Re: (ET) Replace a lift strap, motor, etc....

["Konstanty, Walter (GE Energy Management)" <Walter Konstanty ge com>]

Guys,
   While you are correct that series motors will overspeed if unloaded, 
that is assuming there is amperage available but these smaller motors have 
enough armature reaction and losses that it's hard for them to do so..... 
with that said, don't try it at home... you can damage the motor brushes 
or commutator and make it useless.
  Running a series motor at higher voltage also increases the speed 
proportionately.  Fortunately, the lift motor is a simple motor and the 
increase in loading will hopefully not exceed the thermal rating of the 
motor.  Smelling "odors" is probably varnish and not real good on the 
motor so don't push them.

  I'm retiring from GE after 37 years in the motor division this week.... 
new email will be  Walter Konstanty outlook com  .  It's been a good 
career and plan to do more with Elec-tracs and accessories in the 
future..... 

...Walt

-----Original Message-----
From: RJ Kanary [mailto:rjkanary consolidated net] 
Sent: Sunday, January 25, 2015 10:15 PM
To: Elec >> et
Subject: Re: (ET) Replace a lift strap, motor, etc....

                My Dad told me of horror stories that occurred when some 
of the shaping and processing equipment at American Bridge would suddenly 
unload a series motor. Like large rotating pieces of it taking off and 
leveling several walls before stopping......THEN the motor came apart.

RJ

On 1/25/2015 9:42 PM, David Roden wrote:
> On 25 Jan 2015 at 20:45, Chris Zach wrote:
>
>>   The RPMs really don't go up much under no load, as the series field 
>> plus the series armature cancel each other out.
> No, they don't.  A series motor MUST have a load.  Operated at 
> nameplate voltage without any load, it will speed up until it flies 
> apart.
>
> As a series motor's speed increases, the  current it draws decreases.  
> Since current is the same everywhere in a series circuit, the field 
> current falls, and so does its magnetic flux.  This makes the motor 
> speed up even more.
> And that, folks, is how runaway happens.  If you're lucky, the motor's 
> case will keep the shrapnel from ripping holes in YOU.
>
> For testing an unloaded series motor, you can only apply a fraction 
> (maybe
> 10%) of the rated voltage without danger of runaway.
>
> I suspect that the reason an unloaded ET lift motor doesn't overspeed 
> is that the worm gear wastes a bunch of power, ensuring that the motor 
> always has at least some load.
>
>
> David Roden - Akron, Ohio, USA
>
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