307049 Unit 66 Three Phase Motors and Drives

8/3/2019 307049 Unit 66 Three Phase Motors and Drives

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UNIT 66: THREE-PHASEMOTORS ANDDRIVES

Edexcel Level 3 BTEC Nationals in Engineering Issue 1 May 2007 Edexcel Limited 2007 1

Unit 66: Three-phase Motors and Drives

NQF Level 3: BTEC National

Guided learning hours: 60

Unit abstract

Three-phase motors are used where greater amounts of power are required andsingle-phase motors would not be effective. A good example of this is a largecompressor where a constant speed is required. The two main advantages of three-phase types over single-phase types are the smoother torque they provide and ahigher power to weight ratio, giving smaller frame sizes for comparable poweroutputs.

This unit aims to give learners an understanding of three-phase electric motors thatare used in modern industry and their associated drives.

Learners will develop a broad understanding of the design and operation of three-phase motors which use electrical and electronic control devices to make them work.This will include being able to read and produce simple circuit diagrams andunderstand the principles of installation, commissioning and maintenance. Learnerswill be made aware of the requirements of a drive and the need for the motor anddrive to be matched to the characteristics of the application.

The unit will provide a good foundation for anyone interested in taking up a career in

the manufacturing or processing industry, particularly where large motor drives areinvolved.

Learning outcomes

On completion of this unit a learner should:

1 Understand how squirrel-cage and wound rotor three-phase induction motorsoperate

2 Understand how three-phase synchronous and synchronous-induction motorsoperate

3 Know about the function and operation of motor starters and control gear

4 Know about a range of industrial applications for installing, commissioning andmaintaining three-phase motors.


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Edexcel Level 3 BTEC Nationals in Engineering Issue 1 May 2007 Edexcel Limited 20072

Unit content

1 Understand how squirrel-cage and wound rotor three-phase induction motorsoperate

Principle of operation : production of rotating magnetic field eg distributedwinding, salient pole, frequency, pole pairs, synchronous speed, rotor speed;induction motor types eg squirrel cage, wound rotor (slip ring)

Constructional features : cores eg stator, rotor, laminations, spiders, materials;frame eg cast, fabricated, end covers, materials; rotor eg squirrel cage, woundtype; stator eg distributed windings, single layer, double layer; enclosure egcooling/ventilation, open, totally enclosed, drip proof, flameproof; shaft andfittings eg bearings, slip rings, brushes, brush lifting gear enclosure ratings eg

Ingress Protection, BS 490, BS 5345Characteristics and calculations : characteristics eg frequency, poles, speed/load,torque/speed, torque/slip; calculations eg speed, slip, starting current, load,torque, power, efficiency

2 Understand how three-phase synchronous and synchronous-induction motorsoperate

Principle of operation : production of rotating magnetic field eg distributedwinding, salient pole, frequency, pole pairs, synchronous speed, effect of

excitation; synchronous motor types eg pony motor, synchronous-inductionmotor, synchronising; characteristics eg open circuit, v-curves; reasons forcalculations eg speed, torque, leading/lagging power factor, power, efficiency

Constructional features : rotor eg cylindrical, salient pole; stator eg distributedwindings, single layer, double layer; excitation methods eg DC exciter, ACexciter, brushless

3 Know about the function and operation of motor starters and control gear

Starters : circuit diagrams and operation of induction motor types eg direct online (DOL), star-delta, auto-transformer, soft start, rotor resistance; circuitdiagrams and operation of synchronous motor types eg pony motor, synchronous-induction motor, synchronising; effects of reduced voltage starting eg current,starting torque; protection devices eg short circuit, earth leakage, overload,interlocks, trips

Control gear : speed control eg variable frequency, inverters, pulse widthmodulation (PWM); motor drives eg DC transistor/thyristor, inverter types,braking, soft starting; programmable logic controllers eg simple ladder logic


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4 Know about a range of industrial applications for installing, commissioning andmaintaining three-phase motors

Load characteristics : load characteristics and demands of machinery egcentrifugal fans and pumps, compressors, machine tools, mechanical handlers,plastic extruders, lifts, hoists, conveyors

Ratings and calculations : electrical parameters eg power, KVA, KVAr, powerfactor, voltage, current; mechanical parameters eg power, speed, slip, torque,efficiency, gear ratios, volume, pressure, flow

Installation, commissioning and maintaining : installation procedures egfoundations, mountings, insulation checks, rotation, couplings; commissioningprocedures eg starting, running, load test, temperature monitoring; maintenanceprocedures eg rotor/bearing checks, lubrication, brushes, brushgear, controlgear, insulation tests


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U N I T

6 6 :

T H R E E - P H A S E

M O T O R S A N D

D R I V E S

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l L e v e

l 3 B T E C N a t i o n a

l s i n E n g i n e e r i n g

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4 G r a

d i n g

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d e n c e

t h a

t t h e

l e a r n e r p r e s e n

t s f o r a s s e s s m e n

t n e e

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t e t h a

t t h e y c a n m e e

t a

l l o

f t h e

l e a r n

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o u

t c o m e s

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t h e u n

i t .

