380_Cyclo for GMD Errath01_main

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4 The cycloconverter

Figure 7 Basic circuit diagram of the Cycloconverter fed synchronous motor

S = Circuit-breaker i+,i-,u+,u- = Output current and voltageT1, T2 = Converter transformer

n1 = Stator converterie = Excitation currentn2 = Excitation converter

iR, iS, iT = Stator currentsSM = Synchronous motor uR, uS, uT = Stator voltages

A, B = Three-phase bridges connected in antiparallelf1 = Mains frequencyf2 = Frequency of synchronous motor

f1

n1n2

B

A

i-

i+ u+u-

iR

iS

iT

uRuS

uT

3

SMie

T1T2 T1 T1

f2

S


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4.1 Design and principle

The Cycloconverter is a frequency changer which converts a polyphase voltage with the frequency f1 into

a single or polyphase voltage with a different, lower frequency f2. Energy can be transferred in either

direction directly without a D.C. link. Consequently, the Cycloconverter is classified in the group of line-commutated converters.

By virtue of its design, the Cycloconverter consists of reversible, usually suppressed-half Thyristorconverters as known and used for years with D.C. drive. The basic unit is generally a three-phase bridge

with which a three-phase voltage can be converted into a direct voltage. In this way, the converter outputis a positive, rectified voltage in rectifier operation or a negative, rectified voltage in inverter operation. Bymeans of phase-angle control this voltage can be continuously varied from zero to roughly the maximum

phase-to- phase a.c. voltage, both in the positive and the negative polarity.

Figure 8 Operating range of a Cycloconverter (for one phase)

If the output current of such a converter is controlled to obtain a sinusoidal shape with a givenfrequency like it is shown on Figure 9, the arrangement acts as a frequency converter and is called

a Cycloconverter. The reactive power of commutation required for the current transfer between theindividual legs of each bridge is obtained from the power system. Only one of the antiparallelbridges is in operation at a time, so that circulating currents are entirely excluded. When the current

reverses, i.e. when the current commutates to the antiparallel bridge, a short dead time is observedbefore this antiparallel bridge is fired.

Figure 9 Mains and Output voltage and Mains Output frequency of a Cycloconverter in Sinusoidal operation


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Since the output frequency of the Cycloconverter is derived from the main frequency, it must be lower

than the latter. In practice the output frequency f2 can be continuously varied from zero to about

50 percent of the system frequency. The maximum speed attainable by the drive thus amounts to nearlyhalf the synchronous speed referred to the system frequency.

In view of this physical limit, the Cycloconverter is used for low-speed drives typically like Ball Mills. Themaximum attainable speed can be calculated from the following formula:

f1n max < 60 [r/min]

p

where: f1 = power - system frequency [Hz]

p = number of pairs of poles of the motor

4.2 Methods of Controlling the Cycloconverter

Two different modes of operation are used to control the entire speed range (i.e. frequency range) of the

drive:-Sinusoidal operation-Trapezoidal operation

In the sinusoidal mode for the lower speed range, the machine voltages, and therefore the voltages to begenerated by the Cycloconverter, are low. The converters always operate with partial firing angles and the

output voltages retain their mean sinusoidal characteristic. This operating mode is possible as long as theconverters do not attain their natural control limits as the output voltage amplitudes increase (rectifier andinverter in their end position). One drawback is the relatively high reactive power required for control. At

low speeds, this results in a correspondingly low power factor for the mains.To improve the mains power factor the trapezoidal mode is employed in the upper speed range, wherethe Cycloconverter also has to supply higher voltages. This mode also utilizes the static converters more

effectively with respect to the voltage.

In the trapezoidal mode the static converters are operated at their firing limits for as long as possible inthe low-frequency cycle 1/f2, i.e. during operation as a motor with the rectifier in its end position and

during operation as a brake with the inverter in its end position. The control angle only deviates from thisin the area where the polarity of the converter output voltage changes. Since there is no star connectionbetween the machine and the Cycloconverter, the machine voltages still retain their sinusoidal shape.

Figure 10 Mains and Output voltage and Mains Output frequency of a Cycloconverter in Trapezoidal operation

A

B


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The longer the converters are operated with a full firing angle over one cycle of the machine frequency f2,

the better the mains power factor. The slopes of the trapezoidal characteristic do have a certain limit.

If the voltage system of the synchronous machine is to remain controllable, not more than one converter

may ever be operated with the full firing angle at any one time. This condition is satisfied when eachconverter is in its end position for no longer than /3 in a half-cycle of f2. This makes utilization of theCycloconverter more effective with respect to the voltage. In this mode the peak value of the machine

voltage for the fundamental oscillation is 15% higher than the maximum instantaneous value of theconverter output voltage.

4.3 Physical design of the Cycloconverter

In order to operate in harmful dusty areas, the complete Cycloconverter and the auxiliaries of the motor,

such as cooling, and of the ball mill, such as bearing, lubrication etc., are housed in a closed container.The container provides adequate protection against ingress of dust and water. Picture 2 showsCycloconverter and auxiliaries incorporated in a container with IP55 protection degree.

Picture 2 Shows Cycloconverter and auxiliaries incorporated in a container with IP55 protection degree.

An additional and important advantage of this concept is that all necessary components are already

installed, cabled and tested prior to shipment. This means that installation and subsequent commissioningat site requires only the connection of external cables and checking of external signals. As aconsequence only a very short commissioning time is required.


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4.2.4 Efficiency

Efficiency definition of a drive system is only correct if the overall efficiency of the drive system is defined.

The overall efficiency takes into account all individual efficiencies such as those for the transformer, theconverter, the motor, etc.

Gearless Mill Drives are recognized by their generally high efficiency. The following efficiency calculationis based on an operating point of 95 % of the speed.

The values given in table 3 and used for the following efficiency calculation are based primarily onmeasured values.

P output (kW)

Equipment Losses (kW) (shaft power)

Main transformer 69.1

Excitation Transformer 3.3

Cycloconverter 54

Excitation converter 2.3

Motor ventilating and wind age loss 40

Motor core and stray load loss 36

Motor I2R loss 129

Total: 333.7 5613

Table 3 Losses for efficiency calculation

Poutput 5613 [kW]

h = * 100 h = = 94,3 %Poutput +Plosses 5613 + 333.7 [kW]

Efficiency = 94.3 %


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5. Sistem Advantages

A Gearless Drive offers the following intrinsic advantages compared to other drive systems:

No gear box No lubrication system for gear box

Minimum number of mechanical components Minimum number of electrical components It operates virtually wear free

No inching drive Minimum space requirements Variable speed, base condition for process optimization

Minimum maintenance High availability / reliability Minimum spare parts requirements

Smooth start up

Accurate positioning without auxiliary equipment Bi-directional operation possible without any extra cost

Big air gap between the motor and the stator (low sensitivityfor deformations or displacements of the mill body)

continuous air gap monitoring between rotor and stator gives clear indication of critical operating

conditions, allowing timely shut down before damage occurs

Summary

Bosowa Cement experience is a clear breakthrough leading to the optimal lay-out of Cement Grindinginstallation with capacity of 150 T/H capacity and above. The ideal lay-out from technical, operation and

economical point of view added to friendly environmental installation would be the closed grinding circuitincluding Ball mill with Gearless Motor, high efficiency separator and an Electro-filter Precipitator (EP).

Our experience on Cement Grinding configuration with GMD demonstrates highest operational availability

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380_Cyclo for GMD Errath01_main