1. AC drive – Misc.
AC voltage controller and
Cycloconverter,
PWM control,
Harmonic current & torque
1
2. 1a. AC voltage controller
Example: A 3-phase, AC voltage controller is used to control the speed of a 3-phase, 75-kW, 380-V,
50-Hz,4-pole induction motor driving a high-power ventilation fan. The power factor and the efficiency
of the motor at full-load output power are 0.8 lagging and 70%, respectively. The controller and motor
2
3. are connected in delta as shown in the following diagram. If the motor current is assumed to be
sinusoidal, determine,
(i) the input kVA at full-load output power,
(ii) the rms current of the SCR,
(iii) the peak inverse voltage of the SCR,
(iv) the control range of the firing angle (α),
(v) Sketch the waveforms of the voltage and current of the motor winding if the firing angle α
and the extinction angle β are assumed to be 50o
and 210o
, respectively.
(i) At full-load, kVA134
0.80.7
1075
pfη
P
kVAinputMotor
3
out
=
×
×
=
×
=
(ii)
( )
( )
( )
A83.4
A
2
118
2
I
2
π
π
1
I
2
ωt2sin
2
ωt
π
I
ωtd
2
ωt2cos1
π
I
ωtdωtsin
π
I
ωtdωtsinI2
2π
1
IofvalueRMS
A118
3
204
IcurrentphaseinputMotor
A204
3803
10134
IcurrentlineinputMotor
ph
ph
π
0
2
ph
π
0
2
ph
π
0
2
2
ph
π
0
2
phSCR
ph
3
L
=
=
=
=
−=
−
=
=
=
===
=
×
×
==
∫
∫
∫
(iii) Peak inverse voltage of SCR = √2 x 380 = 537.4 V
(iv)
18036.87:ofrangecontrolThe
36.870.8cos-1
<<∴
==
αα
φ
3
3-phase
AC supply
12. A 1-phase cycloconverter can be constructed by a 3-phase dual converter as shown below. The
converter output voltage is controlled by a closed-loop voltage control. The frequency of the
fundamental in the output wave is equal to frequency of the reference wave.
12
13. 2. PWM control
Why PWM control?
By varying the pulse widths throughout the cycle an improvement in performance is obtained as the
harmonic content of the output waveform is reduced. Emphasis is given usually to the reduction or
elimination of the low-order harmonics (as elimination of particular harmonic content is possible) since
it is both easier and cheaper to filter the higher frequencies.
Production of the pulse-width-modulated waveform is based on the use of a combination of a reference
signal at the required output frequency and a carrier signal. Varying the amplitude of the reference
signal varies the pulse widths and controls the effective amplitude of the output waveform.
Modulation index:
amplitudesignalcarrier
amplitudesignalreference
indexModulation =
Advanced modulation techniques:
Sinusoidal Pulse-Width Modulation (SPWM) is the most commonly used PWM technique, however it
suffers the drawbacks of low fundamental output voltage, some advanced modulation techniques can
offer improved performance.
Trapezoidal modulation
Staircase modulation
Stepped modulation
Harmonic injection modulation
Delta modulation
Example: The gate-signal generation of a multiple PWM inverter is shown in the following diagram. In
the diagram, the reference signal amplitude (Ar) and carrier signal amplitude (Ac) are 5V and 8V
respectively.
(a) Calculate the modulation index.
(b) Sketch the corresponding output voltage waveform.
(a) Modulation index = 6250
8
5
.
A
A
C
r ==
(b) Output voltage for the multiple PWM controlled inverter:
13