incandescent motor start current limiting?

 

In another thread, someone mentioned using incandescent
lights to limit motor starting current.

My reaction is (heel of hand to side of head)
'duh, why didn't I think of that'

It seems such a simple and obvious idea - If it works.
But I've never seen it suggested before (it was mentioned
that it's done on railway equipment)

I'm wondering if anybody has any experience of this that
they'd care to relate?

I'm thinking something like: take a cheap-o 500W modified
sine wave inverter put 1kW of incandescent bulbs in line
and try and run a 1/2 horse motor.

 

I remember mention of that.

Trouble is, as the bulb gets hot the resistance goes up instead of
down. The resultant can be low voltage to a synchronous motor during
running and the current goes up burning out the windings.

Now if the bulb resistance went the other way. There are temp
dependant resistors used in degausing schemes in TVs.

 



What SF said, the bulbs resistance is low when cold which is when you want
the maximum resistance to limit inrush current. (This is why most bulbs pop
when you switch them on). I'd be more tempted to try a HSW600 power resistor
which are expensive at 32 UKpounds.
http://www.mulder-hardenberg.com/update/uploadpdf/doc832_HSW008_4.PDF#search= 'hsw600'

Switch it in whilst the motor is starting for say 1 second or so then switch
over to direct power. The advantage of the resistor is it will last longer
than a collection of bulbs so should prove more economical in the long run.


--
Malc

"Okay, it works now. Or at least it malfunctions in all the expected ways."
 - Mark Edwards, asr

 



I have used incandescent lamps to limit current. It's essentially a type of
resistor that starts out at near-zero resistence, then rapidly rises to the
ohm-value that corresponds to the current load value of the bulb at full
brightness. (see ohm's law)

I used it to convert a momentary-type solenoid into a continous-duty one.
The two types of solenoids look similar, except the monentary-type is used
for starting, and would overheat if left on in a continous circuit.  An
incandescent bulb wired in series with the actuator circuit of the solenoid,
allowed full current to flow through the solenoid for a fraction of a
second, allowing the internal plunger to actuate into the "on" position.
After that, the bulb limited the current to a level sufficient to keep the
solenoid energized, but not overheated.  The bulb also served as an
indicator light for the solenoid.

An incandescent bulb could also be useful in a motor starter circuit. If the
current draw is too great for the power source, it will trip out the breaker
before the motor starts up.  An incandescent bulb in series with the starter
could limit full starter power to a much shorter time, just briefly before
the filament in the bulb heats up.  It may still be sufficient to start the
motor, but it will only use a small fraction of the normal power used for
startup.

A good example of this is a well water pump. A standard electric motor on
such a unit would have sufficient starting power to start under full load.
Most such water pumps are the centrifugal, vane-type, and do not experience
much load when first starting up. They do not need the full measure of
current the starter would normally draw.

I'm not sure how you would wire an incandescent bulb in series with the
starter circuit of a motor. I am sure you couldn't put it in series with the
motor windings itself.  That would starve the motor for current and have
detrimental effects.

I prefer the method of stretching out the springs in the centrifugal starter
switch instead. That essentially does the same thing, limiting the period of
maximum current draw to a much shorter time on startup.

 




The thing is (as I see it) the characteristics of a bulb are inverse to what
the OP needs. He wants to use a 500W invertor to drive a 1/2 HP (373 ish
watts) so what he needs to do is to limit the inrush current and then allow
full current in much the same way as you use star-delta switching with big 3
phase motors. A bulb allows a greater current when cold which is almost
exactly not what he needs.

But I'm open to persuasion that I might be wrong

--
Malc

"Okay, it works now. Or at least it malfunctions in all the expected ways."
 - Mark Edwards, asr