The supply of the machine is three-phase 380V / 50Hz. That voltage is not always stable on site, particularly where the transformer capacity is tight, where several machines start at the same time, or where the incoming line is long. Unstable voltage rarely shows itself as an alarm; instead a number of small faults appear at once: the motor runs hot, the vacuum level will not come up, the heating is slow, and the control trips now and then. If the cause is looked for only in the machine, the root of it is never found.
What a low voltage brings with it
The motor is the most affected. When the voltage falls, the motor has to draw a higher running current to maintain the load torque, the copper loss rises with the square of the current, and the winding temperature rises faster. What is seen on site is that at the same oil feed rate the motor runs hotter at a low voltage. Run like that over a long period, the winding insulation ages faster.
The vacuum pump comes next. The pumping speed of a vacuum pump depends on its rotational speed; if the motor turns more slowly the pumping speed falls and the degassing capability falls with it. The appearance along this chain is very unobtrusive: the vacuum level will not come up, while the pump itself shows no abnormality whatever.
Third is the heating power. The heating power varies with the square of the voltage, so a voltage that is 10% low means nearly a fifth less heating power, the heating-up time is clearly longer, and oil with a high water content is hit hardest.
Fourth is the control circuit. Voltage fluctuations may trigger the phase-loss and phase-sequence protection, and they may also cause spurious operation of the control board, loss of parameters or abnormal jumping of the heating output.
Three places to check on site
- The incoming supply: check whether the cross-section of the incoming cable is adequate, whether the terminals are crimped reliably and whether the terminals have oxidised and blackened. The contact resistance caused by a loose terminal heats up under load, and the voltage fluctuates with it; this is a very common cause.
- The distribution capacity: list the other high-power equipment on the same circuit, because the momentary inrush currents of several machines starting together readily pull the voltage down. Find out whether motors, welding machines, heating furnaces and similar loads share the same transformer.
- The earthing: poor earthing lets the neutral potential drift, the three phase voltages appear unbalanced, and the readings of some measuring instruments drift with it, which makes a voltage problem look like a different problem.
How to confirm that it is a voltage problem and not a machine problem
Measure the three-phase voltages and currents with a clamp meter while the machine is running, and record the range of fluctuation over a period, noting both the lowest and the highest value. Then start up the other equipment at the same voltage level and see whether the voltage falls at the same time. If the voltage drops as soon as the machine is started, the problem is on the distribution side; if the voltage is stable and the machine is still abnormal, the problem is on the machine side.
A second simple comparison is to move the same machine to another supply that is known to be sound and run it there. If everything is normal after the change of supply, the machine itself is sound.
What can be done on the machine side
First, stabilise the voltage: fit an AC voltage stabiliser on the incoming side so that the control circuit and the motor receive a stable voltage. Second, stagger the loads: run the heating and the motor on separate time slots so that the high-power heating elements and the motor never start at the same time. Third, control the start-up sequence: start the vacuum pump and the oil pump at no load first and only bring in the heating once the current has settled, which reduces the voltage drop at the moment of start-up. Fourth, improve the distribution side and move the machine onto a circuit with enough capacity.
| Symptom | What it points to | How to confirm | Action |
|---|---|---|---|
| The motor runs hot and the current is high for the same load | Low voltage | Measure the running voltage and current with a clamp meter and record the lowest value | Check the incoming supply and the terminals, and fit a stabiliser if necessary |
| The vacuum level will not come up and the pump shows no abnormality | Low voltage causes the speed and the pumping speed to fall | Measure the voltage and check whether it is below the rated value | Stabilise the voltage or improve the distribution capacity |
| The heating-up is slow and the oil temperature never reaches the set value | The heating power falls with the voltage | Check the actual power against the set temperature | Stabilise the voltage and extend the preheating time as appropriate |
| A protection operates suddenly while the machine is running | Voltage fluctuation triggers the phase-loss/phase-sequence protection or spurious operation of the control | Record the voltage curve at the moment of operation | Check the incoming terminals and the earthing, and re-test after stabilising the voltage |
| Instrument readings fluctuate up and down | Poor earthing or interference | Re-check the earth connection and the earth resistance | Redo the earthing and check the shielding earth of the instruments |
Safety notes
Measuring voltage and current requires instruments whose range and insulation rating are matched, and the work must be done by qualified personnel. Do not remove or refit terminal connections while the machine is energised. Any work involving the fitting of a stabiliser is to be carried out with the power disconnected and the absence of voltage confirmed.
When a hot motor, a poor vacuum level and slow heating appear together, measure the voltage first. Three symptoms that appear together usually come from a single source.