EPE491_Chapter 6 - DC Machines

Electrical Power and Machines EPE491

D. Johari, FKE UiTM 1

Chapter 6 – DC Machines

Introduction There are 2 types of DC machines:- DC generator and DC motor Although a DC machine can operate either as a generator or a motor, at present its use as a

generator is limited because of widespread use of AC power. The machine is therefore, extensively used as motor in industry.

The study begins with AC generation. Why?o The voltage produced in DC generator is actually AC and only becomes DC after

rectification

Generation of AC Signal Figure below shows an AC generator consisting of a single coil and a permanent magnet. The coil is connected to 2 slip rings mounted on the shaft.

The slip rings are connected to an external load by 2 stationary brushes A & B.

The physical arrangement of AC generator

As the coil rotates (driven by a prime mover), voltage is induced between its terminals, a & dand hence, across the load

The voltage is generated because the coil cut across the flux produced by N & S poles. For a clockwise rotation, the direction of the voltage (i.e. emf) is from a to d for one-half of the

revolution, and from d to a for the other half of revolution Hence, AC signal



Generation of DC Signal Generation of DC signal can be done by replacing the slip rings with commutator.

The physical arrangement of DC generator (also called as dynamo)

Commutator:-o A single slip ring cut in half with each segment insulated from each othero Convert AC to DC mechanically. o Therefore, it is also called as a mechanical rectifier

Using commutator, polarity across the brushes is always constant. Therefore, current in load always flows in same direction. Hence, DC signal Commutation is therefore the process of converting the AC voltages & currents in the rotor of a

DC machine to DC voltages & currents at its terminal.

DC Machine Construction Instead of using permanent magnet to create magnetic field, use a pair of electromagnets An electromagnet is a type of magnet whose magnetic field is produced by the flow of electric

current. The magnetic field disappears when the current ceases. The machine consists of 2 parts:-

a) Stator o Have poleso The poles are excited by DC current to produce magnetic fields



b) Rotoro Has a ring-shaped laminated iron-core with slots.o Coils with several turns are placed in the slots.

DC Machine Construction

It has 2 types of windings:-

DC Machine Winding

a) Armature windingo Voltage is induced on ito Placed on the rotoro Winding arrangement depends on how the coils are connected to form a closed

winding. 2 basic possibilities:-

The armature winding arrangements(a) wave winding (b) lap winding

a) Wave connection

– 2 parallel paths regardless no of poles

– Suitable for high voltage, low current DC machineb) Lap connection

– For a p pole machine, has p parallel path– Suitable for high current, low voltage DC machine

N S

Stator with polesBrush

Rotor

Field



b) Field windingo Carry dc current to create magnetic fieldo Mounted on the poles (stator)o Subdivided into:-

Separately excited winding- No direct connection between field & armature windings

Self-excited winding– Has a direct connection between field & armature windings– It can further be subdivided into: shunt, series, compound excitation

Armature Reaction Since all current-carrying conductors produce magnetic fields, current that flows in the

armature winding will also produce magnetic field. This magnetic field produced by current in the armature affects the flux pattern and distorts the

main field produced by the field winding. This effect is known as armature reaction and affects the commutation process.

To counter the effect of the armature reaction, a set of commutating poles (interpoles) orcompensating windings can be used.

EMF Generated Faraday’s Law states that: - when a current passes through a coil, flux will be induced around

the coil. As the rotor/armature rotates, its winding will cut the flux causing emf to be generated (i.e.

induced voltage) given by:

ggen kNc

ZpEMF

60Where srad

N/

60

2

Z = No of conductors in the armature circuitp = No of polesc = No of parallel path @ no of current path

= 2 for wave-winding= p for lap-winding

N = Speed of armature rotation (rpm)Φ = Useful flux per pole (Wb)

In the case of a motor, the induced voltage is called counter emf because its polarity always acts against the source voltage.

Example 1A 12-pole dc generator has a simplex wave wound armature containing of 2880 conductors. Its flux per pole is 0.05 Wb. The armature is turning at a speed of 200 rpm.

a) How many current paths are there in this machine?b) What is the generated armature voltage of this machine?c) What is the generated voltage if the machine now has lap winding?



