Understanding RMF | The driving force behind every AC machine

every ac motor uses a rotating magnetic field an invention that kicked off the industrial revolution can you guess how the rmf reached this stage in this video we'll travel through the minds of the geniuses behind the development of rmf the greatest contribution can perhaps be attributed to nikola tesla considered by many as the pioneering father of modern engineering to understand how the design theories evolved over time our trip will take us all the way through to a glimpse of modern day winding techniques and rmf production let's go the idea of rotating magnetic fields first started with walter bailey he used dc power supply and this arrangement had electromagnets and commutator rings let's understand this rmf production in a step-by-step manner when a direct current passes through an electromagnet it creates a magnetic field with a specific orientation if you add a second electromagnet which is oppositely wound magnetic field lines get connected between the opposite poles let's represent these newly formed magnetic field lines using an arrow now add one more such pair diagonally opposite to the first pair the interaction between these two magnetic field lines will produce a resultant magnetic field line now let's de-energize the first pair keeping only the second pair energized the resultant magnetic field in this case will be as shown if we energize both pairs again this time reversing the polarity of current in one of the coils the resultant magnetic field changes once again in all four cases the resultant magnetic fields have one thing in common and one thing altered did you spot what those were in each case the field lines have the same magnitude but different angles in short the resultant magnetic field rotates in a step size of 45 degrees we have just seen the design of the first ever rmf further rotation can be induced by repeatedly switching the operation on and off in the same pattern walter added a copper disc on top to physically demonstrate the rotation as the magnetic field changes it induces eddy currents just as michael faraday had once predicted the disk starts to rotate along with the rmf although a genius invention there was a flaw in the design can you see what that might be rather than being a smooth rotation the rmf has a jerky movement it's safe to say this was not a particularly useful feature for an electric motor to have this rocky ride is due to the use of dc power the genius inventor of all time nikola tesla came up with his own idea of producing rmf using two-phase alternating currents this idea is elaborated in his patent for induction motors in 1888 coincidentally the same year an intelligent inventor named galileo ferraris also introduced a similar way of producing rmf tesla's design was a clever modification of walters electromagnets first on his list was to expel the commutators which were notoriously tedious to operate next he supplied alternating currents with an angular difference of 90 degrees between them this means that the field produced by one coil is shifted in position in reference to the other wondering how that works let's take a closer look first of all let's consider the starting point the current from coil a is a small positive value whereas the current from coil b is a larger negative value the individual magnetic fields produced by both the coils is as shown if we add both fields together we get this resultant field in the motor now hold that thought for future reference let's see what happens as the currents vary in this instance coil a carries a positive current of the highest magnitude and the current in coil b is zero at this moment the magnetic field produced by coil a is the only field in operation finally both the coil currents are positive and are at equal magnitude creating a resultant field as shown if you observe these three instances you will find that nikola tesla's two-phase machine design produces a rotating magnetic field while tesla used his ac generator to make this phase shift possible galileo ferraris used an inductor in one of the two coils supplied by a single phase ac dynamo later on nikola tesla's two phase was increasingly popular due to the practical viability of his designs and the efficiency of the ac polyphase generators needed for these motors we'll leave tesla and ferraris to battle this one out meanwhile the story of rmf development is not over just yet back then while the two-phase rmf was just born russian engineer mikhail dobrovolsky gave the world three-phase rmf we will understand the three-phase rmf design logically and by the end of this section we will also understand why three-phase rmfs are superior to nikola tesla's two-phase rmf let's use a simplified winding design to understand the working of three-phase rmf a three-phase current supply will vary with the time as illustrated in this arrangement now we need to find out how the resultant magnetic field varies due to the current variation let's freeze at this instant the magnetic fields produced by all the coils are illustrated here this can be easily deduced using the thumb rule now let's combine all these six magnetic field lines together to get the resultant magnetic field in short this is the shape of the rmf at this instant let's vary the currents for a small time interval and freeze the scene remember that the field density is higher for conductors with larger currents let's trace the resultant path again comparing both instances the resultant field has clearly rotated calculating for all other current instances the magnetic field is seen to rotate one revolution for one cycle of current and there we have it that's how a three-phase rotating magnetic field actually rotates so to round up today's video let's decide who is the ultimate winner of rotating magnetic field systems was it tesla or was it dobrovolsky let's use the modern technique of finite element analysis to settle the vote the simulation results from em works 2d software make a clear case for why three-phase rmf designs are superior to two-phase designs as you can see the two-phase arrangement gives a dip or oscillation in the rotating magnetic field in the three-phase design these oscillations are effectively reduced now for a word on north and south pole notations of the rmf did you notice something a little weird about those here is what we are talking about in a permanent magnet north and south poles are defined as shown yet in the case of rmfs it is defined in an opposite way have you ever wondered why after all the whole purpose of marking north and south poles is to make the analysis easier in a permanent magnet its external area is the useful area however for an rmf the internal area or the magnetic field in the inner region is the most useful if you consider the case of an alternator or an induction motor this fact will be clear to you the outside field of an rmf is never used to get any useful work when you focus on this useful internal area of an rmf it is quite logical to define the north and south poles as shown only if we mark the north and south poles as shown while we get the magnetic field direction correct in the inside area so hopefully that's cleared that up let's move on to the last part of our video modern day windings the modern day winding for the three-phase rmf is able to produce rmf with almost no oscillations this fulfills the actual definition of an rmf which is a magnetic field of constant magnitude revolving in nature one such modern winding which produces a 4-pole rmf is shown here a modern-day winding design is a broad subject we need a separate dedicated video to get into it this concludes our journey through the history of rotating magnetic fields and the minds of their great inventors it is amazing to know how our ancestors developed winding designs which produce perfect rmfs just by using their imagination and precise calculations if you enjoyed this video why not hit 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Channel: Lesics

Views: 400,388

Rating: 4.9469848 out of 5

Keywords: RMF, Rotating magnetic field

Id: wqrGHeuxUvI

Length: 9min 19sec (559 seconds)

Published: Tue Nov 10 2020