Motor With No Load ?

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ibew441dc

ibew441dc

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What is the technical term to describe the condition in which a motor will eventually burn itself up in the event that it continues to run with no load?
 
Well Marc, I don't know the technical name, but it is real!

Every year the earth's orbit decays somewhat and we get a teeny bit closer to the sun. The earth will eventually get close enough to the sun so that the motor will burn up. :D
 
I know you can burn one up pretty quick running it real slow off a VFD for long periods of time if it doesn't have external cooling.
 
ibew441dc said:
What is the technical term to describe the condition in which a motor will eventually burn itself up in the event that it continues to run with no load?
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It is one of the storys in the mother goose collection.
 
That would depend on the type of motor.

A common AC induction motor will happily run with no load.

Some brush commutated can exceed their rated speed if run without a load.

I believe that the technical term for damage in such a state is 'rapid self disassembly' *grin*

-Jon
 
Some of the fancier overload units, particularly one's used on conveyors, will alarm out if the motor suddenly becomes unloaded. That's kinda handy. Shut the whole system down so you don't have boxes piling up all over the place because one section of conveyor broke a coupling or whatever.
 
winnie said:
That would depend on the type of motor.

-Jon
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Good point, I should have been more specific.

Is there a technical term to describe the condition in which a DC motor will eventually burn up in the event that it runs continuously with no load.
 
080516-1658 EST

ibew441dc:

The series type motor has already been mentioned.

A permanent magnetic DC motor unload and at full line voltage only dissipates power from windage, friction, and core losses. This is small compared to full load. The life would be long until the bearings and brushes fail. Also there might be a carbon buildup problem.

On the other hand an unloaded shunt wound DC motor that looses shunt excitation will try to rotate at a much higher speed than normal. This may cause the motor to explode from centrifugal force. The reason is e = df/dt. The counter EMF from the rotation of the armature needs to approximately balance the applied armature voltage and when the field is lowered because of no excitation the rotor has to move faster to produce the same rate of change of flux to provide the necessary counter EMF.

.
 
ibew441dc said:
What is the technical term to describe the condition in which a motor will eventually burn itself up in the event that it continues to run with no load?
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Oops?

Or, 'I hate when that happens!'

The term you are looking for may be 'overspeed'. There are many devices that prevent overspeed in DC motors. Some use centrifugal switches while others are electronic in nature. Some motors, like car starters, have no such protection.

For those that don't think it can happen, take a car starter and let it run without a load while connected to a constant 12 volt supply. I would hazard to guess that it will self destruct in less than an hour. More likely, in a few minutes or so.
 
080521-2058 EST

crossman:

I do not believe there is any normal motor that is more likely to burn up when unloaded vs loaded. Therefore, I had assumed that maybe the question really related to motor runaway instead. This is sort of like sending someone to get a left handed monkey wrench, or maybe some #88 wire.

There is a story of someone wanting some fine wire. I do not know the size now but it was very small, they just placed an order for 1#. They got a call from Belden to inform them that this was not feasible because it would consume a large portion of their resources for some time.

.
 
This one is driving me crazy

This one is driving me crazy

ibew441dc said:
What is the technical term to describe the condition in which a motor will eventually burn itself up in the event that it continues to run with no load?
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This one is driving me crazy.

First, why would an AC motor with no load burn up idling along on a VFD at slow speed? Who cares if the rotor connected motor cooling fan (assumption) is also idling? No Load = No torque = No heat, at least nothing to be concerned about. (constant torque app yes, variable torque no)

Second, a shunt wound field DC motor was mentioned. I would be more concerned about flying shrapnel than heat if some antiquated controller be unable to shut down the motor on field loss.

Third - Permanent magnets? Let off the key perhaps, maybe it’s the “Bendix”

The origin of the thread asked for a term. There is a burn out term for a motor with brushes. I just can’t remember it. But since the material composition of the brushes is to match the design rating of the motor; and if that motor operates continuously at some load significantly lower than that rating, the brushes will fail prematurely due to thermal degradation due to mismatched brush chemistry. It’s true. I’ll find the term, if someone doesn’t find it for me first.
 
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Ranch said:
This one is driving me crazy.

First, why would an AC motor with no load burn up idling along on a VFD at slow speed? Who cares if the rotor connected motor cooling fan (assumption) is also idling? No Load = No torque = No heat, at least nothing to be concerned about. (constant torque app yes, variable torque no)...

.
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But there is a load, that of the motor attempting to run with windings that are designed to be cooled by air flowing past the coils.
 
mengelman said:
Rotational fatigue
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You nailed me, please, no more motors ... pleeeease, ok one more

how about a down hole submerged motor running at 30Hz driving a centrifugal pump that can't overcome TDH and create net flow .... yeah, it would burn up alrighty churning water until it boiled
 
Are you thinking of a large generator on a grid losing its excitation field and then gets driven like a motor from the power off the grid. You see grown men shudder when you talk like that in a powerhouse because it does occasionally happen and its not pretty when it does.
 
quogueelectric said:
Are you thinking of a large generator on a grid losing its excitation field and then gets driven like a motor from the power off the grid. You see grown men shudder when you talk like that in a powerhouse because it does occasionally happen and its not pretty when it does.
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It's not that it is driven like a motor ... it won't overspeed. It is the time for the (typically steam) energy in to be cut off by the overspeed detection. Inertia is our friend here ...

The main and reheat steam valves will close pretty quickly. If they don't, RUN!
 
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