Microelectromechanical Devices

Topic 10: Cycloconverters

Spring 2004

ECE 8830 - Electric Drives

Introduction

Cycloconverters directly convert acsignals of one frequency (usually linefrequency) to ac signals of variablefrequency. These variable frequency acsignals can then be used to directlycontrol the speed of ac motors.

Thyristor-based cycloconverters aretypically used in low speed, high power(multi-MW) applications for drivinginduction and wound field synchronousmotors.

Phase-Controlled Cycloconverters

The basic principle of cycloconversion isillustrated by the single phase-to-singlephase converter shown below.

$C:\My Documents\Spring 2004\ECE 8830\bose4_3.TIF$

Phase-Controlled Cycloconverters(cont’d)

A positive center-tap thyristor converteris connected in anti-parallel with anegative converter of the same type.This allows current/voltage of eitherpolarity to be controlled in the load.

The waveforms are shown on the nextslide.

Phase-Controlled Cycloconverters(cont’d)

$C:\My Documents\Spring 2004\ECE 8830\bose4_4.TIF$

Phase-Controlled Cycloconverters(cont’d)

An integral half-cycle output wave is createdwhich has a fundamental frequencyf0=(1/n) fi where n is the number of inputhalf-cycles per half-cycle of the output. Thethyristor firing angle can be set to controlthe fundamental component of the outputsignal. Step-up frequency conversion can beachieved by alternately switching highfrequency switching devices (e.g. IGBTs,instead of thyristors) between positive andnegative limits at high frequency to generatecarrier-frequency modulated output.

Phase-Controlled Cycloconverters(cont’d)

3 to single phase conversion can beachieved using either of the dualconverter circuit topologies shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_5.TIF$

$C:\My Documents\Spring 2004\ECE 8830\bose4_6.TIF$

Phase-Controlled Cycloconverters(cont’d)

A Thevenin equivalent circuit for the dualconverter is shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_8.TIF$

Phase-Controlled Cycloconverters(cont’d)

The input and output voltages are adjustedto be equal and the load current can flow ineither direction. Thus,

where Vd0 is the dc output voltage of eachconverter at zero firing angle and p and Nare the input and output firing angles. For a3 half-wave converter Vd0 =0.675VL andVd0 = 1.35VL for the bridge converter (VL isthe rms line voltage).

Phase-Controlled Cycloconverters(cont’d)

Voltage-tracking between the input andoutput voltages is achieved by setting thesum of the firing angles to . Positive ornegative voltage polarity can be achievedas shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_9.TIF$

Phase-Controlled Cycloconverters(cont’d)

A 3 to 3 cycloconverter can beimplemented using 18 thyristors asshown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_11.TIF$

Phase-Controlled Cycloconverters(cont’d)

Each phase group functions as a dualconverter but the firing angle of eachgroup is modulated sinusoidally with 2/3phase angle shift -> 3 balanced voltageat the motor terminal. An inter-groupreactor (IGR) is connected to each phaseto restrict circulating current.

Phase-Controlled Cycloconverters(cont’d)

An output phase wave is achieved bysinusoidal modulation of the thyristorfiring angles.

$C:\My Documents\Spring 2004\ECE 8830\bose4_12.TIF$

Phase-Controlled Cycloconverters(cont’d)

A variable voltage, variable frequencymotor drive signal can be achieved byadjusting the modulation depth and outputfrequency of the converter.

The synthesized output voltage wavecontains complex harmonics which can beadequately filtered out by the machine’sleakage inductance.

Phase-Controlled Cycloconverters(cont’d)

A 3 to 3 bridge cycloconverter(widely used in multi-MW applications)can be implemented using 36 thyristorsas shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_14.TIF$

Phase-Controlled Cycloconverters(cont’d)

The output phase voltage v0 can be writtenas:

where V0 is the rms output voltage and 0is the output angular frequency. We canalso write:

where the modulation factor, mf is givenby:

Phase-Controlled Cycloconverters(cont’d)

From these equations, we can write:

and

Thus for zero output voltage, mf=0 and

P= N= /2. For max. phase voltage,

mf=1 => P=0, N= . See below figure

for P and N values for mf=0.5 and 1.

$C:\My Documents\Spring 2004\ECE 8830\bose4_15.TIF$

Phase-Controlled Cycloconverters(cont’d)

The phase group of a cycloconverter can beoperated in two modes:

1) Circulating current mode

2) Non-circulating current (blocking) mode

In the circulating current mode, the currentcontinuously circulates between the +ve and-ve converters. Although the fundamentaloutput voltage waves of the individualconverters are equal, the harmonics willcause potential difference which will result inshort-circuits without an IGR.

Phase-Controlled Cycloconverters(cont’d)

The equivalent circuit of a phase group withan IGR is shown below.

The inclusion of an IGR leads to self-inducedcirculating current as illustrated in the nextslide.

$C:\My Documents\Spring 2004\ECE 8830\bose4_16.TIF$

Phase-Controlled Cycloconverters(cont’d)

$C:\My Documents\Spring 2004\ECE 8830\bose4_17.TIF$

Phase-Controlled Cycloconverters(cont’d)

At t=0, +ve load current is taken by the +veconverter only (iP=i0).

From 0->/2, rising +ve load current willcreate a +ve voltage drop (vL=Ldi0/dt) in theprimary winding of the IGR. This creates a -vevoltage drop in the secondary winding of theIGR -> DN reverse biased.  no currentflow in the -ve converter.

At /2, i0 peaks at Im-> vL=0. After this vLtends to reverse polarity inducing current inthe -ve converter. Voltage across IGR becomesclamped to 0 -> self-induced circulationcurrent between +ve and -ve converters.

