Chapter LIII: Converters

The alternating current must change to a direct current in many cases as in railroad work because the induction motor is not so satisfactory as the direct current series motor and the alternating current series motor is slow in coming into general use.

In all kinds of electrolytic work, transformation must be made, and in many cities where the direct current system was started, it is still continued for local distribution, but the large main stations generating alternating currents and frequently located some distance away from the center of distribution have replaced a number of small central stations.

Transformation may be made by any of the following methods:

1. Rotary converters;
2. Motor generator sets;
3. [A]Mercury vapor rectifiers;
4. [9]Electrolytic rectifiers.

[9] NOTE.--Rectifiers are explained in detail in Chapter LIV.

Strictly speaking, _a converter is a revolving apparatus for converting alternating current into direct current or vice versa_; it is usually called a rotary converter and is to be distinguished from the other methods mentioned above.

Broadly, however, a converter may be considered as _any species of apparatus for changing electrical energy from one form into another_.

According to the standardization rules of the A. I. E. E. converters may be classified as:

1. Direct current converters;
2. Synchronous converters;
3. Motor converters;
4. Frequency converters;
5. Rotary phase converters.

~A direct current converter~ converts from a direct current to
a direct current.

~A synchronous converter~ (commonly called a _rotary
converter_) converts from an alternating current to a direct
current.

~A motor converter~ is a combination of an induction motor with
a synchronous converter, the secondary of the former feeding
the armature of the latter with current at some frequency other
than the impressed frequency; that is, it is a synchronous
converter in combination with an induction motor.

~A Frequency Converter~ (preferably called a _frequency
changer_) converts alternating current at one frequency into
alternating current of another frequency with or without a
change in the number of phases or voltages.

~A Rotary Phase Converter~ changes alternating current of one
or more phases into alternating current of a different number
of phases, but of the same frequency.

~Rotary Converters.~--The synchronous or rotary converter consists of a synchronous motor and a direct current generator combined in one machine. It resembles a direct current generator with an unusually large commutator and an auxiliary set of collector rings.

~Ques. In general, how does a rotary converter operate?~

Ans. On the collector ring side it operates as a synchronous motor, while on the commutator side, as a dynamo.

Its design in certain respects is a compromise between
alternating current and direct current practice most noticeably
with respect to the number of poles and speed.

~Ques. Upon what does the speed depend?~

Ans. Since the input side consists of a synchronous motor, the speed is governed by the frequency of the alternating current supplied, and the number of poles.

Fig. 2,034 is a diagram of a ring wound rotary converter. This
style winding is shown to simplify the explanation. In practice
drum wound armatures are used, the operation, however, is the
same.

With this simple machine the following principles can be
demonstrated:

1. If the coil be rotated, alternating currents can be taken
from the collector rings and it is called an alternator.

2. By connecting up the wires from the commutator segments,
a direct current will flow in the external circuit making a
dynamo.

3. Two separate currents can be taken from the armature, one
supplying alternating current and the other direct current;
such a machine is called a _double current generator_.

4. If a direct current be sent in the armature coil through the
commutator, the coil will begin to rotate as in a motor and an
alternating current can be taken out of the collector rings.
Such an arrangement is called an _inverted rotary converter_.

5. If the machine be brought up to synchronous speed by
external means and then supplied with alternating current at
the collector rings, then if the direction of the current
through the armature coil and the pole piece have the proper
magnetic relation, the coil will continue to rotate in
synchronism with the current. A direct current can be taken
from the commutator, and when used thus, the machine is called
simply a _rotary converter_.

~Ques. What is the relation between the impressed alternating pressure and the direct pressure at the commutator?~

Ans. The ratio between the impressed alternating pressure and the direct current pressure given out is theoretically constant, therefore, the direct pressure will always be as 1 to .707 for single phase converters or if the pressure of the machine used above indicate 100 volts at the direct current end, it will indicate 70.7 volts at the alternating current side of the circuit.

~Ques. Name two different classes of converter.~

Ans. Single phase and polyphase.

~Ques. What is the advantage of polyphase converters?~

Ans. In the majority of cases two or three phase converters are used on account of economy of copper in the transmission line.

