POLARPOLAR EXPLORATION:EXPLORATION: UNDERSTANDINGUNDERSTANDING DCDC ELECTRICELECTRIC MOTORSMOTORS BY BERNIE THOMPSON Vehicles feature more electrical and electronic components every year, with no end in sight. One of these components is the DC electric motor. Here, we explain how these motors work and how to diagnose them when they aren’t working properly.

any years ago the princi- tional machines, there are six compo- charges, have forces that are opposite ples of electricity were nents that comprise the electric motor: in their effects. Electric charges are ei- unknown. Through the axle, rotor or armature, stator, commu- ther positive or negative, whereas mag- brilliance of many scien- tator, field magnets and brushes. netic fields have a north-south orienta- tists, an understanding In order to understand how a direct tion. When magnetic fields are aligned of electricity was gained, current (DC) electric motor operates, a at opposite or dissimilar poles, they’ll and this knowledge is now common- few basic principles must be under- exert considerable forces of attraction Mplace. One of these brilliant individuals stood. Just as in Faraday’s experiment, with one another, and when aligned at was Michael Faraday. In 1821, Faraday the DC motor works with magnetic like or similar poles, they’ll strongly re- took a free-hanging wire and dipped it fields and electrical current. Centuries pel one another. into a pool of mercury in which a per- ago it was discovered that a stone found The magnetic field will pull or put a manent magnet was submerged. He in Asia, referred to as a lodestone, had force upon a ferrous (magnetic) materi- then passed an electrical current an unusual property that would transfer al. If iron particles are sprinkled on a through the hanging wire and, to his an invisible force to an iron object when paper sheet over a permanent magnet, amazement, the wire rotated around the stone was rubbed against it. These the alignment of the iron particles maps the magnet. Although this was the first lodestones were found to align with the the magnetic field, which shows that time that electrical energy was convert- earth’s north-south axis when freely this field leaves one pole and enters the ed to rotational mechanical energy, no hanging on a string or floated on water, other pole with the force field being un- meaningful work was produced. Many and this property aided early explorers broken. As with any kind of field (elec- years later, Faraday’s principle was used in navigating around the earth. tric, magnetic or gravitational), the total to develop the electric motor. It was understood later that this quantity, or effect, of the field is re- An electric motor can be configured stone was a permanent magnet with a ferred to as the flux, while the push as a solenoid, a stepper motor or a rota- field that had two poles of opposite ef- causing the flux to form in space is tional machine. This article covers the fect, referred to as north and south. called a force. This magnetic force field DC rotational machine. In all DC rota- The magnetic fields, just like electric is comprised of many lines of flux, all

38 January 2010 starting at one pole and returning to the can be accomplished by producing a duce a winding. Converting the con- other pole (see Fig. 1 on page 40). magnetic field with an electrical conduc- ductor from a single, isolated straight The modern theory of magnetism tor that has current flowing through it. wire to one that contains many turns states that a magnetic field is produced Nearly all electric motors exploit the forming a winding amplifies the mag- by an electric charge in motion. When use of a current-carrying conductor to netic force many times. The amount of an electric charge is in motion, the elec- create mechanical work. When current is magnetic field amplification is based on trons orbiting the atom are forced to flowing through a conductor and the the number of turns in the winding and align and uniformly spin in the same di- electric charge is in motion, the electrons the amount of current flowing through rection. The more atoms uniformly orbiting the atoms are forced to align the conductor. spinning in the same direction, the and uniformly spin in the same direction. In this configuration, the magnetic stronger the force of the magnetic field. This creates a magnetic field that forms flux is moving through air, which is a When billions of atoms have orbits spin- around the conductor. The larger the poor conductor of magnetic energy, thus ning in the same direction and the ma- current flowing through the conductor, allowing the magnetic flux to spread out terial is capable of holding the atoms’ the more atoms are forced to align and over a very wide area. Therefore, the re- orbits, a permanent magnet is created. rotate in a uniform direction. This rota- luctance from the magnetic field when When two powerful permanent mag- tional alignment of the atoms increases moving through air is quite high. Reluc- nets are moved in close proximity to one the strength of the magnetic field. How- tance is a measure of how difficult it is another, it’s evident that a very real force ever, if one were to place a conductor for the magnetic flux to complete its cir- is exerted that can provide the potential with current flowing through it near a cuit—that is, to leave one pole and enter for work to be done. For work to be ac- permanent magnet, he would be disap- the opposite pole. If the magnetic flux is complished, the relationship between pointed by how feeble this force is. kept close to the magnet, it has less re- the magnetic fields must be controlled What’s needed is a way to amplify the sistance or opposition to flow. properly. The trick here is to control the magnetic force field. This is accom- Reluctance is similar to the way resis- magnetic fields by a means other than plished by taking the conductor wire tance indicates how much opposition

just using the permanent magnet. This and making many turns or wraps to pro- the current encounters in the electric Images Wieck Media & Jupiter Seyfert, Karl photos: Harold Perry; Photoillustration:

