PURPOSE:◦ To consider the machine that converts electrical energy into mechanical energy-

the ELECTRIC MOTOR.

GENERATOR is in operation, it is driven by a MECHANICAL MACHINE, such as an ENGINE

WATER TURBINEELECTRIC MOTOR

the rotation through a magnetic field generates a voltage, w/c in turn, is capable of producing a current in an electric circuit.

MOTOR is in operation, it is “fed” by an electric current from an electrical source of supply.

-produces TWO STATIONARY MAGNETIC FIELDS 1. by the fields poles 2.by rotating armaturew/c react w/ each other to develop TORQUE, w/c in turn, produces

MECHANICAL ROTATION

LOAD ON A GENERATOR-constitutes those electrical devices that convert electrical energy into other forms of energy; loads such as

-electric lighting-electric furnaces-electrical welding-electrical motors-electric battery charging etc.

LOAD ON A MOTOR-constitutes the force that tends to oppose rotation and called a COUNTERTORQUE; loads such as

-fan blades -crushers -churns-pumps -excavators -drills-grinders -elevators -food mixers-boring mills -turntables

-host of other commonly used machines

VOLTAGE OF A GENERATOR-tends to change when the load changes;

SHUNT GENERATORS, a load increase is always accompanied by a DROP IN TERMINAL VOLTAGE

COMPOUND GENERATORS, the voltage may fall, rise, or even remain constant as the load changes.

can always be adjusted1. changing the speed2. changing the strength of the magnetic field

in either case, an increase in speed or flux is accomplished by an increase in voltage.

SPEED OF ROTATION OF A D-C MOTOR-tends to change as the load varies; as will be pointed out later, an increase in load causes the speed of a shunt motor to drop slightly, that of a compound to drop considerably, and that of a series motor to drop very greatly.

can be changed by varying either or both of two things

1.the strength of the magnetic field2.the voltage impressed across the armature

terminalsin general, an increase in the flux decrease the speed, while a higher armature voltage raises the speed.

GENERATORS-operated in parallel w/ others

to supply power to a common load; infrequently, they may be connected in series for the same purpose.

-started w/out electrical loads

MOTORS-operated as single independent

units to drive their individual loads, although in special applications they may be connected in parallel or in series for the purpose of performing particular jobs at varying speed.

-may or may not have a mechanical load when they are started.

THREE GENERAL TYPES OF GENERATORS◦ Series◦ Shunt◦ Compound

THREE GENERAL TYPES OF MOTORSSeriesShuntCompound

The speed of a motor can be controlled by an operator who makes a manual adjustment ADJUSTABLE-SPEED TYPE.

NOTE:VARIABLE SPEED - the speed changes inherently as a result of a

modification of the loading conditions.ADJUSTABLE SPEED - the speed changes only because an operator

or automatic control equipment has made an adjustment of some short.

CONSTANT-SPEED-ADJUSTABLE SPEED MOTOR-a shunt motor w/ a field rheostat

VARIABLE-SPEED-ADJUSTABLE-SPEED MOTOR-a series motor w/ a line rheostat; such as arrangement is used on a hoist.

Neutral Plane

N S

Direction of Rotation

Fig. 1

With the armature rotating as a result of motor action, the armature conductors continually cut through the resultant stationary magnetic field, and because of such flux cutting, voltage are generated in the very the same conductors that experience forces action.

-when a motor is operating, it is simultaneously acting as a generator.

Motor action is stronger than generator action, for the direction of the flow of current in the armature winding is fixed by the polarity of the source of supply. The generated voltage does, however, oppose the impressed emf and, in this respect, serves to limit the current in the armature winding to a value just sufficient to take care of the power requirements of the motor.

COUNTER ELECTROMOTIVE FORCE (counter emf)

-generated voltage opposes the flow of current.

- never be equal to and must always be less than,the voltage impressed across the terminals

because the direction in w/c the current flows determines first the direction of rotation and thus the direction of the counter emf.

The armature current is controlled and limited by the counter emf.

