CGE535

Munawar Zaman Shahruddin

Faculty of Chemical Engineering

Universiti Teknologi MARA, Shah Alam

munawar_zaman@salam.uitm.edu.my

Tel: 03-5544 8019; 019-249 0416

ELECTRICAL AND INSTRUMENTATION TECHNOLOGY

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Week 11-12 CHAPTER 6: BASIC AC AND DC

MOTORS

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Identify the proper motor type for various applications.

State how torque varies with speed for various motors.

Apply the equivalent circuit for dc and ac motors to compute electrical and mechanical quantities.

Lesson Outcome

At the end of class, students should be able to:

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ELECTRIC MACHINE

GENERATOR

MOTOR

AC MOTOR

DC MOTOR

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ELECTRIC MOTOR

The reference of DC or AC refers to how the electrical current is transferred through and

from the motor.

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THE PURPOSE OF ELECTRIC MOTOR

The purpose of a AC/DC Motor is to Convert Electrical Energy into Mechanical Energy

Electrical energy

AC/DC motor

Mechanical energy

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BASIC CONSTRUCTION

A stationary component. Made of copper windings that carry current. The stators coils set up a magnetic field that moves in a circular motion. The stator surrounds the Rotor.

as the name suggests, rotates. It is caused to rotate under the influence of the magnetic field of the stator. The rotor tries to keep up with the stators magnetic field

used to cool the motor.

Allows motor shaft to move smoothly. Reduces energy losses that would occur through friction. The seals keep dust from entering the motor.

Armature, the part of an electric generator or motor that contains the main current-carrying winding.

The armature usually consists of a coil of copper wire wound around an iron or steel core.

The coil and core are placed in a magnetic field produced by one or more permanent magnets or electromagnets. If the armature in a generator or motor is designed to rotate, it is called a rotor; if it is a stationary part, it is called a stator.

In an induction motor (the most widely used type of electric motor), an alternating electric current is supplied to the motor's electromagnets. The oscillating magnetic field produced by the magnets induces a current in the armature, causing it to rotate.

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ARMATURE AND FIELD WINDINGS

A machine may contain several sets of windings (commonly armature and field)

In DC motor, the field winding is on the stator while armature is on the rotor

The armature windings carry currents that vary with mechanical load Small amplitude when load is light Larger amplitude when load is heavier

If machine act as generator, the electrical output is taken from armature

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ARMATURE AND FIELD WINDINGS

TOPIC 1: DC MOTORS

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Common in automotive application (e.g starting, windshield wiper, fans, power window)

Powered from DC source

Difficulty: most electrical energy are AC source-use rectifier to convert to DC, AC machine preferable if they meet needs of application, frequent need for maintenance

Advantage: speed and direction can be controlled more readily than AC motor

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DC MOTOR

Consist of

Rotor (rotating part)

Stator (stationary part)

Brushes

Commutator

Shaft

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ROTATING DC MACHINES

Consider a coil in a magnetic field of flux density, B. When the two ends of the coil are connected across a DC voltage source, current I flows through it. A force is exerted on the coil as a result of the interaction of magnetic field and electric current. The force on the two sides of the coil is such that the coil starts to move in the direction of force.

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DC PRINCIPLES OF OPERATION

In an actual DC motor, several such coils are wound on the rotor, all of which experience force, resulting in rotation. The greater the current in the wire, or the greater the magnetic field, the faster the wire moves because of the greater force created.

At the same time this torque is being produced, the conductors are moving in a magnetic field. At different positions, the flux linked with it changes, which causes an emf to be induced (e = d/dt)

This voltage is in opposition to the voltage that causes current flow through the conductor and is referred to as a counter-voltage or back emf.