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i t e r i a

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d e

d e s c r i

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t h e

l e v e

l o

f a c h

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d t o p

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i t .

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d i n g c r i

t e r i a

T o a c h

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d e

t h e e v

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s h o w

t h a t t h e

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b l e t o :

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h i e v e a m e r i t g r a

d e

t h e e v

i d e n c e m u s t

s h o w

t h a

t , i n a

d d i t i o n

t o t h e p a s s c r

i t e r i a ,

t h e

l e a r n e r

i s a

b l e t o :

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i e v e a

d i s t i n c

t i o n g r a

d e

t h e e v

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u s t s h o w

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t h e p a s s a n

d

m

e r i

t c r

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t h e

l e a r n e r

i s a b l e t o :

P 1

d e s c r i

b e

t h e p r i n c i p

l e o

f o p e r a

t i o n a n

d

t h e c o n s t r u c t

i o n a

l f e a

t u r e s o

f a t

h r e e -

p h a s e s q u

i r r e

l c a g e a n

d a

t h r e e - p

h a s e

w o u n

d r o

t o r

( s l i p r i n g )

i n d u c t

i o n m

o t o r

P 2

c a r r y o u

t c a

l c u

l a t i o n s

i n v o

l v i n g

f r e q

u e n c y ,

p o

l e s , s p e e

d ,

t o r q u e , p o w e r

a n d

e f f i c i e n c y

f o r a

t h r e e - p

h a s e

i n d u

c t i o n m o

t o r

f r o m

g i v e n

d a

t a

P 3

d e s c r i

b e

t h e p r i n c i p

l e o

f o p e r a

t i o n ,

c o n s t r u c t

i o n a

l f e a

t u r e s a n

d e x c i t a

t i o n

m e t

h o

d s o

f a

t h r e e - p

h a s e s y n c h r o n o u s

m o t o r

P 4

d e s c r i

b e ,

w i t h t h e a

i d o

f s u

i t a

b l e c i r c u

i t

d i a g

r a m s ,

t h e o p e r a

t i o n o

f t w o

d i f f e r e n

t

r e d u c e

d v o

l t a g e s t a r t e r s

f o r

i n d u c t i o n

m o t o r s ,

o n e s t a r t

i n g m e

t h o

d f o r a

s y n c h r o n o u s m o

t o r , a n

d a p r o

t e c t

i o n

d e v

i c e

f o r u s e w

i t h a

t h r e e - p

h a s e

i n d u

c t i o n m o

t o r

M 1

e x p

l a i n t h e p r o

d u c t

i o n o

f a r o

t a t i n g

m a g n e

t i c

f i e

l d f r o m

a t h r e e - p

h a s e s u p p

l y

s u i t a

b l e f o r

t h r e e - p

h a s e

i n d u c t

i o n

m o

t o r s

M 2

e x p

l a i n t h e v a r i a

t i o n o

f t o r q u e a n

d s l

i p

f o r a

t h r e e - p

h a

s e i n d u c t

i o n m o

t o r u s i n g

v a l u e s

f r o m

g i v e n

d a

t a

M 3

e x p

l a i n t h e m e

t h o

d s o

f s t a r t

i n g a

t h r e e -

p h a s e s y n c h r o n

o u s m o

t o r a n

d t h e e

f f e c t s

o f v a r i a

t i o n o f e x c i

t a t i o n w

i t h r e

f e r e n c e

t o t h e

V c u r v e c h a r a c t e r i s t

i c s .

D 1

e x p

l a i n t h e s e

l e c t

i o n o

f a t h r e e - p

h a s e

m o

t o r a n

d i t s

d r i v e

f o r a g

i v e n

a p p

l i c a

t i o n u s i n g c a

l c u

l a t i o n s

i n v o

l v i n g

e l e c t r i c a

l a n

d m e c h a n

i c a l

p a r a m e

t e r s

D 2

e v a

l u a

t e t h e s p e e

d c o n

t r o l m e

t h o

d s u s e

d

f o r

t h r e e - p

h a s e

i n d u c t

i o n m o

t o r s a n

d

e x p

l a i n t h e e

f f e c t s o

f c

h a n g

i n g s p e e

d o n

t o r q u e

/ s l

i p c h a r a c

t e r i s t

i c s .


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Essential information for tutors

DeliveryThis unit may be delivered as a stand-alone unit or linked with others in thequalification.

Learning outcomes 1 and 2 will give learners an understanding of the characteristicsand features of induction and synchronous motors. Delivery of these learningoutcomes could be through a programme of lectures, demonstrations and practicalinvestigations. There are some common features which can be covered in general atthe beginning of the unit, eg the distributed three-phase winding and the productionof the rotating magnetic field.

