Understanding Electric Motors
by Bernie C. Thompson
Many years ago the principles of electricity were unknown. Through the brilliance of many scientists, an understanding of electricity was gained and this knowledge is now common place. One of these brilliant individuals was Michael Faraday. In 1821, Faraday took a free hanging wire and dipped it into a pool of mercury in which a permanent magnet was submerged. He then passed an electrical current through the hanging wire, and to his amazement, the wire rotated around the magnet. Although this was the first time that electrical energy was converted to rotational mechanical energy, no meaningful work was produced. Many years later Faraday’s principle was used to develop the electric motor.
In order to understand how the direct current (DC) electric motor operates, a few basic principles will need to be understood. Just as in Faraday’s experiment, the DC motor works with magnetic fields and electrical current. Centuries ago it was discovered that a stone in Asia had an unusual property that would apply an invisible force to an iron object. Later it was found that this stone, referred to as a lodestone, when rubbed against a piece of iron would transfer this same force into the iron. These lodestones were found to align with the earth’s north-south axis when freely hanging on a string or floated on water, and this property aided early explorers in navigating around the earth. It was understood later that this stone was a permanent magnet that possessed a field which was orientated with two poles of opposite effect referred to as north and south. The magnetic fields, just like electric charges, have forces which are opposite in their effects. Electric charges are either positive or negative whereas magnetic fields have a north-south orientation. When magnetic fields are aligned with opposite or dissimilar poles, they will exert considerable forces of attraction with one another and when aligned with like or similar poles, they will repel strongly one another. The magnetic field will pull or put a force upon a Ferris material (magnetic material). If iron particles are sprinkled on a paper sheet over a permanent magnet, the alignment of the iron particles map the magnetic field which shows that this field leaves one pole and enters the other pole with the force-field being unbroken. As with any kind of field (electric, magnetic or gravitational) the total quantity, or effect, of the field is referred to as the flux, while the push causing the flux to form in space is called a force. This magnetic force-field is comprised of many lines of flux all starting at one pole and returning to the other pole (figure 1).
The modern theory of magnetism is that the magnetic field is produced by an electric charge in motion. When an electric charge is in motion the electrons orbiting the atom are forced to align and uniformly spin in the same direction; the more atoms uniformly spinning in the same direction, the stronger the force of the magnetic field. When billions of atoms have orbits spinning in the same direction and the material is correct to hold the atoms orbits, a permanent magnet is created. When two powerful permanent magnets are moved in relationship to one another, it is evident that these forces exert a very real force that can provide the potential for work to be done. In order for work to be accomplished, the relationship between the magnetic fields must be controlled properly. The trick here is to control the magnetic fields by a means other than just using the permanent magnet. This can be accomplished by producing a magnetic field with an electrical conductor that has current flowing through it. Nearly all electric motors exploit the use of a current carrying conductor to create mechanical work. When current is flowing through a conductor and the electric charge is in motion, the electrons orbiting the atoms are forced to align and uniformly spin in the same direction. This creates a magnetic field that forms around the conductor. The larger the current flowing through the conductor, the more atoms are forced to align and rotate in a uniform direction. This rotational alignment of the atoms increases the strength of the magnetic field. However if one were to place a conductor with current flowing through it near a permanent magnet, they would be disappointed in how feeble this force is. What will be needed is a way to amplify the magnetic force-field. This is accomplished by taking the conductor and making many turns or wraps to produce a winding. When the conductor is converted from a single isolated straight wire to a conductor that contains many turns forming a winding, the magnetic force is amplified many times. The amount of magnetic field amplification is based on the number of turns and the amount of current flowing through the conductor. In this configuration, the magnetic flux is moving through air which is a poor conductor of magnetic energy, thus allowing the magnetic flux to spread out over a very wide area. Therefore, the reluctance from the magnetic field when moving through air is quit high. Reluctance is a measure of how difficult it is for the magnetic flux to complete its circuit, which is to leave one pole and enter the opposite pole. If the magnetic flux is kept close to the magnet it has less resistance or opposition to flow. Reluctance is similar to the way that resistance indicates how much opposition the current encounters in the electric circuit. In a low resistance electrical circuit, a high current can move through the conductor with minimal applied voltage. In order for the reluctance to be lowered for further amplification of the magnetic field, a soft iron core is placed in the center of the winding. Since iron is a ferromagnetic material and is denser than air, the magnetic energy moves freely through the iron thus closing the lines of flux and increasing the flux density and decreasing the reluctance. These are very important aspects of the DC motor.