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DC PRINCIPLES OF OPERATION

The value of current flowing through the armature is dependent upon the difference between the applied voltage and this counter-voltage. The current due to this counter-voltage tends to oppose the very cause for its production according to Lenzs law. It results in the rotor slowing down. Eventually, the rotor slows just enough so that the force created by the magnetic field (F = Bil) equals the load force applied on the shaft. Then the system moves at constant velocity

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DC PRINCIPLES OF OPERATION

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DC MOTOR ROTATION CONSIDERATIONS

Speed

Rotation of the shaft

When we supply the specified voltage to a motor, it rotates the output shaft at some speed. This rotational speed or angular velocity, is typically measured in revolutions/minute (rpm)

Torque

Torque is the product of Force x Lever Arm Length (Radius)

Clockwise and Counter-Clockwise efforts are distinguished by differences in sign (+ or -)

The quantitative measure of the tendency of a force to cause or change rotational motion is called torque

The field circuit is represented by resistance RF and inductance LF in series

Consider steady state operation in which current are constant, and neglect the inductance because it behaves as a short circuit for dc current

Thus for DC field

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EQUIVALENT CIRCUIT OF THE DC MOTOR

FFF IRV

The voltage EA shown in the equivalent circuit represents the average voltage induced in the armature due to the motion of the conductors relative to the magnetic field.

The resistance RA is the resistance of the armature windings plus the brush resistance.

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EQUIVALENT CIRCUIT OF THE DC MOTOR

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Type Power range (hp)

Rotor Stator Comments and applications

Wound field Shunt connected

10-200 Armature winding

Field winding

Industrial applications, grinding, machine tools

Series connected

High torque at low speed; dangerous if not loaded; drills, automotive starting motor

Compound connected

Traction motors

Permanent magnet field

1/20-10 Armature winding

Permanent magnet

Servo applications, machine tools, computer peripherals, automotive fans, window motors

TYPES OF DC MOTOR

The field current is in parallel with the armature The field circuit consist of rheostat having a adjustable

resistance (Radj) in series with field coil that can be used to adjust motor speed

If the adjustable resistance is increase while holding the source voltage constant, the speed would also increase

Also, if the voltage source is increase the field current could be hold constant to increase the speed by increasing the value of adjustable resistance

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SHUNT CONNECTED DC MOTOR

The machine is supplied by constant voltage source, VT

Has very high starting torque and draws very large starting currents

Usually, resistance inserted in series with armature during starting to limit the current to reasonable levels

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SHUNT CONNECTED DC MOTOR

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SHUNT CONNECTED DC MOTOR

Mechanical shaft speed

Develop torque

Induced voltage

The armature resistance

Similar to shunt-connected motor except different source are used for the armature and field

Separate as reason to be able to control speed by varying one of these two sources

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SEPARATELY EXCITED DC MOTORS

The field winding is in series with armature Has moderate starting torque and starting current Speed automatically adjust over a large range as the load torque

varies Because it slows down for heavier load, its output power is more

nearly constant than other types of motor Advantageous because the motor can operate within its

maximum power rating for a wide range of load torque In some cases, the no-load speed can be large enough to be

dangerous

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SERIES CONNECTED DC MOTORS

Ex: starter motor in automobiles, when engine is cold the starter motor operate at lower speed, when engine warm the starter spin faster.in either case, the current drawn from battery remains within acceptable limit

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SERIES CONNECTED DC MOTORS

Advantages of DC motor:

Ease of control

Deliver high starting torque

Near-linear performance

Disadvantages:

High maintenance

Large and expensive (compared to induction motor)

Not suitable for high-speed operation due to commutator and brushes

Not readily available for use at home

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SERIES CONNECTED DC MOTORS

The field is supplied by magnets mounted on the stator rather than field coil

Characteristics are similar to those of separately excited machine except field cannot be adjusted

Advantages; No power required to establish the field-leading to better efficiency PM motor can be smaller than equivalent machine with field winding

Disadvantages; The magnet can become demagnetized by overheating/ excessive armature current Flux density magnitude is smaller thus, torque produced per ampere of armature current

is smaller

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PERMANENT MAGNET MOTORS

END OF TOPIC 1

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TOPIC 2: AC MOTORS

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A type of electric motor that runs on alternating current. AC motors are more commonly used in industry than DC motors but do not operate well at low speeds

AC Motors are highly flexible in many ways including their speed control

The components of AC motors have been described in the earlier slides

Can be either single or three phases

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AC MOTORS

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If a 3-phase supply is fed to the stator windings of a 3-phase motor, a magnetic flux of constant magnitude, rotating at synchronous speed is set up.

