ECE 3600 Direct Current (DC) Motors A Stolp 12/5/15

Transcription

1 rmature he rotating part (rotor) Field (Exitation) ECE 3600 Diret Current (DC) Motors Stolp 1/5/15 Provided by the stationary part of the motor (Stator) Permanent Magnet Winding Separately exited Parallel with terminal voltage soure (Shunt exited) Series with terminal voltage soure (Series exited) erminal voltage, Rotor Commutator and ommutation Rotary ontats and brushes whih keep swithing the urrent diretion in the armature so that the motor torque is always in the same diretion. Explained and visualized in lass Eletrial Model Stator Field rmature resistane erminal oltage L rmature Indutane, Important relationships I usually ignored. ω. Bak EMF Indued (developed) torque (NO output or shaft torque) Mehanial speed If the field is onstant ω ω 1 1 n n 1 Simplified Model we will use. 1 If field dereases, speed an inrease to an unsafe level R P onv E.. ind Rotor Copper Loss P onv P onv hese are often substituted in other eq. P meh I. beomes: losses P I. onv P OR 0 I. onv out OR ω P or m onv shaft. OR 0. P. onv shaft drives a load not pitured 1 p1

2 Powers p P in P onv P out. I.. ind ω. m out 1. hp W may inlude:. F I F. I R losses. P rot (& ore & mis) sometimes may inlude: I. F P meh P ore P mis Mehanial Loads and losses P. Constant power If an output or loss power is onstant for all speeds, then the torque is inversely proportional to the speed. his is highly unlikely in real life. Proportional to speed If a power is proportional to speed, then the torque is onstant with speed. Conversely, if a torque is onstant for all speeds, the power is proportional to speed. bit more likely in real life. orque proportional to speed Power is proportional to the square of the speed pproximates more real-life loads, torque required to turn a load usually inreases with speed. Nameplate Operation he Nameplate gives the rated oltage, Current(s), Speed and output Power (often as horsepower, hp). his is onsidered full-load operation. 1. hp W Motor Constant, K Our book defines the motor onstant suh that it does not inlude the field flux,. Often the motor onstant is defined differently, as K, but just alled K, whih is a funtion of the field urrent. K (or K) is most easily found by operating the motor as a generator with no load, then. Spin Diretion: Reverse the leads to either of the windings and the motor will run in the opposite diretion. orque - Speed urves. beomes:. t 0 speed (loked rotor):. R.. K... K. t max speed (no indued torque): K Output orque instead of Indued orque If lost torque is proportional to speed: shaft. shaft fri. ind fri. R.. shaft fri. R K. R. ω m p

3 DC Motor ypes and Charateristis p3 Permanent Magnet Permanent-Magnet motors are typially small. hey an be quite powerful for their size, espeially if made with rare-earth magnets. hese motors are ommon in hildren's toys and servo systems. Some eletri ars use Large permanent-magnet DC motors. he harateristis are like the separately exited motor with a onstant flux. Sine the flux an't be hanged, the motor onstant times flux, K, is usually simplified to just a different motor onstant, usually also alled K, whih inludes the onstant flux. Separately Exited he field flux is by urrent flowing through a field winding, whih is supplied by a separate power soure or at a different voltage than... K slope lower (onstant field) Permanent Magnet or Separately exited half the field flux rmature Simplified Field rmature Winding L I F I F F F L F Field Winding half Output orque instead of Indued orque, If lost torque is proportional to speed: shaft. shaft fri. ind fri. R.. shaft fri. R K. R. ω m t 0 speed (loked rotor):.. K shaft Permanent Magnet or Separately exited no frition lower (onstant field) half the field flux no frition t max speed (no indued torque): fri.. K. K p3

4 Shunt Exited p4 Field winding is onneted in parallel with the armature to the same terminal voltage soure,. I F L F If flux is proportional to field urrent. I F. L t 0 speed (loked rotor):. the "ore onstant" K... so:. K... K. Field Winding rmature Or, simply:.. K I F t max speed (no indued torque):.. K.. I F. K lower. K Output orque instead of Indued orque, If lost torque is proportional to speed: shaft. shaft fri. ind fri. R.. shaft K.. fri. K... K. shaft t max speed (no shaft torque):. fri K.. K.. fri. R... K. lower no 1 fri. R... K frition. K 1 fri. R... K p4

5 Series Exited (Often alled an C/DC or Universal Motor) p5 Field winding is onneted in series with the armature and is designed with muh thiker windings, so it an handle muh larger urrent and has muh less resistane. he resistane of the field winding is now alled R S. Sine R S is in series with, they are often ombined into S. Field Winding Simplified Model erminal oltage R S L S S R R S I. S L P meh loses rmature. S out I. R.. S K or shaft If flux is proportional to field urrent. I. R.. S K. K.. I. S.. K so:. K. R.. ind S K. S. I. Series Exited orque - Speed urve ind. S t 0 speed (loked rotor):.. S. K Max speed (no indued torque) is undefined S max. S S. is usually small, so the maximum indued torque an be huge.. R.. S K lower p5

