Film-Tech Forum ARCHIVE: motor caught on fire! any idea why?

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Posted by Gavin Lewarne (Member # 5397) on 12-15-2009, 05:20 AM:

Hi All,

in the middle of a show last week one of our projectors drive motors literally caught on fire. Lickly nobody was hurt and the only damage seems to be to the motor itself.

My question is this, why did it do it?

As soon as i recieved the phone call i rushed in to do a damage assesment. When i got there the fire service were letting people back into the building and doing a safety check. Once they left, i isolated all power to that machine and removed the motor. Interestingly enough, the projector mechanism is undamaged and rotated freely with no roughness or binding. The drive belt was intact and in good condition. The motor housing had not mooved and even the motor itself was freely rotating. So what caused the blaze?

apparently the flames were about 4 feet high and green, as if the coper in the windings itself was burning. The machine was running fine for 20 minutes when the motor just stopped and caught ablaze.

I have checked the inverter output and all electrical wiring to the motor controls and within the motor itself and cannot find a problem. The motor is less than 2 years old and very clean.

Got a new one on the way so will be up and running tomorrow, but i just dont understand why it happened [Frown]

Is worrying for the future considering the other machine has an identical motor on it

Posted by Rick Raskin (Member # 1561) on 12-15-2009, 07:21 AM:

Without dis-assembly it would be difficult to tell. You could do a visual of the internal components and look for damage. If there is a warranty, let the manufacturer or supplier deal with it.

Posted by Richard Fowler (Member # 893) on 12-15-2009, 07:49 AM:

You may have had a motor that was not rated for invertor control, overheated windings and breakdown of insulation can be the end result.

Posted by Robert Minichino (Member # 3495) on 12-15-2009, 09:24 AM:

How long is the wiring run between the inverter and the motor, and is there a reactor (inductor) installed between the inverter and the motor? Reiterating what Richard said, was this motor rated for inverter use?

An inverter connected to a motor with a short length of wiring (under 10 feet or so) won't cause any trouble, regardless of the motor's insulation rating. Problems start popping up with longer lengths of wiring between the inverter and motor, where the distributed capacitance in the wiring combines with the inductance of the motor windings to form a resonant tank circuit, resulting in high voltage spikes at the inverter output's waveform transitions (the inverter sends out a 6kHz or so square wave). These spikes may arc over between windings, eventually causing insulation breakdown, shorted turns, overheating, and finally, fire.

That having been said, the overcurrent protection in the inverter should have protected against fire. The only case where it wouldn't have would be if you are using an inverter meant for a larger motor without properly reducing the inverter's overcurrent setting based on the reduced motor size.

Posted by Jim Cassedy (Member # 4115) on 12-15-2009, 09:49 AM:

A couple of years ago, I had a Century drive motor
catch fire after the centrifugal switch on the
starting winding went bad and "stuck".

Since the start winding was never meant for
continuous duty, the motor overheated and began
melting the insulation off of the motor windings.

Of course, this happened on my night off.
(Staff Quote: "Gee, we smelled something burning
but didn't think it was anything important.")

For some reason it took a while for the main
circuit breaker to kick off, and by that time
even some of insulation on the wires inside the
BX type cable running to the motor had started
to melt. From what I was told, smoke and blue-green
sparks began shooting from the motor before the
whole thing seized and the breaker tripped.

It took over a week to get the stench out of the booth.

I had to replace the motor and a wholebunca wiring,
including an external "slow start" circuit which
totally melted.

Posted by Robert Minichino (Member # 3495) on 12-15-2009, 09:52 AM:

If he's running an inverter, then this is a 3 phase motor (induction or synchronous), and there's no centrifugal switch.

Posted by Gordon McLeod (Member # 33) on 12-15-2009, 10:28 AM:

The older OEM motors on cnemecanica did hve issues with invertor control especially the low RPM ones as the little fan on the end of the shaft doesn't keep it cool enough
I have usually put a small muffin fan on the end of those motors

Posted by Gavin Lewarne (Member # 5397) on 12-15-2009, 11:43 AM:

i only have a 4ft cable run between inverter and the motor, so thats not the issue. Inverter still checks out just fine, as does an insulation test on the motor wiring. The circuit breaker never tripped! (but that too checks out OK)

both motorts are rated for inverter use.

The motor was only 2 years old and has never given us a bit of trouble until saturdays fiasco. The motor on the other machine is of similar age.

and yes, the bloody stench is taking ages to go! [Frown]

Posted by John Hawkinson (Member # 1135) on 12-15-2009, 12:05 PM:

What're the manufacturer and model of both the inverter and the motor?

--jhawk

Posted by Robert Minichino (Member # 3495) on 12-15-2009, 12:11 PM:

It could just have been a manufacturing defect that took a while to become evident. There doesn't seem like anything about the installation that could have caused the motor to burn (reasonable wiring, no locked shafts), although it is odd that the inverter kept the motor running during what should have been an overcurrent condition. Then again, it could have been a high-resistance fault that generated enough heat to start the fire.

