Magnets, Magnetic Field, Electromagnet, Electromechanical Solenoid, Stator, Armature, DC Electric Motor and Automobile Starter Motor

Posted by PITHOCRATES - April 18th, 2012

Technology 101

Electric Current flowing through a Wire can Induce Magnetic Fields Similar to those Magnets Create

We’ve all played with magnets as children.  And even as children we’ve observed things.  If you placed a bar magnet on a table and approached it with another one in your hand one of two things would happen.  As the magnets approached each other the one on the table would either move towards the other magnet.  Or away from the other magnet.  That’s because all magnets are dipoles.  That is, they have two poles.  A north pole.  And a south pole. 

These poles produce a magnetic field.  Outside of the magnet this field ‘flows’ from north to south.  Inside the magnet it ‘flows’ from south to north.  So imagine this magnetic force traveling through the magnet from south to north and right out of the north pole of the magnet.  Where it then bends around and heads back to the south pole.  Something most of us saw as children.  When we placed a piece of paper with iron filings over a bar magnet.  As we placed the paper over the magnet the iron filings moved.  They formed in lines.  That followed the magnetic field created by the magnetic dipole.  You can’t see the direction of the field but it only ‘flows’ in one direction.  As noted above.  If the north pole of one magnet is placed near the south pole of another the magnetic field ‘flows’ from the north pole of one magnet to the south pole of the other magnet.  Pulling them together.  If both north poles or both south poles are placed near each other they will repulse each other.  Because the magnetic field is ‘flowing’ out from each north pole.  Or into each south pole.  The magnets repulse each other because the magnetic field is trying to flow from north to south.  If one magnet was able to rotate this repulsion would rotate the magnet about 90 degrees.  To try and align one north pole with one south pole.  As the momentum pushed the magnet past the 90 degree point the force would reverse to attraction.  Rotating the magnet about another 90 degrees.  Where it will then stop.  Having aligned a north and a south pole. 

It turns out this ability to move things with magnetic fields is very useful.  Both in linear motion.  And rotational motion.  Especially after we observed we could create magnetic fields by passing an electric current through a wire.  When you do a magnetic field circles the wire.  To determine which direction you simply use the right-hand rule.  Point your thumb in the direction of the current flow and wrap your fingers around the wire.  Your fingers point in the direction of the magnetic field.  Fascinating, yes?  Well, okay, maybe not.  But this is.  You can wrap that wire around a metal rod.  Creating a solenoid.  And all those induced magnetic fields add up.  The more coils the greater the magnetic field.  That ‘flows’ in the same direction in that metal rod.  Creating an electromagnet out of that metal rod.  If you ever saw a crane in a junk yard picking up scrap metal with a magnet this is what’s happening.  The crane operator turns on an electromagnet to attract and hold that scrap metal.  And turns off the electromagnet to release that scrap metal.

A DC Electric Motor is Basically a Fixed Magnet Interacting with a Rotating Magnet

If that metal rod was free to move you get something completely different.  For when you pass a current through that coiled wire the magnetic force it creates will move that metal rod.  If it’s not restrained it will fly right out of the coil.  Which is interesting to see but not very useful.  But the ability to move a restrained metal rod at the flick of a switch can be very useful.  For we can use a solenoid to convert electrical energy into linear mechanical movement.  As in a transducer.  An electromechanical solenoid.  That takes an electrical input to generate a mechanical output.  Which we use in many things.  Like in a high-speed conveyor system that sorts things.  Like a baggage handling system at an airport.  Or in an order fulfillment center.  Where things fly down a conveyor belt while diverter gates move to route things to their ultimate destination.  If the gate is not activated the product stays on the main belt.  When a gate is activated a gate moves across the path of the main conveyor belt and diverts the product to a new conveyor line or a drop off.  And the things that operate those gates are electromechanical solenoids.  Or transducers.  Things that convert an electrical input to a mechanical output.  To produce a linear mechanical motion.  To move that gate.

Solenoids are useful.  A lot of things work because of them.  But there is only so much this linear motion can do.  Basically alternating between two states.  Open and closed.   In or out.  On or off.  Again, useful.  But of limited use.  However, we can use these same principles and create rotational motion.  Which is far more useful.  Because we can make electric motors with the rotational motion created by magnetic fields.  The first electric motors were direct current (DC).  And included two basic parts.  The stator.  And the rotor (or armature).  The stator creates a fixed magnetic field.  With permanent magnates.  Or one created with current passing through coiled wiring.  The armature is made up of multiple coils.  Each coil insulated and separate from the next one.  When an electric current goes through one of these rotor coils it creates an electromagnet. 

So a DC electric motor is basically a fixed magnet interacting with a rotating magnet.  Current passes to the rotor winding through brushes in contact with the armature.  Like closing a switch.  Current flows in through one brush.  And out through another.  When current goes through one of these rotor coils it creates an electromagnet.  With a north and south pole.  As this magnetic field interacts with the fixed magnetic field produced by the stator there are forces of attraction and repulsion.  As the ‘like’ poles repel each other.  And the ‘unlike’ poles attract each other.  Causing the armature to turn.  After it turns the brushes ‘disconnect’ from that rotor wiring and ‘connect’ to the next rotor winding in the armature.  Creating a new electromagnet.  And new forces of repulsion and attraction.  Causing the armature to continue to turn.  And so on to produce useful rotational mechanical motion.

An Automobile Starter Motor combines an Electromechanical Solenoid and a DC Electric Motor

Everyone who has ever driven a car is thoroughly familiar with electromechanical solenoids and DC electric motors.  Because unlike our forefathers who had to use hand-cranks to start their cars we don’t.  All we have to do is turn a key.  Or press a button.  And that internal combustion engine starts turning.  Fuel begins to flow to the cylinders.  And electricity flows to the spark plugs.  Igniting that compressed fuel-air mixture in the cylinder.  Bringing that engine to life.

So what starts this process?  An electromechanical solenoid.  And a DC motor.  Packaged together in an automobile starter motor.  The other components that make this work are the starter ring gear on the flywheel (mounted to the engine to smooth out the rotation created by the reciprocating pistons) and the car battery.  When you turn the ignition key current flows from the battery to the electromechanical solenoid.  This linear motion operates a lever that moves a drive pinion out of the starter (while compressing a spring inside the starter), engaging it with the starter ring gear.  Current also flows into a DC motor inside the starter.  As this motor spins it rotates the starter ring gear on the flywheel.  As combustion takes place in the cylinders the pistons start reciprocating, turning the crankshaft.  At which time you let go of the ignition key.  Stopping the current flow through both the solenoid and the DC motor.  The starter stops spinning.  And that compressed spring retracts the drive pinion from the starter ring gear.  All happening in a matter of seconds.  So quick and convenient you don’t give it a second thought.  You just put the car in gear and head out on the highway.  And enjoy the open road.  Wherever it may take you.  For getting there is half the fun.  Or more.

Electric motors have come a long way since our first DC motors.  Thanks to the advent of AC power distribution and polyphase motors.  Brought to us by the great Nikola Tesla.  While working for the great George Westinghouse.  Pretty much any electric motor today is based on a Tesla design.  But little has changed on the automotive starter motor.  Because batteries are still DC.  And before a car starts that’s all there is.  Once it’s running, though, a polyphase AC generator produces all the electricity used after that.  A bridge rectifier converts the three phase AC current into DC.  Providing all the electric power the car needs.  Even charging the battery.  So it’s ready to spin that starter motor the next time you get into your car.


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