~Ques. How is the armature of a polyphase converter connected?~

Ans. Similar to that of an alternator with either delta or ~Y~ connections.

Figs. 2,037 to 2,041 show various converter connections between
the collector rings and commutator.

Fig. 2,037 indicates how the armature is tapped for two phase
connections.

Fig. 2,038 shows three phase delta connections, and fig. 2,039
the three phase ~Y~ or star connections.

Six phase delta and ~Y~ connections are frequently used as
shown in fig. 2,040 and fig. 2,041, both of which require
two secondary coils in the transformer, one set of which is
reversed, so as to supply the current in the proper direction.

~Ques. With respect to the wave, what is the relation between the direct and alternating pressures?~

Ans. The direct current voltage will be equal to the crest of the pressure wave while the alternating voltage will depend on the virtual value of the maximum voltage of the wave according to the connections employed.

Table of

Alternating Current and Voltage in Terms of Direct Current

(According to Steinmetz)

------------------------------------------------------------------------
| | DIRECT| SINGLE | TWO | THREE |
| |CURRENT| PHASE | PHASE | PHASE |
|-----------------|-------|-------------|-------------|----------------|
|VOLTS BETWEEN | | | | |
|COLLECTOR RING | 1 | | | |
|AND NEUTRAL POINT| |1/(2√2̅)=.354|1/(2√2̅)=.354| 1/(2√2̅)=.354 |
|-----------------|-------|-------------|-------------|----------------|
|VOLTS BETWEEN | | | | |
| ADJACENT | 1 | 1/√2̅=.707 |½=.5 | √3̅/(2√2̅)=.612|
|COLLECTOR RINGS | | | | |
|-----------------|-------|-------------|-------------|----------------|
| | | | | |
| AMPERES | 1 | √2̅=1.414 | 1/√2̅=.707 | (2√2̅)/3=.943 |
| PER LINE | | | | |
|-----------------|-------|-------------|-------------|----------------|
| | | | | |
|AMPERES BETWEEN | 1 | √2̅=1.414 |½=.5 |(2√2̅)/(3√3̅)=.545|
|ADJACENT LINES | | | | |
------------------------------------------------------------------------

-------------------------------------------------------------
| | SIX | TWELVE | n |
| | PHASE | PHASE | PHASE |
|-----------------|-------------|-----------|---------------|
|VOLTS BETWEEN | | | |
|COLLECTOR RING | | | |
|AND NEUTRAL POINT|1/(2√2̅)=.354|1/(2√2̅)=.354|1/(2√2̅)=.354|
|-----------------|-------------|-----------|---------------|
|VOLTS BETWEEN | | | |
| ADJACENT |1/(2√2̅)=.354| .183 | (SIN(π/n))/√2̅|
|COLLECTOR RINGS | | | |
|-----------------|-------------|-----------|---------------|
| | | | |
| AMPERES | √2̅/3=.472 | .236 |(2√2)̅/n |
| PER LINE | | | |
|-----------------|-------------|-----------|---------------|
| | | | |
|AMPERES BETWEEN | √2̅/3=.472 | .455 |(√2̅SIN(π/n))/n|
|ADJACENT LINES | | | |
-------------------------------------------------------------

In a single phase rotary, the value of the direct pressure is 1
to .707, therefore a rotary which must supply 600 volts direct
current must be supplied by 600 × .707 = 424 volts alternating
current. For three phase rotaries the ratio is 1 to .612, or
in order to produce 600 volts direct current, 600 × .612 = 367
volts on the alternating current side of the rotary is required.

Fig. 2,034 shows a complete diagram of the electrical
connections. A single phase rotary is illustrated so as to
simplify the wiring.

The table of Steinmetz on page 1,464 gives the values of the
alternating volts and amperes in units of direct current.

~Ques. How is the voltage of a rotary varied on the direct current side?~

Ans. Pressure or potential regulators are put in the high tension alternating current circuit and may be regulated by small motors operated from the main switchboard or operated by hand.

~Ques. What is the advantage of unity power factor for rotary converters?~

Ans. It prevents overheating when the rotary is delivering its full load in watts.