Phase-Controlled Cycloconverters(cont’d)

The +ve and -ve converter currents can beexpressed as:

The self-induced circulating current issimply iP-iN.

In practice, the waves will not be puresine waves but include a ripple current.Practical waveforms are shown on thenext slide.

Phase-Controlled Cycloconverters(cont’d)

$C:\My Documents\Spring 2004\ECE 8830\bose4_18.TIF$

Phase-Controlled Cycloconverters(cont’d)

Advantages of circulating current mode ofoperation, over blocking mode include:

Output phase voltage wave has lowerharmonic content than in blocking mode.

Output frequency range is higher.

Control is simple.

Disadvantages

Bulky IGR increases cost and losses.

Circulating current increases losses inthyristors.

Over-design increases cost.

Phase-Controlled Cycloconverters(cont’d)

In the blocking mode of operation, noIGR is used and only one converter isconducting at any time.

Zero current crossing detection can beused to select +ve or -ve converterconduction as shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_19.TIF$

Phase-Controlled Cycloconverters(cont’d)

Since the cycloconverter is usuallyconnected directly to a motor, theharmonics from the converter will inducetorque pulsations and machine heatingresulting in increased machine losses.Also, since the cycloconverter is essentiallya matrix of switches without energystorage (neglecting IGR) Pin=Pout . Thusdistortions in the output voltage waveformreflect back into the line input. See text fora discussion of the load voltage and lineharmonics.

Phase-Controlled Cycloconverters(cont’d)

A major disadvantage of cycloconvertersis poor DPF (displacement power factor).To calculate DPF, consider a phase groupof an 18-thyristor cycloconverter shownbelow.

$C:\My Documents\Spring 2004\ECE 8830\bose4_25a.TIF$

Phase-Controlled Cycloconverters(cont’d)

Assume the +ve converter is operating incontinuous conduction and is connected to ahigh inductance load and assume that thecycloconverter is operating at low frequency.Segments of the output current and voltagewaves are as shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_25b.TIF$

Phase-Controlled Cycloconverters(cont’d)

The Fourier series of the line current isgiven by:

where i0 is the load current and  is thesupply frequency. The current wave has adc component and a fundamentalcomponent with a lagging phase angle, P.

Phase-Controlled Cycloconverters(cont’d)

Since the supply’s active and reactivepower components are contributed only bythe fundamental current, the instantaneousactive Pi’ and reactive power Qi’ for thepositive converter is given by:

where Vs =rms line voltage.

Phase-Controlled Cycloconverters(cont’d)

These equations can be rewritten as:

If the firing angle is constant, the converteracts as a rectifier and Vd=Vd0cosP and i0=Id.

In a cycloconverter P and i0 vary sinusoidallyand so Pi’ and Qi’ are also modulated. Weneed to average these parameters todetermine loading on the source.

Phase-Controlled Cycloconverters(cont’d)

$C:\My Documents\Spring 2004\ECE 8830\bose4_26.TIF$

Phase-Controlled Cycloconverters(cont’d)

The expression for the average reactivepower 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 outputpower per phase, respectively.

Phase-Controlled Cycloconverters(cont’d)

P0 and Q0 are given by:

and

Since the real output power = real inputpower, we can write:

The input DPF can be expressed as:

DPF = cosi =

Phase-Controlled Cycloconverters(cont’d)

 DPF =

where tan = Q0/Pi (=Q0/P0)

Phase-Controlled Cycloconverters(cont’d)

This equation for DPF applies whenadditional phase groups are added or if a36-thyristor implementation is considered.

mf=1 was assumed in this derivation.For mf 1:

The maximum value of line DPF is 0.843.

Phase-Controlled Cycloconverters(cont’d)

See Bose text pp. 180-184 for methodsto improve DPF.

Phase-Controlled Cycloconverters(cont’d)

The control of a cycloconverter is verycomplex. A typical variable speed constantfrequency (VCSF) system is shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_28.TIF$

Phase-Controlled Cycloconverters(cont’d)

Generator bus with regulated voltage butvariable frequency (1333-2666 Hz) is fedto the cycloconverter phase groups. (Agenerator speed variation of 2:1 isassumed). The dual converter in eachphase group uses a low-pass filter togenerate a sinusoidal signal.

 modulator receives biased cosine wavesfrom generator bus voltage and sinusoidalcontrol signal voltages to generatethyristor firing angles.

Phase-Controlled Cycloconverters(cont’d)

3 sinusoidal control signals are generatedthrough the vector rotator. The feedbackvoltage Vs is generated from the outputphase voltages.

Phase-Controlled Cycloconverters(cont’d)

Details of  modulator are shown below:

$C:\My Documents\Spring 2004\ECE 8830\bose4_29.TIF$

Matrix Converters

These types of cycloconverters use high-frequency, self-controlled ac switches (e.g.IGBTs). A 3 to 3 converter is shownbelow:

$C:\My Documents\Spring 2004\ECE 8830\bose4_36.TIF$

Matrix Converters (cont’d)

A matrix of nine switches where any inputphase can be connected to any outputphase. The switches are controlled byPWM to fabricate an output fundamentalvoltage whose amplitude and frequencycan be varied to control an ac motor.

The output waveform synthesis is shownon the next slide.

Matrix Converters (cont’d)

$C:\My Documents\Spring 2004\ECE 8830\bose4_37.TIF$

Matrix Converters (cont’d)

Matrix converters offer the advantageover thyristor cycloconverters of beingable to produce unity PF line current.

However, compared to PWM voltage-fed converters, the parts count issignificantly higher.

High-Frequency Cycloconverters

See Bose text pp. 186-189