Maintenance Detective: 240V Fan Motor

Let's play a game for all the maintenance players!

You get a call on a 240V fan motor that keeps throwing its overload.

You do your homework. You pull out your meter and check the resistance across all three winding sets - balanced. You check winding to ground - no short. The motor runs. It just trips. Over and over.

So what's going on?

First:  check your mechanical load before you go electrical. Spin the fan by hand with the motor de-energized. It should turn freely with minimal resistance. If it's got mechanical drag, that'll heat up at speed and get worse which could explain your problem.

Second: verify your supply voltage at the motor terminals under load. Low incoming voltage forces the motor to draw more current to maintain output. Even a 10% voltage sag can push a properly loaded motor into nuisance trip territory. 

Third: open the junction box on the motor and verify the voltage configuration. Dual-voltage motors can be wired for either 240V or 480V internally. If someone rewired this motor and set the jumpers for 480V while it's sitting on a 240V supply, the motor will run, especially with light loading, but it only has 25% of its rated torque available.  Stalling under this condition could throw the overload.

Fourth: Megohm the windings. Your ohmmeter doesn't have the voltage to detect arcing which will be present at run condition. Degraded winding insulation can read perfectly clean on a standard meter and still leak significant current at 240V AC. A megohmmeter applies 500V or 1000V DC test voltage to the winding. A healthy motor should read over 1 megaohm (the old rule of thumb is that it'll run if it has at least 1000 Ohms per volt).

Running wet or hot will tend to make motors fail from insulation failure.

I recommend testing from the cabinet instead of testing from the motor junction box for your first test.  That way you're testing the whole system at once and a negative test allows you to move on.  However, a positive test (shorting detected) means you need to keep testing to localize it.  Water in a junction box on the way to the motor can cause this sort of trip. 

Megging wires is relevant... wires can arc to ground just like motor windings.

For the motor itself: A reading of less than 1 Megaohm means a motor's on its last legs.  A reading of less than 1000 Ohms/volt (240k Ohms for a 240 Volt Motor) means it'll likely be throwing that overload when you run it.

Here's what happens when insulation degrades: leakage current bleeds from the winding to the motor frame at operating voltage. Your overload relay sees total current....useful work current plus leakage. It doesn't know the difference. It just knows current is high, and it trips. The motor isn't being overloaded mechanically. The insulation is failing, and the overload is doing exactly what it's supposed to do.

Why does insulation fail? Heat cycling over time, moisture intrusion, oil or coolant mist contamination, dust accumulation, or a previous winding event that was never fully repaired. In fan applications it tends to be gradual.  The motor trips progressively more over weeks or months as the insulation continues to break down.

The fix: rewind or replace the motor.

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