~Ques. What greatly influences the power factor of the high tension line?~

Ans. The strength of the magnetic field.

~Ques. Does variation of the field strength materially affect the voltage?~

Ans. No.

Since variation of the field strength does not materially
affect the voltage, by adjusting the resistance in series with
the magnetic circuit, the strength of the field can be changed
and the power factor kept 1 or nearly 1 as different loads are
thrown on and off the rotary.

~Ques. What is the effect of a field too strong or too weak?~

Ans. If too strong, a leading current is produced, and if too weak, the current lags, both of which reduce the power factor and are objectionable.

Usually there is a power factor meter connected up in the main
generating station and one also in the rotary substation, and
it is the duty of the attendant at the substation to maintain
the proper power factor.

~Ques. What is the ordinary range of sizes of rotaries~?

Ans. From 3 kw. to 3,000 kw.

~Ques. What is the general construction of a rotary converter?~

Ans. It is built similar to a dynamo with the addition of suitable collector rings connected to the armature windings at points having the proper phase relations.

Standard rotary converters have been developed for 25 and 60
cycles. The standard railway machines are compound wound, the
series field being designed for a compounding of 600 volts at
no load and full load when supplied from a source of constant
pressure with not more than 10 per cent. resistance drop and
with 20 to 30 per cent. reactance in the circuit. The large
size machines are usually wound for six phase operation.

~Compounding of Rotary Converters.~--Compounding is desirable where the load is variable, such as is the case with interurban railway systems. The purpose of the compounding is to compensate automatically for the drop due to line, transformer, and converter impedance.

On account of the low power factor caused by over compounding, and the fact that substations are customarily connected to the trolley at its nearest point without feeder resistance, over compounding is not recommended. An adjustable shunt to the series field is provided with each machine.

Shunt wound converters are satisfactory for substations in large cities and similar installations where due to the larger number of car units demanding power, the load is more nearly constant.

~Ratio of Conversion.~--The relation between the alternating and direct current voltages varies slightly in different machines, due to differences in design. The best operating conditions exist when the desired direct current voltage is obtained with unity power factor at the converter terminals when loaded.

~Ques. Upon what does the ratio of conversion depend?~

Ans. Upon the number of phases and method of connecting the windings.

For single phase or two phase machines it is 1 to .7; for three
phase, 1 to .612, or six phase, 1 to .7 or 1 to .613 depending
upon the kind of connection used for the transformer.

For example, a two phase rotary receiving alternating current
at 426 volts will deliver direct current at 600 volts, while a
three phase rotary receiving alternating current at 367 volts
will deliver direct current at 600 volts.

~Ques. What difficulty would be encountered if other ratios of conversion than those given above were required?~

Ans. An armature with a single winding could not be used.

It would be necessary to use a machine with two distinct
armature windings or else a motor generator set.

~Ques. What change in voltage is necessary between a converter and the alternator which furnishes the current?~

Ans. The voltage must be reduced to the proper value by a step down transformer.

~Voltage Regulation.~--As the ratio of the alternating to the direct current voltage of a converter is practically constant, means must be provided to compensate for voltage variation due to changes of load in order to maintain the direct current pressure constant.

There are several methods of doing this, as by:

1. Shifting the brushes (objectionable);
2. Split pole method;
3. Regulating pole method;
4. Reactance method;
5. "Multi-tap" transformer method;
6. Synchronous regulator.

~Shifting the Brushes.~--Were it not for the difficulties encountered, this would be a most convenient method of voltage regulation, since by this procedure the direct current voltage may be varied from maximum to zero. It is, however, not practical because of the excessive sparking produced when the brushes are shifted out of the neutral plane.

~Split Pole Method.~--In order to overcome the difficulty encountered in shifting the brushes the split pole method was devised by Woodbridge in which each field pole is split into two or three parts.

The effect of this is the same as shifting the brushes except that no sparking results.

The other part is arranged so that its excitation may be varied, thus shifting the resultant plane of the field with respect to the direct current brushes.

One of these parts is permanently excited and it produces near its edge the fringe of field necessary for sparkless commutation.