Principle of Operation1. DC Generator

Generator converts mechanical energy into electrical energy Its operation can be described as follows:

o When current is supplied to the field winding (on stator), flux will be produced o As the rotor rotates (turned by a prime mover), voltage EG will be induced on the

armature windingo If a load is connected across the terminals, IA will flow and voltage across the load is

given by VT

2. DC Motor Motor converts electrical energy into mechanical energy

Its operation can be described as follows:o The armature winding is connected to a dc power supplyo Current flows through the armature windingo Since armature is within a magnetic field, a force is exerted on the windings.o The force causes the shaft to rotate.o As the rotor/armature rotates, its winding will cut the flux causing emf to be generated

on the armature winding (i.e. counter emf with its polarity acting against the source voltage).

Types of DC Machines The type of dc machines is classified according to the way in which their fields are excited:

separately excited and self-excited.

Separately Excited Winding When the dc field current is supplied by an external source, the machine is said to be

separately excited. There is no direct connection between field & armature circuits

1. Separately Excited DC Generator Equivalent circuit for a separately excited dc generator is given as below:

Stator RotorSeparately excited dc generator

AATG RIVE



2. Separately Excited DC Motor Equivalent circuit for a separately excited dc motor is given as below:

Stator RotorSeparately excited dc motor

AATA RIVE

Example 2If the no-load voltage of a separately excited generator is 135V at 850 rpm, what will be the voltage if the speed is increased to 1000rpm? Assume constant field excitation.

Example 3A separately excited generator has a no-load voltage of 140V when the field current is adjusted to 2A. The speed is 900rpm. Assume a linear relationship between the field flux & current. Determine:a) The generated voltage when the field current is increased to 2.5 Ab) The terminal voltage when the speed is increased to 1000rpm with the field current set at 2.2 A



Self-Excited Winding There is a direct connection between the armature and the field winding. The armature winding will provide the current to create the magnetic field. It may be classified as:

a) Series excitation - field winding is connected in series with the armature windingb) Shunt excitation - field winding is connected in parallel with the armature windingc) Compound excitation - field winding is connected in series & parallel with the armature

winding

1. Series Excitation

a) Series DC generator b) Series DC motor

)( fAATG RRIVE )( fAATA RRIVE

2. Shunt Excitation

a) Shunt DC generator b) Shunt DC motor

AATG RIVE AATA RIVE



3. Compound Excitation

a) Compound DC generator b) Compound DC motor

)( 2fAATG RRIVE )( 2fAATA RRIVE

Example 4Find the armature current and the counter emf for a shunt motor running at 1500 rpm at 51A with a 120 V source, 120 field winding and 0.1 armature resistance.



Power Flow Diagram Illustrates power flow from input to output Represented as a fishbone. The branches indicate losses

In general, Pin = Pout + losses and the losses are given as follows:

Losses in DC machine

o For DC machine, there are also losses that occur across the brushes called as brush losses, given by PBD = VBDIA. Since it is often only very tiny, it may be neglected.

DC Generator

Power flow diagram of series generator

Power flow diagram of shunt generator

Power flow diagram of compound generator

Power equation for DC generator:-

lossesPP outin Total losses = Pca + Pcf + Pμ

Pout = VTIL output power on the load



DC Motor

Power flow diagram of series motor

Power flow diagram of shunt motor

Power flow diagram of compound motor

Power equation for DC motor:-

lossesPP outin Total losses = Pca + Pcf + Pμ

Pin = VTIL input power taken from dc supplyPconv = EAIA developed mechanical powerPconv = Pμ + Pout

Motor Torque The general equation for torque is defined as

N

PPT

2

60

Where T- Torque (Nm), N- Speed (rpm), P- Power (W), - sradN

/60

2

For load torque (or shaft/net/load torque): N

PT out

out 2

60

For loss torque: N

PTL

2

60



For mechanical torque (or induced torque): N

PT m

m 2

60

Efficiency

It is given by: %100xP

P

in

out

o For DC generatorlossesIV

IV

P

P

LT

LT

in

out

o For DC motorLT

m

in

out

IV

PP

P

P

Voltage Regulation Voltage regulation is a measure on how much the terminal voltage changes from no-load

condition. It is usually expressed as a percentage of full-load voltage of the generator given by:

