Scr Unit Chapter 3

SCR UNIT

REVISION HISTORY Rev. A B C D E F G H J K L M

Description Initial Release Formatting and content revisions. Added Revision History page and drawing number in footer. Corrected text, table explanation letter, and procedure step numbering. Remove references to Ansaldo Ross Hill on pages 3-15 and 3-28. Correct figure placement on pages 49 through 51. Update figures. Add photo numbers, improve appearance of Figure 3-21, and correct errors in text. Convert to Word 97 format. Add Table of Contents codes. Correct Level 5 and Level 6 styles and errors. Correct Figure 3-13 and accompanying text on page 3-19. Correct Figure 3-1, and note on page 3-7 (change “50 Hz” to “60 Hz”). Edit two images in SCR Removal and Reassembly Procedure for Front and Rear Access, Horizontal Bridge.

REVISION HISTORY PAGE 20605-49 Rev. M

ERO/ECN # ---C23357 C24068 C24670 C25640 C25719 C25990 C28670 C29222 C29718 C29857 C31629

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3-1

SCR UNIT SPECIFICATIONS

OPERATION The SCR Unit rectifies the three-phase AC supply to provide continuously variable DC power to the traction motors. The SCR bridge, which performs the rectification, is isolated by a circuit breaker from the Main AC Bus. The bridge output is assigned to one of the motors via contactors. The contactors are closed in pairs (DC+ and DC-)(refer to Figure 3-1). The contactor logic and the DC voltage level are controlled from the DRILLER'S CONSOLE (refer to Section 4 of this manual). Electronic circuits in the DC Control Module regulate the voltage and current within preset limits. All SCR Units are identical. If one unit is not working, another is usually available to maintain power to the motor. Similarly, the electronic DC Control Modules and SCR cells of the bridge are interchangeable. SCR 1

ELECTRICAL Three Phase AC Input Voltage: 600 VAC Frequency: 60 Hz DC Output Per SCR Unit Current

Zero to current-limit-value Amps continuous at stall throughout maximum voltage. Current Limits

Different limits are selected to allow maximum horsepower and torque to be obtained from the Rig Equipment without exceeding the Manufacturers ratings. Refer to the label on top of each DC Control Module to verify current limit settings. SCR 2

AC BUS

AC BUS

SCR BRIDGE

SCR BRIDGE

MOTOR 20601-34 Rev. B

Figure 3-1. Parallel Connection of SCR Bridges

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3-2 CURRENT LIMITS (CONCLUDED) The circuit breaker connecting the SCR bridge to the Main AC Bus may be replaced by a molded case switch.

constructed from 14-gauge steel. The module has its own heat sink. Size: Weight:

On older units, the circuit breaker has an overcurrent magnetic trip circuit. Newer units use ceramic fuses mounted on the vertical bridge for overcurrent protection. There is also an undervoltage (UV) or shunt trip circuit which is triggered for fuse failure and SCR overtemperature.

4" (10 cM) Wide by 12" (30 cM) Deep by 12" (30 cM) High. 21 Pounds (9.5 KG).

Forced-Ventilation System The ventilation system consists of either two or six air blowers located below the SCR bridge. Air Flow Rating

Current Rating

These values are available on the system one line printer. Overtemperature Rating

The overtemperature switch, associated with each SCR is set for 165°F (74°C) on older units. Newer units are set for 190°F (88°C).

Each bridge requires either one or three motors to power the blowers. Each motor turns two blowers. One is connected at either end of the motor’s shaft. Voltage: Current: Speed:

MECHANICAL Cubicle

600 VAC. ≈1.0 Amp (see blower name plate) 1,765 RPM (see blower nameplate)

Intake Air Filter Rating

The assemblies are mounted within the SCR Cubicle, on the side and door panels and a slide pan located below the blowers. Controls and Indicators These are mounted on the door panel of the SCR Cubicle. DC Control Module Many of the SCR rectification and control electronic circuits are assembled on one printed circuit (PC) card. The PC card One Line Diagram shown in Figure 1-2 (refer to Section 1) are housed in a module

SCR UNIT 20605-49 Rev. M

1,500 cubic meters per minute through each SCR cell.

Filters are mounted on the ventilation openings. They consist of expanded aluminum gauze enclosed in a metal frame. They have an MBS (Master Bureau of Standards) rating of 12 to 15% (sufficient to trap common dust particles). SCR Enclosure The SCR is mounted within a set of enclosures designed to provide insulation and heat transfer, and to dampen mechanical vibration. The entire assembly is called the SCR Enclosure. The innermost assembly consists of the SCR enclosed on either side by aluminum heatsinks. A twoSCR DRIVE SYSTEM TECHNICAL MANUAL

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3-3 bolt clamp is tightened to press the heatsinks into the SCR. Pressure exerted on the SCR is indicated by a gauge located on the top side of the enclosure.

MAINTENANCE This Section contains specific functional tests to assure proper operation of the SCR unit. •

There are two separate tests: one for SCRs and one for SCR Bridges.

•

Perform the appropriate test(s) after repairing or replacing any of the unit assemblies.

•

If the unit under test fails to perform as indicated, refer to troubleshooting later in this section to locate the malfunction.

