Chapter 1 ac commutator motors1
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The document discusses AC commutator motors, specifically AC series motors. It provides details on their construction, operation, characteristics, and applications. Some key points: - AC series motors are similar to DC series motors but with modifications like a laminated magnetic circuit and fewer turns in the field winding to operate on AC supply. - They have high starting torque, variable speed depending on load, and power output higher than induction motors of similar size. - Common applications include electric traction, power tools, appliances like vacuums and blenders utilizing their high speed ability.
![The series field winding uses as few turns as possible to reduce the reactance of
thefield winding to a minimum. This reduces the voltage drop across the field
winding.3.
A high field flux is obtained by using a low-reluctance magnetic circuit.
4.
There is considerable sparking between the brushes and the commutator when
themotor is used on a.c. supply. It is because the alternating flux establishes
highcurrents in the coils short-circuited by the brushes. When the short-circuited
coils break contact from the commutator, excessive sparking is produced. This
can beeliminated by using high-resistance leads to connect the coils to the
commutator segments.5.
The operating voltage should be made low for reducing the inductance. A
largenumber of turns will be required for high voltage motors with
proportionally lowcurrent.6.
In some special cases AC series motors are designed at Low frequency.
Thisautomatically reduces inductive reactance. In case of traction motors this
procedureis adopted.Thesingle phase ac series motor has practically the same
operating characteristics as dcseries motors. The torque or tractive effort varies
nearly as the square of the current andthe speed varies inversely as the current
nearly. The inductively compensated ac seriesmotor also operates satisfactorily
on dc system and has increased output andefficiency. The speed of the motor
while working on ac system can be controlledefficiently by taps on a
transformer. The most important application of ac series motor isin electric
traction service up to 1600 kW, 200 to 600 volts using 15-25 Hz.
1.2.1CONSTRUCTION
The construction of en a.c. series motor is very similar to a D.C. series motor
except thatabove modifications are incorporated [See Fig. (1.1)].Such a motor
can be operated either on a.c. or D.C. supply and the resulting torque-speed
curve is about the same in eachcase. Forthis reason, it is sometimes called a
universal motor.
Fig 1.1 AC Series Motor
1.2.2OPERATION
When the motor is connected to an a.c. supply, the same alternating current
flowsthrough the field and armature windings. The field winding produces an
alternating flux](https://image.slidesharecdn.com/chapter1accommutatormotors1-200526171625/85/Chapter-1-ac-commutator-motors1-2-320.jpg)
Chapter 1 ac commutator motors1
- 1. CHAPTER1ACCommutatorMotors1.1 INTRODUCTION AC commutator motors, like comparable DC motors, have higher starting torque andhigher speed than AC induction motors. The series motor operates well above thesynchronous speed of a conventional AC motor. AC commutator motors may be either single-phase or poly-phase. Since a commutator motor can operate at much higher speedthan an induction motor, it can output more power than a similar size induction motor.However commutator motors are not as maintenance free as induction motors, due to brush and commutator wear. Themorecommoncommutatormotorsaretheseriesmotors,universalmotors, repulsionmotorsandtherepulsion-induction motors,withvariousmodificationsand combinations . 1.2 AC SeriesMotors Direct current shunt or series motors rotate in the same direction regardless of the polarity of the supply. Thus, it might be expected that either motor would operate onalternating current. It has been found, however, that the shunt motor develops but littletorque when it is connected to an ac supply. The reason of it is that the field winding,owing to its high inductance, causes the field current to lag the armature currents beingthe same, the main field and armature currents are in phase, therefore, theoretically sametorque is developed with a given alternating currents as with a like amount of directcurrent in a series motor. If an ordinary dc series motor were connected to an ac supply, itwould operate, but not very satisfactorily. The reasons are (i) pulsating torque due toreversal of armature and field current every half cycle, (ii) excessive eddy current lossesin the field and yoke due to alternations in the field flux, (iii) heavy sparking due toinduced voltages and currents in the armature coils short-circuited by the brushes whenundergoing commutation and (iv) abnormal voltage drop and low power factor due toinductance of field winding. Hence some modifications are necessary for satisfactoryoperation on ac. These modifications are1. The entire magnetic circuit is laminated in order to reduce the eddy current loss.Hence an a.c. series motor requires a more expensive construction than a D.C.series motor.2.