~Regulating Pole Method.~--As applied to the rotary converter regulating poles fulfill the same functions as commutating or interpoles (see page 385) on motors and dynamos, that is, they insure sparkless commutation from no load to heavy overloads with a fixed brush position.

The regulating poles are used in order to vary the ratio between the alternating current collector rings and the direct current side without the use of auxiliary apparatus such as induction regulators or dial switches which involve complicated connections and many additional wires. The regulating poles are arranged with suitable connection so that the current through them can be raised, lowered or reversed.

The characteristics of the regulating pole converter being novel, a detailed explanation of the principles involved is given to facilitate a clear understanding of its operation.

Consider a machine with a field structure as shown in fig.
2,056 resembling in appearance a machine with commutating
poles, but with the brushes so set that one of the regulating
poles adds its flux to that of one main pole, cutting the
inductors between two direct current brushes. The regulating
pole is shown with a width equal to 20 per cent. of that of the
main pole.

To obtain definite figures, it will be assumed that the machine
at normal speed, with the main poles excited to normal density,
but with no excitation on the regulating poles, gives 250 volts
direct current pressure. Then with each regulating pole excited
to the same density as the main poles, and with a polarity
corresponding to that of the main pole in the same section
between brushes, the direct current pressure will rise to 300
volts at the same speed, since the total flux cutting the
inductors in one direction between brushes has been increased
20 per cent.

If, on the other hand, the excitation of the regulating poles
be reversed and increased to the same density as that of the
main poles, the direct current pressure will fall to 200
volts, since in this case the regulating poles give a reverse
pressure, that is, a pressure opposing that generated by the
main poles.

Now, if the machine be equipped with collector rings, that is,
if it be a converter, this method of varying the direct current
voltage from 200 to 300 volts does not give nearly as great
a variation of the alternating current voltage; in fact, the
latter voltage will be the same when delivering 200 volts as
when delivering 300 volts direct current pressure, if the field
excitation be the same.

This may be seen by reference to fig. 2,057, which is a diagram
of the alternating current voltage developed in the armature
windings by the two sets of poles.[10]

[10] NOTE.--In the Burnham split pole rotary converter, each pole is divided into only two sections, one larger than the other. A main shunt winding is arranged on the large sections, and a winding for providing the voltage regulation is placed on the other section. When the current is sent through this latter winding in one direction the voltage is raised, when in the other direction the voltage is lowered.

The horizontal line OA represents the alternating current
voltage generated by the main poles, alone, with the
regulating poles unexcited, that is, when delivering 250 volts
direct current pressure.

For a six phase converter OA measures about 180 volts
diametrically, that is, between electrically opposite collector
rings.

If now the regulating poles be excited to full strength,
to bring the direct current pressure up to 300 volts, the
alternating current voltage generated by the regulating poles
will be 90 degrees out of phase with that generated by the main
poles (since they are placed midway between the main poles),
and will be about 40 volts as shown by the line AB.

The resultant alternating current volts across the collector
rings will be represented by the line OB with a value equal to
184.

Again, if the regulating poles be reversed at full strength,
to cut the direct current pressure down to 200 volts, the
alternating current voltage of the main and regulating poles
will be OA and AC respectively, giving the resultant OC equal
to OB with a value of 184 volts. Accordingly, the direct
current pressure may be either 200 or 300 volts with the same
alternating current pressure, and if the main field be kept
constant, the direct current pressure may range between 200 or
300 volts, while the alternating current pressure varies only
between 180 and 184 volts.

The alternating current pressure can be kept constant through
the full range of direct current voltage by changing the main
field so as always to give an equal and opposite flux change to
that of the regulating field. A constant total flux may thus
be obtained equal to the radius of the arc BC, fig. 2,057. In
this case the line OA, representing the main field strength,
will equal OB when the regulating field is not excited, and 250
volts can only be obtained at this adjustment.

This method of operation gives unity power factor with a
constant impressed pressure of 184 volts alternating current
with a range of direct current voltage from 200 to 300 volts.

~Ques. Where should the regulating poles be located for best results?~

Ans. A better construction is obtained by placing them closer to the corresponding main pole, as in fig. 2,060, than when spaced midway between the main poles as in fig. 2,056.

