ECE 8830 - Electric Drives

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ECE 8830 - Electric Drives
Topic 10 Cycloconverters
Spring 2004
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Introduction

• Cycloconverters directly convert ac signals of
  one frequency (usually line frequency) to ac
  signals of variable frequency. These variable
  frequency ac signals can then be used to directly
  control the speed of ac motors.
• Thyristor-based cycloconverters are typically
  used in low speed, high power (multi-MW)
  applications for driving induction and wound
  field synchronous motors.

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Phase-Controlled Cycloconverters

• The basic principle of cycloconversion is
  illustrated by the single phase-to-single phase
  converter shown below.

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Phase-Controlled Cycloconverters (contd)

• A positive center-tap thyristor converter is
  connected in anti-parallel with a negative
  converter of the same type. This allows
  current/voltage of either polarity to be
  controlled in the load.
• The waveforms are shown on the next slide.

5
Phase-Controlled Cycloconverters (contd)

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Phase-Controlled Cycloconverters (contd)

• An integral half-cycle output wave is created
  which has a fundamental frequency f0(1/n) fi
  where n is the number of input half-cycles per
  half-cycle of the output. The thyristor firing
  angle can be set to control the fundamental
  component of the output signal. Step-up frequency
  conversion can be achieved by alternately
  switching high frequency switching devices (e.g.
  IGBTs, instead of thyristors) between positive
  and negative limits at high frequency to generate
  carrier-frequency modulated output.

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Phase-Controlled Cycloconverters (contd)

• 3? to single phase conversion can be achieved
  using either of the dual converter circuit
  topologies shown below

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Phase-Controlled Cycloconverters (contd)

• A Thevenin equivalent circuit for the dual
  converter is shown below

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Phase-Controlled Cycloconverters (contd)

• The input and output voltages are adjusted to
  be equal and the load current can flow in either
  direction. Thus,
• where Vd0 is the dc output voltage of each
  converter at zero firing angle and ?p and ?N are
  the input and output firing angles. For a 3?
  half-wave converter Vd0 0.675VL and Vd0
  1.35VL for the bridge converter (VL is the rms
  line voltage).

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Phase-Controlled Cycloconverters (contd)

• Voltage-tracking between the input and output
  voltages is achieved by setting the sum of the
  firing angles to ?. Positive or negative voltage
  polarity can be achieved as shown below

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Phase-Controlled Cycloconverters (contd)

• A 3? to 3? cycloconverter can be implemented
  using 18 thyristors as shown below

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Phase-Controlled Cycloconverters (contd)

• Each phase group functions as a dual converter
  but the firing angle of each group is modulated
  sinusoidally with 2?/3 phase angle shift -gt 3?
  balanced voltage at the motor terminal. An
  inter-group reactor (IGR) is connected to each
  phase to restrict circulating current.

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Phase-Controlled Cycloconverters (contd)

• An output phase wave is achieved by sinusoidal
  modulation of the thyristor firing angles.

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Phase-Controlled Cycloconverters (contd)

• A variable voltage, variable frequency motor
  drive signal can be achieved by adjusting the
  modulation depth and output frequency of the
  converter.
• The synthesized output voltage wave contains
  complex harmonics which can be adequately
  filtered out by the machines leakage inductance.

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Phase-Controlled Cycloconverters (contd)

• A 3? to 3? bridge cycloconverter (widely used
  in multi-MW applications) can be implemented
  using 36 thyristors as shown below

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Phase-Controlled Cycloconverters (contd)

• The output phase voltage v0 can be written as
• where V0 is the rms output voltage and ?0 is
  the output angular frequency. We can also write
• where the modulation factor, mf is given by

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Phase-Controlled Cycloconverters (contd)

• From these equations, we can write
• and
• Thus for zero output voltage, mf0 and
• ?P ?N ?/2. For max. phase voltage,
• mf1 gt ?P0, ?N ?. See below figure
• for ?P and ?N values for mf0.5 and 1.

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Phase-Controlled Cycloconverters (contd)

• The phase group of a cycloconverter can be
  operated in two modes
• 1) Circulating current mode
• 2) Non-circulating current (blocking) mode
• In the circulating current mode, the current
  continuously circulates between the ve and -ve
  converters. Although the fundamental output
  voltage waves of the individual converters are
  equal, the harmonics will cause potential
  difference which will result in short-circuits
  without an IGR.

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Phase-Controlled Cycloconverters (contd)

• The equivalent circuit of a phase group with
  an IGR is shown below.
• The inclusion of an IGR leads to self-induced
  circulating current as illustrated in the next
  slide.

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Phase-Controlled Cycloconverters (contd)

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Phase-Controlled Cycloconverters (contd)

• At t0, ve load current is taken by the ve
  converter only (iPi0).
• From 0-gt?/2, rising ve load current will
  create a ve voltage drop (vLLdi0/dt) in the
  primary winding of the IGR. This creates a -ve
  voltage drop in the secondary winding of the IGR
  -gt DN reverse biased. ? no current flow in
  the -ve converter.
• At ?/2, i0 peaks at Im-gt vL0. After this vL
  tends to reverse polarity inducing current in the
  -ve converter. Voltage across IGR becomes clamped
  to 0 -gt self-induced circulation current between
  ve and -ve converters.