Va – Ec Ia= armature current

Ia = Va= impressed voltage across

Ra armature windingEc= counter emf generated in

armatureRa= resistance of armature

Since the counter emf is a generated voltage, it depends, for a given machine, upon two factors:

1. the flux per pole (0)

2. the speed of rotation (S) in revolutions per minute.

Ia =Va – k S

Ra

0

At the instant a d-c motor is started, the counter emf (Ec) is ZERO because the armature is not revolving.

As the armature accelerates to full speed, the value of Ec rises to a value that causes the proper value of armature current (Ia) to flow; the proper armature current is that required by the armature to permit it to drive its load at speed S.

Since the counter emf (Ec) limits the current in the low-resistance armature winding, it should be understood that at the instant of starting, when Ec is ZERO, the armature current would be extremely high unless some resistance were added to offset the lack of Ec.

If Ec is ZERO, or very small As the motor is coming up to speed, a resistance must be

inserted to take the place of Ec.

As the speed increases, the resistance may be cut out gradually because Ec rises.

When the motor has attained normal speed, all resistance can be cut out of the armature circuit.

In order for DC motors to function properly, they must have some special control and protection equipment associated with them. The purposes of this equipment are:

1. To protect the motor against damage due to short circuits in the equipment;

2. To protect the motor against damage from long-term overloads;

3. To protect the motor against damage from excessive starting currents;

4. To provide a convenient manner in which to control the

operating speed of the motor.

Shunt Field

Movable Arm

Variable Starting Resistor

To Power Source

ab

Fig. 2

Series field

Variable Starting Resistor

To Power Source

ab

Fig. 3

To Power Source

Series field

Shunt Motor

a

b

Fig. 4

Motors must be started w/ the movable arm at a to be gradually moved to b as the armature accelerates to full speed.

In the case of very small motors, usually the fractional-horsepower sizes up to about ¾ hp, no starting resistor is necessary.

Such motors may be started by simply closing the line switch.

Two reasons for this practice:

1. The resistance and the inductance of the armature winding are generally sufficient high to limit the initial rush of current to values that are not particularly serious.

2. The inertia of a small armature is generally so low that it comes up to speed very quickly, thereby minimizing the serious effect that might otherwise result from high sustained current.

Two standard types motor starter for shunt and compound motors:

Three Point Starter

Starting Resistor(R)

Holding Coil

Soft Iron Keeper

Starter Arm

OFF

____________

____________

Shunt MotorField RheostatMain Switch To D-C

Source

ab

b’

1

23 4 5

6

L F A

Fig. 5

Note:◦ Terminal L must be connected to either side, positive or

negative, of the d-c source on the main switch (wire a)◦ Terminal F is connected to one field to one field terminal on

the motor (wire b)◦ Armature Terminal A must be connected to either one of the

motor armature terminals (wire c)◦ The final connection must then be made from the second line

terminal on the main switch to a junction of the remaining two armature and field terminals of the motor.

If it is desired that the speed of the motor be controlled, a field rheostat should be inserted in series between the field terminal F on the box and motor field terminal (wire b).

FUNCTION OF THE STARTER1. if the power fails and the starter arm is not restored to the OFF position, the motor might be damaged should the power come on again2. if the shunt field circuit were opened accidentally and the starter arm did not return to the OFF position, the motor speed might become dangerously high.

Starter

Shunt Motor

Holding Coil

Field Rheostat

(2)

(1)

R

Fig. 6

NOTE: ◦ The Main Circuit, in heavy lines, consists of the

variable resistors R and the armature.

◦ The second circuit includes the shunt field, the holding coil, and the field rheostat.

◦ Last circuit, it should be noted that the current through the field is the SAME current that flows through the holding coil.

Holding Coil

Field Rheostat

------------------------

-------------------

-------------------------

Starter

Compound Motor

(1)

(2)

(3) R

r

Fig. 7

When the starter arm is on first stud, the line current divides into THREE PARTS:

1. The main circuit is through the starting resistor (R), the series field and the armature.

2. The second circuit is through the shunt field and its field rheostat.

3. The third circuit is through the holding coil and a current-protecting resistor(r).

NOTE: The arrangement permits any change in current in the shunt-field circuit w/out affecting the current through the holding coil; in this regard it overcomes the objection to the holding coil will always be sufficient and will prevent the spiral spring from restoring the arm to the OFF position, no matter how the field rheostat is adjusted.