%100xV

VVVR

fl

flnl

T

TG

V

VEVR

Speed Regulation Speed regulation in a motor is good if the speed of the motor is relatively constant over its

normal range. It is usually expressed as a percentage of full-load speed given by:

%100xN

NNSR

fl

flnl

Example 5A 150V shunt motor has the following parameters:-

Ra = 0.50Ω Rf = 150Ω rotational loss = 250 WOn full-load, the line current is 19.5A and the motor runs at 1400 rpm. Determine:

a) The developed powerb) The output powerc) The output torqued) The efficiency at full-load



Starting of DC Motor At the instant of start-up, counter emf is zero because the armature is not rotating. So, full voltage is applied at this moment, the armature current could be very high which can

blow fuses & disconnecting itself from the supply. It is therefore necessary to insert some resistance in series with armature circuit to limit this

current. This practical arrangement is known as starter. There are several types of starter, such as manual starter as shown in Figure 1 and automatic

starter as shown in Figure 2.

Figure 1: Manual starter for shunt motor

Figure 2: Automatic starter connected to a shunt motor



DC Motor Speed Controlo There are three basic means of controlling the speed of a DC motor and each means has

different applicable range of effectiveness.

1. Armature resistance controlo Adding a resistor in series with the armature effectively increases the armature circuit

resistance. This results in a reduction of the steady-state speed, since the numerator becomes smaller. The no load speed is not affected since IA = 0. In this method of control the field current is kept constant. This method is relatively simple and inexpensive. However, there are some drawbacks. By adding a resistance means increase losses; I2R and reduce the motor’s speed. Therefore the speed of the motor that has resistance in the armature circuit is always lower comparing to the motor that don’t has resistance in the armature circuit.

2. Field controlo This method of speed control is by changing the flux. To do so we connect a resistance in

series with the field winding. Normally this is a variable resistor and it is generally called a field rheostat. As can be appreciated, we can add resistance only. Therefore the field current is decreased and the speed increases with a reduction in flux. This method also has some disadvantages. One of that is; we can only raise the speed at which the motor normally runs at a particular load. Another disadvantage is that the speed is increased without a corresponding reduction in shaft load, so we will be overloading the motor. However at light loads or no-load conditions, the speed can be above normal operating speed by about 300%.

3. Terminal voltage controlo This method of speed control is by changing the terminal voltage of the motor. This

normally the most frequent application of control at least for shunt motors where the field winding is then separately excited. The voltage control method lowers the speed in a similar fashion as the armature circuit resistance speed control method. However it does not have its drawbacks. The no-load speed and full-load speed can be reduced all the way down to zero if desired.

o There are various ways to obtain a variable DC voltage. In classical method, a Ward-Leonard system with rotating machines is used for speed control. Recently, solid-state converters have been used for this purpose.

1. Ward-Leonard Systemo In this system, the armature voltage is controlled by varying the field current of DC

generator. By using this system, the speed of DC motor can be smoothly varied. Other advantage of the system is it allows the DC motor to run in either direction of rotation by reversing the field current (which changes the polarity of armature voltage of DC generator). The disadvantages of this system are include forcing users to buy three machines of equal rating which is quite expensive and relatively have low efficiency.



Figure 3: Ward-Leonard system

2. Solid-state Speed Controlo In recent years, solid-state converters have been used as a replacement for Ward-Leonard

system. The converters used are controlled rectifier or choppers.

Controlled Rectifiers If the supply is AC, controlled rectifiers can be used to convert a fix AC supply voltage into avariable-voltage DC supply. If all the switching devices in the converter are controlled devices, such as silicon-controlled rectifiers (SCRs), the converter is called a full converter. Of some devices are SCRs and some are diodes, the converter is called a semi converter.

Figure 4: Speed control of DC motors by controlled rectifiers

EPE491_Chapter 6 - DC Machines

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