Kallam Haranadhareddy Institute of Technology, Guntur

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Utilization of Electrical Energy

Unit-I

Selection of Motor

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JNTUK SYLLABUS: Choice of motor, type of electric drives, starting and running characteristics–Speed control– Temperature rise–Applications of electric drives–Types of industrial loads–continuous– Intermittent and variable loads–Load equalization.

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TEXT BOOKS:

1. 1. Utilization of Electric Energy – by E. Openshaw Taylor, Orient Longman.
2. 2. Art & Science of Utilization of electrical Energy – by Partab, DhanpatRai& Sons.

REFERENCE BOOKS:

1. Utilization of Electrical Power including Electric drives and Electric traction – by N.V.Suryanarayana, New Age International (P) Limited, Publishers, 1996.
2. 2. Generation, Distribution and Utilization of electrical Energy – by C.L. Wadhwa, New Age International (P) Limited, Publishers, 1997.
3. WEBSITES: www.ieee.org, www.iitm.ac.in,

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Introduction

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Drive

Conventional Drive

Electric Drive

Components:

1.Prime Mover (I/C Engine)

2.Transmission Equipment

3.Mechanical Load

Components:

1.Prime Mover (Electric Motor with controlling Device)

2.Transmission Equipment

3.Mechanical Load

Drive : The combination of prime mover, transmission equipment and mechanical load is a drive.

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Block Diagram of Electric Drive

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Factors Governing the Selection of Motors

• Nature of supply
• Ac supply
• Dc supply
• Rectified ac supply

• Electrical characteristics
• Starting characteristics
• Running characteristics
• Speed control
• Braking characteristics

• Mechanical characteristics
• Type of encloser
• Method of power transmission
• Type of bearing
• Type of cooling
• Noise level

• Nature of drive
• Individual drive
• Group drive multi drive

• Size and rating of motor
• Depending upon load cycle
• Depending upon over load capacity

• Cost
• Capital cost
• Running cost

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Nature of Supply

Pure DC or Rectified DC

DC Series Motor

DC Shunt Motor

DC Compound Motor

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A.C

1-ø Motors

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3- ø

Squirrelcage Motors

Slipring Motor

Scharge Motor

Pure DC Waveform

Rectified or modulated DC Waveform

1-ø AC Waveform

3-ø AC Waveform

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• D.C. motors are not used so widely as A.C. motors.

1. Additional equipment is required for converting existing A.C. supply into D.C. supply.

2. D.C. motors have commutators that are subject to trouble and resulting in sparking, brush wear and tear and the presence of moisture and destructive fumes in the surrounding air.

3. D.C. motors are generally more expensive than A.C. machines for similar working conditions.

•   In some cases, such as in electric excavators, steel mills and cranes the speed control is important so DC supply is used by converting A.C. supply in to D.C. supply.

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Classification of Electric Drive Based on Nature of Supply

AC Drive

D C Drive

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Classification of Electric Drive

Individual Drive

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Group drive

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Multi Motor Drive

In individual drive a single electric motor is used to drive one individual machine.

Ex: single-spindle drilling machines and various types of electrical hand tools and simple types of metal working tools

• The group drive is a drive in which a single electric motor drives the group of working machines.
• It comprises of a single lengthy shaft, to the shaft the different working machines are attached by means of pulleys and conveyor belts.
• Sometimes group drive is also known as LINE SHAFT DRIVE.
• Group drive is often used in industries where successive operations are required like car manufacturing industries e.t.c.

In a mullimotor drive. separate motors are used for operating different parts of the same mechanism. Thc system contains several individual drives, each of which is used to operate its own mechanism. This type of drive finds application in complicated machine tools, rolling mills, / overhead crane, paper making machines etc. Automatic control methods can be employed.

Comparison Between Group Drive and Individual Drive

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Types of Loads

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The various. types of loads that occur in industrial practice can be classified depending upon their variation with time and duty cycle, which are specified by the load diagram

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Classification of Loads with respect to time

1.Continuous constant loads: The load on the motor remains same for a longer time.

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Ex: Fans and pumps,

2.Continuous variable loads : The load on the motor varies continuously over a period of time, but occurs repetitively for a longer duration.

Ex: Conveyors, metal cutting lathes, hoist winches etc.

3.Pulsating loads: Certain types of loads exhibit a torque behavior which can be thought of as a constant torque superimposed by pulsations.

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Ex: Compressors, reciprocating pumps, frame saws, textile looms and all loads having crank-shafts.

4.Impact loads: The load on the motor increases to a maximum level suddenly, This peak loads occur at regular intervals of time, Motors driving these loads are equipped with fly wheels for load equalisation .

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Ex: Forging hammers, rolling mills) shearing machines, presses etc.

6.Short time loads: The load on the motor remain constant and occurs for a short time and then remain idle or off for a sufficiently long time,

Example: Mixies, battery chargers and most of the household equipments,

5.Short time intermittent loads: The load on the motor occurs periodically in identical duly

cycle, each consisting of a period of application of load and rest.

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Ex: Excavators, cranes, roller trains, hoisting mechanisms etc.

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Classification of Loads with respect to Duty Cycle

1.Continuous Duty Loads

2.Short Time Duty loads

3.Intermittent Duty Cycle loads

Continuous duty at constant load

Continuous duty with variable load cycle

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Electrical characteristics

Starting characteristics

Speed Control characteristics

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Running Or Operating characteristics

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Starting Characteristics:

• The study of starting characteristics of a motor is essential to know whether the starting torque that the motor is capable of

developing is sufficient to start and accelerate the motor. The torque for accelerating depends upon the load torque.

