How to Dry Out Rotating Electrical Machines after a Flood: Part 2

1 April 2019


Last time we discussed how to dry out windings after your equipment has become flood damaged. This time, we will give you step-by-step instructions on how to dry out rotating electrical machines.

Dismantle your machine

Although it may seem obvious, you must first completely dismantle all parts to your machine.  You can’t just simply ‘spot clean and dry’ certain areas of your equipment and think it will be ready to use. 

In an effort to remove silt or dirt from all parts they must all be carefully cleaned with a suitable grease solvent.  These parts include the windings; however, they DO NOT include the ball or roller bearings.  DO NOT clean these as that may cause damage.

Next, thoroughly clean all bearings and housings, paying particular attention to oil grooves and oil reservoirs, which may collect silt and dirt.  Be sure to disconnect and swab any oil lines. You can also stem clean and dry them.

Once that step is complete dismantle the brush rigging and clean insulators.  Pay special attention to his, as some insulators will retain water and must be dried thoroughly. 

Don’t apply voltage until you test insulation resistance

Just as we stated in the previous blog, do not apply any voltage to a winding until an insulation resistance test has been completed and an insulation resistance reading of at least 100 kohm is obtained for several hours.  Once you have ensured that the reading can be achieved and maintained, only then is it safe to apply low voltage. 

Be careful! If the windings are very wet, even the low voltage of hand-cranked ohmmeters can puncture the insulation.  In an effort to avoid this, the crank should be turned at low speeds until some knowledge of insulation resistance value is obtained.  If you are using a modern microelectronic tester, the voltage is regulated, eliminating the problem.

Commutators can be difficult to dry out

Commutators be slightly difficult to dry out.  If you are finding this to be the case you may want to consider loosening or even removing some of the V-ring bolts or clamping nuts to let the water out from the inside of the commutator. On large commutators, you may need to increase the drying temperature to as high as 266°F if lower temperatures don’t produce satisfactory results after a reasonable amount of time.

Replacing bands and slot-wedges

Bands on armatures or rotors must be checked carefully for tightness following drying out equipment.  There is a good chance that they will need to be replaced because of looseness resulting from drying out the insulation underneath.

Same goes with slot-wedges.  Some slot-wedge materials will be affected by moisture.  All wedges should be carefully examined and new wedges installed where necessary.

DC motor or generator field coils and field coils from synchronous machines can be problematic

DC motor or generator field coils and field coils from synchronous machines can sometimes present problems.  It may be necessary to remove these coils from the machines for proper treatment if it is impossible to bring the insulation resistance up.

After drying the parts in a suitable oven, they should be immersed completely in insulation varnish while they are still hot and must be allowed to cool while still immersed.  By doing this, the varnish will be drawn into the layers. 

When cooled, the coils should be checked for short circuits by making a comparative test of their resistance.  A low-resistance ohmmeter will indicate open coils and can also be used to determine shortened turns by reduction in resistance that turn-to-turn shorts produce.

Treat windings with insulting varnish

After a thorough cleaning and drying, most windings should be treated with insulating varnish, especially if cleaning solvents have removed any of the varnish coatings.  Dipping- and baking-type varnish, followed by a suitable baking period, is preferred, but air-drying varnish may be used to expedite the return of equipment to service.  The latter is only recommended if the original varnish is in reasonably good condition.

Before starting the machine back up

Before restarting any machine, be sure you check the entire insulation, paying close attention to lubrication and electrical connections.  Ensure that no tools or materials are accidentally left behind in or on the machine. 

Remember that three-phase motors being restored to service can have proper connection assured for correct rotation with a motor and phase rotation tester.  This instrument will check for correct phase rotation for motor and live lines, while three-phase lines can be separately checked with a phase sequence indicator.

When put back into service, the rotational speed of motors can be readily checked with contact and photoelectric hand tachometers. These handy instruments are particularly useful, as flooding may have made digital online indicators inoperative.

Check out our next blog where we will be discussing how to clean and dry out switchboards and electrical controls after a flood, or review part 1.