Spiral Spring

Starter ArmSoft Iron Keeper

Protecting Resistor for holding Coil

Holding Coil

Main Switch

Compound Motor(long Shunt)

Field RheostatTo D-C Source

Starting Resistor(R)

2

1

3 4 56

L L- F A

r

OFF

Fig. 8

CONTROLLER- whenever a starter is equipped w/ some means for varying the speed of the motor to w/c it is connected.-may be also designed to permit reversing the direction of rotation and may include protective features such as overload relays, undervoltage relays, and open-field devices.-a device used in connection w/ the starting of a series motor because it usually serves also for reversing and speed-control purposes.

FIG. 98:As the resistance is cut in, the speed increases; also, at a comparatively high speed, the field must be weakened considerably. Should the motor be stopped w/ a high value of field resistance and then started again before the rheostat is set at the all-out position, the motor would attempt to start too rapidly; furthermore, the motor would draw an excessive armature current to compensate for the low field current because the required load torque depends upon the product of both the flux and the armature current.

Soft IronCopper Wiper

Field Rheostat Arm

Armature Resistor Arm

Holding Coil

Copper Segment

Protecting ResistorMain Switch

Compound Motor(short-shunt)

Series Shunt

Shunt Field To D-C Power

L L- A F

12 3 4 5 6

Field Rheostat Starting Resistor(R)

OFF

h

r

a b

c

Fig. 9

Operations of controller:1. There are two arms, the longer one moving over a set of field-rheostat contact points (upper) and the shorter one moving over a set of armature-resistor contacts.2. The handle for moving both arms clockwise simultaneously is on the upper arm.3. The spiral spring is fastened to the armature-resistor arm only.4. A copper wiper is mounted on the armature-resistor arm and wipes over a copper segment as it moves forward.5. In the final position of the armature-resistor arm, the copper wiper makes contact w/ one end of the holding coil at point h, the copper wiper leaving the copper segment.6. In the final position, the armature-resistor arm is held by the holding coil, while the field-rheostat arm is free to be moved counterclockwise to any point on the field.

Automatic starting of motors is preferable to manual operation because, when properly designed and adjusted, the starting resistor are timed to be cut out so that the acceleration is uniform and the maximum allowable armature current is not exceeded.

Manual Starters, although cheaper, may be operated improperly at times, in w/c case damage may be done to both motor and starter..

Relays – 1AX, 2AX, 3AX

Contactors – M, 1A, 2A,

3AOL- oevrloadll – normally open

contactsll – normally closed

contacts

Shunt Field

Interpole Field

OL

OL

R1 R2 R3

1A 2A 3A

M

1AX

2AX

3AX

M1

Start Stop

1A

2A

3A

1AX

2AX

3AX

Fig 10

The COUNTER-EMF method, the shunt motor is started by pressing the START BUTTON. This energizes the main contactor (M), w/c instantly closes the auxiliary contacts M1 ( to seal the START BUTTON) and the main contacts M. The motors then starts w/ resistors R1, R2, and R3 in series in the armature circuit.

NOTE: Relays 1AX, 2AX, and 3AX are connected across

the armature terminals, where the voltage drop changes as the motor accelerates; since these relays are adjusted to pick up at preset and increasingly larger values of voltage, contacts 1AX, 2AX, and 3AX will close in a definite sequence.

Relay – CRContactor- M, 1A,

2A, 3All – normally open

contactsll – normally

closed contactsOL- overload

Shunt Field Field Rheostat

3A

3A

3A

3A

R1

R2 R3

1A

1A

1A

2A2A

2AOL

M

Series Field

CR

CR1

CR2

T.C.

T.C.

T.C.

T.C.

M1M2

r

Stop

Start

I.F.

Fig. 11

There are a group of three contactors 1A, 2A, and 3A, each of w/c has one pair of simultaneously closing contacts across a block of armature resistance and another pair of timed contacts that close w/ a time delay after the coil is energized.