An electric motor can be configured as a solenoid, stepper motor or a rotational machine. This article will cover the DC rotational machine. In all DC rotational machines, there are six components that comprise the electric motor; axle, rotor or armature, stator, commutator, field magnetic, and brushes. In order for the magnetic fields to interact with one another and produce work in the form of rotation, the magnetic fields must have the correct configuration (figure 2).
In this basic example, the magnet is of a permanent type and the circuit carrying conductor is formed into a single loop which is referred to as an armature. Current is fed from an outside source, such as a battery to the brushes, one brush has a positive potential and one brush has a negative potential. The current passes through the negative brush, which is stationary, to the commutator bar. The commutator bars keep the motor from reversing as the armature changes its polarity from positive to negative as it rotates through the magnetic flux. These metal bars constructed in a split ring configuration are usually made of copper and turn the alternating current in the armature into direct current in the circuit by only allowing current to pass when the armature is in a specific position (figure 3).
The commutator is directly connected to the armature so that the current flows through the armature winding back to the positive commutator and then to the stationary positive brush which is connected to the battery. As current moves through the armature, the electric charge is in motion which produces an electromagnetic field around the conductor of the armature. This armature magnetic field interacts with the stationary permanent magnetic field just as when two powerful permanent magnets are moved in relation with one another they produce a very real force. This same force occurs when an electromagnetic field interacts with a permanent magnetic field, or another electromagnetic field. This magnetic force is one that can produce work by the attraction of opposite poles and repulsion of similar poles. The permanent magnetic fields produced by the north and south poles cross the magnetic armature field, thus producing a force at right angles to the permanent magnetic field. Since the armature conductor is bent into a loop, the current is moving in opposite directions in each of the legs. Current flow in one leg of the loop is moving away from the commutator while the current in the other leg of the loop is moving toward the commutator. When the current direction is changed in a magnetic field the direction of the force is also changed in the opposite direction. The direction of force is at right angles to both the current and the magnetic flux density. This means that the forces on the two legs of the armature in the permanent magnetic field are applied in right angles in opposite directions. One leg of the armature is forced upward while the other leg of the armature is forced downward. This force that is applied on the armature produces a turning action on the armature. This turning action or torque is what turns the armature in the DC motor. In practical applications of a DC motor, there is not one armature loop rather multiple loops. This allows the armature to produce an even output of torque and allows self starting in any armature position.
The DC electric motor is used extensively by the automotive industry from starting the engine, to moving the fuel from the containment system, to window operations, to moving the seats. All these are just a few examples of where DC motors can be used in the modern vehicle. Widespread use of DC motors in vehicles makes it necessary to check these DC motors for proper operation. An oscilloscope is used in conjunction with an amperage clamp. Since the DC motor works with current flow, the current will show the operating condition of the electric motor circuit. Let us gather data with the oscilloscope so we can analyze operation of the DC motor (figure 4).
In this example, a fuel pump amperage waveform is shown that uses permanent magnets for the field generation. At point “A” the fuel pump relay has just been commanded on and the current starts to flow through the armature loop of the DC motor. At point “B” is the peak current or “rush in current” achieved in the circuit, which is 14.6 amps. The rush in current is a very important point in that this is the only place in the current waveform that will show the true current flow of the circuit. This is due to the nature of the DC motor; as soon as the current flow moves through the armature winding, the magnetic field puts the armature under a torque condition that starts the armature rotating. Induction in the armature is created when the rotating armature winding cuts through the permanent magnet’s force field. Induction occurs when a magnetic field moves across a conductor. As the magnetic field moves across the armature winding, the voltage that is induced into the armature winding frees electrons. However, since current is flowing through the armature winding these free electrons impede the current flow. This induced current opposes the current flowing through the armature winding. For example; imagine a school hallway packed shoulder to shoulder with children running down the hallway as fast as they can run. Now imagine children entering the hallway from the classrooms located along this hallway. The children leaving the classrooms can’t change the flow of children already running down the hallway without increasing the pressure (resistance). Just like the children entering the hallway, the induced voltage (pressure) in the armature winding creates resistance to the change in current flowing through the armature circuit. This resistance is called counter electromotive force (CEMF for short), or when this occurs in a motor or generator it is referred to as reactance. The faster the magnetic field moves across the armature, the higher the inductive current that occurs within the armature. This can be seen in the current waveform at “C”. The rush in current drops considerably as the armature rotation increases in speed or RPM. As the armature RPM increases the current in the armature decreases until the working RPM of the motor is obtained at “D”, which is an average of 6 amps. It is very important to check the rotational speed of the armature by speeding up the oscilloscope so there are twenty or so armature current loop humps visible on the screen. Now look through the armature loop humps until you find a signature hump, which is one that is different from the others. This hump will repeat it self seven to nine humps later as most automotive fuel pumps have six or eight armature loops, so to get back to the hump that you started with, there will be one more hump added. Take the oscilloscope cursors and mark the two signature humps, the oscilloscope cursors will now display the frequency in hertz. The hertz is the number of complete revolutions the armature made in one second. To convert this to RPM, multiply the hertz by 60. So in (figure 4), the cursor hertz are 96 multiplied by 60 sec (96Hz x 60sec = 5760 RPM). Most automotive fuel pumps should have a rotational speed of 5000 to 6000 RPM. If the fuel pump is working correctly the current waveform will show a current draw of about 4 to 10 amp depending on the motor design, with a pump RPM speed between 5000 and 6000 RPM. If the fuel pump has cavitations, such as an empty fuel tank, the average current will be low at about 2 amps and the RPM will be high at about 8500 RPM. If the fuel pump is binding, the current will be high and the RPM will be low. Looking at the armature RPM is important, as seen in figure 5; this is the same fuel pump as in figure 4.