6 Output orque instead of Indued orque, If lost torque is proportional to speed: p6. S shaft shaft. fri _ shaft. fri shaft. R.. S K. fri lower C/DC or Universal Motor (Series Exited DC motor) Beause the same urrent flows through both the field and the armature, the series-exited motor will turn the same diretion even if the urrent flows the opposite diretion-- or even if goes bak and forth (C). hese motors are ommon in C devies beause they an provide a lot of power and torque in small, lightweight motor. hey are very ommon in handheld power tools like drills, saws, grinders, weed eaters, hedge trimmers, et.. hey are also found in vauum leaners, blenders and food proessors. If you look at the motor of an C devie and see brushes and a ommutator, then it is a universal motor. S L S It is easy to vary the speed of these motors by hanging the average voltage, usually with a thyristor-based ontrol similar to a light dimmer. Universal motors tend to be very noisy. When used with C supply, the indutane of the windings beomes important. Compounded Motor motor with both shunt and series field windings. Covered in your book, starting on p. 40. Brushless Many Brushless DC motors simply replae the ommutator with a rotor position sensor (magneti (Hall effet) opti, indutive, et.) and power-eletroni iruitry whih swithes the winding urrent. he field is usually on the rotor (permanent magnet or a winding fed through slip rings) and the armature windings are stationary. hese motors are analyzed just like DC motors with brushes. Many DC fans are made this way. Many Brushless DC motors are atually 3-phase Synhronous or Indution motors driven by power-eletroni iruitry whih produes variable-frequeny, Pulse-Width-Modulated (PWM), 3-phase power to operate the motor. tually, they don't have to be 3-phase, as long as they have at least phases., 3, 4, and 6 phases are ommon. Brushless motors have some important advantages. hey are mehanially simpler and more reliable. hey an be operated in environments where the sparking between brushes and ommutator would be undesirable or unsafe. hey are relatively quiet. Some sample waveforms are shown on the next page. p6

7 ariable-frequeny, Pulse-Width-Modulated (PWM), 3-phase Power p7 Lower power phase Higher power v( t) v( t) t t B C Higher speed Pulse Width Modulation (PWM) for speed ontrol lower speed s you an see by the torque - speed urves above, regulating the terminal voltage,, is a very effetive way to ontrol the speed of a DC motor. Unfortunately, it is often an ineffiient proess. Pulse Width Modulation, shown here, is a very effiient. It is also more linear, espeially at low speeds. he torque - speed urves do not show these non-linearities-- due largely to the differene between stati and dynami frition. (Motors are often a bit stiky at startup.) Higher speed t If you do use PWM to ontrol a motor, it is important to remember that the indutane within the windings will not allow the urrent to go to zero instantaneously. diode (alled a flybak, flywheel, or freewheel diode when used like this) provides a path for the urrent still flowing through the motor when a pulse is swithed off. I D t H-bridge: Of ourse, if you want to make the motor turn in both diretions you'll need a more omplex iruit. Look at the iruit at right, it's has the shape of an H, hene the name. If transistors Q 1 and Q 4 are on, then the urrent flows as shown, left-to-right through the motor. If transistors Q and Q 3 are on, then the urrent flows the other way through the motor and the motor will turn in the opposite diretion. (he motor here is a permanent-magnet DC motor.) In my iruit, the top two transistors are PNPs, whih makes the iruit more effiient. he H-bridge ould also be made with all NPNs or with power MOSFE transistors. n H-bridge requires four inputs, all operated in onert. o turn on Q 1 and Q 4, as shown, in1 would have to be low and in4 would have to be high. t the same time, the other two transistors would have to be off, so in would have to be high and in3 would have to be low. p7

8 If the ontrol iruit makes a mistake and turns on Q 1 and Q 3 (or Q and Q 4 ) at the same time you'll have a toaster instead of a motor driver, at least for a short while. he iruit at left requires only two inputs. ransistors Q 5 and Q 6 work as inverters, when their inputs are high, their outputs are low and vie-versa. he resistors are known as pull-up resistors. he H-bridge should also inlude flybak diodes. p8 Regenerative Braking Eletri motors are not limited to onverting eletrial energy to mehanial. hey an also onvert energy from mehanial to eletrial. If that is done for the purpose of mehanial braking, say in an eletri ar, then it's alled Regenerative Braking. It is a way reover kineti energy when slowing a moving mass, or potential energy of a mass moving from a higher to a lower elevation. Examples: a ar oming to a stop at a traffi light or driving down Parley's anyon. his reovered energy an be used to reharge batteries or simply be wasted in resistors. Eletri lawnmowers and some eletri drills use this tehnique to stop the moving parts very quikly for safety. rmature Reation his phenomenon is well explained in your book, starting on p.37. One effet is a shift of the neutral plane of the field flux. his is a small twisting of the overall North - South orientation and an inrease the sparking at the brushes. If the load and rotation diretion are known and onstant, a small twist of the brush loation (in the same diretion) will help mitigate the sparking. Interpoles (shown on p. 379) are a better solution. Please note that Fig on p. 374 is for a generator and that ω will be in the opposite diretion for a motor. he other effet is an overall flux weakening due to ore saturation. few series windings to shore up the flux at high loads an help with this. Brush Loss Sometimes the voltage drop aross the brush-ommutator onnetion is also onsidered. his voltage drop is usually estimated at about for both brushes, regardless of the armature urrent. (p. 384 in book.) Charaterizing an Unknown DC motor For a motor that an be operated as separately exited and as a generator; Motor Constant: Operate the motor as a generator with no load ( 0), then. Calulate K from speed and measurements. You may wish to alulate this at various field urrents. : Hook an eletrial load to your still-spining generator. djust input power to return to no-load speed. Measure and and alulate. You may wish to repeat at several loads and take an average. lternatively, measure,, and ω at different mehanial loads, solve equations for unknowns, K and. 1 I. 1 1 I. 1. ω 1 I.. ω You may wish to alulate this at various field urrents. If you an't measure the rotational speed, but an measure the time required to move something a fixed distane and whih would be inversely proportional to speed:. K 1 I. 1. t 1. K t K is just another onstant whih is found together with K as KK. p8