January 2010 39 UNDERSTANDING DC ELECTRIC MOTORS circuit. In a low-resistance electrical cir- nent magnets are moved near one an- opposite direction. The direction of force cuit, a high current can move through other. This same force occurs when an is at right angles to both the current and the conductor with minimal applied electromagnetic field interacts with ei- the magnetic flux density. This means voltage. In order for the reluctance to ther a permanent magnetic field or an- that the forces on the two legs of the ar- be lowered for further amplification of other electromagnetic field. This mag- mature in the permanent magnetic field the magnetic field, a soft iron core is netic force is one that can produce work are applied at right angles in opposite di- placed in the center of the winding. rections. One leg of the armature is Since iron is a ferromagnetic material forced upward while the other is forced and denser than air, the magnetic ener- downward. These forces applied on the gy moves freely through the iron, thus armature produce a turning action on closing the lines of flux and increasing the armature. This turning action, or the flux density and decreasing the re- Single-Bar Magnet torque, is what turns the armature in the luctance. These are very important as- DC motor. In practical applications of a pects of the DC motor. DC motor, there’s not one armature loop In order for the magnetic fields to in- but, rather, multiple loops. This allows teract with one another and produce the armature to produce an even output work in the form of rotation, they must of torque and allows self-starting in any have the correct configuration (see Fig. Repulsion of Similar Poles armature position. 2 below). In this basic example, the DC electric motors are used exten- magnet is of a permanent type and the sively by the automotive industry, for circuit-carrying conductor is formed in- starting the engine, moving the fuel from to a single loop, which is referred to as the containment system, operating the an armature. Current is fed from an windows and moving the seats, to cite outside source, such as a battery, to the just a few examples. Widespread use of brushes; one brush has a positive poten- DC motors in vehicles makes it neces- tial and one has a negative potential. Attraction of Dissimilar Poles sary to check them for proper operation. Illustrations: Blue Soroos Illustrations: The current passes through the nega- Fig. 1 An oscilloscope is used in conjunction tive brush, which is station- with an amp clamp. Since a ary, to one of the commuta- Brush Flux Lines Forces DC motor works with cur- tor bars. The commutator Axle rent flow, the current will bars keep the motor from re- show the operating condition versing as the armature of the electric motor circuit. changes its polarity from pos- Let’s gather data with the itive to negative as it rotates oscilloscope so we can ana- through the magnetic flux. lyze the operation of a DC These metal bars con- Brush motor (Fig. 4, page 42). In structed in a split-ring config- this example, a fuel pump uration are usually made of Battery Permanent Magnet amperage waveform is copper and turn the alternat- Commutator (Stator) shown that uses permanent ing current in the armature Fig. 2 magnets for the field genera- into direct current in the cir- tion. At point A, the fuel cuit by allowing only current to pass by the attraction of opposite poles and pump relay has just been commanded when the armature is in a specific posi- repulsion of similar poles. The perma- on and the current starts to flow tion (Fig. 3, page 41). The commutator is nent magnetic fields produced by the through the armature loop of the DC directly connected to the armature so north and south poles cross the magnet- motor. Point B indicates the peak cur- the current flows through the armature ic armature field, thus producing a force rent, or rush-in current, achieved in the winding back to the positive commutator at right angles to the permanent mag- circuit, which is 14.6 amps. and then to the stationary positive brush, netic field. Since the armature conduc- The rush-in current is a very impor- which is connected to the battery. As tor is bent into a loop, the current tant point in that it’s the only place in the current moves through the armature, the moves in opposite directions in each of current waveform that shows the true electric charge is in motion, which pro- the legs. Current flow in one leg of the current flow of the circuit. This is due to duces an electromagnetic field around loop moves away from the commutator, the nature of the DC motor. As soon as the conductor of the armature. while the current in the other leg of the the current flow moves through the ar- This armature magnetic field inter- loop moves toward the commutator. mature winding, the magnetic field puts acts with the stationary permanent mag- When the current direction is the armature under a torque condition netic field, just as a very real force is changed in a magnetic field, the move- that starts the armature rotating. Induc- produced when two powerful perma- ment of the force is also changed, in the tion in the armature is created when the