• The loads which are usually met with may be divided according to accelerating torque requirements into the following categories:
• Load requiring very small accelerating torque in comparison with full load torque such as when the motor is to be run light.
• Load requiring the torque which may increase with speed and it may be proportional to (speed)² as in case of fan.
• Load having constant load torque at all speeds in case of lifts.
• When the motors have to start and accelerate against full load torque and in addition to accelerate since some heavy moving parts as in case of rolling mills.

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Running Characteristics Or Operating Characteristics:

• While studying electrical behavior of a machine under normal operating conditions, the speed-torque characteristic, speed-current characteristic, and torque-current characteristic, losses, efficiency, magnetizing current and power factor at various loads are to be kept in the view.
• The last two factors i.e. magnetizing current and power factor are to be considered in case of A.C motors.

Electrical characteristics

Starting characteristics

Speed Control characteristics

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Running Or Operating characteristics

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Speed Control:

While selecting a motor for a particular drive special care has to be taken for the speed variations. If we consider the entire range of loads out of which some loads may require constant speed drives, some may require smooth variation on speed and some may require step changes in speed

Electrical characteristics

Starting characteristics

Speed Control characteristics

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Running Or Operating characteristics

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1(a)Continuous Duty at Constant Load cycle

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In this case the load torque remain constant for a sufficiently longer period.

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Selection of Motor:

While driving such type of load, the motor should have a rating sufficient to drive it without exceeding the specified temperature. The rating of the motor selected for this duty is called its continuous rating. By continuous rating one means that it is the maximum Load that the motor can give continuously over a period of time, without exceeding the temperature limit. The motor so selected also should be able to withstand momentary overload capacity. The temperature rise of the motor is represented by dashed curve, This type of motors will have high efficiency because they will be operating almost at its full-load and also have good power factor.

Ex: Centrifugal pumps, conveyors, compressors, fans etc.

1(a)Continuous Duty at Constant Load cycle

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In this type of duty, the load on the motor is not constant, but has several steps in one cycle. This cycle of loading repeats for a longer time.

Selection of Motor: If the load variations are slight, a motor of continuous rating of the highest load may be chosen for the purpose. If the variations in the load cycle are large) the motor temperature changes continuously. After several cycles of operation the motor temperature attains a steady-state value. The selection of motor for this type of duty involves thermal calculation which is a difficult task, Suppose the motor is selected according to lowest load, it will not drive the highest load satisfactorily and the temperature rise will be exceedingly high, Suppose the motor is selected according to the highest load, it will be overrated and the efficiency of the motor will be poor. The motor is under utilised and the power factor also will be poor.

2.Short Time Duty Loads

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In this class of duty constant load occurs on the motor for a short interval of time and then off for sufficiently long time allowing the motor to cool down to ambient temperature. Such load,occurs in mixies (house hold appliances), some crane drives, opening and closing of weirs, lock gates, bridges etc.

3.Intermittent Duty or Periodic Intermittent Duty Loads

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The load on the motor is a sequence of identical duty cycles as shown in the below figure. The motor is loaded for some time t_on and shut off for a period of t_off .

Duty Cycle =(t_on / t_on+ t_off)

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Duty Cycle=(t_start+ t_on)/( t_start+ t_on+ t_off)

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Duty Cycle=(t_start+ t_on+t_br)/( t_start+ t_on+t_br + t_off)

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Standard Motor ratings

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1. Continuous Rating: The output which a motor can give continuously without exceeding the temperature beyond the specified limit. A continuously rated motor can also give 25% overload for 2 hours.

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2. Continuous Maximum Rating: It is similar to continuous rating without permitting any overload. It is used only for motors larger than 1.5 kW.

3. Short Time Rating: The output, which a motor can give for a specified period say 15 minute, 30 minute, I hour etc., without exceeding the specified temperature rise, the motor being assumed to start from cold.

Load Equalization using Fly Wheel

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Type of Enclosures

Open type

Protected or Guarded type

Screen Protected type

Drip Proof type

Splash Proof type

Totally Enclosed type

Pipe Ventilated type

Flame Proof type

Weather Proof type Enclosure

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Types of Bearings

Sliding Contact/

Plain Bearing

(Friction Bearing)

Rolling Contact Bearing

(Anti-friction Bearing)

Right Line or Guide Bearing

Journal bearing

Thrust Bearing

Pivot or Sleep Bearing

Collar bearing

Ball Bearing

Roller bearing

Radial Bearing

Thrust bearing

Sliding Contact Bearings

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Rolling Contact Bearings

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Types of Mountings

Foot Mounting type

Ceiling Mounting Type

Wall Mounting Type

Flange Mounting Type

Cradle Mounting Type

Body Turned Mounting Type

Noise

• Noise is a sort of disturbance, which pollutes the atmosphere.
• Noise may be either mechanical or electrical. Extreme silence is of course necessary in motors for domestic purposes, in hospitals, theaters & other similar situations. Even for industrial purposes reduction of motor noise is often desirable.
• Below table shows the source of noise & methods to reduce the same.

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Types of Transmission Drive

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The mechanical power developed by the motor is available at the shaft. This power has to be transmitted to the machine or load

The power transmission is classified into 2 types.

1. Direct transmission 2. Indirect transmission

In direct power transmission, the power is transmitted to the load by means of fixed coupling or flexible coupling. This drive has high efficiency

• Belt drive

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• V-belt drive

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• Rope drive

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• Chain drive

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• Gear drive

Kallam Haranadhareddy Institute of Technology, Guntur

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