Single Phasing Faults In Marine & Land Induction Motors
Single phasing is the term used when one of the three stator windings in an induction motor is not ‘energised’.
Three reasons for this happening in a Marine (& Land) environment are:-
- Mechanical vibration – causing the terminal nuts to become loose, and therefore disconnecting a ‘phase’ wire.
- Physical Damage to the supply cable – this can be minimised by using appropriate cable protection systems, such as trunking.
- ‘Open Circuit’ Stator Winding – an internal break in one of the motor windings will cause single phasing. The motor windings are known as the ‘Stator’.
Testing a Marine 3 Phase Induction Motor for Single Phasing
‘Single Phasing’ is the term used to describe when one of the three Stator windings in a three phase electrical motor, goes ‘open circuit’.
Symptoms seen will be rough & noisy running, and an increase in the current being drawn by the induction motor.
Firstly, separate all six of the Stator wires (there are 2 wires per phase, hence 6 wires for a three phase motor).
Secondly, using a low range Ohmmeter, measure across each pair of the three phase wires (one pair at a time!).
The Ohmmeter should show no/very low resistance (depending on the sensitivity of your Ohmmeter).
If you get no change in the meter reading, when you connect it across a pair of windings, then you have an ‘Open Circuit’ winding.
Ensure that your Ohmmeter is calibrated, and working.
With the meter set ready to measure Ohms, you should touch the two meter probe leads together.
If using a digital multimeter (which most are these days), the display should show a low resistance value.
The value displayed on your digital Ohmmeter (multimeter) may be a few Ohms (less than 5), rather than zero.
This is because the meter ‘probe’ leads have ‘resistance’, and therefore show it on the display.
To be totally accurate if recording a measurement, you should subtract this ‘resistance’, from any actual measurement you make of the motor windings (though probably a bit OTT to do it).
If you are using an older style Analogue Ohmmeter, commonly referred to in the UK, as an ‘AVO’ (after the famous brand), then you may need to calibrate the meter first (will talk about this elsewhere on the blog, or ask me about it).
Having a single phasing fault on an Induction Motor will cause the current in the other two phases to rise.
To prevent damage to the cabling and / or motor windings, an overcurrent protection relay is normally fitted in the motor starter circuit.
The overcurrent device trips during a single phasing fault.
However if the motor is located in a location that is not visible, the fact that the motor has tripped, may not be immediately obvious.
A better way would be to use sensors to measure motor parameters.
Parameters such as current and voltage in the supply cables.
If clip on current sensors were attached to each of the three phase supply wires, then faults would be quickly identified.
The sensors would relay data via wireless links or fixed data cables to local or cloud networks.