The SCR bridge can be phased up for testing by setting the MANUAL VOLTAGE SWITCH (Item 5 on Figure 3-2) to ON and rotating the MANUAL VOLTAGE KNOB (Item 4 on Figure 3-2) clockwise. In the ON position, the MANUAL VOLTAGE SWITCH opens all of the assignment contactors so that power is not applied to the DC traction motor. Bridge voltage can be monitored at the SCR VOLTS TEST PINS (Item 7 on Figure 3-2). The voltage ratio is 16:1, such that the SCR VOLTS TEST PINS (Item 7 on Figure 3-2) will represent a bridge voltage of 750 VDC when the SCR VOLTS TEST PINS register 46.8 VDC.

Refer to Troubleshooting for an explanation of the various SCR Unit circuits, and Removal and Repair (found at the end of this section) for identification of the components. See Figure 3-2 for a front panel view of the DC Control Module. The POWER ON LIGHT (Item 1 on Figure 3-2) illuminates when power is applied to the DC Control Module. The TEST SELECT SWITCH (Item 3 on Figure 3-2) permits the operator to check the Contactor (CONT) and Throttle Reference (REF) signals received from the SCR Control Cubicle. The TEST SELECT SWITCH and TEST VOLTMETER (Item 2 on Figure 3-2) are color coded. For example, if the TEST SELECT SWITCH is set to one of the yellow CONT positions, the TEST VOLTMETER needle will deflect to the yellow band to indicate a normal condition.

Item 1. 2. 3. 4. 5. 6. 7. 8.

Description Power ON light Test Voltmeter Test Select Switch Manual Voltage Knob Manual Voltage Switch Zero Throttle Interlock Light SCR Volts Test Pins SCR Amps Test Pins FS-026-19

Figure 3-2. DC Control Module Indicators and Controls

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3-4 1. SCR Unit is ON but not assigned to any of the DC functions.

MAINTENANCE (CONTINUED) An indication of Bridge current can be monitored at the SCR AMPS TEST PINS (Item 8 on Figure 3-2). The current/voltage ratio is 376:1, such that the SCR AMPS TEST PINS represent 1,000 Amps of bridge current for each 2.66 VDC present on the SCR AMPS TEST PINS.

2. SCR Unit is assigned to a function before the throttles are set to ZERO. The light is not illuminated when the throttles are set to ZERO and assigned contactors PULL IN.

The ZERO THROTTLE INTERLOCK LIGHT (Item 6 on Figure 3-2) indicates the operational status of the module circuit which suppresses the SCR bridge Firing Reference. When the ZERO THROTTLE INTERLOCK LIGHT is illuminated, the bridge voltage will be ZERO. It is illuminated under the two following conditions: Table 3-1. SCR Unit Test ACTION

RESULT

PRELIMINARY A. Ensure that SCR Unit to be tested will not be assigned from the Driller's Console.

A. This step is performed for safety purposes.

B. Remove the wire from Terminal 134 of the DC Control Module.

B. This opens the contactors.

C. Energize the AC bus, if required.

C. On DC Control Module, POWER ON and ZERO THROTTLE INTERLOCK lights will illuminate (the ZERO THROTTLE light will extinguish if a DC motor is assigned AND the REFERENCE SIGNAL was zero at that time).

D. Close the SCR circuit breaker.

D. SCR ON light will illuminate and blowers will switch ON.

SCR BRIDGE CHECK A. Set the DC Control Module MANUAL VOLTS SWITCH to ON.

A. There are no measurable results from this step.

Rotate the DC Control Module MANUAL VOLTS KNOB slowly clockwise to MAXIMUM and counter-clockwise to MINIMUM. This should cause the bridge voltage to go from zero to ≈800 VDC. When the SCR bridge is not loaded, the capacitors in the bridge circuitry tend to raise the maximum bridge voltage. The bridge voltage will initially go negative.

SCR UNIT 20605-49 Rev. M

Observe the reading on the DC Voltmeter mounted on the DC Control Module. The unloaded bridge will initially read a negative voltage and then go positive to ≈800 VDC. Voltage Feedback from the Voltage Feedback Board can be monitored at the DC VOLTS terminals located on the front of the DC Control Module. Voltage Feedback should be 46.8 VDC @ 750 VAC.

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3-5 Table 3-1. SCR Unit Test (Concluded) ACTION

RESULT

CONTACTOR AND THROTTLE CHECK A. Trip the SCR Circuit Breaker.

A. This step is performed for safety purposes.

B. Reconnect the wire to DC Control Module Terminal 134.

B. This closes the contactors.

C. Jumper the Circuit Breaker auxiliary contacts TB8-4 to TB8-5.

C. This closes the ASSIGNMENT CONTACTORS without applying power to the motors.

D. Set the Driller's Console ASSIGNMENT SWITCH to various positions. In each position, check the Driller's Console contactor (CONT) and Throttle Reference (REF) signals.

D. Example: Suppose RT is assigned to the SCR Unit in 1 o'clock position. Then RT CONT (pin 129) and RT REF 1 (pin 130) can be checked.

