Before a machine is brought to site, the floor space, the foundation and the oil supply are confirmed over and over, and the power supply tends to be settled last. Someone says there is power on site, and the mismatch only shows up when the machine is unloaded and connected. This article uses the power supply of the ZYD-100 as the example and sets out what single phase and three phase mean on site, and why the question is worth asking earlier.
The supply a machine needs is written down
The power supply of the ZYD-100 is three phase 380V / 50Hz and the total power is 72 + 5 kW. Both are stated on the nameplate and in the technical documents, and they are the fixed conditions of the machine rather than parameters that can be negotiated. A machine designed and built for three phase has its incoming line, its contactors, its fuses and its internal wiring arranged for a three phase circuit, and it cannot be used directly where the site has single phase supply only.
The difference between single phase and three phase is not only the number of conductors. A three phase circuit shares the power across three phase lines, and the current in any one of them is smaller than on a single phase circuit. A single phase circuit puts the whole load on one line, the current rises accordingly, and the incoming line size, the fuse rating, the contactor capacity and the heating of the cable all have to be worked out again for that one line. A thicker cable does not solve this, and the three phase components inside the machine cannot be replaced by a single line either.
There is a second effect that is easily overlooked. On a three phase machine the load is broadly balanced at runtime and the current readings on the phases are broadly similar. Under single phase supply the whole load sits on one phase, and the conductors, the terminals and the protective devices on that phase run at a higher temperature for longer, which also means a shorter inspection interval. Even if three phase supply is provided later, the earlier operating record stays with the machine.
Four questions to settle before the machine arrives
- Is the supply at site single phase or three phase, how many wires, and how are the earth and the neutral conductor arranged.
- How much capacity is available. Does it cover the total power of the machine or only part of the circuits.
- Where the distribution board is, and how long the run from the board to the machine is and along which route.
- Whether the ambient conditions on site meet the requirements. The ambient temperature range of the machine is -20–45 °C.
These four questions should be confirmed by the site and the equipment supplier together and written into the delivery checklist. The power supply belongs to the civil works and the distribution, but whether the machine can be energised depends entirely on it, and when the boundary is not drawn clearly the usual outcome is a machine standing on site and waiting.
Why altering the supply on site rarely pays
When a site has single phase supply, the two usual answers are to change the distribution or to change the machine. Changing the distribution means a new distribution board, cable runs, an outage to be arranged and a wait; changing the machine means going through selection, purchase and transport again. Both cost time and money, and the workload of changing the distribution is usually only visible once the work is under way on site.
The safer course is to write the power supply into the selection as a hard constraint. The supply requirement of each model should be checked line by line against its technical documents rather than inferred from the model name or from experience. The technical parameters of a machine are fixed before it leaves the factory, so conditions that can be confirmed in advance should not be left to the site.
There is a common mistake in the order of work: the model is chosen on throughput first, and the supply is dealt with afterwards. The result is often that the model is already fixed and the cost of modifying the supply exceeds the cost of a machine that fits. The correct order is to rule out the models whose supply conditions are not met, and only then compare throughput, footprint and functions among the rest. A different order of screening can lead to a different conclusion.
The power supply is not a closing item after the machine reaches site. It is a hard constraint that belongs to the selection stage.
