InductionMotors 2016 annotated notes- AULA MOTORES INDUÇÃO

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Variable Speed Induction Motor Drives –
Operation from a Voltage Source Converter
Induction Motors
Variable Speed Induction Motor Drives 1
H63EDR
Introduction
V/F Controlled Induction Motor Drives
• Most popular variable drive system currently used by industry;
•Requires power converter which can provide variable
amplitude, variable frequency voltage supply;
•Performance – poor dynamic response, speed droops with
load (slip);
•Control requirements are very simple;
•High performance “vector controlled” induction motor drives
are covered in H64ANA (level 4 module)
•Starting point for the analysis – per phase equivalent circuit;
• See revision notes on Moodle on induction motor basics
and how to derive per phase equivalent circuit
Variable Speed Induction Motor Drives 5
Simplified Per Phase equivalent Circuit
Variable Speed Induction Motor Drives 2
Variable Frequency
• To operate with a good rotor efficiency, 
s must be kept small. 
• Variable frequency operation
 If you want to change wr, you must change we
Variable Speed Induction Motor Drives 3
Pmech = PAG – PSLIP
Pmech = PAG (1-s)
Pslip = sPAG → rotor losses
Why Variable Voltage?
• Consider φG - the airgap flux level.
• φG is controlled by the MMF i.e. by
the magnetising current im.
• iron saturates as im is increased
above its rated value, ie a large
increase in im will only give a small
increase in φG when saturated
Variable Speed Induction Motor Drives 4
Flux
MMF (ampere turns)
Why Variable Voltage?
Torque α φG . ir
•Ideally we want to control φG to be as large as possible.
•As we increase φG beyond its rated value, the iron saturates,
i.e. a large increase in im gives only a small increase in φG
– We get a disproportionate increase in i2R losses due to larger Im
– We get harmonics in the AC current causing extra i2R losses
– Also get extra losses in the iron e.g. hysteresis
•We therefore control φG to be its rated value
– The rated magnetising current is usually much smaller than the
rated rotor current
– This keeps the total stator current (Im and Ir) low and also the
copper losses (I2R losses) low
–At flux levels below rated flux, more total stator current is required for
a given torque, therefore increasing the copper loss and reducing the
maximum torque capability of the drive
Variable Speed Induction Motor Drives 5
Why Variable Voltage?
• Therefore for most applications it is desirable
to operate at rated flux so that the iron loss
and copper loss (at rated current) are
minimum.
• Im must therefore be controlled to be
constant at all frequencies. Noting
• Vm is controlled to be proportional to we.
Variable Speed Induction Motor Drives 6
Voltage Control (High Speed)
• At high values of vm and we, the voltage drop across the stator 
resistance and the stator leakage inductance (Vrl) is very small 
compared to vm and therefore
Vm ≅ Vs the applied stator voltage
• |Vs| is controlled to be proportional to we
 VF Control – applied voltage is proportional to frequency
• Above we (rated), Vs is held at its rated value resulting in field 
weakening
Variable Speed Induction Motor Drives 7
Voltage Control (Low Speed)
• At low values of vm (low speed/frequency)
– vrl becomes significant 
– Vs has to be boosted to maintain constant Vm / we
Variable Speed Induction Motor Drives 8
VF Characteristic
Variable Speed Induction Motor Drives 9
Torque Speed Characteristic of a VF Drive
• A variable speed 
drive only operates 
in the linear region
Variable Speed Induction Motor Drives 10
Torque Speed Characteristic of a VF Drive
Variable Speed Induction Motor Drives 11
Torque
Speed (rpm)
Torque Speed Characteristic of a VF Drive
If we control vs to be 
proportional to we then we 
have constant flux 
operation at most speeds, 
and a series of torque-
speed curves where each 
line represents a different 
applied we.
Variable Speed Induction Motor Drives 12
Open Loop VF Control
•Use the fact that the slip is small at rated torque 
(4% at 10kW, <1% for 100kW motors) and w
r
≈ w
e
Variable Speed Induction Motor Drives 13
VF Characteristic 
from page 9
Open Loop VF Control
• No speed feedback required
• Dynamic response slow – we have to include a 
rate of change limiter for the speed to prevent 
the slip s increasing beyond its rated value
– High slip will cause an overcurrent
• Based on steady state equivalent circuit – does 
not take dynamic response of real motor into 
account
• Speed drops by a small amount with load 
(i.e. up to rated slip frequency)
Variable Speed Induction Motor Drives 14
Closed Loop Speed Control with Slip 
Compensation for a V/F Drive
• Incorporates speed measurement and feedback
• Works on the principle that torque is proportional 
to slip frequency (page 10)
• The derivation of this scheme is based on 
steady-state analysis which is not valid under 
rapidly changing conditions – the dynamic 
performance is poor.