E. Remove the AUXILIARY CONTACT JUMPER when all CONT and REF signals are checked out.

E. There are no measurable results from this step.

Table 3-2. SCR Bridge Test ACTION

RESULT

MODULE POWER SUPPLY CHECK A. Power the Main AC Bus (if necessary).

A. There are no measurable results from this step.

B. Check the voltage present at the following DC Control Module pins:

B. The voltage at each pins should be as follows:

Pin(s)

Voltage

103 through 108

12 VAC

153

+14 VDC

154

-14 VDC

FIRING PULSE CHECK

20601-35 Rev. B

Figure 3-3. SCR Firing Pulse A. Open the SCR Circuit Breaker of the SCR unit under test.

A. This step is performed for safety reasons.

B. Place the MANUAL VOLTAGE SWITCH (Item 10 in Figure 3-2) in the up position.

B. No observerable results are monitored during this step.

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3-6 MAINTENANCE (CONTINUED) Table 3-2. SCR Bridge Test (Concluded) ACTION

RESULT

C. Rotate the MANUAL VOLTAGE KNOB (Item 4 on Figure 3-2) and check the firing pulses to each of the six SCRs with an Oscilloscope.

C. The observed SCR Firing Pulse should be similar to that shown in Figure 3-3. This procedure allows all points in the firing circuit to be monitored without having high voltage present.

When making the checks on the SCR firing pulses: the chassis of the Oscilloscope must be floated, the Oscilloscope ground is connected to the SCR’s Cathode, and the Oscilloscope probe to the SCR’s Gate. FEEDBACK CHECK A. Check the ripple of the Voltage Feedback signal waveform across the SCR VOLTS TEST pins on front of the DC Control Module.

A. The waveform should be similar to that shown in Figure 3-4.

The Voltage Feedback waveform provides an indication of firing of the SCR's. Note that there are six peaks. Each SCR contributes a peak.

GOOD

BAD

BAD

20601-36 Rev. B

Figure 3-4. Voltage Feedback Waveform

SCR UNIT 20605-49 Rev. M

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3-7 Table 3-3. SCR Test ACTION

RESULT This is accomplished by comparing the Current Feedback ripple and SCR firing pulses on a dual-trace Oscilloscope while the SCR Unit is driving a load. During each 60 Hz cycle, there are six peaks in the ripple. Each of the six SCRs in the bridge contributes a peak. If one of the SCRs does not fire, a peak will be missing. If the SCR misfires, one of the peaks will be distorted.

ISOLATE FAULTY SCR

A. To check the six SCRs, connect Channel 2 of the Oscilloscope to the DC Control Module SCR AMPS TEST PINS and connect Channel 1 of the Oscilloscope to the DC Control Module Test Points listed below: DC Control Module Test Point

A. The waveform will look like the one shown in Figure 3-5. The firing pulse that is in sync with the missing or distorted peak is the one going to the faulty SCR (refer to Figure 3-6). In Figure 3-6, the B+ firing pulse lines up with the distorted peak, indicating that the B+ SCR is misfiring.

SCR

TP1

A+

TP2

A-

TP3

B+

TP4

B-

TP5

C+

TP6

C-

Ch. 1 on Test Point 4 (B-)

Ch. 2 on SCR Amps Test Pins

10223-18 Rev. A

Figure 3-6. Comparison of SCR Amps and Firing Sync Waveform

SCR MISFIRING

SCR NOT FIRING 20601-37 Rev. A

Figure 3-5. Current Feedback Waveform

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3-8 MAINTENANCE (CONCLUDED) Table 3-3. SCR Test (Concluded) ACTION

RESULT

SCR RESISTANCE TEST A. Trip the SCR Circuit Breaker.

A. There are no measurable results from this step.

B. Switch Multimeter polarity to + and the scale to R x 10,000.

B. There are no measurable results from this step.

C. Test the Multimeter by connecting the + (red) and – (black) leads.

C. The meter needle will swing to indicate Zero Ohms.

D. Measure resistance across the SCR by touching one Multimeter lead to the AC bus bar and the other Multimeter lead to the DC bus bar.

D. The resistance measured should be in the >10 KΩ range. The SCR is leaking if the measured resistance is less. The charging action of snubber capacitor that is across SCR will be observed (the resistance will not instantaneously be in the >10 KΩ range, but will instead rise over a period of seconds). Let the Multimeter needle stabilize before taking final reading.

E. Measure resistance across the SCR in the reverse direction by swapping the Multimeter leads.

E. Again, the resistance should be >10 KΩ.

AC LEAKAGE TEST A. Reactivate an SCR bridge, but do not assign it.

A. There are no measurable results from this step.

B. Use a Multimeter to check for AC from each leg of the DC Bus to ground.

B. A reading of >50 VAC indicates excess AC leakage. Most likely a snubber network has opened.

C. If excess AC leakage was detected in either Step B or C, remove power to the SCR Bridge and check all components in the snubber networks.

C. Repair/replace any defective components.

TROUBLESHOOTING Troubleshooting consists of locating a malfunctioning component in the SCR Unit. The Troubleshooting Index in the back of this section provides specific instructions.

THEORY OF OPERATION Figures 3-7 and 3-8 are the schematic diagrams of two different SCR Units. The circuits in the units can be grouped as follows: A. B.

