Troubleshooting Generator Starting Problems (Ross Hill, Hill Graham SCR Systems)
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Generator starting problems can be caused by any number of electrical or mechanical problems. This article describes a systematic approach to getting to the root of the problem.
Caterpillar Engine and Generator
Overview
Early Ross Hill and Hill Graham SCR systems were not equipped with "Hands-Off-Starting" (HOS), which means that the engine actuator had to be manually lifted to get the engine to pick up.
Later systems were fitted with AC Modules which were able to detect when the engine is turning over and provide throttle current to lift the actuator which allows fuel to flow to the engine so that the engine revs increase. The HOS system includes a pair of 12V batteries and a battery charger to provide throttle current from a black start.
If the actuator is lifting and the engine doesn't start the cause is more likely to be fuel, air or mechanical.
The second aspect of getting a generator up and running is voltage build-up. The initial voltage build-up relies on there being some residual magnetism in the generator which generates a few volts. During start-up the exciter control is completely bypassed and this small voltage is fed back into the exciter (via the exciter transformer), which then causes a higher voltage to be generated. This cycle repeats until there is sufficient voltage for the exciter controls to take over and regulate.
Systems Without HOS
For systems without HOS the problem will typically be that, when the actuator is released, the control system does not take over. This means that there is no current to the actuator, or the actuator is failing to respond to the current applied.
Check the following:
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Check that the generator section front panel controls are in the correct position, and the emergency stop has not activated. The Engine Governor control should be in the IDLE position (or RUN if there is no IDLE position).
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The actuator arm will be difficult to lift with the engine stationary, but with it turning there is some hydraulic assistance. If the actuator remains difficult to lift, even with the engine turning over, there may be a problem with the hydraulics.
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Use a DVM set to measure up to 200mA DC and insert this in series between the throttle (actuator) and one of the AC Module throttle connections. As the engine is turned over you should see a minimum of 20mA. Re-connect the actuator to the AC Module and measure the DC voltage across the throttle terminals. The resistance of the actuator is usually around 30 Ohms, so for every 10ma of throttle current there should be 0.3V (V = I x R), so 20mA requires only 0.6V into a 30 Ohm load.
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If current is present and the voltage is lower than expected then suspect a short circuit in the wiring to the actuator, or a fault in the actuator itself. Check the actuator coil resistance at the actuator terminals (disconnect all wiring) to confirm. If the voltage looks OK, check the voltage at the actuator terminals, and if present then the actuator is likely to be faulty.
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If current is NOT present but the voltage is as expected then suspect an open-circuit between the AC Module terminals and the actuator. If the voltage is lower than expected then check that the generator voltage is building up sufficiently to power the AC Module. Check the AC Module +/-16V power supplies during starting. These need to reach at least 10V to drive the throttle. If not there then look at the voltage build-up as a possible source of the problem.
Systems With HOS
For systems HOS the problem will typically be that, when the engine is turned, the control system does not take over and lift the actuator. This means that there is no current to the actuator, or the actuator is failing to respond to the current applied.
The HOS system relies on two things to provide current to the actuator during starting:
- Battery voltage supply
- A signal from the engine flywheel pulse-pick-up
Check the following:
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Check that the generator section front panel controls are in the correct position, and the emergency stop has not activated. The Engine Governor control should be in the IDLE position (or RUN if there is no IDLE position). Note that on some systems the battery voltage is only connected to the module when the switch is in the IDLE position, so HOS will not work in RUN.
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Check the battery voltage supply at the AC Module terminals with a DVM. During a black start the voltage is provided by the batteries only, but with one generator on line the batteries are (or should be) supported by the charger. A lower than expected voltage is indicative of flat batteries (or a faulty charger if the voltage is low with at least one generator on line).
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Check the pulse pick-up input to the AC Module. Ideally use an oscilloscope for and AC waveform at least 5V peak-to-peak. The frequency will increase as the engine speed increases. If an oscilloscope is not available you may be able to use a DVM set to measure AC voltage however some DVMs may be filtered for AC voltages of 50-60Hz. If the pulse pick-up voltage is too low or non-existent the sensor may be faulty, dirty, poorly adjusted or may be disconnected.
