The Cycloconverter·fed Synchronous Motor

Basic Principle:• Figure shows the basic principle: the stator of the three-phase motor is connected to

the AC system via a three-phase cycloconverter, the input transformer of which has three different secondary sides.

• The motor can be an induction motor. In most cases, however, synchronous motors are preferred. They are more robust because of their wider air gap. The rotor is excited via slip rings by a rectifier.

Mode of Operation

Converter.

• Cycloconverters for large drives are made up of line-commutated controlled six-pulse

rectifier bridges (AC VSRs), the "DC" output of which is sinusoidally controlled.

• They consist of two antiparallel bridges per phase, one for each direction of the generated

AC current. Three single-phase cycloconverters form a three-phase cycloconverter.

• The cycloconverter of each motor phase cuts individual sections out of the 50 Hz

(respectively, 60 Hz) AC supply voltage and recombines them into a new voltage, the

short-term mean value of which has a sinusoidal form.

• This voltage generates a sinusoidal motor current with low harmonic components. The

bridge, which conducts the current, also generates the output voltage.

• The other one is blocked until the polarity of the current changes. Since the voltage is

composed of sections of the AC supply voltage its frequency is limited to about 40% of

this frequency.

• The AC side transformer needs three separated secondary sides with the leakage

inductances mainly concentrated at the secondary sides .

• This is to decouple the commutation loops of the three single-phase cycloconverters.

Cycloconverters are able to deliver output currents ia,b,c with any phase angle

relative to the output voltage ucyc ab.c: Therefore, energy transfer in both directions

is possible, enabling the drive to be operated in the four-quadrant torque speed

mode. As we will see, the line power factor is proportional to the speed.

Motor. • In synchronous motors fed by externally commutated current source converters with

intermediate DC link, the output voltage is generated by the motor and impressed to

the motor-commutated rectifier. The motor, therefore, must be equipped with rotor

damper windings. They reduce the commutation inductance and keep the duration

of the motor commutation short.

• In complete contrast to the foregoing, cycloconverters are line commutated and

generate an output voltage, which is impressed to the stator of the motor.

• The motor must be built without rotor damper windings. Damper windings would

reduce the leakage inductance. The harmonics in the voltage of the cycloconverter-

for which the equivalent circuit of the motor consists (nearly) only of its leakage

inductance would generate increased current harmonics.

• Further harmonics reduction can be achieved with a star connection in

cycloconverter and motor ( as in figure above) the zero-sequence component Uo in

the cycloconverter output voltage "eye a,b,c can be kept away from the motor and

shifted between both star points S, and Sm. There it cannot generate current

harmonics, if the star points are not interconnected.

Ua,b,c = ueye a,b,c - Uo

Uo = 1/3 (Ucyc a + Ueye b + Ueye c)

• To understand the way the drive system works, it is useful to regard the

cycloconverter and the excitation rectifier as current sources which impress their

currents onto the motor by means of their controls.

• The excitation DC current ie = I, generates a magnetic field in the stator which is

turning with the rotor.

• The angular frequency of the rotor is w = 21r' n . zp' where n is the speed and zp the

number of pole pairs.

• The flux linkage of this field is represented in Figure below as a rotating phasor I, Me

Jwt given by the current I, and the stator-rotor coupling inductance M. The phasor is

in line with the excitation field axis.

• The three-phase currents ia ,b,c also form a magnetic field in the stator, which turns

synchronously at the same angular frequency w. The flux linkage of this field is

represented in Figure as a rotating phasor ILe jw' with I= Ief pi' .

• It is determined by the amplitude I of the stator current multiplied with the stator

inductance L and has a phase shift <Pie to the excitation field axis .

• The triangle formed by the flux linkage phasors Ie Mejw' and IL ejwt, and of their

resultant phasor 'ᵠejwl encloses an area, which is proportional to the electro

magnetic torque. That means, the torque T; can be influenced by the currents I, Ie'

and the phase angle <Pie‘.

• The relation between the resultant flux linkage and the stator voltage is given by the

stead y-state phasor equation 2below in which the stator resistors are neglected.