~Ques. When the regulating poles are spaced as in fig. 2,060, what is the effect on the direct current voltage?~

Ans. The effect is the same as for the midway position (fig. 2,056) except for magnetic leakage from the main poles to the regulating poles when the latter is opposed to the former, that is, when the direct current voltage is being depressed.

~Ques. What is the effect on the alternating current voltage?~

Ans. It is somewhat altered as explained in figs. 2,058 and 2,059.

~Reactance Method.~--This consists in inserting inductance in the supply circuit and running the load current through a few turns around the field cores. This method is sometimes called _compounding_, and as it is automatic it is generally used where there is a rapidly fluctuating load.

If a lagging current be passed through an inductance, the collector ring voltage will be lowered, but will be raised in case of a leading current. The degree of excitation governs the change in the phase of the current to the converter, the excitation, in turn, being regulated by the load current. Accordingly with series inductance, the effect of the series coils on the field of the converter is quite similar to that of the compounding of the ordinary railway dynamo.

~Multi-tap Transformer Method.~--The employment of a variable ratio step down transformer for voltage regulation is a non-automatic method of control and, accordingly, is not desirable except in cases where the load is fairly constant over considerable periods of time. It requires no special explanation.

~Synchronous Booster Method.~--This consists of combining with the converter a revolving armature alternator having the same number of poles.

~Ques. How is the winding of the booster alternator armature connected?~

Ans. It is connected in series with the input circuits on the converter.

~Ques. How are the field windings connected?~

Ans. They are either fed with current regulated by means of a motor operated field circuit rheostat, or joined in series with the commutator leads of the converter.

~Ques. For what service is the synchronous booster method desirable?~

Ans. For any application where a relatively wide variation in direct current voltage is necessary.

It is particularly desirable for serving incandescent lighting
systems where considerable voltage variation is required for
the compensation of drop in long feeders, for operation in
parallel with storage batteries and for electrolytic work where
extreme variations in voltage are required by changes in the
resistance of the electrolytic cells.

~Motor Generator Sets.~--The ordinary rotary converter is the most economical machine for converting alternating currents into direct currents, and where slight variations in the direct current voltage is necessary, they are mostly used on account of their high efficiency, and because they are compact.

In many central stations where they supply a great variety of apparatus, the motor generator sets are employed as the generator is independent of the alternating current line voltage and any degree of voltage regulation can be performed.

~Motor Generator Combinations.~--The following combinations of motor generators are made and used to suit local conditions:

Synchronous motor dynamo
Induction motor dynamo
Direct current motor dynamo
Direct current motor alternator
Synchronous motor alternator
Induction motor alternator

Standard practice has adopted high tension alternating current for transmission systems, but direct current distribution is very frequently used. This is particularly true where alternating current apparatus has been introduced in old direct current lighting systems.

The synchronous motor or the induction motor connected to a generator stands next in importance to the rotary converter because it is easy to operate and the pressure may be changed by a rheostat placed in the field circuit of the generator.

The line wires carrying full voltage can usually be connected direct to the motor and thus do away with the necessary step-down transformer required by the rotary.

~Ques. What is the behavior of a rotary converter when hunting?~

Ans. It is liable to flash over at the direct current brushes, which is common in high frequency converters where there are a great number of poles and the brushes are necessarily spaced close together around the commutator.

~Ques. Is this fault so pronounced with motor generator sets?~

Ans. The motor generator operating on a high frequency circuit, the generator can be designed with a few poles and the brushes set far apart which will greatly reduce the chance of flashing over.

A synchronous motor will drive a generator at a constant speed
during changes in load on it, and by having a field regulating
resistance it can be used to improve the power factor of the
system.

When an induction motor is used its speed drops off slowly
as the load comes on the generator, and it is necessary to
regulate the voltage of the generator by means of a field
rheostat, or compound wound machines may be used.

While an induction motor requires no separate excitation of the
field magnets like the synchronous motor, its effect on the
power factor of the system is undesirable.