SCR Bridge Surge Suppression Circuit

SCR UNIT 20605-49 Rev. M

C. D. E. F.

Ground Detection Circuit Contactor Control Logic DC Control Module Sprocket Slip Circuit

SCR BRIDGE See Figure 3-7 or 3-8. Three-phase AC from the Main AC Bus is applied to the SCR bridge through a circuit breaker. Each AC phase is connected to two SCRs. One SCR feeds the positive AC portion to the +DC Bus and the other SCR feeds the negative AC portion to the -DC Bus. For example, Phase A is connected to the A+ and A-

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3-9 SCRs. The A+ SCR feeds the +DC bus while the A- SCR feeds the -DC bus. The +DC and -DC buses are connected to traction motors via assignment contactors. The SCRs are switched on and off to vary the DC level through firing pulses applied across the GATE and CATHODE terminals of each SCR. The firing pulses are generated in the DC Control Module (refer to Figures 3-7, 3-8, and 3-9). Refer to the Unique Devices section of this manual for a general description of an SCR. CIRCUIT BREAKER The Circuit Breaker is actually a power switch. It has a UV Trip coil and is tripped open automatically if 24 VDC power to the coil is interrupted. The positive terminal of the coil is permanently connected to +14 VDC power. The negative terminal is connected to -14 VDC power through various normally-closed switches which signal the following hazardous conditions:

Emergency Off The UV Trip circuit is also activated by the normally closed EMERGENCY OFF pushbutton on the DRILLER'S CONSOLE. CURRENT FEEDBACK Three Current Transformers (CT1, CT2 and CT3) are used to sense current flowing into the SCR bridge. On PC1 board, the CT signals are rectified and the resulting DC output divided through a resistor circuit. One signal is used to drive the front panel DC AMMETER. The other signal is applied to the DC Control Module as SCR Amps (Pin 131). The SCR Amps signal is 2.66 Volts per 1,000 Amps out of the SCR bridge. RC FILTER A ferrite core is used for each SCR to reduce the rate of change of current (dI/dT) through the SCR. The RC filter (a resistor and capacitor) is designed to reduce the rate of change of voltage (dV/dT) across the SCR. Excessive dV/dT or dI/dT can cause the SCR to misfire or fail.

SCR Overtemperature There are two temperature sensors. One is mounted on the top of each heat sink column. The sensor contact is designed to open when the heatsink temperature exceeds 195°F (91°C). Blown SCR Fuse Fuse protection for the SCR consists of two 600 Amp fuses mounted in parallel. The UV Trip circuit is wired through blown-fuse indicator microswitches.

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3-10

Figure 3-7. SCR Unit Schematic Diagram

SCR UNIT 20605-49 Rev. M

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6281-028-20 Rev. A

3-11

Figure 3-8. SCR Unit Schematic Diagram (Continued)

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3-12

Figure 3-8. SCR Unit Schematic Diagram (Concluded)

SCR UNIT 20605-49 Rev. M

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3-13 FIRING PULSE A pulse transformer isolates SCR voltages from control module firing circuits. The current pulse rises very quickly to ≈1 Amp to HARD FIRE the SCR. It then descends to a 0.5 Amp BACKPORCH (refer to Figure 3-9) to force more and more of the SCR to turn on through a regenerative process.

The star winding of transformer T4 supplies six 12 VAC three-phase voltages to the module (VCA, VCB, VAB, VBA, VBC, and VAC to pins 103 through 108). These signals are used to synchronize firing pulses for the six SCRs and derive ±14 VDC power supplies.

There are two pulses, Main and Backup, which are 60° apart. The Main turns on one SCR. The Backup keeps an oppositepolarity SCR turned on. The previous opposite-polarity SCR allowing conduction was turned on 60° before the SCR turned on by the Main pulse. The Main pulse has a minimum duration of ≈300 mS. The Backup has a minimum duration of ≈200 mS. Main and Backup pulses are separated by 2.77 mS. Main and Backup pulses reoccur every 16.66 mS for each device. VOLTAGE FEEDBACK The +DC and -DC buses are tapped to develop a 16:1 ratio analog of the SCR bridge volts. DC+ and DC- signals from the DC buses dropped through a set of 3.9 KΩ resistors are used to drive the front panel SCR Voltmeter. The other is a differential voltage signal (+Vbr) - (-Vbr). This is applied to the DC Control Module (Pins 101-102) for use in the DC Regulator circuit. When the SCR bridge voltage is 750 VDC, the differential voltage signal is 46.8 VDC.

20601-38 Rev. A

Figure 3-9. Firing Pulse

CONTACTOR POWER SUPPLY Transformer T5 (see Figures 3-7 or 3-8) supplies three-phase 46 VAC, to the PC1 board. On PC1, the 46 VAC is rectified to 60 VDC for contactor power supply.

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3-14 RECTIFIER-TYPE SURGE SUPPRESSION PANEL

MOV-TYPE SURGE SUPPRESSION PANEL

This is an RC circuit which filters transient spikes on the AC bus. Circuit loss does not disable the drive system but does increase the likelihood of damage to the SCR cells (refer to Figure 3-10).