At this point, the rotor is stationary. The rotating magnetic flux passes through the air gap between the stator & rotor and sweeps past the stationary rotor conductors.

This rotating flux, as it sweeps, cuts the rotor conductors, thus causing an e.m.f to be induced in the rotor conductors.

As per the Faradays law of electromagnetic induction, it is this relative motion between the rotating magnetic flux and the stationary rotor conductors, which induces an e.m.f on the rotor conductors.

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AC MOTORS PRINCIPLES OF OPERATION

Since the rotor conductors are shorted and form a closed circuit, the induced e.m.f produces a rotor current whose direction is given by Lenzs Law, is such as to oppose the cause producing it.

In this case, the cause which produces the rotor current is the relative motion between the rotating magnetic flux and the stationary rotor conductors. Thus to reduce the relative speed, the rotor starts to rotate in the same direction as that of the rotating flux on the stator windings. The frequency of the induced e.m.f is same as the supply frequency.

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AC MOTORS PRINCIPLES OF OPERATION

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Type Power range (hp)

Rotor Stator Application

Induction 1-5000 Squirrel cage Three- phase armature windings

Simple rugged construction: very common; fans, pump

Wound field Adjustable speed using rotor resistance; cranes, hoists

Synchronous 1-5 Permanent magnet Precise speed; transport sheet materials

1000-50,000 Dc field winding Large constant load

TYPES OF AC MOTOR

Phase - defines the type of electrical power being supplied to the motor

Each phase is displace 120

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3-PHASE AC POWER

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PARTS OF AN AC MOTOR

Electro-Magnets

Stator

Rotor

Operating speed of synchronous motor is constant

The number of magnetic poles P is always an even integer

If some other speed other than presented in Table 1 is required, a synchronous machine is usually not a good choice

The starting torque is zero

One approach is to operate the motor as an induction motor with reduced load until the speed approaches synchronous speed and then switch to synchronous operation

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AC SYNCHRONOUS MOTOR

Synchronous Speed - The speed of the stators magnetic field rotation.

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SYNCHRONOUS SPEED

f is Applied Frequency

P is magnetic poles that rotate at synchronous speed

P

fns

120

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Table 1: Synchronous Speed

Synchronous Speed (60 Hz) =

.

P ns

2 3600

4 1800

6 1200

8 900

10 720

12 600

Synchronous speed versus number of poles for f=60 Hz

The motor has good starting torque

In normal operation, the speed of induction motor is only slightly less than synchronous speed

Ex: at full load, a typical four pole (P=4) induction motor runs at 1750 rpm and at no load it speed approaches 1800rpm

During startup, the current drawn by induction motor can be many times larger than its rated full-load current

To avoid excessive current, large induction motors are usually started with reduced voltage

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AC INDUCTION MOTOR

TOPIC 2: Small Motors

Manufacturers will be required to comply with the Department of Energys (DOE) energy conservation standard for small electric motors beginning 2015.

A small commercial or industrial electric motor converts electrical energy to rotating mechanical energy. When operating, the electrical energy is transferred as useful mechanical energy to some driven device such as a fan, pump, blower, compressor, or conveyor.

Small electric motors include single phase and polyphase motors built in a two-digit National Electrical Manufacturers Association (NEMA) frame and are rated from to 3 horsepower.

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SMALL ELECTRIC MOTORS

US Department of Energy

The current standard will save approximately 2.6 quads of energy and result in approximately $35 billion in energy bill savings for products shipped from 2015-2044.

The standard will avoid about 133.6 million metric tons of carbon dioxide emissions, equivalent to the annual greenhouse gas emissions of about 26.2 million automobiles.

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SMALL ELECTRIC MOTORS

US Department of Energy

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END OF TOPIC 2

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TOPIC 3: Torque, Starting and Speed Control

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BASIC THEORY

Torque is rotating EFFORT, speed is rotating FLOW

Torque = force x radius

Voltage is electrical EFFORT, current is FLOW of electrons

Power = EFFORT x FLOW

Mechanical power P(mech) = torque x speed

Electrical power P(elec) = voltage x current

Torque is the tendency of a force to rotate an object about an axis, fulcrum, or pivot.

Torque is very important element of Electric machine.