Although it is seldom necessary to convert direct current to
alternating, such an arrangement of a direct current motor
driving an alternator is often justified in place of an
inverted rotary converter, as in this case the alternating
current voltage can be changed independent of the direct
current voltage.

The racing of an inverted rotary under a heavy inductive load
or short circuit does not take place in motor generator set
mentioned above.

~Frequency Changing Sets.~--A frequency of 25 cycles is generally used on railway work and in large cities using the Edison three wire system, and as a 25 cycle current is not desirable for electric lighting it is necessary to change it to 60 cycles by means of a frequency changer shown in fig. 2,069 for distribution in the outlying districts.

The two machines in this combination are of the same construction, only the synchronous motor would have eight poles and have the 25 cycle current passing through it, while the generator would have 20 poles and produce 62½ cycles per second at 300 revolutions per minute. By supplying the motor with 24 cycles, the generator would produce 60 cycles.

It will be seen from the figure that the separate exciter is fastened on the base plate and has its armature directly connected to the shaft.

~Parallel Operation of Frequency Changers.~--It is very difficult to construct two or more frequency changers and join them to synchronous motors so that the current wave of one machine will be in phase with the other, since the speed of the motor will depend on the frequency of the line and be independent of the load thrown on it.

When alternators are run in parallel, if one machine lag behind, the other carries the load with the result that the lightly loaded machine will speed up and get in step with the other, or in other words a synchronizing current will flow between the two alternators and tend to keep them in proper relation with respect to phase and load.

~Cascade Converter.~--This piece of apparatus was introduced by Arnold and La Cour. Briefly, it consists of a combination of an induction motor having a wound armature and a dynamo, the armatures being placed on the same shaft. The windings are joined in cascade, that is, in series with those of the armature of the induction motor. The line supplies three phase currents at high voltage direct to the field of the induction motor and drives it, generating in it currents at a lower voltage depending on the ratio of the windings.

Part of the current thus generated in the armature passes into the armature of the dynamo and is _converted_ by the commutator into direct current as in a rotary converter, but is also increased by the current induced in the winding of the dynamo armature.

~Ques. At what speed does the machine run?~

Ans. Assuming equal numbers of pole, the armatures rotate at a speed corresponding to one half the circuit frequency.

Thus if the motor have six poles and the frequency be 50, the
rotary field revolves at 50 × 60 ÷ 3 = 1,000 R.P.M. and the
motor will revolve at one-half that speed or 1,000 ÷ 2 = 500
R.P.M.

Since the connections are so arranged that these currents
tend to set up in the armature a revolving field, rotating at
half speed in a sense opposite to that in which the shaft is
rotating at half speed, it follows that by the super-position
of this revolving field upon the revolutions of the machine,
the magnetic effect is equivalent to a rotation of the armature
at whole speed, so that it operates in synchronism, as does the
armature of a rotary converter.

Half the electric input into the motor part is, therefore,
turned into mechanical energy to drive the shaft, the other
half acts inductively on the armature winding, generating
currents therein.

As to the dynamo part it is half generator, receiving
mechanical power by transmission along the shaft to furnish
half its output, and it is half converter, turning the currents
received from the armature into direct current delivered at the
brushes.

~Ques. What action takes place in the motor armature winding?~

Ans. Since it runs at one-half synchronous speed, it generates alternating current of half the supply current frequency, delivering these to the armature of the dynamo.

~Ques. What claim is made for this type of apparatus?~

Ans. The cost is said to be less than a motor generator set, and it is claimed to be self-synchronizing and to require no special starting gear, also to be 2.5 per cent. more efficient than a motor generator.

~Ques. How is the machine started from the high pressure side?~

Ans. The field winding is connected directly to the high pressure leads. The three slip ring brushes are connected with external resistances which are used while starting, the external resistances being gradually cut out of the circuit as the machine comes up to speed (the same as with an ordinary slip ring motor).

~Ques. How does a cascade converter compare with a synchronous converter?~

Ans. It is about equally expensive as the synchronous converter with its necessary bank of transformers, but is about one per cent. less efficient. It is claimed to be more desirable for frequencies above 40 on account of the improved commutation at the low frequency used in the dynamo member. For lower frequencies the synchronous converter is preferable.