There are a number of operating conditions and problems that may lead to the generation of voltage transients. The energization of a transformer primary, a fuse that blows during a power fault, and switch arcing will all cause voltage transients. These voltage transients can be responsible for damage to circuit components. Most semiconductor devices are not able to withstand voltage transients in excess of their voltage ratings.

The line input is fused and then rectified through a diode bridge. DC output from the bridge charges a capacitor bank to 1,000 VDC. A 25Ω, 225 W resistor limits the charging current to 35 Amps. About 30 mS after power is turned on, relay K1 closes to short out the resistor. The excess charge caused by a spike is discharged through the resistor bank. When power is turned OFF, the capacitors are also discharged through the resistor bank. THE CAPACITOR DISCHARGE TAKES 10 SECONDS. DO NOT TOUCH ANY PART OF THE CIRCUIT DURING THIS PERIOD. The front panel SURGE SUPPRESSION LIGHT is normally illuminated. It will extinguish if any of the lines fuses are blown. The line indicator-type fuses are linked via indicator fuse switches (F4, F5, and F6) to a microswitch (S1) whose normally closed contacts are in series with the SURGE SUPPRESSION LIGHT circuit. When a line fuse blows, it’s indicator button opens the indicator switch, thus turning off the SURGE SUPPRESSION LIGHT.

SCR UNIT 20605-49 Rev. M

Transient protection has been provided by three-phase delta-connected Metal Oxide Varistors (MOVs). An MOV device exhibits high resistance until the terminal voltage exceeds its rated clamping voltage. The resistance of the device then substantially reduces, causing it to pass the voltage transient-induced current to one of the other phases, thus protecting the devices in the circuit (refer to Figure 3-11). The MOV-Type Surge Suppression Panel has been made possible due to the improved devices in the varistor family. The varistor combines the desired characteristics of the ideal voltage clamp provided by solid-state diode devices with the superior energy-absorbing ability of a metal oxide element. This MOV-Type Surge Suppression Panel is made of MOV devices connected line-to-line on the 600 VAC bus in each SCR bridge. The varistors are fused with 60 Amp indicating fuses. A blown fuse is indicated when the green SURGE SUPPRESSION LAMP is extinguished.

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20601-39 Rev. A

3-15

Figure 3-10. Rectifier-Type Surge Suppression Circuit

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3-16 SUPPRESSION

6281-028-20 Rev. A

MOV-TYPE SURGE PANEL (CONCLUDED)

Figure 3-11. MOV-Type Surge Suppression Circuit SCR UNIT 20605-49 Rev. M

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3-17 The SURGE SUPPRESSION LAMP extinguishing may be an indication of a failed MOV device. It is possible that these devices may completely rupture upon failure. They are therefore covered with a safety shield to prevent equipment damage or personnel injury. DO NOT OPERATE THIS PANEL WITHOUT THE SAFETY SHIELD INSTALLED. TO INSPECT THE MOVS, OPEN THE FUSES BEFORE REMOVING THE SAFETY SHIELD. BE SURE TO INSTALL THE SHIELD BEFORE CLOSING THE FUSES TO REAPPLY POWER TO THE MOVS.

The lights will dimly illuminate during normal operation. If there is a DC ground fault, it will complete the circuit through all the phases of the Main AC Bus, so all the three lights will brightly illuminate. A deflection reading on the %DC GROUND METER indicates a ground on one of the DC buses. THE DC GROUND DETECTION CIRCUIT ONLY PROVIDES AN INDICATION OF A DC GROUND. IT DOES NOT CORRECT THE PROBLEM. THE GROUND FAULT MUST BE QUICKLY LOCATED AND CORRECTED TO PREVENT FURTHER DAMAGE TO THE EQUIPMENT.

PERIODIC MAINTENANCE CONTACTOR CONTROL LOGIC It is unlikely that the MOVs protection will be downgraded. It is recommended that the MOVs be inspected on a periodic basis (every three months) for loose leads and evidence of overheating. Ensure that you observe the precaution shown just above this subsection (PERIODIC MAINTENANCE) before performing the inspections. DC GROUND FAULT DETECTION CIRCUIT This circuit (refer to Figure 3-12) consists of three lights. Each light is connected on one side to a phase of the AC bus and grounded on the other. A series-connected meter indicates the percentage of the fault.

The bridge output is assigned to one of several traction motors by closing the appropriate contactors. The contactor logic is set through the DRILLER'S CONSOLE ASSIGNMENT SWITCH. Single-pole contactors are used to assign motors which turn in only one direction. For reversing motors, the outputs of the singlepole contactors are applied to the motor armature via a double-pole contactor. The double-pole contactor reverses the armature leads to reverse the motor direction. The power contactor coils require 74 VDC to energize. The positive terminal of all coils is connected to +60 VDC. -14 VDC is connected to the negative terminals of the coils by a number of wired-in-series contacts. These monitor and ensure that all conditions are satisfactory to power the assigned motor. If any of the contacts in this control logic open, the power contactors trip and the SCR bridge is phased down.