Series Relays- SR1, SR2, SR3

Contactors- M, 1A, 2A, 3A

ll –normally open contacts

ll – normally closed contacts

OL- overload relay

Shunt Field

Series Field

OL

OL

I.F. R1R2R3

ll

3A

3A

2A

2A

1A

1A

SR1

SR1

SR2

SR2

SR3

SR3

2A

1AM2

M1

Start Stop

M

M

Fig. 12

The current-limit acceleration starter is in another way, depending for the motor’s increase in speed upon the current slowly when the load is heavy and more rapidly under light-load conditions.

NOTE: There are three relays, SR1, SR2, SR3, and three contactors, 1A, 2A, 3A.

When a generator delivers electrical power to a load, its terminal voltage tends to change.

Electric motor generally receives its electrical power (E X I) at substantially constant voltage.

It is then converts this electrical power into mechanical power, by doing so by developing torque as it rotates its mechanical load.

When a load is applied to a motor, the natural tendency of the latter is to slow down because the opposition to motion is INCREASED.-the counter emf DECREASES, for the reason that Ec is proportional to the speed. This reduction in the speed immediately results in an increase in armature current

this increase in armature current must be exactly that required by the motor to drive the

increased load because any increase in mechanical driving power must be met

by a corresponding increase in electrical power in put to the armature.

Electrical power input = Va x IaIa must increase, for the reason that Va is substantially constant.

Va – EcIa =

Ra

TWO CHANGES IN LOADING A MOTOR:1. a reduction in speed 2. an increase in armature current

NORMAL SPEED-the speed at w/c a motor operates

when it is driving its rated load, its so-called rated horsepower.

The TORQUE developed by a motor, i.e., the tendency of a motor to produce rotation, depends on two factors:

1. the flux created by the main poles2. the current flowing in the armature winding

The torque is independent of the speed rotation.

T = k X O X Ia lb-ftWhere:

T = torque (lb –ft)o = flux per pole (maxwells)Ia = total armature currentk = proportionality constant

Shunt Motor

Series Motor

Compound Motor (Long-Shunt)

Ish

Ish

Il

Il

Ia

Ia

Ia

Fig. 13

Shunt Motor

The torque of a shunt motor depends only upon the armature current; assuming that the shunt-field current is not changed by field-rheostat adjustment, the torque is independent of the flux

Ish

Ia

Il

Fig. 14

Series Motor

The torque developed by a series motor depends upon the armature current and the flux that this current produces in passing through the series field.

AT LIGHT LOADS:when the magnetic circuit iron is net saturated, the field flux is directly proportional to the load current. T= k( k2Ia) X Ia = k2Ia2

AT THE HEAVY LOADS:when the magnetic circuit iron is saturated, the flux will change very little or not at all w/ changes in Fig. 15

load.

Ia

Fig. 15

Compound Motor (long-shunt)

The torque of a compound motor (cumulative only, where the shunt-field and series-field ampere-turns aid each other) combines the torque-load characteristics of the shunt and series-motor. As the load on the motor increases, the armature, or load, current passing through the series field creates flux that adds to the constant shunt- field flux.

Ish

Ia

Il

Fig. 16

Torque

T

Rated Armature Amp.

Armature Current (Ia)

Rated Torque

S1

C1

S2

C2Overload Range

Shunt Compound(Cumulative)

Series

Fig. 17

1. The speed of a shunt motor rises about 2 to 8 percent when the rated load is completely removed

2. The speed of a compound motor rises approximately 10 to 25 percent when the rated load is completely removed.

3. The speed of a series motor rises very rapidly when the load is removed and must

Therefore, always drive some load if it is to prevented from racing dangerously, i.e., “running away”.

S = rpmVa –Ia Ra k O

1.The speed of a shunt motor is substantially constant and has a very definite no-load value.

2. The speed of a compound motor varies considerably and also has a very definite no-load value.

3. The series motor operates over an extremely wide speed range and tends to “run away” at light loads--- it should never be used w/ a belt drive or when the load is such that the torque might drop to approximately 15 percent of the full-load torque.

