Lecture PPT

1. Unit 1: Selection of Motor Power Ratings:
Thermal Model of Motor for Heating and Cooling, Classes of Motor Duty,
Determination of Motor Rating.
Direct Current Motor Drives: Controlled Rectifier Fed dc Drives, Single and
three Phase Half and Fully Controlled Rectifier Control of dc Separately
Excited Motor, Rectifier Control of dc Series Motor, Supply Harmonics, Power
Factor and Ripple in Motor Current, Chopper Control of Separately Excited dc
Motor, Chopper Control of Series Motor.
Industrial Drives

2. Selectionof motor power rating
- operatebelow rated powerfactor
• Thepower rating of a motor for a specificapplicationmustbe
carefully chosento achieveeconomywith reliability.
- useof a motor having insufficient rating
- it failsto drive theload;
- lowersthe productivityandreliability through
frequent damagesandshut-downsdueto
overloadingof the motor andpowermodulator;
- useof motor havingextra power than needed
- extra initialcost
- extra lossof energy uneconomical
Especially,IMs &
Synch.motors

3. • It isclearthat, heat isproduceddueto lossesin copper,iron and
friction duringmotoroperation.
• Asthe temperature of the motor increasesbeyondambient value
part of the heat flowsto the surroundingmedium.
• Asoutflow of heat increases,equilibrium setswhenthe heat
generatedbecomesequal to heat dissipated.
• Themotor temperature reachessteadystate andit dependson
power loss,whichintern dependsonthe output power of the
machine;andthis dependenceof temperature riseonoutput
power whichistaken asthermal loadingonthemachine.

4. • Steadystate temperature isnot the sameat variousparts of the
machine.It isusuallyhighest in the windingbecauseloss
density in conductorsishighanddissipationrate islow. (since
windings are embeddedin slots)
• Among the variousmaterialsusedinmachines, insulation
has the lowest temperature limit.
• Dependingonthe temperature limits, insulatingmaterials are
dividedintoclasses.
Insulationclass temperature limit
γ 90oC
A 105oC
E 120oC
B 130oC
F 155oC
H 180oC
C Above180oC
Insulationtemperature limits

5. • Foraspecificoperation, motor rating shouldbe chosensuchthat
the insulationtemperature never exceedsthe prescribed limit.
Otherwise, it will lead to its immediate thermal break down
causingshortcircuitanddamageof the winding.
• Forloadswhichoperate at aconstantpower andspeed,
determination of motor power rating issimple.
• Usually,mostloadsoperate at variable power andspeed,and
the patterns of these variations are different for different
applications

6. Electric Motor Power Rating
• Power rating for electrical machines indicates the required supply voltage for
smooth running of that machine, it also shows the permissible maximum
amount of current which can easily flows through the machine and there will
be a chance of breakdown in the machine if those parameters goes beyond
this limit.
• Similarly when we discuss about motor power rating, we are looking for the
suitable conditions where maximum efficiency is obtained from the electric
motor. When the motor have insufficient rating, there will be frequent
damages and shut downs due to over loading, and this is not intended.

7. Cont…
• Another essential criteria of electrical motor power rating is that,
during operation of motor, heat is produced and it is inevitable due to
I2R loss in the circuit and friction within the motor. So, the ventilation
system of the motor should be designed very carefully, to dissipate
the generated heat as quickly as possible. The output power of the
motor is directly related with the temperature rise, that’s why it is
also called thermal loading.
• The thermal dissipation will be ideal when the ventilation system is
designed in such a way that the heat generated during the operation
is equal to or less then heat dissipated by the motor to the
surrounding.

8. Contt…
• Now, due to the design of motors, temperature is not same
everywhere inside the motor. There is a high amount of heat
produced in the windings because, windings cause higher heat
generation. The insulating materials used in the winding are also
chosen depending on the amount of heat generated inside the motor
during operation. So in the end it can be said that the main objectives
of selecting and finding out motor power rating are-
• To obtain the suitable thermal model of motor and design the machine
properly.
• Finding out motor duty class.
• Calculating motor ratings for various classes of duty.

9. • Economical Operation: can work continuously and take the place of
2 or more laborers
• Dependable: always ready to work
• Efficient: efficiency ranges from 65 to 95 percent
• Long Life: 20 to 30 years with proper care
• Ease of Operation: special skills not required
• Safe: if properly used
• Low Service Requirement: minimum amount of attention required
• Quiet Operation: when properly applied and installed
• Automation: can be automatically and remotely controlled
• Adaptable: light, compact, easily moved
• Available: throughout the country; standard bases, sizes, wiring
equipment, etc.
Advantages of Using Electrical Motors

10. 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
Factors To Consider In Selecting Electric
Motors

11. Type of Power Available
• 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:
Single-Phase Three-phase
115 208
208 230
230 460
240 480
460
480

12. Size of Motor
• 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 PTO with an electric motor of approximately the
same Hp

13. Starting Load
• Motor selected must produce adequate starting torque to start
the load
• Commonly–used 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

14. Speed Requirement
• 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

15. Bearing Type
• Sleeve bearings
OR
• Anti-friction bearings
• Require less maintenance and can be mounted in any
position

16. Base Type
•Rigid base
•Sliding adjustable base
•Cushion mount
• Reduces vibration & wear
•Determined by application of motor

17. Environment
• 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)

18. Power Availability
Motor Size
Service Entrance
Amperage &
Wiring
Voltage Phase
1/3 hp or less 30 A; 2-wire 120 V Single
1/3 hp-1½ hp 30 A; 3-wire 120-240 V Single
1½ hp-5hp 60 A; 3-wire 120-240 V Single
5 hp-7½ hp 100 A; 3-wire 120-240 V Single
Over 7 hp Over 100 A; 4-wire 277-480 V Three
Two-wire: one conductor & one neutral wire
Three-wire: two conductors & one neutral wire
Four-wire: three conductors & one neutral wire