• To obtain good magnetic utilization of the motor and to work under all load conditions

with the lowest possible current in converter and motor, the currents I and I, and the

phase angle <Pie should be combined in such a way, that

• The resultant magnetic field (Ill ) is always kept constant at its nominal value.

• The sta to r current I = lei"''' is a purely ac tive current, the phasor of which is in

line with the voltage phasor !l. = uei"'·' (U: amplitude).

• In this case the electromagnetic torque T, is given by equation.

• This equation follow s from the equation for the electric power P supplied by the

converter

which is transferred to the rotor as below equation. (The losses are neglected .)

Controls.

• The main objective of the controls is speed control by means of torque control. This

has to be achieved with rated stator flux and a purely active stator current in order to

keep converter and motor as small as possible at the given power and speed ratings.

• The simplified basic control structure shown in Figure below should be regarded

rather as an illustration of the control tasks than as a real control structure.

• The output of the superimposed speed controller n is used to derive the set values

for the rotor and stator current controls, which have to act upon the torque. The

rotor current controls have to impress the DC excitation current I, on the rotor via a

controlled rectifier.

• The stator current controls, which are not shown in detail in Figure 7-16, have to

impress the stator ampere turns to the motor with a given intensity (I) and position

relative to the shaft ('Pie) via the cycloconverter.

• Both currents I and I, and the phase angle 'Pie determine the torque Te.

• But they have to be coordinated (e.g., by means of function generators Ie(i) and

'Pie(I)) in such a way that they keep the stator flux constant at its rated value and

that they generate a purely active stator current.

• In this way, with the converters operated as current sources, the torque can be

controlled independently from the speed. High torque at low speed and even at

standstill is easily attainable.

• Mechanical oscillations of the rotor relative to the rotating magnetic field in the

stator, known from synchronous motors directly connected to the AC system,

cannot occur, when impressing the phase angle 'Pie of stator current phasor I ==

Ie}<Pje relative to the excitation field axis of the rotor!

• The controls which are in practical use are very different and sophisticated. But

basically all of them have to follow the foregoing rules.

• The vector control structure in Figure above is excitation field oriented and has

parameter sensitive function generators Ie(I) and 'Pie(I).

• This structure proved to be good enough for high-torque, low-speed drives

with low dynamic requirements.

• For high dynamic requirements, which can also include heavy load torque

peaks and operation in the field-weakening range, this structure needs

additional loops to ensure

• A more direct field orientation of the stator current

• A more direct stator flux control with a less parameter sensitive flux

estimation.

Line Power Factor.

• Keeping the magnetic field at the nominal value means that the amplitude of the

motor voltage is proportional to the frequency.

• Adjusting the motor voltage proportional to the frequency means that the control

angles Q of the three-phase cycloconverter, starting from Q ~ 90° at standstill,

decrease with increasing frequency.

• Therefore, the line power factor is proportional to the frequency . At maximum

frequency the power factor comes close to "1" if the cycloconverter then operates in

the trapezoidal mode with control angles α -> 0°.

Practical Implementations of the System

The following has to be considered following:

• Frequency variable cycloconverters produce frequency variable interharmonics

at the AC system side. Passive LC filters are not well suited to suppress interharmonics

with variable frequencies. If necessary, active filters have to be used for this purpose.

• When feeding two different three-phase winding systems at the same stator

with two three-phase cycloconverters, one has to take into account that the

cycloconverters impress their voltages onto the windings.

Therefore, both three-phase winding systems have to be decoupled magnetically to

such an extent that their voltages do not produce high interacting current harmonics.

Applications

Four-quadrant torque-speed operation at a high power level with high torque at low speed and

at standstill but with a rather low maximum speed are drive requirements for which

cycloconverter-fed synchronous motors are best suited:

• Gearless cement mill drives were the first applications of this kind. The mill tube is driven

from a low-speed wrap around motor with a high number of poles (Figure 7-19).

• Drives for mine hoists with similar high-power ratings became a typical application of this

system.

• Reversing rolling mills are applications with extremely high dynamic requirements for torque

and speed reversal.

• Many ice breakers and some other ships are equipped with diesel generator fed

cycloconverter synchronous motors with power ratings up to about 20 MW per unit.

Thank you