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If the pulse pick-up signal is OK use a DVM set to measure up to 200mA DC and insert this in series between the throttle (actuator) and one of the AC Module throttle connections. As the engine is turned over you should see a minimum of 20mA. Re-connect the actuator to the AC Module and measure the DC voltage across the throttle terminals.
The resistance of the actuator is usually around 30 Ohms, so for every 10ma of throttle current there should be 0.3V (V = I x R), so 20mA requires only 0.6V into a 30 Ohm load. -
If current is present and the voltage is lower than expected then suspect a short circuit in the wiring to the actuator, or a fault in the actuator itself. Check the actuator coil resistance at the actuator terminals (disconnect all wiring) to confirm. If the voltage looks OK, check the voltage at the actuator terminals, and if present then the actuator is likely to be faulty.
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If current is NOT present but the voltage is as expected then suspect an open-circuit between the AC Module terminals and the actuator. If the voltage is lower than expected then check that the generator voltage is building up sufficiently to power the AC Module. Check the AC Module +/-16V power supplies during starting. These need to reach at least 10V to drive the throttle. If not there then look at the voltage build-up as a possible source of the problem.
Voltage Build-Up
Voltage build-up problems normally fall into one of the following categories:
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Lack of Residual Magnetism
Voltage build-up relies on there being some residual magnetism in the generator to get the process started. New generators, or generators which have been stationary for a long time may not have any residual magnetism, so voltage can not build up. Normally on start-up a small deflection can be seen on the generator section panel voltmeter (up to 100V). If this deflection is very small or there is none at all it may be necessary to 'flash' the exciter with a voltage source such as a 12V car battery.
To do this disconnect the exciter control connections, connect the negative terminal of a battery to the negative exciter terminal and strike the battery positive terminal lead wire several times on the positive exciter terminal (there will be sparks).
Re-connect the control wires and try starting again
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No Exciter Build-Up
The small voltage generated by the residual magnetism should pass through the exciter transformer, then through a bypass relay (called the build-up relay), rectified, then injected back into the exciter terminals to facilitate further voltage build up.
During starting, check the DC voltage on the exciter terminals. If no DC voltage appears ther may be a problem with the exciter transformer fuses, the transformer itself, the build-up relay or the rectifying diodes.
Check the exciter transformer and fuses with a DVM set to measure AC volts. Measure the primary voltage during starting and compare to the secondary voltage input to the rectifying diodes. The ratio of the two should match the transformer ratio (typically 600:100).
In all but the earliest Ross Hill systems the exciter build-up relay and rectifying diodes are all mounted on the Generator Exciter PCB which can easily be replaced if a fault is suspected.
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Voltage Collapse
In some instances the generator voltage can be observed to build up nicely then suddenly collapse. This occurs when the bypass relay (build-up relay) energises to hand over control to the AC Module exciter regulator. This indicates a fault with the relay itself, the exciter control thyristors, or the AC Module exciter regulator and firing pulses. Further fault-finding requires the use of an oscilloscope to determine if the firing pulses are correctly synchronised, so try replacing the Exciter PCB (or thyristors and build-up relay if the exciter is comprised of individual components), then the AC Module if this does not fix the fault.
Comments (2)
Can we use MPU sensor for Engine speed feedback to AC Module (P/N: 0521-2500-04 REV.W1) Sr No: 10472, Job No: 5047.
Please give me advise.
Hi Good day!
We have a steam power generation unit. We have rewind it's exciter and after that, when tried to excite the generator. But generator is not picking up the voltage. We increased the excitation voltage up to 35V but the generator's voltage was only around 20V. What could be the reason of this problem?
Check the exciter current. You may have voltage at the terminals, but no current path. You don't mention whether or not this is an exciter with brushes or brushless, so check either brushes or diodes.