It is estimated that electrical drives and other rotating equipment consume about 50% of the total electrical energy generated in the world today. Other estimates are that pumps, fans, blowers and compressors consume as much as 65% of this total, a large proportion of these applications are powered by fixed or constant speed drivers whose load demands often fluctuate. This poor match of speed and demand results in considerable wasted energy and significantly increased wear of system components. Variable speed drive technology is a cost effective method to match driver speed to load demands and is an excellent opportunity to reduce operating costs and improve overall efficiencies in your application. This workshop gives you a fundamental understanding of the installation, operation and troubleshooting of variable speed drives. Typical practical applications of VSDs in process control and materials handling, such as those for pumping, ventilation, conveyers, compressors and hoists are covered in detail. You will learn the basic setup of parameters, control wiring and safety precautions in installing a VSD. The various drive features such as operating modes, braking types, automatic restart and many others will be discussed in detail. You will learn the four basic requirements for a VSD to function properly with emphasis on typical controller faults, their causes and how they can be repaired. The concluding section of the workshop gives you the fundamental tools in troubleshooting VSDs confidently and effectively. Even though the focus of the workshop is on the direct application of this technology, you will also gain a thorough understanding of the problems that can be introduced by VSDs such as harmonics, electrostatic discharge and EMC/EMI problems. MORE INFORMATION: http://www.idc-online.com/content/practical-variable-speed-drives-instrumentation-and-control-systems-38

1. Practical Variable Speed Drives for
Instrumentation and Control Systems
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2. BASIC CONSTRUCTION
 Basic Design unchanged in over 50 years, but
now have smaller physical size and lower cost per
kW due to:
• Modern insulation materials
• Computer based design optimisation
techniques
• Automated manufacturing methods
• International standardisation physical
dimensions
 AC Induction Motor comprises 2 main parts :
• Stationary part called the Stator
• Rotating part called the Rotor
 Both Stator and the Rotor are made up of :
• Magnetic circuit - laminated grain oriented
steel
• Electric circuit - insulated copper or
aluminum
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3. BASIC CONSTRUCTION
 Two types of Rotor Construction
• Wound Rotor type, which comprises 3 sets of windings with connections
to 3 slip rings on the shaft
• Squirrel Cage Rotor type, which comprises a set of copper or aluminium
bars installed into the slots, which are connected to an end-ring at each end
 Other parts
• Two end-flanges to support the DE and NDE bearings
• Two Bearings to support the rotating shaft
• Steel shaft for transmitting the torque
• Cooling fan at NDE for cooling of stator and rotor
• Terminal box for external electrical connections
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4. BASIC CONSTRUCTION
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5. PRINCIPLES OF OPERATION
 3-phase AC Voltage connected to the Stator windings
• Currents establish magnetic field (flux pattern)
• Rotates around the inside of the stator
• Rotation Speed in synchronism with the power frequency
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6. PRINCIPLES OF OPERATION
 In its simplest form:
• 3-phase Stator windings connected to power supply
• flux completes one rotation for every cycle of mains
• On 50Hz, the stator flux rotates at 50 revs per second
• Rotor turns at 50 x 60 = 3,000 revs per minute.
• Called a 2 pole motor (2 poles 1-North, 1-South)
 The design of the Stator windings can be changed to be suitable for 4-pole
operation:
• Therefore rotates at half the speed ... 1,500 rev/min
• Called a 4 pole motor (4 poles 2-North, 2-South)
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7. PRINCIPLES OF OPERATION
 Flux distribution in a 4 pole motor at any one moment
• Shows the 2-North and 2-South poles
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8. SPEED OF AC INDUCTION MOTOR
 AC Induction motors can be designed and manufactured with the number
of stator windings to suit speed requirements
• 2 pole motors .... stator flux rotates at 3,000 rev/min
• 4 pole motors .... stator flux rotates at 1,500 rev/min
• 6 pole motors .... stator flux rotates at 1,000 rev/min
• 8 pole motors .... stator flux rotates at 750 rev/min etc
 Speed of Stator Flux is called Synchronous Speed
rev/ min
= f x 60
p/2
= f x 60 no
pole - pairs
rev/ min
= f x 120 no
p
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9. ACTUAL ROTOR SPEED
 Air-gap Magnetic Flux cuts across the rotor conductors
• Faraday's Law - voltage induced in the rotor windings
• Voltage depends on the rate of change of flux
• Current in the rotor windings sets up own magnetic field that interacts with
Stator flux to produce the rotational force
• Lenz's Law - Direction of the force tends to Reduce the changes in flux field
rotor accelerates to follow the direction of the rotating flux
 At starting, the rotor is stationary
• Magnetic flux cuts the rotor at synchronous speed and induces the highest
rotor voltage and rotor current
• As rotor accelerates, rate at which the magnetic flux cuts the rotor windings
reduces … and the induced rotor voltage decreases proportionately
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10. ACTUAL ROTOR SPEED
 When Rotor Speed approaches synchronous speed:
• Magnitude and frequency of rotor voltage becomes small
• If rotor reached synchronous speed, the rotor windings would be moving at
the same speed as the rotating flux
• Induced voltage (and current) in the rotor would be zero
• Without rotor current, no rotor field and no Torque
 To produce Torque:
• Rotor must rotate at a slower (or faster) speed
• So, the rotor settles at a speed less than rotating flux called the Slip Speed
• The difference in actual speed to synchronous speed is called the Slip
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11. ROTOR SLIP
 The slip will vary according to Load Torque
• As load torque increases, the slip increases
• More flux lines cut the rotor windings
• Increases rotor current and magnetic field
• Consequently increases rotor torque
• Typical slip between 1% (no-load) to 6% (full-load)
 Slip (in per unit) is given by :
per - unit
Slip = s = ( n o
- n)
n
o
 Actual rotational speed is
n = no (1 - s) rev/ min
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12. EQUIVALENT CIRCUIT OF AC MOTOR
 Electrical circuit can be represented by an equivalent circuit
 Sketch shows ... motor does not have separate field windings
 Stator current therefore serves a double purpose
• Carries Magnetising current for rotating magnetic field IM
• Carries Rotor current that provides shaft torue IR
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13. SIMPLIFIED EQUIVALENT CIRCUIT
 Equivalent circuit simplified by taking out 'transformer'
• adjusting XR and RR values by the turns ratio N = NS/NR
i.e. 'transferring' them to the stator side
• So, must also adjust for frequency ... which depends on slip
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14. 3-PHASE AC INDUCTION MOTORS
 AC Induction Motors are one of the most successful inventions, consume > 50%
of all electrical energy generated
 They are very popular for Industrial Applications
• Simplicity easy to manufacture
• Reliability very little maintenance
• Relatively low cost more kW per $
 Work well even in a bad environment
• Dust-proof
• Water-proof
 Can be used for Variable Speed Control
• Speed proportional to frequency
 Need to clearly understand how they work
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15. MORE SIMPLIFIED CIRCUIT
 Rotor Resistance is Variable
• Rotor current IR …. depends primarily on the slip (s)
 Magnetising Inductance is roughly Constant
• Magnetising Current IM ...... depends on voltage (V)
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16. CURRENT VECTORS
 Stator current IS represents the vector sum of :
• Magnetising current IM ... generates rotating magnetic field
• Rotor current IR ... which produces the rotor Torque
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17. MOTOR PERFORMANCE
 The Power Equations are as follows :
• Angle between IS and IR is power factor angle f
• Total apparent motor power S is given by
S = P + jQ kVA
• Active Power P is given by
P = 3 x V x I R kW
P = 3 x V x I S x Cosf kW
• Reactive Power Q is given by
Q = 3 x V x I M kVAr
Q = 3 x V x I S x Sinf kVAr
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18. MOTOR PERFORMANCE
 Torque-Speed Curve is the basis of all motor applications
• Curve derived from the equivalent circuit
 Fundamental equation for a 3-phase AC induction motors,
• Refer to any standard textbook
• Represents the equivalent circuit
 Output Torque of the motor is given by
T = 3 x s x V x R
2
¢
M ¢ 2
¢
R
2
[( R S + R R
) + s( X S + X R
) ] n
 Output Torque proportional to V2
o
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19. TORQUE-SPEED CURVE
A : Breakaway Starting Torque
B : Pull-up Torque
C : Pull-out Torque or Breakdown Torque
D : Synchronous Speed (Zero Torque)
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20. MOTOR ACCELERATION
 During Starting - Current is High
• usually about 6 times rated current
• Manufacturers specify a Maximum Starting Time
• Avoid overheating of the motor windings
 Acceleration time depends on
• Motor torque (TM) characteristic
• Load torque (TL) characteristic
• Total Moment of Inertia (JTot) of rotating parts
 Acceleration torque is the difference between TM & TL
T A = (T M - T L ) Nm
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21. MOTOR ACCELERATION
 Acceleration time of a drive system depends on load inertia
sec
( n 2 - n 1
)
T
= J 2 t
p
60
A
d
 Inertia can be calculated using the formula
J = G x D 2
kgm
4
2
 On geared drives Inertia "referred" to the motor shaft
2
(Load Speed ) J = J kgm
2
2
(Motor Speed )
M L
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22. EFFICIENCY OF MOTOR
 Overall Efficiency of a machine .... is a measure of how well it converts
electrical energy into mechanical output energy
 Efficiency roughly depends on:
• Constant losses independent of load
• Load dependent losses mainly copper losses
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23. THERMAL RATING OF MOTORS
 Motor Life depends on the integrity of Insulation
• Mechanical Loads must be within thermal rating
• Duty cycle of the Load: continuous or cyclical
 Temperature in motor windings should not rise to a level which exceeds
the Critical Temperature.
 Classified by standards such as IEC 34.1 and AS 1359.32 based on an
Ambient Temperature of 40OC
Insulation Class E B F H
Max Temperature 1200C 1300C 1550C 1800C
Rated Temp Rise 700C 800C 1000C 1250C
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24. THERMAL RATING OF MOTORS
 Motors are designed with a Thermal Reserve
• Operating continuously at maximum rated temperature
• The life expectancy of the insulation is about 10 years
• Class-B rating, use Class-F insulating materials at higher ambient
temperatures
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25. THERMAL DE-RATING OF MOTORS
 When motors are operated in abnormal conditions:
• need to apply a de-rating factor
 Typical de-rating tables as follows :
Ambient
Output
Temp
% of Rated
Altitude
above Sea
Output
% of Rated
30oC
40oC
45oC
50oC
55oC
60oC
70oC
107 %
100 %
96 %
92 %
87 %
82 %
65 %
1000m
1500m
2000m
2500m
3000m
3500m
4000m
100 %
96 %
92 %
88 %
84 %
80 %
76 %
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26. DO YOU WANT TO KNOW MORE?
If you are interested in further training or information,
please visit:
http://idc-online.com/slideshare
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