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3-18 CONTACTOR CONTROL LOGIC (CONCLUDED)

0601-40 Rev. A

Figure 3-12. Ground Fault Detection Circuit During the following discussion, refer to Figure 3-13 (a typical Rotary Table Control circuit). 1. The Rotary Table is assigned to SCR 3 at the DRILLER'S CONSOLE ASSIGNMENT SWITCH. 2. The Rotary Table FORWARD/REVERSE SWITCH is set to REVERSE. Refer to the assignment switch configuration at the top-right corner of the schematic. The rotary table can be run on either SCR 2, 3, or 4. The Rotary Table motor connections are at the bottom-left corner of the schematic. SCR 3 bridge is connected to the Rotary Table motor through single-pole contactors K1 and K6 and reversing contactor K5. SCR UNIT 20605-49 Rev. M

The coil connections for K1, K5 and K6 are shown in the middle of the circuit. The positive terminals of all the coils are connected to +60 VDC. Trace the -14 VDC control signal. The -14 VDC power supply in the DC Control Module is first passed through the normallyclosed MANUAL VOLTAGE SWITCH. This switch is placed in the TEST position to phase up the SCR bridge without applying power to the traction motors. The signal emerges from Pin 134 of the DC Control Module as CONT PS. It is routed through a normally-open auxiliary contact of the SCR 3 circuit breaker. This normallyopen auxiliary contact closes when the circuit breaker is closed, thereby assuring that the SCR Unit is turned on. SCR DRIVE SYSTEM TECHNICAL MANUAL

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3-19 The control signal is then sent to the DRILLER'S CONSOLE where it turns on the SCR 3 ON light. The control signal is also connected to one side of the DRILLER'S CONSOLE ASSIGNMENT SWITCH. The control signal emerges on the other side at the 3 o'clock contact since the DRILLER'S CONSOLE ASSIGNMENT SWITCH is set to the 3 o'clock position. The contactor signal now passes through the contacts of the RT vane switch relay RL2. A vane switch was not installed in the Rotary Table equipment; therefore, a jumper was installed so that RL2 will always be energized. If a vane switch is installed in the Rotary Table sometime in the future, the jumper can be removed. The signal next passes through REV contact of the ROTARY TABLE LOCKOUT SWITCH energizing the coil of contactor K5.

The control signal is then routed through all the normally-closed auxiliary contacts of the power contactors in the SCR 3 Unit other than K1, K5 and K6. This assures that the bridge output is not connected to two motors at one time. At this point in time, the coils of K1 and K6 are energized. To ensure that their contacts have closed, the control signal is passed through the normally-open auxiliary contacts of K1 and K6. The control signal is then returned to the DC Control Module as RT CONT (Pin 129). In the DC Control Module, the reference for the SCR firing circuits is disabled as long as all the CONT signals (RT CONT, MP1 CONT, etc.) are not -14 VDC. The Rotary Table reference and Current Limit signals which originate in the DRILLER'S CONSOLE are also shown in Figure 3-13.

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6222-012-01 Rev. C

3-20

Figure 3-13. Typical Rotary Table Control Circuit

SCR UNIT 20605-49 Rev. M

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3-21

10197-22 Rev. A

Figure 3-14. DC Regulator Circuit

DC CONTROL MODULE

DC REGULATOR

The DC Control Module (refer to Figure 314) contains electronic circuits for the SCR Unit. These can be grouped into three assemblies:

The DC REGULATOR is a feedback control circuit which automatically matches the motor speed and torque to the throttle command from the Control Console in command (Mud Pump or Driller’s). The DC REGULATOR circuit is shown in Figure 314.

1. DC Regulator 2. SCR Firing Circuits 3. DW Dynamic Brake Figure 3-14 shows the DC Control Module block diagram and pin designations.

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3-22 DC REGULATOR (CONCLUDED) The DC REGULATOR output is a FIRING REFERENCE (TP7) to the SCR firing circuits. Inputs to the circuit consist primarily of the SPEED REFERENCE, SPEED FEEDBACK, and CURRENT FEEDBACK. The regulator consists of two control loops, an outer VOLTAGE LOOP (Speed) and an inner CURRENT LOOP (Torque). The SPEED REFERENCE signal from the Control Console is summed with the SPEED FEEDBACK to derive a CURRENT COMMAND signal which, in turn, is summed with the CURRENT FEEDBACK signal to derive the FIRING REFERENCE signal. DC FOOT THROTTLE OPERATION The Drawworks has a Foot Throttle to allow quick response to the SCR bridge during tripping (refer to Figure 3-14). The DW Foot Throttle reference signal (pin 114) is applied directly to the Current Limit Summing Junction, thus skipping the Speed Feedback Junction. It, therefore, it acts as a Current Command. When the Driller presses the Foot Throttle, current to the Drawworks motors rises quickly and the voltage follows. The DW Speed Reference from the Driller's Console Hand Throttle (pin 117) and the DW Foot Throttle Reference are auctioneered through D10 and D59 to select the greater value (more negative) throttle command. When the Driller begins a tripping operation, he first sets the Drawworks to cathead speed by slightly cracking (moving) the Driller’s Console Hand Throttle. At this time, the Speed Reference and Foot Throttle are enabled. When the Driller presses the Foot Throttle to lift a heavy load, the Foot Throttle Reference quickly supersedes the DW Speed Reference. The Foot Throttle Reference goes to Zero when the Driller removes pressure from the Foot Throttle. SCR UNIT 20605-49 Rev. M