Speed rpm Rated Hp

Hp output

Rated Speed

--- Maximum Safe Speed

Fig. 18

per cent speed regulation=

The greater the countertorque, the lower the speed.

Shunt motors are generally regarded as constant-speed motors because their percent speed regulation is very small.

Compound motors are properly considered to be variable-speed motors because their percent speed regulation is comparatively high.

Snl - Sfl

Sfl

X 100

Whenever the variable series-field ampere-turns of a compound motor “buck” the constant shunt-field ampere-turns, the total flux tends to diminish w/ increasing values of load.

AT LIGHT LOADS: the series-field current is low, so that it has little demagnetizing effect upon the shunt field.

AT HEAVY LOADS: the series-field current is comparatively high, w/c means that the demagnetizing series may be considerable.

THREE DIFFERENT WAYS OF ADJUSTING THE SPEED:1. inserting a field rheostat in the shunt-field circuit of a shunt or compound motor

2. inserting a resistance in the armature circuit of a shunt, compound, or series motor

3. varying the voltage across the armature circuit of a shunt or compound motor while, at the same time, maintaining constant the voltage across the shunt field

Speed rpm

HP outputFig. 20

Wiring connections of a WARD LEONARD variable-voltage system of control for a shunt motor

To A-C source

separately-excited controlled motor

separately-excited controlling generator

A-C driving motor for controlling generator and controlled motor

coupling

field rheostat

exciter for controlling generator and generated controlled motor

Fig. 21

Arrangement of machines and wiring connections of WARD LEONARD method of control.

Shaft Mechanical load

Fig. 22

NOTE: the fundamental interconnection of the two armatures of the main machines- called loop circuit.

A-C driving motor

Main exciter

Control Rheostat

I.F I.F

Controlling

generator Intermediate exciter

Gen. Field

Va

Loop

Motor Field

Controlled motor

-

+

Fig. 23

The controlling generator is driven by a prime mover, usually a constant-speed a-c motor, and the speed control of the controlled motor is affected by shunting the series field of the generator w/ a variable resistance.

Rheostat

Series Field Loop

generator

I.F.I.F.

I

Series Field

Motor

Fig. 24

OPERATION OF THE SYSTEM:

1.The terminal voltage of a series generator (operating at constant speed) depends upon the series-field current or excitation, and this, in turn, is a function of the loop(or laod) current, or as here, of that part of the current that is not shunted.

2.The speed of a series motor varies inversely as the load, w/c in turn, also depends upon the loop current

Since the characteristics curves of the two series machines are complementary, in the sense that a generator-current rise attempts to increase the motor speed (the voltage is higher), while the same motor-current rise has an inverse effect upon the speed, the resulting action is to keep the motor speed constant, for a given rheostat setting.

Where: RM = equivalent resistance of motor OM = flux produced by motor

But:V= Eg –I Rg

Where: Eg = generated voltage of generator Rg = equiv. resistance of generator

SM =

V - I RM

K OM

The magnetic neutral tends to shift when a motor is loaded. The reason for this tendency is the fact that the armature current creates a magnetic filed of its own, apart from that created by the stationary poles, the magnetic axis of w/c is exactly halfway between the centers of the main poles.

Fig. 25

TWO GENERAL METHODS:1. changing the direction of current flow through the

armature2. changing the direction of current flow through the

field circuit on circuits.

The direction of rotation of a D-C motor cannot be reversed by interchanging the connections to the starting switch, because this reverses the current flow through both the armature and the field.

The reversing switch is connected to the shunt field

The reversing switch is connected to the armature

Ia

--

++

Reversing Switch Reversing Switch Ish

Fig. 26

ARMATURE REVERSING FIELD REVERSING

Fig. 27

F- forward contactor

R- reversing contactor

1A,2A-contacts

T-timing relay

CR- control relay

T.C.- time closing contacts

Fig. 28

NOTE: 1.It is provided w/ two acceleration

contactors and resistors, designated by 1A, 2A and R1, R2

2.Arrangement is made for ARMATURE REVERSING through forward contacts F and reversing contacts R.

3.The push-button station is equipped w/ FOR and REV button, each of w/c, when pressed, closes one set of contacts and simultaneously opens another set.