- 2. The series field winding uses as few turns as possible to reduce the reactance of thefield winding to a minimum. This reduces the voltage drop across the field winding.3. A high field flux is obtained by using a low-reluctance magnetic circuit. 4. There is considerable sparking between the brushes and the commutator when themotor is used on a.c. supply. It is because the alternating flux establishes highcurrents in the coils short-circuited by the brushes. When the short-circuited coils break contact from the commutator, excessive sparking is produced. This can beeliminated by using high-resistance leads to connect the coils to the commutator segments.5. The operating voltage should be made low for reducing the inductance. A largenumber of turns will be required for high voltage motors with proportionally lowcurrent.6. In some special cases AC series motors are designed at Low frequency. Thisautomatically reduces inductive reactance. In case of traction motors this procedureis adopted.Thesingle phase ac series motor has practically the same operating characteristics as dcseries motors. The torque or tractive effort varies nearly as the square of the current andthe speed varies inversely as the current nearly. The inductively compensated ac seriesmotor also operates satisfactorily on dc system and has increased output andefficiency. The speed of the motor while working on ac system can be controlledefficiently by taps on a transformer. The most important application of ac series motor isin electric traction service up to 1600 kW, 200 to 600 volts using 15-25 Hz. 1.2.1CONSTRUCTION The construction of en a.c. series motor is very similar to a D.C. series motor except thatabove modifications are incorporated [See Fig. (1.1)].Such a motor can be operated either on a.c. or D.C. supply and the resulting torque-speed curve is about the same in eachcase. Forthis reason, it is sometimes called a universal motor. Fig 1.1 AC Series Motor 1.2.2OPERATION When the motor is connected to an a.c. supply, the same alternating current flowsthrough the field and armature windings. The field winding produces an alternating flux
- 3. φ that reacts with the current flowing in the armature to producea torque. Since botharmature current and flux reverse simultaneously, the torque always acts in the samedirection. It may be noted that no rotating flux is produced in this type of machines; the principle of operation is the same as that of a D.C. series motor. 1.2.3CHARACTERISTICS The operating characteristics of an a.c. series motor are similar to those of a D.C. seriesmotor.1. The speed increases to a high value with a decrease in load. In very small seriesmotors, the losses are usually large enough at no load that limits the speed to adefinite value (1500 - 15,000 r.p.m.).2. The motor torque is high for large armature currents, thus giving a high startingtorque.3. At full-load, the power factor is about 90%. However, at starting or when carryingan overload, the power factor is lower.
- 4. 1.2.4APPLICATIONS The fractional horsepower a.c. series motors have high-speed (and correspondingsmall size) and large starting torque. They can, therefore, be used to drive:(a) High- speed vacuum cleaners (b) Sewing machines(c) Electric shavers (d) Drills similar to that of a small dc motor. The compensating winding is used to reduce thereactance voltage present in the armature when motor runs on AC supply. This voltage iscaused by the alternating flux by transformer action.In a 2 pole non compensated motor, the voltage induced by transformer action in a coilduring its commutation period is not sufficient to cause any serious commutation trouble.Moreover, high resistance brushes are used to aid commutation. Fig 1.11 Universal Motor 1.6.1SPEED/TORQUECHARACTERISTICS The speed of a universal motor varies just like a DC series motor (i.e) low at full load andhigh at no load. In fact on no load the speed is limited only by its own friction andwindage load. Fig 1.12 shows the typical torque characteristics of universal motor bothfordc and ac supply .Usually gear trains are used to reduce the actual load speeds to propervalues. Fig 1.12 Speed Torque Characteristics of Universal motor DCACSpeedTorque
- 8. 2MARKSQuestionsandAnswers1. What is meant by AC commutator motors? The commutator is a feature of dc motors, but ac motors having wound rotor with brushes and commutator arrangements, are called commutator motors which work onsingle phase AC supply. Such motors are called AC commutator motors. 2. What are the different types of AC commutator motors? The more common commutator motors area. The series motors, b.
- 9. Universal motors,c. Repulsion motorsd. Repulsion-induction motors, with various modifications and combinations3. Mention few applications of AC series motors (oruniversalmotor) Electric Traction, Hoists , Locomotives , High HP applications, Vacuum cleaners,food processors and mixers, Hair driers. Coffee grinders, electric shavers etc.4. What modifications have to be done on DC series motor to make it to work withsingle phase AC supply?a. The entire magnetic circuit is laminated in order to reduce the eddy current loss.Hence an a.c. series motor requires a more expensive construction than a D.C.series motor. b. The series field winding uses as few turns as possible to reduce the reactance of the field winding to a minimum. This reduces the voltage drop across the fieldwinding.c. A high field flux is obtained by using a low-reluctance magnetic circuit.d. There is considerable sparking between the brushes and the commutator whenthe motor is used on a.c. supply. It is becausethe alternating flux establisheshigh currents in the coils short-circuited by the brushes. When the short-circuited coils break contactfrom the commutator, excessive sparking is produced. This can be eliminated by using high-resistance leads to connect thecoils to the commutator segments.5. Draw the equivalent circuit diagram of AC series motor
- 10. 6. Draw the phasordiagram of AC series motor 7. Draw the speed torque characteristics of AC series motor (orUniversalMotor) 8. What are the types of AC series motor a. Conductively compensated AC series motor b. Inductively compensated AC series motor 9. Draw the equivalent circuit diagram of Repulsion motor 10. Draw the torque Vs Brush Position characteristics of Repulsion motor