The constructional features of different types of motor would ideally bedemonstrated through inspection of stripped-down examples. The types of motorused throughout the unit should be normal production types in preference tospecially manufactured demonstration types. The voltage rating of the supplies usedshould be 400V 50Hz, although it is appreciated that local requirements andavailability of equipment may dictate the use of different values. Although simulationand computer packages can be used to demonstrate some features andcharacteristics, these methods should not be used to deliver the whole unit.

When delivering starters and control gear in learning outcome 3, learners should seepractical examples of as many types as possible. They should have opportunities toconnect them to the appropriate motor, helping them know about their function and

appropriate selection. The starting and synchronising of three-phase synchronousmotors may prove difficult to demonstrate. Where this and other features cannot bedemonstrated in the centre, efforts should be made to arrange industrial visits sothat learners can see the appropriate equipment.

Learning outcome 4 could also be partly delivered in local industry. The coverage of load characteristics, installation, commissioning, maintenance, ratings andcalculations should be supported by manufacturers and suppliers literaturewherever possible.

Manufacturers manuals that relate to all aspects of the unit content will be of tremendous help and should be freely available to learners.

Assessment

This unit could be assessed through the use of four assignments.

The first assignment could be about induction motors and cover criteria P1, P2, M1and M2. A written task should be given to cover P1. Learners will need to considerthe production of the rotating magnetic field when describing the principle of operation. They will also need to consider cores, frame, rotor, stator, enclosure andshaft and fittings when describing the constructional features.


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Learners should be given data to use when carrying out the calculations required byP2. This data could be varied for each learner to aid authenticity, or the task couldbe time-constrained. A further task should be set asking them to explain theproduction of a rotating magnetic field and the variation of torque and slip usinggiven values to achieve criteria M1 and M2 respectively. Learners evidence will bewritten and will include their calculations.

The second assignment could cover synchronous motors and cover criteria P3 and M3.A written task could cover the construction, operation and excitation of synchronousmotors. A practical exercise or demonstration could be done for starting andvariation of excitation. From this information, learners can formulate relevantdescriptions and explanations. Starting voltage, current and torque are usefulparameters to be measured.

A third assignment on starting and control could cover criteria P4 and P5. While awritten task could be used to cover both criteria, it would be more interesting forlearners if they could operate starters and drives and take appropriatemeasurements. Description of the control gear could be as a result of stripping downor opening up starters, drives and other control gear. In doing so it is important thatlearners also consider the effects of reduced voltage such as current and startingtorque.

The fourth assignment on applications could cover criteria P6, P7, P8, D1 and D2.Tasks should require learners to refer to details of typical loads and machinery to bedriven (P6). Learners must then consider the types of motor and control gearappropriate for driving different loads and select and describe the features (P7). ForD1 choices need to be supported by detailed calculations, with relevant explanationsof some depth using calculated parameters. The evaluation of speed control (D2)must be linked to the torque/slip characteristics and include details of more than

one method, eg thyristor drives and inverter drives. A further written task could asklearners to describe the installation, commissioning and maintenance procedures fora three-phase motor (P8).

Manufacturers literature and specifications are a useful source of practicalinformation for the above tasks.

Links to National Occupational Standards, other BTEC units, other BTECqualifications and other relevant units and qualifications

This unit is linked to Unit 5: Electrical and Electronic Principles , Unit 52: ElectricalTechnology and Unit 64: Electrical Applications .

The unit may contribute to the knowledge requirements of some of the units withinthe Level 3 NVQ in Electrical and Electronic Engineering, particularly:

Unit 26: Assembling Rotor and Armature Windings Unit 27: Assembling Stator Windings Unit 28: Assembling and Fitting Commutators Unit 29: Balancing Assembled Rotors or Armatures Unit 30: Assembling and Fitting Electrical Rotating Equipment.


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Essential resources

Centres delivering this unit must have access to industrial standard three-phaseelectric motors, starting/control gear and associated drives. In addition, appropriateand adequate testing instruments and fault-finding assemblies should be provided.European and British Standards, health and safety and other publications should also

be available.

Indicative reading for learners

Textbooks

Hughes A Electric Motors and Drives: Fundamentals , Types and Applications(Newnes, 2005) ISBN 0750647183

Wildi T Electrical Machines, Drives and Power Systems (Prentice Hall, 2005)ISBN 0131969188


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Key skills

Achievement of key skills is not a requirement of this qualification but it is

encouraged. Suggestions of opportunities for the generation of Level 3 key skillevidence are given here. Staff should check that learners have produced all theevidence required by part B of the key skills specifications when assessing thisevidence. Learners may need to develop additional evidence elsewhere to fully meetthe requirements of the key skills specifications.

Communication Level 3

When learners are: They should be able to develop thefollowing key skills evidence:

describing the principle of operationand the constructional features of motors

describing the principle of operation,constructional features and excitationmethods of a motor

describing the control gear requiredfor a three-phase induction motor anda three-phase synchronous motor

describing the installation,commissioning and maintenanceprocedures for a motor.

C3.3 Write two different types of documents each one givingdifferent information aboutcomplex subjects.

One document must be at least1000 words long.

Documents

307049 Unit 66 Three Phase Motors and Drives