MPA Logo, San Francisco Maritime National Park Association, USS Pampanito, Historic Ships at Hyde Street Pier, Education Programs Maritime Park Association Home Page Maritime Park Association Home Page Events Maritime Park Association Home Page Maritime Park Association Home Page Maritime Park Association Home Page Volunteer Membership Donate Maritime Park Association Home Page USS Pampanito Submarine Historic Ships at Hyde Street Pier Education Programs About Maritime Park Association Home Page Directions to Maritime Jobs at Maritime Facility Rental at Maritime Trustees of the Association Calendar Press Room Store Maritime Map
9
AUTOMATIC A.C. FREQUENCY
AND VOLTAGE CONTROL
 
A. MOTOR GENERATOR SPEED REGULATORS
 
9A1. Automatic motor speed regulator for d.c.-a.c. motor generator set. In order to maintain the frequency of the a.c. output of the d.c.-a.c. motor generator set within closely controlled limits, a speed regulator for the d.c. motor is required. The speed regulator is essentially an automatic, mechanical, governor-operated field rheostat. Its principal elements are a mechanical governor which acts as the sensitive element, and a field rheostat which is in series with the shunt field of the motor to be controlled.

The governor is coupled to the motor shaft and consists of balanced weights or flyballs, rotating about a common shaft. These weights are prevented from flying outward by pressure of the governor spring. As the speed of the motor increases, the pressure of the governor spring is overcome and the flyballs move outward until a balanced condition is again reached between the spring pressure and flyballs. The outward motion of the flyballs and the movement of the governor spring actuate an operating rod which in turn transmits its motion, through a lever and bracket, to the sliding contact bar of the regulator field rheostat.

The field rheostat is of segment assembly (commutator) design. The segments are electrically connected to resistor plates. As the V-shaped carbon contact bar is moved across the segments, the resistance included between the two points of the V contacting the segments is short circuited, thereby increasing or decreasing the strength of the motor shunt field, depending upon the direction in which the contact bar is moved.

The speed at which the regulator operates is principally dependent upon the tension setting of the governor spring. This setting, how ever, cannot be sufficiently accurate to operate the regulator within the desired regulation

  motor speed. To provide a finer adjustment, a range spring that permits adjustment to within 5 percent of normal speed is connected to the actuating lever. This spring, due to its length and the location of its pivot pin, aids the action of the governor spring. It does not, however, exert enough force to interfere with the movement of the flyballs. Normally, the range spring should be set halfway between zero and maximum tension; and further adjustment, to obtain normal motor speed, should be made at the governor control spring.

To reduce hunting and provide stability to the operation of the regulator, an oil dashpot and a droop spring are coupled to the actuating lever. The dashpot consists of an oil-filled cylinder in which a piston operates. It is coupled to the actuating lever by means of a piece of flat spring steel. Whenever the actuating lever moves, the flat spring forces the piston to move, and since the oil tends to retard this movement, the tendency of the actuating lever to hunt is appreciably reduced.

The droop spring is attached to the actuating lever by means of a bracket. The position of the bracket and droop spring in relation to the actuating lever is such that the tension in the spring opposes the tension provided by the governor spring with a varying force, dependent upon the position of the actuating lever. A similar effect would be obtained if the droop spring were omitted, and the governor spring tapered (conical) in cross section. This is the case in many types of governors.

Regulator adjustments should be attempted only after a thorough study of the manufacturer's instruction book which outlines the correct procedures to be followed.

This regulator is shown mounted on the motor shaft on a motor generator set in Figure 9-1.

 
124

This type of governor is also used to regulate the speed of the d.c.-d.c. motor generators used for lighting on some vessels. Speed regulation in this case is used to control the voltage regulation and it becomes therefore a voltage regulator as well.

9A2. Operation. Prior to starting the motor generator set, the operator should make certain that the manual motor field rheostat is at the lowest speed (all resistance cut out), and that the manual generator field rheostat is at the lowest voltage end (all resistances cut in), also that the automatic speed and voltage regulator switches are in the OFF position. The motor

  generator set should then be started and the speed of the set increased, using the manual motor field rheostat, until the frequency of the a.c. output voltage is approximately 62 cycles. The automatic speed regulator switch should then be put in the ON position. This places the automatic, speed regulator in operation. The manual motor field rheostat should now be returned to the lowest speed position.

In stopping, the load should first be removed from the generator, and the automatic speed regulator switch then turned to the OFF position.

9A3. Maintenance. Moving parts of the

Figure 9-1. Speed regulator for lighting motor generator sets and interior communication a.c. motor generator sets.
Figure 9-1. Speed regulator for lighting motor generator sets and interior communication a.c. motor generator sets.
 
