my invisible friends hello and welcome back to our series in these the most omit two dish of the videos I've included in our series about DC motors whose flocks is not created by currents in a coil but rather by permanent magnet an introduction equivalent electromotive force of a magneto motive force how is that when we have a coil with a current in it applying magneto motive force to a magnetic circuit n times I we can represent that in an equivalent electric circuit by an electromotive force with this value and I and ideal the source what about a magnet will a magnet in its equivalent electric circuit behaves as a current source a current source and that delivers flux so it is a Florence orce but not an ideal one no a non ideal current source that is the electric circuit equivalent of a permanent magnet let's have a look as a refresher of how it is that we analyze a circuit with an ideal current source it is current source with the value is connected to a resistor with the resistance R X ohms graphically if we have the current versus the voltage in either of those elements we see that for the current source the representation is a horizontal line and the value is the value of the current source of course the reason is the current in that source is the same is for any value of the voltage in the circuit but their relationship between VX the voltage in the resistor and I XOR is I the current in the circuit is given by Ohm's law graphically that relationship is given by this red slanted line that has a slope 1 over R the intersection of both characteristics gives us what is the actual voltage in the resistor but I've said that our permanent magnet does not behave as an ideal current source but as the real nonlinear carrying source low let's analyze the circuit an electric circuit with a current source that has not the ideal characteristic but rather at least far from ideal IV characteristic that is a current source and as before the voltage VX in the resistor and its currents are related to one another by Ohm's law graphically given by the write line that has again the slope 1 over R X the intersection of the current source no linear characteristic and the linear characteristic of the resistor gives us what is the actual voltage in the resistor and what is the current in the circuit now let's create a flux instead of a current a creating a flux with a current is something that we've learned how to do we apply a care into a coil in the magnetic circuit and that current in that coil will apply an MMF that will be responsible for establishing a flux in the circuit the magnetic core will take the flux over to the air-gap on the right which is where we need it we have learned that under certain conditions we can represent that magnetic circuit with this equivalent electric circuit instead of the coil we have a voltage source an ideal one with a value NR a the same value of the MMF applied by the coil and in the circuit we've represented the magnetic core Bites reluctance our Fe and the Aragon Bites reluctance our Sablan well you know what is the catch here that that electric circuit is valid only if the permeability of the magnetic core is caused or which is more likely to happen if their reluctance of the air-gap are not is far greater than their reluctance of the magnetic core in that case we neglect the hydrants reluctance on half that circuit we say the MMF applied by the coil is applied directly to the air a gap when we shown the circuit what we get is not the current what we get is the flux in the original magnetic circuit but now let's take our attention to the subject of permanent magnets let's begin with that MMF applied not to the air gap but to a piece of iron with a very wide hysteresis loop what we have nicknamed hard iron a hard iron a piece of iron with the white hysteresis loop is perfect for fabricating a permanent magnet when we apply an excitation wait a minute that graphic is B versus H but we know that B is proportional to the flux in the circuit and that H is proportional to the current in the coil until the total MMF applied by the coil well when we increase latinum F starting here at zero the flux of course will increase until we reach saturation but when we reduce the MMF we reduce the current in that coil the flux of course decreases a ban at the point in which the current in the coil is zero the flux in the circuit is not zero anymore it a piece of iron on the right has become permanently magnetized it is a permanent magnet at that point there is known as their remanent flux and if we wanted to demagnetize that piece of iron we would have to apply a negative MMF with the current in the coil all the way down to this point the coercive force at this point the flux in the circuit is zero the piece of iron on the right has been demagnetized on the right magneto motive for supplying to the magnet on the left the one who supply the MMF actually is the magnet itself this is the area that will be of interest to us when we analyze the behavior of the magnet how do we create a flock with a permanent magnet well located the magnet has replaced on the coil with a current and that we have the air-gap we've said represent a lad magnet with an equivalent current source current source who want which one well observe again in these characteristic of flux versus MMF but when we analyze a that circuit and now we care what happens between the remanent flux and the coercive force we don't care about MMF applied to the magnet but rather about MMF applied by the magnet this is the region of operation of the circuit right there so let me flip that curve horizontally like so this is the area where we're gonna be working this is MMF applied by the magnet here well in that circuit the MMF of the air-gap the one applied the drop if you will the magnetic potential drop MMF not in the air gap is reluctance of the air gap multiplied by the flux in the air gap but that is just a line on this the flux versus MMF and graphic right and that is the line of the air gap at the intersection with the characteristic of the permanent magnet will give us that point of operation of that magnetic circuit which gives us what is the flux in the air gap on this side on this axis and down here what is the MMF applied by the permanent magnet to the air gap please observe one thing if the air gap is too big the reluctance will be will be much higher and then the slope of this line one over reluctance will be even lower and then the point of operation is down here and the MMF applied by the permanent magnet it gets dangerously close to the coercive force at that point you run the risk of the magnetizing the permanent magnet that is one reason why permanent magnets are stored short circuited like so they are stored and their remnant flux point now how do we create the flux in a DC motor normally in our classes we've done that with coils and caring's in this way we have these two thousand current coils being split in two halves a thousand turns on the top and a thousand turns on the bottom and the current of two amps is flowing through them creating the magnetic flux in the air gap mm-hmm but let's replace all those coils by a permanent magnet remove the coils like so on the figure on the right and then in the middle of the magnetic circuit we include our permanent magnet that magnet will create the flux in the air gap the rest of the machines behavior is as seen in class now thanks to www.engvid.com the left and one on the right between those two permanent magnets of course the North Pole of the one on the right on the right faces to the shaft and the South Pole on the magnet on the Left fascias the shaft have and the coils of the rotor here is a cross-section of that motor and in it I have included a part from the magnets on the rotor I have included also what is in the core the core that closes the magnetic flux path observe that each one of those magnets has a North Pole and the South Pole show that the rotor sees a North Pole on one side and a South Pole permanently on the other side let's have a look at this one in motion look notice one thing the only two points of connections the only two cables we will get out of these permanent magnet DC motor are the ones connected to those two brushes they two terminals of the armature of the coils rotating in that rotor of that machine the terminal a1 armature connector one and armature connector - those are the ones you would find available if you have a permanent magnet DC motor and that is all my invisible friends thank you very much for watching and I hope to see you again in our next movie and cut