40 January 2010 rotating armature winding cuts through armature loops, so to get back to the pared to that in Fig. 4 (14.6 amps). The the permanent magnet’s force field. In- hump you started with, there will be one rush-in current is based on the overall duction occurs when a magnetic field more hump added. Take the oscilloscope resistance of the circuit and will vary moves across a conductor. cursors and mark the two signature among different motor designs. Usually, As the magnetic field moves across humps; the cursors will now display the the higher the fuel pressure, the more the armature winding, the voltage that’s frequency in hertz (Hz). Hertz refers to work the DC motor will have to provide. induced into the armature winding frees the number of complete revolutions the So the armature loops are constructed of electrons. However, since current is armature makes in one second. To con- larger diameter wire that will lower the flowing through the armature winding, vert hertz to rpm, multiply by 60. overall resistance of the circuit, thus al- these free electrons impede the current Staying with Fig. 4, the cursor hertz lowing a higher rush-in current to be ob- flow. This induced current opposes the of 96 is multiplied by 60 seconds to get tained. Lower fuel pressure motors usu- current flowing through the armature an rpm of 5760 (96 ϫ 60 ϭ 5760). Most ally have a rush-in current of about 10 winding. For example, imagine a school automotive fuel pumps should have a amps, while higher fuel pressure motors hallway packed shoulder to usually have a rush-in current of shoulder with children running 12 Volts about 16 amps. down the hallway as fast as they Since the DC motor in Fig. 4 could. Now imagine children en- is the same as in Fig. 5, it shows Spring tering the hallway from the class- there is resistance in the circuit. rooms located along the side. Brush We need to check the rpm of The children leaving the class- the motor in Fig. 5. The hertz rooms can’t change the flow of reading is 73, so the rpm is 4380 children already running down To Ground (73Hz ϫ 60 sec = 4380 rpm). the hallway without increasing This clearly shows that the fuel the pressure (resistance). Just like Commutator pump is turning much slower; the children entering the hallway, however, the average amperage the induced voltage (pressure) in of the motor at its operating the armature winding creates re- rpm is only .4 amp different. sistance to the change in current Insulator This indicates that checking the flowing through the armature average current of the circuit circuit. This resistance is called may not show the problem. It counterelectromotive force Commutator will be necessary also to check (CEMF). When this occurs in a Switch the difference between the top Cuts Off motor or generator, it’s referred Lower Wave of the current draw from the ar- to as reactance. mature loop and the bottom of The faster the magnetic field the current draw where the moves across the armature, the brush moved to the next com- higher the inductive current that mutator bar. This should be occurs within the armature. This Amperage within about 1 amp. The bot- Time can be seen in the current wave- tom portion of the current form at point C in Fig. 4. The Fig. 3 waveform should be sharp and rush-in current drops consider- clean, indicating a clean current ably as the armature rotation increases rotational speed of 5000 to 6000 rpm. If transfer as the brush moves from one in speed, or rpm. As the armature rpm the fuel pump is working correctly, the commutator segment to the next. increases, the current in the armature current waveform will show a current When a DC motor is used as a starter decreases until the working rpm of the draw of about 4 to 10 amps, depending motor, the current waveform can also be motor is obtained at point D, which is on the motor design, with a pump rpm viewed on an oscilloscope. Many of the an average of 6 amps. speed between 5000 and 6000 rpm. If same principles can be used to diagnose It’s very important to check the rota- the fuel pump has cavitations, such as this circuit as were used on the fuel tional speed of the armature by adjusting an empty fuel tank, the average current pump. Fig. 6 on page 42 illustrates the the oscilloscope time base so there are will be low, about 2 amps, and the rpm starter motor data on the oscilloscope. 20 or so armature current loop humps will be high, about 8500 rpm. If the fuel In this example, the rush-in current is visible on the screen. Now look through pump is binding, the current will be 539 amps, which is a little low; it’s usual- the armature loop humps until you find a high and the rpm will be low. ly 700 to 1500 amps, depending on the signature hump—one that’s different Looking at the armature rpm is im- design of the motor. Larger diameter from the others. This hump will repeat portant, as seen in Fig. 5 (the same fuel wire will provide more work capability, itself every seven to nine humps, as most pump as in Fig. 4). In Fig. 5, the rush-in but will also have high rush-in currents. automotive fuel pumps have six or eight current is low at 9.5 amps when com- As the starter armature is put under

January 2010 41 UNDERSTANDING DC ELECTRIC MOTORS Screen captures: Bernie Thompson captures: Screen Fig. 4 Fig. 5

Fig. 6 Fig. 7 torque, the armature starts to rotate, amps different from one cylinder to the the starter armature but increased its which in turn rotates the crankshaft. next. Suspect a problem if the amper- speed, thus decreasing the current The pistons attached to the crankshaft age change from the top of the current draw. As cylinder 1 came up on the also start their up & down movements. hump to the bottom where the brush compression stroke, the starter arma- The crankshaft is loaded by cylinder changes commutator bars is not within ture slowed down, causing an increased compression and slows down on each of this 30- to 50-amp range. Higher com- current draw. Remember to always dis- the engine’s compression strokes. As the pression engines will have a larger dif- able the ignition or fuel to the engine starter armature slows down, the induc- ference between the top and bottom of during this test. You want only the com- tion also decreases, which causes the the current hump. This current draw pression to affect the starter rpm. current to increase. As the crankshaft can be used to quickly check the me- The magic of Faraday’s principle is speeds up after the compression stroke, chanical condition of the engine. now commonplace. Every time you the armature also speeds up, causing The screen capture in Fig. 7 shows start an engine or roll down a window in the induction to increase, which, in an exhaust valve is bad. By completing a a vehicle, you’re surely a magician, for turn, causes the current to decrease. quick cranking amperage test, the low- you’ve released the sheer magic of the Each of the current humps in Fig. 6 compression cylinder can be identified. electric motor. represents an individual cylinder under In the zoom window, the current draw compression. When an engine is in representing each cylinder can clearly This article can be found online at good condition, these humps, peak to be identified. Cylinder 6 has no current www.motormagazine.com. valley, should be no more than 30 to 50 rise, indicating this cylinder did not load

42 January 2010