– More complex control schemes based on full 
transient representations (differential equations) have 
been developed. These are known as 'vector control' 
schemes and achieve better performance than the 
equivalent dc drive 
Variable Speed Induction Motor Drives 15
Variable Speed Induction Motor Drives 16
Closed Loop Speed Control with Slip 
Compensation for a V/F Drive
Practical Realization of Variable Voltage Variable 
Frequency Supply
• Uses pulsewidth modulation to create variable AC 
voltage from a fixed DC voltage (the DC link)
– High frequency switching (0.5 – 20kHz)
– Fixed switching frequency (depends on output power)
– Varying duty cycle
• Use diode rectifier to create the DC Link from the AC 
mains
Variable Speed Induction Motor Drives 17
Variable Speed Induction Motor Drives 18
Vax
Practical Realization of Variable Voltage Variable 
Frequency Supply
Consider one leg only – care must be taken not to switch Q1 and 
Q2 simultaneously – a lockout delay is introduced between the 
switching off of 1 device and the switching on of its partner
Variable Speed Induction Motor Drives 19
Sa Sb Ia Vax Conducting Device
On Off + E/2 Q1
On Off - E/2 D1
Off On + -E/2 D2
Off On - -E/2 Q2
Practical Realization of Variable Voltage Variable 
Frequency Supply
Vax
• When the machine is motoring the phase current 
drawn is (roughly) in phase with the phase voltage. 
Note that when D2 or D1 is conducting current flows 
through the DC link capacitor - no current can flow 
'into' the diode bridge. 
Variable Speed Induction Motor Drives 20
Practical Realization of Variable Voltage Variable 
Frequency Supply
Pulsewidth Modulation
• IGBT acts as a switch
– Requires logic signal to tell it to turn on or off
– Requires gate drive circuit to “amplify” the logic 
signal and switch the IGBT
• Need to create pulse train for each IGBT to tell it 
when to switch on and off
– Pulses occur at fixed switching frequency, but the 
pulse widths need to be changed
– Pulsewidth modulation (PWM) creates the pulses 
based on desired amplitude and frequency
Variable Speed Induction Motor Drives 21
Pulsewidth Modulation (Analogue)
Variable Speed Induction Motor Drives 22
Modulation frequency = fm
Carrier frequency = fc
Natural Sampling
Pulsewidth Modulation
• The relationship between the amplitude of 
the modulation wave (variable) and the 
amplitude of the carrierwave (fixed) is called 
the modulation depth (m) where
– m = modulation amplitude / carrier 
amplitude
• Asynchronous PWM occurs when the 
carrier frequency is fixed – typically between 
0.5 kHz and 20 kHz depending on thepower rating of the drive.
• The figure shows a sinusoidal modulation 
wave (10Hz, 0.9 amplitude)) and a 
triangular carrierwave (500Hz, 1.0 
amplitude). 
Variable Speed Induction Motor Drives 23
Voltage Spectrum
• A typical spectrum of VAX (see 
diagram page 18) would look 
like →
• How does the motor work 
with such an awful voltage 
waveform?
• The motor is inductive - its 
impedance at high frequencies 
is high, therefore voltage 
components associated with fc 
give rise to much less current 
than the fundamental. The 
machine reacts predominantly 
to the fundamental 
components.
Variable Speed Induction Motor Drives 24
fc
fm
2fc
Motoring and Regeneration
• To reverse speed, in motoring mode, simply change 
the phase order 
– normally phase a leads phase b lead phase c. 
– To reverse the machine make phase a lead phase c lead 
phase b. 
• If the machine is driven above we by the load, the 
machine will regenerate and the phase current will be 
(roughly) 180° out of phase with the phase voltage. 
– there is a large transfer of energy to the capacitor and VDC
charges up. 
– Something must be done to prevent VDC rising 
uncontrollably
Variable Speed Induction Motor Drives 25
Motoring and Regeneration
• Mean Iinverter +ve when motoring
• Mean Iinverter –ve when regenerating
Variable Speed Induction Motor Drives 26
I inverter
Regenerative Braking
Use a Dynamic Braking 
Resistor
When the regenerative 
current causes the DC 
link capacitor voltage to 
rise above a preset 
value, a power electronic 
switch is activated which 
discharges the capacitor 
through a large resistor. 
The resistor acts as an 
energy dump, and 
regenerative energy is 
dissipated in the resistor 
as heat.
Variable Speed Induction Motor Drives 27
True 4 Quadrant Drive
Variable Speed Induction Motor Drives 28
Conclusions
• The Induction motor is the most popular motor 
for variable speed applications
– Motor is rugged, spark free and requires little 
maintenance
– V/F control is simple to commission, and its poor 
dynamic performance and close “speed holding” are 
acceptable for many applications
– IGBT inverters are now relatively cheap and easy to 
use at powers up to 100kW
• BUT
– Diode bridge draws poor supply current
– High dynamic response requires the added 
complexity of vector control.
Variable Speed Induction Motor Drives 29