*t/m), H = = l Permeability Where μ is the permeability of the material the Flux is flowing Permeability of free space is the reference for all other materials and has its own symbol, μ o = 4π 10 ( ) The permeability of other materials is related to the permeability of free space by the relative permeability, μ r = μ r is essentially 1.0 for all materials other than ferromagnetic materials could be several 1000 3
Reluctance In an electrical circuit I and V are related by R = In a magnetic circuit MMF and Flux are related by Reluctance = = l Right Hand Rule I and ϕ are related by the Right Hand Rule 4
Ferromagnetic Materials Metals are crystalline in structure Crystals are magnetic domains, random until MMF is applied, then they line up. Two Classes: Soft easy to magnetize. Used in transformers and motors as a flux channel Hard difficult to magnetize and demagnetize. Used for permanent magnets. The Magnetic Circuit can be used two ways: Transformer Action Generator Action 5
Transformer Action As Magnetic Flux varies in a Sinusoid Voltage is induced Induced Voltage opposes the source Voltage Motor and Generator Action Because of the physical movement of the machine, we need to look closer to the external forces magnetic fields exert on current carrying conductors. This schematic shows two parallel conductors carrying This schematic shows two parallel conductors carrying opposite currents. Each conductor produces a magnetic field that are oriented in the same direction. 6
Motor and Generator Action Now turn the conductors and look a them from the end. The flux adds between the conductors. This creates a magnetic pressure that enacts a force on the conductors. Note: the X is current going into the page and the is coming out of the page. Magnetic Forces are REAL (and Imaginary) This picture is from the IEEE Buff Book. On the left, the bus bar was protected with the correct current protection. On the right, the bus bar experienced excessive current which created a large magnetic force that bent the copper bars. 7
Flux Bunching The principal shown in previous slides is what is known as flux bunching. If a conductor s flux combines with the flux of an external field, the force on the conductor will push it in the direction of the lower flux. We can use this force to create motion, called Motor Operation. Lets look at a Linear Motor as an example. Linear Motor Direction of Fem is determined by the flux bunching Fem, B, and I are mutually perpendicular (Left Hand Rule) 8
Left Hand Rule Assists in determining the direction of force Remember, F-B-I Flux Bunching If a conductor moves so as to cut lines of flux, or if the flux passes thru a coil, an EMF will be induced in the conductor. (Faraday s Law). The direction of the induced EMF will oppose any change in the flux. This is called Generator Operation. 9
Linear Generator The induced voltage is given by = B, V, and E are mutually perpendicular (Right Hand Rule) Right Hand Rule Determine the direction of induced voltage/current 10