As a result, the DW Speed Reference is enabled, and the motor speed and torque return to cathead values. MANUAL OPERATION During testing, it would often be convenient if it were possible to phase up the SCR bridge without applying power to the motor. A Manual Operation circuit makes this feasible. When the MANUAL VOLTS switch (S1) is set to the ON position, the -14 VDC CONT Power Supply to the ASSIGNMENT CONTACTOR logic is disconnected and the MANUAL VOLTAGE rheostat is connected into the regulator circuit. The bridge can now be phased up by rotating the MANUAL VOLTAGE rheostat knob clockwise. Power is not applied to the traction motors since the ASSIGNMENT CONTACTORS remain open. These circuits generate firing pulses for the SCR bridge (refer to Figure 3-14). CURRENT FEEDBACK This is an analog of motor torque (torque is directly proportional to the armature current). SPEED REFERENCE This signal originates in the Control Console in command (Mud Pump or Driller’s). These consoles are equipped with handwheels which the operator rotates to control the traction motor speed. Each handwheel is linked to a rheostat which outputs a Zero to -8 VDC Speed Reference signal for Zero (zero speed) to 8 VDC (maximum speed). The regulator may receive the Speed Reference signal from more than one location. SCR DRIVE SYSTEM TECHNICAL MANUAL

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3-23 SCR FIRING CIRCUITS These circuits generate firing pulses for the SCR bridge (refer to Figure 3-14).

Interlock circuit is closed, the selected function (i.e., Drawworks, Mus Pump, etc.) will be able to run. ZERO THROTTLE INTERLOCK

There are six identical firing circuits, one for each SCR. The Firing Pulse waveform actually consists of two pulses, a main pulse followed by a backup pulse. The backup pulse is essential for refiring the SCR at low DC output when current is not continuous. The time difference between the main and backup pulses is constant. The main pulse is synchronized with one of the six phase signals from the AC bus (VAB, VBC, etc.) and Firing Reference from the DC Regulator. The backup pulse is synchronized with a main pulse signal from one of the remaining firing circuits. SPEED FEEDBACK This is an analog of the motor speed. It’s range is zero (no feedback) to +5 VDC (maximum feedback) at 1,000 RPM. In a shunt motor, the speed is directly proportional to the armature voltage. Therefore, the differential voltage feedback signals (VBR+) - (VBR-) are simply compared to Op Amp Z701 to derive the single level 'N' (refer to Figure 3-10). In a series motor, speed is a function of the armature voltage divided by the magnetic flux. The flux, in turn, is a function of the armature current. N (the symbol for speed inconventional motor speed equations) for a series motor is obtained by dividing the Voltage Feedback signal by the shaped Current Feedback signal in Z703. CONTACTOR INTERLOCK There are several Contactor Interlock circuits (i.e., Drawworks, Mud Pump, etc.). Each is completely separate. When all the interlocks on a particular Contactor

This circuit protects the SCR bridge and the traction motor from unintended starts. It disables the Firing Reference if the CONT signal switches to -14 VDC while the corresponding Speed Reference is also great (negative). Thus, the Driller must set the throttle to ZERO before switching the assignment. CURRENT LIMIT This signal prevents the Speed Reference signal from demanding excess current. It is simply a negative current flow produced by applying -10 VDC across a selected resistor. To lower the Current Limit, the negative current flow is decreased by selecting a resistor with higher value. If the current limit desired is 1,000 Amps, the resistor selected will be ≈390 KΩ. SPEED LIMIT This signal prevents the Speed Reference from demanding excess speed. It is particularly useful for series motors. In shunt motors, an adequate field current prevents overspeeding. POWER LIMIT This signal prevents the Current Command from demanding excessive power, and thus overloading the engines. It is effective at about 90% to 95% of the engine-generator capacity on line. In other assemblies, the Power Limit signal is derived by processing the KVA Feedback and KW Feedback from all generators connected to the Main AC Bus.

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3-24 THREE-PHASE DIODE BRIDGE RECTIFIER

SCR OPERATION An SCR conducts (turns on) when two conditions are satisfied: •

When it is forward biased.

•

It’s Gate terminal is FIRED with a current pulse. This pulse is just that, a pulse - it is not a constant voltage which remains applied to the Gate terminal.

If the Gate terminal is fired as soon as the SCR is forward biased, the SCR is working just like an ordinary diode. The SCR Gate terminal firing is delayed to vary the DC output. In this SCR firing circuit: •

The six-phase Reference will indicate when the SCR is forward biased.

•

The Firing Reference will indicate when the SCR should be fired to achieve the desired DC output level.

A. SINGLE-PHASE HALF-WAVE RECTIFIER

Before discussing a three-phase diode bridge rectifier, let us review several simple one-phase rectifiers. The most basic is a single-phase half-wave rectifier (refer to Figure 3-15). This consists of a diode mounted in-line with a one phase AC input. The output is a pulsating DC voltage that varies between zero and maximum voltage. The diode conducts for 180°. For 180° (when the diode is not conducting), the output will be zero. During the other 180°, the output will rise from zero to maximum and then back to zero. A single-phase diode full-wave rectifier bridge (refer to Figure 3-15) has four diodes. Each of the AC input lines has two diodes connected to it. Each of these diodes is connected to a different DC output line. The diodes turn on and off automatically as the bias changes. This process is called commutation.