In DC motor:

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TORQUE

= , ka = constant for a particular machine Ia = Armature current

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TORQUE

The torque developed by AC induction motor is given by Td = nV1

2R2 / S s[(R1+R2/S)

2+(X1+X2)2]

Where

n= no. of poles V1 is supply voltage S is slip (difference between the synchronous speed and the shaft rotating

speed) R1 and R2 stator and rotor resistance respectively X1 and X2 stator and rotor inductance respectively s synchronous speed (depends on the input power frequency and the number of

electrical magnetic poles in the motor)

A torque speed curve shows how a motor's torque production varies throughout the different phases of its operation.

Starting torque, also called locked rotor torque, is produced by a motor when it is initially turned on. Starting torque is the amount required to overcome the inertia of a standstill.

Pull-up torque is the minimum torque generated by a motor as it accelerates from standstill to operating speed. If a motor's pull-up torque is less than that required by its application load, the motor will overheat and eventually stall.

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TORQUE-SPEED CHARACTERISTICS

Breakdown torque is the greatest amount of torque a motor can attain without stalling. High breakdown torque is necessary for applications that may undergo frequent overloading . One such application is a conveyor belt. Often, conveyor belts have more product placed upon them than their rating allows. High breakdown torque enables the conveyor to continue operating under these conditions without causing heat damage to the motor.

Full load torque is produced by a motor functioning at a rated speed and horsepower. The operating life is significantly diminished in motors continually run at levels exceeding full load torque.

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TORQUE-SPEED CHARACTERISTICS

Synchronous speed is the speed at which no torque is generated by a motor. This occurs in motors that run while not connected to a load. At synchronous speed, the rotor turns at exactly the same rate as the stator's rotating magnetic field. Since there is no slip, there is no torque produced

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TORQUE-SPEED CHARACTERISTICS

m=Pm/T=nm x 2/60

since T= kaIa m=Pm/kaIa

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SPEED IN DC MOTOR

= , ka = constant for a particular machine Ia = Armature current nm= rotational speed in rpm

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USEFULL RELATIONSHIP IN AC MOTOR

Many applications require the speed of a motor to be varied over a wide range. One of the most attractive features of DC motors in comparison with AC motors is the ease with which their speed can be varied.

We know that the back emf for a separately excited DC motor:

Rearranging the terms,

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DC MOTOR SPEED CONTROL

From this equation, it is evident that the speed can be varied by using any of the following methods:

Armature voltage control (By varying VT)

Field Control (By Varying )

Armature resistance control (By varying Ra)

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DC MOTOR SPEED CONTROL

This method is usually applicable to separately excited DC motors. In this method of speed control, Ra and are kept constant.

In normal operation, the drop across the armature resistance is small compared to Eb and therefore:

Eb VT

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ARMATURE VOLTAGE CONTROL

Since Eb=kam, the equation can be arranged as follows:

m = VT/ka

So, we can simply relate m with the VT in a simple linear equation.

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ARMATURE VOLTAGE CONTROL

In this method of speed control, Ra and VT remain fixed.

Therefore,

m 1/ and

If as a result of magnetic linearity

So, it will result-in inversely proportional relationship between speed and magnetic flux.

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FIELD CONTROL

The voltage across the armature can be varied by inserting a variable resistance in series with the armature circuit.

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ARMATURE RESISTANCE CONTROL

From speed-torque characteristics, we know that:

Rearranging the equation, it will relate m and Ra in a form of linear relationship with negative slope and intercept of VT/K.

Note that VT and in this case are remained constant.

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ARMATURE RESISTANCE CONTROL

With the reference of the previous equation (page 55), we should have a brief idea on which variable should we manipulate to control the speed of AC motors.

It is related to either power/voltage sources or torque. There are several ways or methods to control the speed which

are: Change the number of poles (in discrete increments -inefficient &

rarely done) torque Change the frequency of the AC signal power/sources Change the slip torque

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AC MOTORS SPEED CONTROL

Variable speed AC Motor adjustable speed drives are known as

inverters,

variable frequency drives (VFD) , or

adjustable speed drives (ASD).