In figure 5, the rush in current is low at 9.5 amps when compared to figure 4 where rush in current is 14.6 amps. The rush in current is based on the over all resistance of the circuit. This rush in current will vary between different motor designs. Usually the higher the fuel pressure the more work the DC motor will have to provide, so the armature loops are constructed of larger diameter wire which will lower the over all resistance of the circuit, thus allowing a higher rush in current to be obtained. Lower fuel pressure motors usually have a rush in current of about 10 amps where higher fuel pressure motors usually have a rush in current of about 16 amps. Since the DC motor in figure 4 is the same as in figure 5, it shows there is resistance in the circuit. What will be needed is to check the RPM of the motor in figure 5, which is a hertz reading of 73Hz, so the RPM is (73Hz x 60sec = 4380 RPM). This clearly shows the fuel pump is turning much slower, however the average amperage of the motor at its operating RPM is only 0.4amps different. What this shows is that checking the average current of the circuit may not show the problem. It will be necessary also to check the difference between the top current draw from the armature loop and the bottom of the current draw where the brush moved to the next commutator bar. This should be within about 1 amp. The bottom portion of the current waveform should be sharp and clean showing a clean current transfer as the brush moves from one commutator to the next.
When a DC motor is used for the starter motor, the current waveform can also be viewed on the oscilloscope. Many of the same principles will be used to diagnose this circuit as were used on the fuel pump. Let us gather the starter motor data on the oscilloscope (figure 6).
In this example, the rush in current is 539 amps, which is a little low considering that it is usually 700amps to 1500amps, depending on the design of the motor. Larger diameter wire will provide more work capability, but it will also have high rush in currents. As the starter armature is put under torque, the armature starts to rotate, which in turn rotates the crankshaft. The pistons attached to the crankshaft also start their up and down movements. This means the crankshaft is loaded by the cylinder compression and slows down on each of the engine’s compression strokes. As the starter armature slows down, the induction also decreases which causes the current to increase. As the crankshaft speeds up after the compression stroke, the armature also speeds up causing the induction to increase which causes the current to decrease. Each of the current humps in figure 6 represents an individual cylinder under compression. On a good engine these humps, peak to valley, should be about 30-50amps different from one cylinder to the next. Suspect a problem if the amperage change from the top of the current hump to the bottom where the brush changes commutator bars is not within 30amps to 50amps. Higher compression engines will have a larger difference between the top and bottom of the current hump. This current draw can be used to check quickly the mechanical condition of the engine. In figure 7 the exhaust valve is bad.
By completing a quick cranking amperage test the low compression cylinder can be identified. In the zoom window, located in figure 7, each current draw representing each cylinder can clearly be identified. Cylinder 6 has no current rise, indicating this cylinder did not load the starter armature but increased its speed thus decreasing the current draw. As cylinder 1 came up on the compression stroke the starter armature slowed down causing an increased current draw. Remember to always disable the ignition or fuel to the engine during this test; you only want the compression to affect the starter RPM.
The magic of Faraday’s principle is now common place. Every time you start your engine or roll down your window in your vehicle you are surely a magician; for you have released the shear magic of the electric motor.
AUTOMOTIVE TEST SOLUTIONS, INC. © 2011
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