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Single phasing or overload?

Single phasing or overload?

Single phasing or overload?

I'd be grateful for a quick second opinion (or two).

Photos below from the terminal box of a 440 V, 60 Hz, 30 kW, 6 pole TEFC motor (S1 duty, Class F insulation). Apart from the two obviously cooked wires, the rest look and feel normal.

It let some (a lot of) smoke out. The rotor turns freely by hand. Continuity/IR checks don't reveal a great deal.

One pole of the 60 A contactor is melted (the one that switched the red phase - see the pencilled markings on the motor terminal box), while the other two are distressed to a lesser extent.

I'm being told this has all the hallmarks of an overloaded motor, but am thinking personally that it looks a lot more like single phasing. Motors aren't really my thing, so I'm wondering whether to go with what I'm being told, or to put my foot down.

Comments gratefully received.


RE: Single phasing or overload?

Single-phasing certainly looks credible as a cause. The two undamaged windings were effectively connected in series across the two healthy phases of the supply when one phase was lost, so they didn't pass as much current.

The underlying question is, why did the overload protection not clear the fault before the motor burnt out? Most modern O/L relays (within the last 30 years) can detect imbalance as well as just current magnitude. Is this supplied from a very old installation?

RE: Single phasing or overload?

When you open the motor (you may need to, to change the leads) and if you see only partial winding burnouts spread evenly, it's single phasing.


RE: Single phasing or overload?


Thanks for the observation. The installation is pretty old - and it's an application where we prefer an eventual burnout over an unexpected trip.


RE: Single phasing or overload?

I've worked with a few run-to-destruction motors where the O/L relay generated a critical alarm but didn't trip the drive. These were the DC emergency pumps protecting turbo-alternators and their prime movers, and potentially damaging a pump worth tens of thousands of pounds was a lot better prospect than wrecking a multi-million pound generator train and potentially venting hydrogen coolant on loss of seal oil. Venting hydrogen into a bearing fire would be the start of an exceptionally bad day.

On the basis that it is nice to understand what happened and why, the other thing you might want to check is the overload 'class' which determines how quickly it responds to an overload - generally it's a Class 10, which translates as 10 seconds to trip at a current of 6x setting.

In Europe the manufacturers expect standard general-purpose motors to be protected with a Class 10 overload unless the motor has been specially designed for severe starting duty on things like large fans or conveyors. Overloads for severe start applications are available as the much slower Class 20 and Class 30 (20 seconds and 30 seconds respectively at 6x setting), and if the overload is a slower class than the motor was designed to be protected by then bad things can happen to the motor even if the relay setting current is correct.

RE: Single phasing or overload?

The MCCBs are definitely on the list of things to look at. Can't imagine these motors were designed for severe starting (nothing in the documentation or the application to suggest it).


RE: Single phasing or overload?

A badly melted contact may cause single phasing in several of ways.
It may hold another pole open;
it may not be making contact itself.
it may hold in two but not three poles.

"Why not the best?"
Jimmy Carter

RE: Single phasing or overload?

Even if you have a bi-metal OL relay that provides some level of single phase protection, if the overall load on the motor is very low at the time of the phase loss, it still may not trip. The bi-metal OLs have a differential mechanism that biases the trip point to he lower, but if the other two phases don't reach even that biased trip point, the motor will continue to run in a single phase condition and that may damage the motor windings. But I doubt it would damage the leads like that.

My guess is that the contact was going bad on the contactor and was arcing, causing the OL relay to trip from the single phase condition when fully loaded. But rather than address the issue, someone kept production going by turning up the dial on the OL relay until that high current caused the conductor to fail.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

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