125

regulator should be kept free and clean. The contact bar and the contact surface of the segment assembly should be inspected periodically. If the regulator has remained idle for any length of time, or if the contact surfaces have become rough for any reason, polish these surfaces. Use very fine sandpaper (8/0) and finish with crocus cloth. If the contact surfaces are so rough that the sandpaper alone will not produce a smooth finish, use a very fine file such as a contact point file, follow up with a fine-grade sharpening stone, and then finish with sandpaper and crocus cloth. The contact surface across the segments   must be kept absolutely straight. Care should also be taken not to burr the segments while polishing, and no particles should remain in the undercut sections between adjacent segments. Neither the contact bar nor the segment assembly should ever be oiled.

In the event that it becomes necessary to renew any parts of the regulator that might disturb its operating adjustment, the manufacturer's instruction pamphlet should be studied carefully and the adjustment procedure outlined therein followed precisely.

 
B. ROTARY SOLENOID TYPE AUTOMATIC A.C. VOLTAGE REGULATOR
 
9B1. Description. In order to maintain a constant a.c. line voltage in the output of the a.c. generator, a voltage regulator is required. The rotary solenoid type regulator is essentially a solenoid-operated, sliding contact, field rheostat. It consists principally of a sensitive rotary solenoid with a spring balanced plunger, a transformer, a dry disk rectifier, and a field rheostat which is in series with the shunt field of the a.c. generator whose voltage is to be controlled.

The rotary solenoid is energized by means of rectified current obtained from the generator through the transformer and rectifier. The plunger of the solenoid controls the excitation of the generator field by moving a V-shaped sliding contact bar across segments which are connected to the voltage regulator resistors. As the V-shaped contact bar is moved across the segments, the resistance between the two points of the V making contact is short circuited, thereby increasing or decreasing the strength of the generator shunt field, depending upon the direction in which the contact bar is moved.

Any change in the generator a.c. voltage causes a variation in the magnetic field strength of the solenoid and this in turn causes the solenoid plunger to move the sliding contact bar. At the normal voltage output of the generator, the magnetic field is of such strength as is necessary to keep the plunger approximately in midposition.

A spring is attached to the plunger arm. When the circuit is deenergized this spring keeps

  the plunger in its maximum clockwise position. When the circuit is energized, the magnetic field of the solenoid tends to move the plunger in a counterclockwise direction, putting the spring under tension. The spring tension, however, is adjusted so that at normal voltage, or midposition of the plunger, a balanced condition is attained between the tension of the spring and the strength of the solenoid magnetic field.

In the event that the a.c. voltage decreases in the circuit, the following action takes place:

1. The magnetic strength of the solenoid is decreased, allowing the spring to pull the plunger in a clockwise direction.

2. As the plunger moves in a clockwise direction, more resistance is shorted out of the generator field circuit. This increases the generator field strength and raises the voltage.

If, on the other hand, the load voltage rises, an action opposite to that described above takes place.

To reduce hunting in the system, two features are installed. One is a sealed oil dashpot arrangement mounted so that its piston retards any rotary motion of the plunger arm. The other is an electrical circuit consisting of an adjustable resistor so connected across the solenoid main winding that the direction of current ow through the circuit opposes that of the current supplied by the dry disk rectifier. Whenever the terminal voltage of the generator suddenly decreases, the potential across this circuit is momentarily decreased, reducing the effect of the anti-hunt current opposing the main current.

 
126

This results in a less sudden reduction in the strength of the magnetic field of the solenoid than would be obtained if the anti-hunt circuit were not installed. Therefore, the effect of the circuit is to retard any sudden tendency of the plunger to move in a clockwise direction. If the terminal voltage of the generator suddenly increases, the effect of the anti-hunt circuit is to retard the sudden movement of the plunger in a counterclockwise direction.

In addition to the main winding on the solenoid, there is a compound winding. This winding carries a portion of the generator field current and is connected so that its magnetic effect opposes that of the main winding, resulting in a compounding effect. A bypass resistor

  is connected across the winding to serve as a means of adjusting the amount of compounding.

The range of voltage through which the regulator must operate is controlled by adjustment of the range control resistor. To attain approximately normal voltage, the initial adjustment of the range control resistor should be made with the regulator control rheostat in midposition. Final adjustment is then made by positioning the control rheostat.

It is to be noted that the tap on the secondary of the transformer is deliberately made off-center in order to introduce a slight unbalance of the magnetic field of the solenoid in operation and thus reduce the effect of static friction on the plunger.

Figure 9-2. Schematic diagram of I.C. motor generator voltage regulator, rotary solenoid type.
Figure 9-2. Schematic diagram of I.C. motor generator voltage regulator, rotary solenoid type.
 
127

9B2. Operation. After the motor generator set has been started and the frequency of the a.c. voltage regulated as outlined in Section 9A2, the output voltage of the generator should be manually adjusted to its rated value by means of the generator field rheostat. Then, with the automatic regulator control rheostat set approximately in its midposition, the regulator switch should be turned on and the manual generator field rheostat turned to the ALL RESISTANCE OUT position. The regulator now has control of the voltage and final readjustment of the   voltage can be made by means of the regulator control rheostat.

In removing the regulator from control of the generator, the load should first be removed from the generator. The manual generator field rheostat should next be moved to cut all resistance into the generator. The automatic regulator switch may then be turned to OFF.

