The most natural reaction after the overload protection has tripped is to reset and try again. The problem is that the cause of the overload is still in the machine, and the reset only pushes the same load once more; in most cases it trips again at the same point and a few minutes are wasted. The approach that actually saves time is to rule out the possible causes one by one in a fixed order, without opening the machine at any point.
First step: distinguish an overload from a phase loss and phase sequence fault
The two kinds of trip look different. An overload is the protection operating after the current has stayed above the set value for a while, by which time the machine has run for some time and the temperature has risen; phase loss and phase sequence protection refuses to start, or operates instantly, when the supply conditions are not met, so the machine often will not run at all or stops as soon as it moves. Phase loss and phase sequence protection is part of the standard configuration of the ZYD-100; if a three-phase 380V / 50Hz supply is missing a phase or has a reversed phase sequence, the protection operates first. The first thing is therefore to look at when the trip happened: if it stopped at the instant of start-up, check the supply; if it stopped after running for a while, look in the direction of the load.
Second step: check whether the oil feed is over the limit
The flow rate of the ZYD-100 is 6000 L/h, and that is the value under rated conditions. Opening the oil inlet valve wider, or raising the flow rate on site to keep to the schedule, puts the actual load on the pump above the design value. In addition, a tank level below the suction port and air in the inlet pipework both put the pump into an abnormal running state, which shows up as unusual noise and a rising load. Neither of these two requires opening the machine; the valve position and the oil level are enough to judge them.
Third step: check whether the filter element differential pressure has risen
The resistance of the filtration section lands directly on the pump load. The differential pressure alarm value for the 150-500-5 fine filter element is 0.35 MPa; in the arrangement of five elements in parallel, one element whose differential pressure is markedly higher than the others usually means that the flow through that element is too large, that more contaminants are being carried in from upstream, or that the element itself has reached the end of its life. A differential pressure that rises across the board raises the pump load with it. The judgement is made from the pressure gauge readings, without opening the filter housing.
Fourth step: check the oil temperature and viscosity
If the oil temperature does not come up, the viscosity cannot come down either, and the pump has to develop more torque at the same flow rate, with the current rising accordingly. The temperature control range is 45–80 °C; when the reading is low, first confirm that the heating is working and that the probe position reflects the real oil temperature, and only then decide whether the problem lies in the filtration or the flow rate. The order of this step matters: do not touch the temperature control before the flow rate and the differential pressure have been ruled out.
Fifth step: check the three-phase supply
Loose terminals, poor contact and unbalanced phase-to-phase voltage all cause abnormal motor current. Phase loss and phase sequence protection blocks the most severe class of these, but mild imbalance does not necessarily operate the protection and can only be found by measuring the voltage and inspecting the terminals. This step has to be done after the power is switched off and the absence of voltage verified, and it belongs at the end of the investigation sequence.
| Step | What to check | Readings to record | Whether the machine has to be opened |
|---|---|---|---|
| 1 | Overload operation or phase loss and phase sequence operation | Moment of the trip, how long it had been running | No |
| 2 | Oil inlet valve opening and tank level | Valve position, level, air in the pipework | No |
| 3 | Differential pressure of each branch | Inlet and outlet differential pressure, difference between branches | No |
| 4 | Oil temperature and temperature control circuit | Panel temperature, heating operation | No |
| 5 | Three-phase voltage and terminals | Phase-to-phase voltage, state of terminal tightening | Verify absence of voltage after switching off |
Before resetting, write these down
- Whether the trip happened at the instant of start-up or after the machine had been running for a while
- The oil inlet valve opening, the tank level and the panel temperature at the moment of the trip
- The inlet and outlet differential pressure of the filtration section, and the difference between the branches
- Whether the current reading climbs again after the reset
A trip is not the cause, it is the result; not resetting and not opening the machine, but narrowing the field with the five steps above and waiting for concrete readings before deciding whether to open the machine, saves both time and parts compared with resetting again and again.