B. SINGLE-PHASE FULL-WAVE RECTIFIER BRIDGE 20601-41 Rev. A

Figure 3-15. Single-Phase Half-Wave Rectifier and Full-Wave Bridge Rectifier SCR UNIT 20605-49 Rev. M

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3-25 The left-most part of Figure 3-16 shows a three-phase diode bridge rectifier and several input waveforms. Compare the circuit of the singlephase full-wave bridge rectifier to the three-phase bridge rectifier. The only major difference is the addition of the two diodes for the C phase and the C phase winding. If these are disregarded and the A and B phase voltages are summed, the two rectifier bridges are (for all intents and purposes) the same. In a three-phase diode bridge rectifier, things get a little more interesting. Even though the diode is capable of conducting for 180°, it will only do so for 120°. This is because the diode connected to a second AC phase becomes more positively biased than the diode connected to the first phase. At this time, the diode connected to the first phase stops conducting and the diode connected to second phase starts conducting. This process happens on both output voltage legs of the rectifier. The only difference is that the events on the two output voltage legs occur 60° apart. Look at the top waveforms (VA, VB, and VC). Anytime VA is +VAC, the A+ diode may conduct. Anytime VA is -VAC, the A- diode may conduct. However, the diode will not conduct until 60° after it can start conducting because a diode connected to another phase has a larger potential voltage. Once the + diode starts conducting, it will be paired with a - diode on the output DC voltage leg for 60°. At that point, a - diode on another phase will start conducting and the first - diode will stop. VL will thus be the output of diodes A+ and Bfor 60°, then diodes A+ and C- for 60°, then diodes B+ and C- for 60°, etc. Note that each diode conducts for 120°.

The six-diode bridge waveform shows the commutation process through a single 60 Hz three-phase cycle. The 360° cycle is divided into vertically-divided 30° sections (refer to waveform A in Figure 3-16). Observe that between 30° to 150°, the A+ diode is more positively biased than either B+ or C+ and is therefore conducting. Similarly, the C- diode is more negatively biased than either A- or B- during 90° to 210° and is therefore conducting. Each diode conducts for 120° and is turned off for 240°. Table 3-4 gives the commutation sequence. Table 3-4. Diode Commutation DEGREES

TURNS ON

TURNS OFF

30

A+

C+

90

C-

B-

150

B+

A+

210

A-

C-

270

C+

B+

330

B-

A-

SCR RECTIFIER BRIDGE Figure 3-16 also shows a three-phase sixSCR rectifier bridge. In an SCR rectifier bridge, commutation does not occur automatically. It must be forced through firing pulses. Observe that SCR A+ is forward biased between 30° and 150°. All the SCRs are forward biased for current flow during 120° of each AC cycle. The SCR can be fired (conduction started) anytime during this 120° period. This 120° period is defined as the range of the firing angle (α).

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3-26 SCR RECTIFIER BRIDGE (CONCLUDED)

DIODE BRIDGE

A. DIODE BRIDGE

SCR BRIDGE

B. SCR WAVEFORMS

= 0°

C. SCR WAVEFORMS

D. SCR WAVEFORMS

= 90°

= 60°

20601-42 Rev. A

Figure 3-16. SCR Bridge Operation Waveforms

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3-27 When α = 0° (waveform B in Figure 3-16), the SCRs are fired as soon as they are forward biased. The SCR bridge output is maximum. In this case, the SCR bridge is functioning just like a Diode Bridge. This α gives maximum average VDC (we can call it our reference VDC or 1).

Overspeed protection for series motors is normally provided through the OVERSPEED CIRCUIT in the DC Control Module.

If α is 60° (waveform C in Figure 3-16), the SCRs are not fired until 60° after they are forward biased. In this case, the average VDC is 0.5.

The OVERSPEED CIRCUIT works for all configurations of series motors except where two motors are driven in parallel from a single SCR bridge (refer to Figure 3-17).

If α is 90° (waveform D in Figure 3-16), the SCRs are not fired until 90° after they are forward biased. In this case, the average VDC is Zero.

Suppose MP1A breaks its chain drive. The unloaded motor will overspeed. It will draw full voltage, but little current. Most of the current will flow into MP1B. The OVERSPEED CIRCUIT will not detect the overspeed because the Current Feedback signal indicates the total current drawn by the two motors.

SPROCKET SLIP CIRCUIT This circuit provides overspeed protection for two series motors that are driven in parallel from a single SCR bridge. Such an arrangement is normally used for Mud Pumps. If either one of the motors exceeds a preset speed limit, due to a malfunction in the chain or belt drive, the circuit cuts off power to both motors by tripping the ASSIGNMENT CONTACTORS and turning on the front panel SPROCKET SLIP LIGHT. In the system, overspeed protection for shunt motors is achieved through a FIELD LOSS RELAY. A shunt motor cannot overspeed unless the motor field is low. The FIELD LOSS RELAY monitors the field current. It opens to trip the assignment contactors of the motor if the current is