Common ways to vary AC frequency:

Six-step inverter

Pulse-Width-Modulation

Vector Flux 64

CHANGE AC FREQUENCY

AC rectified to DC, then switched to imitate a sine wave

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SIX-STEP INVERTER

DC voltage (rectified AC) rapidly switched to match "area under curve"

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PULSE-WIDTH MODULATION

Vector control implies that an ac motor is forced to behave dynamically as a dc motor by the use of feedback control.

Always consider the stator frequency to be a variable quantity.

Think in synchronous coordinates.

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VECTOR FLUX

Important to match the motor to the load

ensure that a change in motor power gives a desired change in load speed

Load should have a substantial inertial components

inertial torque can "carry" the load through brief periods when motor torque cannot

Best used with motors designed for high slip

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CHANGING ROTOR SLIP

Additional series resistance reduces voltage across main windings

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VARIABLE SERIES RESISTANCE

More efficient than previous method, no power wasted in the series

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VARIABLE VOLTAGE TRANSFORMER

Commonly used with 3-speed fan motors (like the one in AC Motor Lab)

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TAPPED WINDING

END OF TOPIC 3

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TOPIC 4: Motor Selection

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A motor must do three things:

1. Start the equipment load

2. Drive the load once it is started

3. Survive the abuse of the surroundings in which it operates

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MOTOR SELECTION

230-volt motor should not be used if only 115-volt circuits are available

Three-phase motor cannot be operated on electrical system with only single-phase service

Typical Operating Voltages:

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TYPE OF POWER AVAILABLE

Single-Phase Three-phase

115 208

208 230

230 460

240 480

460

480

Rated in HP

Refers to the power that it will develop when the motor is turning at full speed

Rules of Thumb for estimating size needed:

If equipment can be operated by hand, a 1/4 HP motor will usually be adequate

If gasoline engine is to be replaced by electric motor, an electric motor approximately 2/3 the HP rating of the engine will be adequate

Replace tractor power take-off (PTO) with an electric motor of approximately the same HP

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SIZE OF MOTOR

Motor selected must produce adequate starting torque to start the load

Commonlyused motors:

Split phase

Capacitor start-induction run

Capacitor start-capacitor run

Repulsion start-induction run

Series or universal

Shaded pole

Three-phase

Capacitor start-induction run & Three-phase are the most common and produce highest starting torque

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STARTING LOAD

Rated at the speed the shaft will turn in revolutions per minute (rpm) when motor is operating at full speed

Rpm of motor should be speed needed to operate equipment at proper speed

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SPEED REQUIREMENT

Sleeve bearings

OR

Anti-friction bearings

Require less maintenance and can be mounted in any position

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BEARING TYPE

The type of base or method of mounting an electric motor may depend upon the load it drives.

Some may have a resilient mounting allowing for some flexibility

Some are mounted directly to the machine.

Still others may have a mounting bracket welded to the motor housing.

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TYPE OF MOUNTING

Rigid base

Sliding adjustable base

Cushion mount

Reduces vibration & wear

Determined by application of motor

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BASE TYPE

Easy Starting Loads Difficult Starting Loads

Shaded Pole Induction Capacitor-Start, Induction-Run

Split Phase Repulsion-Start, Induction-Run

Permanent-Split, Capacitor-Induction

Three-Phase, General-Purpose

Soft-Start Perkey Concept: use tractor PTO to start

Repulsion-Start, Capacitor-Run

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STARTING LOADS

Motor Duty = amount of time the motor is operating under full load, and how much time it is stopped

Continuous Duty: constant full load for over 60 minutes at a time

Intermittent Duty: fully loaded for 5, 15, 30, or 60 minutes

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MOTOR DUTY

Motors produce heat

Cooling: fan on shaft, openings in end

Must protect from dust, water etc

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ENCLOSURES

Provide proper protection from surroundings

Typical motor enclosures: Open drip proof

Splash proof

Totally enclosed-fan cooled (TEFC)

Explosion proof

Totally enclosed-air over (TEAO)

Totally enclosed-non ventilated (TENV)

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ENVIRONMENT

END OF TOPIC 4

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CONCLUSION

Define machine and motor

Explain on DC and AC motors

Explain on small motors

Explain on torque, speed and starting

Explain on motor selection

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