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Phase-Controlled Cycloconverters (contd)

• The ve and -ve converter currents can be
  expressed as
• The self-induced circulating current is simply
  iP-iN.
• In practice, the waves will not be pure sine
  waves but include a ripple current. Practical
  waveforms are shown on the next slide.

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Phase-Controlled Cycloconverters (contd)

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Phase-Controlled Cycloconverters (contd)

• Advantages of circulating current mode of
  operation, over blocking mode include
• Output phase voltage wave has lower harmonic
  content than in blocking mode.
• Output frequency range is higher.
• Control is simple.
• Disadvantages
• Bulky IGR increases cost and losses.
• Circulating current increases losses in
  thyristors.
• Over-design increases cost.

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Phase-Controlled Cycloconverters (contd)

• In the blocking mode of operation, no IGR is
  used and only one converter is conducting at any
  time.
• Zero current crossing detection can be used to
  select ve or -ve converter conduction as shown
  below

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Phase-Controlled Cycloconverters (contd)

• Since the cycloconverter is usually connected
  directly to a motor, the harmonics from the
  converter will induce torque pulsations and
  machine heating resulting in increased machine
  losses. Also, since the cycloconverter is
  essentially a matrix of switches without energy
  storage (neglecting IGR) PinPout . Thus
  distortions in the output voltage waveform
  reflect back into the line input. See text for a
  discussion of the load voltage and line
  harmonics.

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Phase-Controlled Cycloconverters (contd)

• A major disadvantage of cycloconverters is
  poor DPF (displacement power factor). To
  calculate DPF, consider a phase group of an
  18-thyristor cycloconverter shown below.

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Phase-Controlled Cycloconverters (contd)

• Assume the ve converter is operating in
  continuous conduction and is connected to a high
  inductance load and assume that the
  cycloconverter is operating at low frequency.
  Segments of the output current and voltage waves
  are as shown below

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Phase-Controlled Cycloconverters (contd)

• The Fourier series of the line current is given
  by
• where i0 is the load current and ? is the
  supply frequency. The current wave has a dc
  component and a fundamental component with a
  lagging phase angle, ?P.

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Phase-Controlled Cycloconverters (contd)

• Since the supplys active and reactive power
  components are contributed only by the
  fundamental current, the instantaneous active Pi
  and reactive power Qi for the positive converter
  is given by
• where Vs rms line voltage.

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Phase-Controlled Cycloconverters (contd)

• These equations can be rewritten as
• If the firing angle is constant, the converter
  acts as a rectifier and VdVd0cos?P and i0Id.
• In a cycloconverter ?P and i0 vary
  sinusoidally and so Pi and Qi are also
  modulated. We need to average these parameters to
  determine loading on the source.

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Phase-Controlled Cycloconverters (contd)

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Phase-Controlled Cycloconverters (contd)

• The expression for the average reactive power
  contributed by the line, Qi, is given by
• where ? load power factor angle.
• Performing the integration above yields
• where P0, Q0 are the real and reactive output
  power per phase, respectively.

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Phase-Controlled Cycloconverters (contd)

• P0 and Q0 are given by
• and
• Since the real output power real input
  power, we can write
• The input DPF can be expressed as
• DPF cos?i

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Phase-Controlled Cycloconverters (contd)

• ? DPF
• where tan? Q0/Pi (Q0/P0)

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Phase-Controlled Cycloconverters (contd)

• This equation for DPF applies when additional
  phase groups are added or if a 36-thyristor
  implementation is considered.
• mf1 was assumed in this derivation. For
  mf ?1
• The maximum value of line DPF is 0.843.

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Phase-Controlled Cycloconverters (contd)

• See Bose text pp. 180-184 for methods to
  improve DPF.

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Phase-Controlled Cycloconverters (contd)

• The control of a cycloconverter is very
  complex. A typical variable speed constant
  frequency (VCSF) system is shown below

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Phase-Controlled Cycloconverters (contd)

• Generator bus with regulated voltage but
  variable frequency (1333-2666 Hz) is fed to the
  cycloconverter phase groups. (A generator speed
  variation of 21 is assumed). The dual converter
  in each phase group uses a low-pass filter to
  generate a sinusoidal signal.
• ? modulator receives biased cosine waves from
  generator bus voltage and sinusoidal control
  signal voltages to generate thyristor firing
  angles.

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Phase-Controlled Cycloconverters (contd)

• 3? sinusoidal control signals are generated
  through the vector rotator. The feedback voltage
  Vs is generated from the output phase voltages.

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Phase-Controlled Cycloconverters (contd)

• Details of ? modulator are shown below

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Matrix Converters

• These types of cycloconverters use
  high-frequency, self-controlled ac switches (e.g.
  IGBTs). A 3? to 3? converter is shown below

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Matrix Converters (contd)

• A matrix of nine switches where any input
  phase can be connected to any output phase. The
  switches are controlled by PWM to fabricate an
  output fundamental voltage whose amplitude and
  frequency can be varied to control an ac motor.
• The output waveform synthesis is shown on the
  next slide.

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Matrix Converters (contd)

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Matrix Converters (contd)

• Matrix converters offer the advantage over
  thyristor cycloconverters of being able to
  produce unity PF line current.
• However, compared to PWM voltage-fed
  converters, the parts count is significantly
  higher.

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High-Frequency Cycloconverters

• See Bose text pp. 186-189