*Direct Current (DC) Machines Fundamentals Generator action: An emf (voltage) is induced in a conductor if it moves through a magnetic field. Motor action: A force is induced in a conductor that has a current going through it and placed in a magnetic field Any DC machine can act either as a generator or as a motor.

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*DC Machine The direct current (dc) machine can be used as a motor or as a generator. DC Machine is most often used for a motor. The major advantages of dc machines are the easy speed and torque regulation. However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors. In the past, automobiles were equipped with dc dynamos to charge their batteries.

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*Application of DC Motor Even today the starter is a series dc motor However, the recent development of power electronics has reduced the use of dc motors and generators. The electronically controlled ac drives are gradually replacing the dc motor drives in factories. Nevertheless, a large number of dc motors are still used by industry and several thousands are sold annually.

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*DC Machine ConstructionFigure 8.1 General arrangement of a dc machine

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*DC MachinesThe stator of the dc motor has poles, which are excited by dc current to produce magnetic fields. In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current. Compensating windings are mounted on the main poles. These short-circuited windings damp rotor oscillations.

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*DC Machines The poles are mounted on an iron core that provides a closed magnetic circuit.The motor housing supports the iron core, the brushes and the bearings. The rotor has a ring-shaped laminated iron core with slots. Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees.

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*DC MachinesThe coils are connected in series through the commutator segments. The ends of each coil are connected to a commutator segment. The commutator consists of insulated copper segments mounted on an insulated tube. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

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*DC MachinesThe rotor has a ring-shaped laminated iron core with slots. The commutator consists of insulated copper segments mounted on an insulated tube. Two brushes are pressed to the commutator to permit current flow. The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing.

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*DC MachinesThe commutator switches the current from one rotor coil to the adjacent coil, The switching requires the interruption of the coil current. The sudden interruption of an inductive current generates high voltages . The high voltage produces flashover and arcing between the commutator segment and the brush.

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*DC Machine Construction

Figure 8.2 Commutator with the rotor coils connections.

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Shaft

Brush

Copper segment

Insulation

Rotor Winding

N

S

Ir_dc

Ir_dc/2

Rotation

Ir_dc/2

Ir_dc

1

2

3

4

5

6

7

8

Pole winding

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*DC Machine Construction

Figure 8.3 Details of the commutator of a dc motor.

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*DC Machine Construction

Figure 8.4 DC motor stator with poles visible.

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*DC Machine Construction

Figure 8.5 Rotor of a dc motor.

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*DC Machine Construction

Figure 8.6 Cutaway view of a dc motor.

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* DC Motor Fundamentals

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*DC Motor OperationIn a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field. The rotor is supplied by dc current through the brushes, commutator and coils. The interaction of the magnetic field and rotor current generates a force that drives the motor

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Shaft

Brush

Copper segment

Insulation

Rotor Winding

N

S

Ir_dc

Ir_dc/2

Rotation

Ir_dc/2

Ir_dc

1

2

3

4

5

6

7

8

Pole winding

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*8.2.1 DC Motor OperationThe magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S). The poles generate a magnetic field that is perpendicular to the current carrying conductors. The interaction between the field and the current produces a Lorentz force,The force is perpendicular to both the magnetic field and conductor

(a) Rotor current flow from segment 1 to 2 (slot a to b)(b) Rotor current flow from segment 2 to 1 (slot b to a)

90

Vdc

30

N

S

B

v

v

a

b

1

2

Ir_dc

90

Ir_dc

30

N

S

Vdc

a

b

1

2

B

v

v

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*8.2.1 DC Motor OperationThe generated force turns the rotor until the coil reaches the neutral point between the poles. At this point, the magnetic field becomes practically zero together with the force. However, inertia drives the motor beyond the neutral zone where the direction of the magnetic field reverses. To avoid the reversal of the force direction, the commutator changes the current direction, which maintains the counterclockwise rotation. .

(a) Rotor current flow from segment 1 to 2 (slot a to b)(b) Rotor current flow from segment 2 to 1 (slot b to a)

90

Vdc

30

N

S

B

v

v

a

b

1

2

Ir_dc

90

Ir_dc

30

N

S

Vdc

a

b

1

2

B

v

v

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*8.2.1 DC Motor OperationBefore reaching the neutral zone, the current enters in segment 1 and exits from segment 2, Therefore, current enters the coil end at slot a and exits from slot b during this stage. After passing the neutral zone, the current enters segment 2 and exits from segment 1, This reverses the current direction through the rotor coil, when the coil passes the neutral zone. The result of this current reversal is the maintenance of the rotation.

