Practical Variable Speed Drives for Instrumentation and Control Systems

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Practical Variable Speed Drives for Instrumentation and Control Systems


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


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


BASIC CONSTRUCTION


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



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)



Flux distribution in a 4 pole motor at any one moment

• Shows the 2-North and 2-South poles


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



• 8 pole motors .... stator flux rotates at 750 rev/min etc Speed of Stator Flux is called Synchronous Speed

minrev/ p/2

60 x f =

pairs-pole

60 x f = no

minrev/ p

120 x f = no


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


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


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 :

Actual rotational speed is

unit-per n

n) - n( = s= Slip

o

o

minrev/ s)- (1n = n o


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


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


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


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)


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


MOTOR PERFORMANCE

The Power Equations are as follows :

• Angle between IS and IR is power factor angle

• Total apparent motor power S is given by

• Active Power P is given by

• Reactive Power Q is given by

kVA jQ + P = S

kW I x V x 3 = P R

kW x I x V x 3 = P S Cos

kVAr I x V x 3 = Q M

kVAr x I x V x 3 = Q S Sin


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

Output Torque proportional to V2

n])X + X s(+ )R + R[(R x V x sx 3

= To

2RS

2RS

R2

M


TORQUE-SPEED CURVE

A : Breakaway Starting TorqueB : Pull-up TorqueC : Pull-out Torque or Breakdown TorqueD : Synchronous Speed (Zero Torque)


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

Nm )T - T( = T LMA


MOTOR ACCELERATION

Acceleration time of a drive system depends on load inertia

Inertia can be calculated using the formula

On geared drives Inertia "referred" to the motor shaft

sec T

)n - n(

60

2 J = t

A

12d

kgm 4

D xG = J 2

2

kgm ) Speed(Motor

) Speed(Load J = J

2

2

2

LM


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


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 1800CRated Temp Rise 700C 800C 1000C 1250C


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


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 :

AmbientTemp

Output% of Rated

Altitudeabove Sea

Output% of Rated

30oC40oC45oC50oC55oC60oC70oC

107 %100 %96 %92 %87 %82 %65 %

1000m1500m2000m2500m3000m3500m4000m

100 %96 %92 %88 %84 %80 %76 %


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Practical Variable Speed Drives for Instrumentation and Control Systems

Engineering

rotor field

speed of stator flux

rotor windingsreduces

second rotor

rotor windings voltage

flux current

frequency of rotor voltage

slip speed