Flooded or Wet Induction Motors
First of all, try not to!
When designing a marine of industrial installation, you need to consider the IP, or ingress protection rating of your Induction motor.
By choosing a motor with a high enough IP rating, it is possible that when you submerge your Induction Motor, nothing will happen.
This is because the higher the IP rating, the better the motor casing is at keeping out dust & moisture, from the motors delicate ‘internals’.
Lets imagine a scenario, that an incompetant person has decided to save a few ‘quid’ by specifying an IP54 rated Induction Motor.
This motor will withstand dust & a splash of water, but not submersion, and is therefore suited to dry factories on land, but not a typical marine environment.
So what would actually happen if it gets submerged?
Inside the induction motor are tightly wound coils of copper wire, called a ‘stator’.
It is important for the motor to work, that there is a high ‘Insulation Resistance’ between each of the three ‘Stator Windings’.
Despite being wound tightly together, the stator windings are in fact insulated from each other by the coating on the copper wires.
What happens if water gets into the stator windings, is that the ‘insulation resistance’ between the three separate windings that make up the stator, goes down to very low levels.
The insulation resistance between the ‘phases’ can be measured using an insulation resistance, or IR tester.
Insulation Resistance (IR) testers, often refered to as ‘Meggers’ after a famous brand of IR testers, inject a high voltage through two separate stator windings, or between a winding and the casing of the Induction Motor.
The display on the IR tester shows the resistance between the separate stator windings.
The insulation resistance should be very HIGH.
On ships, SOLAS (Marine regs) specifies a legal minimum of 0.5 Mega Ohms.
In practical terms you should not accept an Insulation resistance of less than 1M (Mega) Ohm on board ships, and not less than 2M Ohms for land based installations.
Low IR on board ships will cause an Earth Fault alarm to be triggered.
How to Dry Out a Flooded Induction Motor on-Board Ship.
If you find yourself in the situation of having a flooded motor, with resulting very low insulation resistance, the situation can be remedied.
Firstly consider safety, as water and electricity are not a good combination, and water can conduct electricity.
Follow all correct electrical supply isolation procedures (Valid Permit to work, Locks & Tags etc).
Check and re-check that you have correctly isolated the supply, using an appropriate tester, such as a ‘live line tester’. Also check that the tester works (using ‘Proving Unit, or known test supply) , both before, and after testing the isolated supply.
Once you are satisfied that the motor is safe to work on, then drying out the stator windings of the flooded motor, can be done in two ways.
The first method is to dismantle the Induction Motor, so that the stator windings are exposed, and to place the motor in a warm environment.
As getting the motor back in service quickly is important, a warm air source, such as a hairdryer can be used to warm the windings, and chase out moisture.
If using something like a hairdryer be careful, as hairdryers have a low IP rating, as they are not designed to come into contact with water.
Therefore keep it at a safe distance from the motor.
Never stick one inside the stator of a damp motor for instance (disclaimer: this blog is for information only, and should not be relied on legally, as its free. If you injure yourself, due to lack of proper formal training, then I accept no liability)
Periodically you can re-check the Insulation Resistance readings, until they have risen back up to acceptable levels, and the motor is dry.
Important note: Sea water is corrosive, and the motor is likely to need washing / cleaning out, with fresh water etc, before you start the drying process.
The second method that can be used, is to gently heat the windings up using a lectrical welding set.
A low current from the welding set is passed through the windings, which causes them to gently heat up, which helps dry them out.
I am not going to elaborate on the second point too much, as you really need ‘hands on’ training on how to do it, and I accept no liability / nor want you to injure yourself.
Your marine training provider can show you how to do this.
Once the windings have totally dried out, and the insulation resistance is back to the pre ‘flooded’ levels, then the motor can be reassembled and re-connected.