Posted by Frank Angel (Member # 248) on 12-17-2009, 02:46 PM:

quote: Gavin Lewarne
in the middle of a show last week one of our projectors drive motors literally caught on fire. Lickly nobody was hurt and the only damage seems to be to the motor itself.

My question is this, why did it do it?

Simple: It's the perversity of inanimate objects.

Posted by Gavin Lewarne (Member # 5397) on 12-17-2009, 04:23 PM:

frank, do you mean how things that are not meant to break do so in a terrible way?

yesterday, the battery on my dads car burst spewing acid all over the engine compartment.

its a 6 month old beemer that now needs totally new engine electronics.

Posted by Ian Parfrey (Member # 5122) on 12-17-2009, 05:10 PM:

Is there a possibility the problem is in the inverter and NOT the motor itself?

A spontaneously combusting motor seems a rarity.

Posted by Rick Raskin (Member # 1561) on 12-17-2009, 05:41 PM:

Frank:

I had a college prof who used that exact phrase. Today I guess it would be [bs]

happens.

Posted by Mark Gulbrandsen (Member # 72) on 12-17-2009, 10:30 PM:

Was the motor properly fused, was the invertor fused? If the motor was thermally protected (internally fused) then the manufacturer may be liable.

Posted by Frank Angel (Member # 248) on 12-17-2009, 10:48 PM:

Manufacturers never think they are liable and they'll give you a thousand reasons why they are not. It's the perversity of manufacturers' escape clauses. [Big Grin]

Posted by Gavin Lewarne (Member # 5397) on 12-18-2009, 04:18 AM:

yes, the inverters and motors have seperate fuses as well as the entire circuit being on a MCB.

i have removed the inverters now, and am using our old rheostats instead. give a nice soft start.

i have taken th old motor to a re-winding specialist in the area (with the hopes of it being rebuilt for a spare) but alas, he said its beyond hope. Reckons the windings had a fault from manufacture given what is damaged and the nature of the charring inside.

Posted by Rick Raskin (Member # 1561) on 12-18-2009, 07:01 AM:

Given your diagnosis it appears the motor's windings may have had a internal insulation breakdown. I suspect that there was a heat related issue that caused the fault to manifest itself. I'd certainly query the manufacturer to see if there is any possibility of compensation.

I had a similar experience with another type of motor that was long out of warranty. However, the manufacturer replaced the unit and wanted the carcass returned for testing.

Posted by Ian Parfrey (Member # 5122) on 12-18-2009, 07:10 AM:

Are the charred windings the field or armature?

Posted by Tony Bandiera Jr (Member # 2365) on 12-18-2009, 10:15 PM:

Ian, 3 phase motors do not have windings on the armature. [Big Grin]

(In fact, other than "Universal" motors like ones used for vacuum cleaners, mixers, etc, no modern motors have true energized windings on the armature.)

quote: Mark Gulbrandsen
If the motor was thermally protected (internally fused) then the manufacturer may be liable.

Mark the vast majority of 3-phase motors DO NOT have built-in thermal protectors. You can special order them with "winding thermostats" which require extra leads brought back to the motor controller to trip the controller's control circuit to shutdown. The thermostats don't directly interrupt power to the windings as the opening of only one thermostat would "single-phase" the motor and burn it up quickly.

Gavin, it is very important that the inverter drive is correctly programmed for the motor it is controlling, the most important parameters that need to be set are incoming line voltage, motor rated operating voltage, motor current, motor nameplate frequency, motor nameplate rpm, and carrier (or PWM) frequency (6,000Hz or 8,000Hz is normal, but I have run older motors with success by setting the PWM to 4,000Hz)

The total flameout should not have happened with a properly programmed drive, but a motor can still fail even with a properly set up drive (but it still would not be damaged beyond repair. )

Posted by Gavin Lewarne (Member # 5397) on 12-19-2009, 09:24 AM:

im not running the inverters any more. FYI, our motors are single phase with a custom made inverter to ramp up rather than a programmable one. it was more of a "soft start" system.

it was three phase into the inverter, and single phase out. But im running the old rheostats now so no problems there.

The field windings were totally buggered, nothing left really but charcoal like stuff. the armature contacts were melted and the rear bearing was badly scorched (looked like the lubricant in it also caught fire).

Since installing new motor, absolutely no problems [Smile]

and it is much cooler while running. old one used to get very toasty.

Posted by Tony Bandiera Jr (Member # 2365) on 12-19-2009, 04:07 PM:

Ah, that explains a lot.

First off, since the "inverters" were a custom deal you can forget about getting any warranty claim from the motor maker. The inverters probably didn't have any type of overload protection designed in anyways, so you're much better off going back to the rheostats.

Make SURE that your motors have built-in thermal overload protection (since they're single phase) to avoid a repeat performance.

I would also install fuses (time-delay or "slow-blow" type) rated at or just above the motor's nameplate current.

Posted by Gavin Lewarne (Member # 5397) on 12-20-2009, 11:14 AM:

yup, have already installed inline slow blow 2.5a fuses in line with the live feed going to the motor. The rheostat is also protected inline with a 5a fuse.

not sure about the motors having thermal overload protection in them, doubt it considering the age of the machines they are driving.

Still no problems / heat etc with new motor! [Smile]