Troubleshooting Mud Pump Field Supplies (Ross Hill, Hill Graham SCR Systems)
One of the most common problems encountered with Ross Hill or Hill Graham type Mud Pump Field supplies is field unbalance tripping, which causes the main DC contactors to open and requires a reset. The problem is more likely to occur under heavy load conditions, and resluts in the complete loss of the Mud Pump, so is a particular nuisance. This article describes some of the measures you can take to identify and resolve the problem.
Theory of Operation
The active field supply is used to achieve balanced load sharing between two shunt DC motors connected in parallel to a single SCR. It actually consists of two field supplies: one which is fixed (a diode rectifier), and the other (an SCR rectifier) which is varied to achieve load balancing. The field current controls the back EMF produced in each motor armature, which opposes the forward drive voltage from the SCR. The bigger the difference, the more current that flows in the armature. A weaker field will produce less back EMF (for a given speed) and hence more armature current. A stronger field will produce more back EMF which will oppose the armature current thus reducing current flow.
Fig 1: Current Flow in the Active Field Supply HED
The Active Field Supply Regulator PCB can only control one of the field supplies, and it takes as it's basic current reference the current in the fixed field supply, and will initially simply match it. The difference in the current flowing in each motor armature is measured using a Hall Effect Device (HED). One of the armature connections to each motor is passed through the HED in opposite directions so that when the currents are matched the net output from the HED is zero, i.e. the currents cancel each other out. If the currents are unbalanced the resulting output from the HED is used to provide a trim to the active current regulator to adjust the field current in one of the motors.
Under normal operation, the output from the HED will be close to zero, but if the regulator circuit is unable to correct the imbalance the output from the HED will rise until it exceeds the threshold level set on the Active Field Regulator PCB which then causes the drive to trip.
While nuisance tripping is a problem, it occurs because the system is working to protect itself, so disabling the tripping interlock is not advisable.
After checking connections and tightness of terminations, the first thing to check is the output from the HED. Generally, on Ross Hill or Hill Graham systems the HED output is scaled either at 650mV/1000A, or 360mV per 1000A depending on which type of HED is fitted. The 650mV HEDs were fitted on older systems (pre 1980-ish) and are black or dark grey in appearance. Later rigs had 360mV/1000A HEDs and are light grey, or bluish-grey. Note that the Active Field Regulator should be calibrated for the type of HED fitted. The only sure-fire way of checking if the HED is working correctly is to verify it with another instrument such as a DC clamp meter,
The trip level is around 200A current unbalance. If the HED is tripping before this level is reached there may be a problem with the calibration of the Active Field Regulator PCB. If, however, the trip is occurring because of a current imbalance then the cause must be investigated. Here are some possibilities:
Current flow in armature cables through HED wrong
This would only occur if the cables were disconnected or replaced. If the direction of current flow is the same in both cables the HED signal would reach the 200A tripping level with only 200A on showing on the SCR door. This is an unlikely cause if no major work has been undertaken.
Output from the HED inverted or crossed
This causes the field regulation to work the wrong way, so that the active field is weakened when it should be strengthened and vice-versa, The result of this is that at no load the fields operate without tripping, but as soon as a small load is applied and a small current unbalance is detected the Active Field Regulator PCB acts to make the unbalance worse. Tripping occurs at low loads.
The motors which are connected in parallel must be of a similar make and model. The regulation system can only work to compensate for variations in the manufacture of similar motors, not compensate for a total mismatch. If balancing problems are experienced after changing one motor this may be the cause.
Faulty cables, connections or motor
If the Mud Pump works for long periods of time at high loads before suddenly tripping there may be a problem with the cables, connections or motor, which is exacerbated by temperature as the equipment warms up. This can be seen by monitoring the HED output. You should see the output from the HED gradually creep up. To work out which motor is likely to be faulty, measure the DC voltage on the field terminals. The one with the higher voltage is likely to be the suspect one.
If the tripping occurs with little or no output from the HED there may be an intermittent connection in either the HED circuit, the motor field or armature connections. If the trip occurs without latching or indication the fault may be in the trip interlock circuit to the contactors (note that some systems are fitted with non-indicating, non-latching circuits).
Faulty active field
The active field supply is a half-controlled SCR bridge and as such, is suspect to failures of thyristors, pulse transformers, etc in the same way that the main SCR bridge is. It is also not uncommon for excessive supply waveform distortion to cause the field thyristors to spontaneously switch off. If this is occurring there will be a split second between the field shutting down and the trip occurring because the field supply current takes a while to collapse. Another symptom of this is when tripping only occurs when there are additional loads on the system such as another Mud Pump under load or Rotary Table or Top Drive loads.
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