Episode #36: What Is An Electric Motor?

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The Motion Control Show

Today we're talking about what an electric motor is.  Electric Motors have actually been around for a long time, so there isn't anything really new and earth shattering here.  I'm going to be using a lot of old graphics I've been using for a while, but they just help to explain the basic concepts.  If you have any applications or any questions, anything you'd like to discuss with us, reach out to us at www.TheMotionControlShow.com, or this e-mail address.  We're happy to help.  I'm Corey Foster at Valin Corporation.  Let's see what we can learn.

A motor is just an electric machine that converts electrical energy into mechanical energy.  That's why we use the term electro-mechanical.  It's an electromechanical energy converter.  If the electricity is coming in, as shown here from the left-hand side into the motor, it then converts it into mechanical energy and there are some losses, of course.  The power in is going to equal the power out plus the power of the losses.  Now, if we turn a motor around, like a generator or alternator in a car, and you run it backwards and you turn the shaft of the motor, that is going to create an electrical output.  Again, there are going to be losses.  One direction it's motor and the other way it's a generator.  Now keep in mind that without any movement, no change occurs in the mechanical energy.  Electrical energy can still be going in, but all it's doing is heating the motor and that's part of the losses.  So, motor one way; a generator the other way.

The motor operates on a few electromagnetic principles.  They are permanent magnets, so opposites attract: North, South.  The likes repel: North, North repels; South, South repels.  Because they’re electro-magnets, the magnet field is based on the direction of the current flow and the number of turns of the copper wire influences the strength of that field.  The direction of it is defined by the right-hand rule.  So, if you have a coil of wire and the current is going that way through the wire, that field, the current it generates, goes that way.  Likewise, if the current goes this way, the field goes that way. 

If you take a look at these three windings here, watch just one winding.  Watch the L2 for example and watch these arrows.  These arrows indicate which way the current is going, and the strength of it.  If you watch just one arrow, first it's going one way, and it builds, then that changes directions and goes the other way.  The other arrow with it corresponds to that direction because it's just the other end of the wire.  As I was just saying, as the current goes through this winding, it creates a magnetic field that goes around it and which way that field goes is defined by the direction of the current.  These arrows right here indicating that the magnetic field this is generating is changing back and forth.  First, the direction of the field is going one way, then it's going the other way.  Each time that field changes, it goes from North to South.  So, one way it's North, the other way its South. North, South, North, South.  And it gets larger and smaller in each direction.  Now if you stare at a spot in between these windings, notice that these arrows are not going at the same time.  They aren’t synchronized exactly at the same time.  In fact, there are about 120 degrees off electrically, so you'll see that one is a little bit ahead of the other, which is a little bit ahead of the other.  If you stare at the middle, you can see that they are off by just a little bit.  This one turns North and then as it starts to fade into South, this one turns North and then as it starts to fade into South, this one starts to turn North.  So, if we put a magnet in the middle that has one end North and one end South, as this one is turned from South into North, it's going to repel that North over here and is going to look like this.  And that is how an electric motor is controlled by the current in the windings. 

An ideal motor has a torque/speed curve that looks like this.  As the torque goes up, the speed goes down and as the speed goes up, the torque goes down.  That's really an ideal motor, if there's no mechanical losses, no electrical losses; it's all based on Ohm’s law, V=IR.  The current is directly related to the torque and the voltage is directly related to the velocity.  However, because this isn't a perfect world, we have all sorts of losses.  We have frictional losses in the bearings and air.  We have winding resistances.  We have eddy currents in the magnetism.  We have hysteresis.  We have magnetic saturation.  And most of all, we have heat.  Heat is a big loser!  So, what that ends up doing is just cutting off the curve.  This is going to be more like what a speed/torque curve ends up looking like, where there is kind of a flat on the top, kind of a constant power curve, and then there's ultimately a non-infinite speed that it can go. 

In every electric motor there's the rotor, which is what is around the shaft.  Here's the shaft of motor and these are all just layers of steel lamination's that are stacked up against each other.  These are all laminations.  And then there are aluminum rotor bars up and down here that direct the current from the magnetic field.  Then the current that goes through those magnetic bars is turned around by the little cap on the end and that current goes up and down that router.  Then there's the stator which goes around it.  The stator is also a bunch of steel laminations stacked together.  It has some windings and then this particular design has three phases, A, B and C as denoted by the different colors, and then in between, there are slots.  That's where the copper wire goes around those teeth.  This is one type of motor. 

I have a rotor that then turns the shaft here on the end.  We have bearings that allow that shaft to turn.  It is controlled by the stator, which is powered with the windings going in and out of the stator here.  If we need to cool off the motor because heat is a loss, there is a fan on the back.  This is just one type of motor in particular. 

Let's take a look at another type of motor here.  This has one winding here, which is connected to the other side here.  Notice that the wire is wound one way here and wound the other way here.  Therefore, you get North and South.  Now, this is a very simplified version of this particular motor, but because the current is going through this one and creating North here and South here, the North is going to attract the South part of the rotor, and the South is going to attract the North part of the rotor.  This is a more realistic version of this particular type of motor where there's a lot more of these teeth.  It's just a higher quantity of the number of teeth and poles that we're talking about here.  But let's go back and take a closer look at what happens when we change the current from one to the next.  OK, so we have current going through here, so we have North and South.  And you can see the South is connected or is attracted to the North.  You can see the current is still going through here, but now we have current going through here as well, so this is South and this is North.  The South has gone away from the North a little bit because this North has been pulled toward the South, so you can't see it really obviously, but this South Pole has turned a little bit here, so now they're balanced between these two poles.  So, if we go one more step here, the current is turned off here.  It's turned on here, and left on there, so now the North Pole is lined up straight with this one and the South Pole has moved a little bit more.  If you look at this South Pole here, it's moved a little bit and then a little bit.  And that is how a rotor is moved incrementally from one pole to the next in a controlled fashion. 

I hope that helps you to understand what an electric motor is, at least the basics.  There are a lot of different types.  We're going to be talking about those in the future.  I am Corey Foster at Valin Corporation.  I hope this helps.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.