(a) Rotor current flow from segment 1 to 2 (slot a to b)(b) Rotor current flow from segment 2 to 1 (slot b to a)

90

Vdc

30

N

S

B

v

v

a

b

1

2

Ir_dc

90

Ir_dc

30

N

S

Vdc

a

b

1

2

B

v

v

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*Equivalent circuit.The equivalent circuit of DC Motors (and Generators) has two components: Armature circuit: it can be represented by a voltage source and a resistance connected in series (the armature resistance). The armaturewinding has a resistance, Ra. The field circuit: It is represented by a winding that generates the magnetic field and a resistance connected in series. The field winding has resistance Rf.

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*Classification of DC Motors Separately Excited and Shunt Motors- Field and armature windings are either connected separate or in parallel. Series Motors- Field and armature windings are connected in series. Compound Motors- Has both shunt and series field so it combines features of series and shunt motors.

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Series DC Motors The armature and field winding are connected in series. High starting torque.

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Separately Excited Compound DC Motor

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*DC Field Excitation MethodsThere are four different methods for supplying the dc current to the motor or generator poles:Separate excitation;Shunt connectionSeries connectionCompound

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*Shunt DC Motor Equivalent circuitFigure 8.14Equivalent circuit of a shunt dc motor

DC Power supply

Vdc

Eam

Iam

Fmax

Rf

If

Ra

Vbrush

Im

Pout

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*Series DC Motor Equivalent circuitFigure 8.15Equivalent circuit of a series dc motor

Vdc

Eam

Rf

Ra

Vbrush

Im

Fmax

DC Power supply

Pout

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*Compound DC Motor Equivalent circuitFigure 8.16Equivalent circuit of a compound dc motor

Vdc

Eam

Rfs

Ra

Vbrush

Im

Fmax

DC Power supply

Rfp

Iam

Ifp

Pout

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*Separately Excited DC Motor Equivalent circuitFigure 8.13Equivalent circuit of a separately excited dc motorEquivalent circuit is similar to the generator only the current directions are different

Rf

Ra

Vbrush

Vdc

Eam

Vf

Fmax

If

Iam

Mechanical power out

Electrical power in

DC Power supply

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*Counter EMF in DC MotorCounter Emf in DC Motor

k armature constant Z no. of conductors N speed in rpm

Ec = Va - IaRa Va Voltage applied to armature resistanceRa Armature resistanceIa Armature current

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Shunt DC motor equivalent circuit

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*Series DC motor equivalent circuit

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Torque Developed by Shunt DC Motor

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*Speed of Motor

S speed in revs/min.

Speed

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*Motor Torque Equation Motor Torque, a

Zno. of conductorsflux/poleano. of parallel paths of armature windings

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*Derivation of Motor Torque Equation

The interaction of fields produced in armature and field windings, causes a torque to be developed in the armature. Average torque in a conductor is expressed as, Tp = BlIa r/a Ia is the armature current.

Total developed torque in a machine is given by,

Z is no. of conductors and Ka is armature constant

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*Motor Torque Equation

Torque developed in a machine depends on flux armature current Ia and armature constant

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*Example 1 The armature of a permanent magnet DC generator has a resistance of 1 ohm and generates a voltage of 50 V when the speed is 500 r/min. If the armature is connected to a source of 150V, calculate the following:The starting current (Ans:150A)The counter emf when the motor

runs at 1000r/min and at 1460 r/min. (Ans: 100V and 146 V)The armature current at 1000 r/min and at

1460 r/min. (Ans: 50A and 4 A)

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*Example 2The following details are given on a 225 kW (300hp),250 V, 1200 r/min DC motor.Armature coils = 243Turns per coil = 1Type of winding = lapArmature slots = 81Commutator segments = 243Field poles = 6Diameter of armature = 559 mmAxial length of armature = 235 mm Calculate:i) The rated armature current (Ans:900 A)ii) The number of conductors per slot (Ans: 486 conductors)iii) The flux per pole (Ans: 25.7 mWb)

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*Example 3A 2000kW, 500 V, variable speed motor is driven by a 2500 kW generator using a Ward Leonard control system shown as follows:

The total resistance of the motor and generator armature circuit is 10 mohm. The motor turns at a nominal speed of 300 r/min when Eo is 500 V. Calculate a) the motor torque and speed when Eo=400V and Eo= 380 V. (Ans: 31.8 kNm, 228 r/min) b) the motor torque and speed when Eo=350V and Eo= 380 V. (Ans: 47.8 kNm, 228 r/min)

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