9B3. Maintenance. The same general procedures as outlined in Section 9A3 should be followed in maintaining this voltage regulator.

 
C. REACTOR TYPE AUTOMATIC VOLTAGE REGULATOR
 
9C1. Description. A reactor type automatic voltage regulator is essentially a series of electrical and magnetic circuits so connected that they automatically control the field current of the a.c. generator to maintain a constant line voltage in the output circuit of the a.c. generator. It consists principally of a number of reactors, transformers, and rectifiers. A schematic diagram of this regulator is shown in Figure 9-3. For simplification the relays and controls are not shown.

When the voltage regulator is not energized, the a.c. voltage can be controlled manually by the generator field rheostat. To put the regulator in operation, the voltage regulator switch is closed. This supplies power to the main power transformer and, through relays, transfers generator field control to the voltage regulator.

The main rectifier, No. 1, is supplied from the winding A on the main power transformer, and from the current transformer. The reactor L2 is connected across the current transformer so that the voltage supplied by this unit will have proper phase relationship to the voltage supplied by the main power transformer. This phase relationship is such that the vector sum of the voltages across the two units is approximately proportional to the field current required by the generator to maintain constant voltage, regardless of power factor.

The saturable reactor L3 is connected in series with the main rectifier supply, and its impedance is automatically adjusted to maintain

  the exact field current required to produce constant voltage. The impedance of the saturable reactor is controlled by the two d.c. exciting coils, B and C. Under normal operation, the magnetizing forces provided by these two coils are approximately equal but in opposite directions. The current in coil C, which tends to saturate the reactor, is supplied by transformer No. 3 and rectifier No. 3, and is proportional to the output voltage of the generator. The current in coil B tends to reduce the saturation of the reactor and is supplied by rectifier No. 2. The power for this rectifier is supplied by the main power transformer windings and is controlled by the saturated reactor L1 and the voltage control.

Since reactor L1 is saturated, a small change in the voltage supplied to the circuit causes a relatively large change in the current flowing through the rectifier and through the reactor coil B. By adjusting the voltage control and the tap on the main power transformer winding, the ratio between the load voltage and the voltage supplied to the reactor L1 can be varied.

Coil B is used to control the degree of saturation in the reactor L3. Coil C is used to provide a base magnetizing force so that the total magnetizing force will be extremely low. This is done because reactor L3 is most sensitive to a change in magnetizing force when the total magnetizing force is nearly zero.

In operation, the voltage control is set so that the load voltage is of the desired magnitude. If the load voltage becomes momentarily too large, the current through reactor L1 is

 
128

increased greatly, and the d.c. winding B reduces the saturation of the saturable reactor. This increases its impedance and thus reduces the field current supplied to the generator, returning the load voltage to its normal value. If the load voltage becomes momentarily too low, the current in reactor L1 decreases greatly, and the load voltage is brought up to its normal value in a similar manner.

When a load is applied to the generator, the increased current in the current transformer supplies a greater voltage to rectifier No. 1, thus instantly increasing the field voltage to the value required to maintain constant load voltage. When the load is decreased, the opposite occurs. To clarify the operating principle of the saturable reactor, Figure 9-4 shows rectifiers No. 2 and No. 3 of Figure 9-3 replaced by batteries. Coil C of the saturable reactor receives a constant potential from one of the batteries while a rheostat inserted in the battery circuit supplying coil B permits voltage to that coil to be varied. Variation of the voltage in coil B controls the impedance of the saturable reactor and this in turn regulates the generator field through

  the main rectifier. The rheostat in the circuit of coil B takes the place of the saturable reactor L1 and the voltage control shown in Figure 9-3.

9C2. Operation. The operating procedure for this voltage regulator is the same as that given for the rotary solenoid type described in Section 9B2.

9C3. Maintenance. The only parts of this regulator that require maintenance are the relays, which are located on the control panel. Contact burning is kept at a minimum by having the relays electrically interlocked to prevent arcing. The contacts are of pure silver and are not affected by blackening. However, if the points become badly pitted, they should be dressed with a fine point file or replaced. The contact gap should be set between 1/8-in. and 3/16-in. The relay bearings should be kept free from dirt to insure satisfactory operation.

The manufacturer's instructions should be studied and followed in replacing or adjusting any parts of the regulator that may require such service.

Figure 9-3. Schematic diagram of I.C. motor generator voltage regulator, reactor type.
Figure 9-3. Schematic diagram of I.C. motor generator voltage regulator, reactor type.
 
129

Figure 9-4. Equivalent schematic diagram of I.C. motor generator voltage regulator, reactor type.
Figure 9-4. Equivalent schematic diagram of I.C. motor generator voltage regulator, reactor type.
Previous chapter
Previous Chapter
Sub Elec. Home Page
Sub Elec. Home Page
Next chapter
Next chapter


Copyright © 2013, Maritime Park Association
All Rights Reserved
Legal Notices and Privacy Policy
Version 1.10, 22 Oct 04