Voltage/Current Relation That I Having Problems With

[wirebiter]

I know I knew this many years ago and I am sure the answer is simple but I would really appreciate the help.

Event That Brought Up The Question:
Working on a complany vehicle, there was a heater fan that would keep failing. As we troubleshot, we saw that system voltage was low (bad alternator) causing the fan to draw more current (possibly, at least this is what I was told and I seem to remember that this is in fact true).

If this is correct, my question is how?

In my head, I am thinking that the fans resistance (at least before being damaged) would have stayed the same...therfore with:

I = E/R

Resistance staying the same and system voltage droping, current would then drop and not overheat/damage the fan. But then, that doesn't fit the example.

If I look at it as having to maintain the same power, then:

P = I x E

To maintain the same power, and voltage decreases, then current must increase which does fit the above example.

But, then if current increases, resistance had to decrease (Im pretty sure that wouldn't have happened to the fan, again..at least not at first) OR, voltage had to increase, which does not fit the above example. (Using I = E / R again)

I know I am just missing a simple part or concept, or I am over nuking things, but if someone could clear this up for me I would appreciate it. (Hopefully I explained it well)

 

[sophiecentaur]

It's not just the resistance of a motor that defines how much current it draws. When it spins, it produces a 'back emf' which limits the current when the motor is actually turning. Before the motor is up to speed, the current will be higher (maximum when the motor is stationary or stalled) However, normal operating current shouldn't go 'up' as the volts drop - all things being equal. Is it possible that there is a lot of friction somewhere? This could be overloading the motor, limiting the top speed, which induces less back emf so you may be getting a near-stall current. Bottom line is that I think the low volts may be a red herring.

 

[jim hardy]

A heater fan in my day was a shunt-wound DC motor though they might be permanent magnet now.
Speed was controlled via resistance in series with the motor's field winding.
A DC motor has the property that the less the field the faster it wants to turn.
So the higer speed position had more resistance, just backward from what you'd expect.

IF a shunt wound motor loses its field current it will try to turn VERY fast and the armature current will go high as it accelerates.
If it has a fan blade connected to its shaft it will draw enormous current trying to overspeed that fan.

I suspect that was the root of your trouble.
If you still have the truck, look around heater fan speed switch for chrome wire in a spiral - that's the resistor in series with field. Usually there's two or three , one ofr each speeed. If one is broken the motor will either run away when selected to that switch position or blow its fuse (typically a 20A fuse).

I had an old Ford in which mice built a nest in air duct behind heater switch where those resistors were located.. One day i smelled smoke and embers came out the dashboard vents - they'd blocked airflow to the field resistors...heat plus fuel plus air, you know..

 

[sophiecentaur]

IF a shunt wound motor loses its field current it will try to turn VERY fast and the armature current will go high as it accelerates.
If it has a fan blade connected to its shaft it will draw enormous current trying to overspeed that fan.

Where would the force come from to make it turn? I don't understand. No magnetic field.

 

[jim hardy]

residual magnetism can overspeed an unloaded motor.

a loaded motor with no field will pull very high armature current as it cannot make much counter emf on the meager residual field.

i will back up on 'overspeed the fan'

i should have said 'trying to to drive the fan'

indeed you won't make a lot of torque without a field.
current will be high, perhaps limited by resistance of armature and connecting wires.

Sorry for the mis-statement. i typed before completing thought process. thanks !

 

[sophiecentaur]

So the armature rotating in a low field won't produce as much back emf. That figures. Cheers - I must say, it read all wrong at first!

 

[jim hardy]

Cheers - I must say, it read all wrong at first!

thank you for the catch Mr Sophie.

i spoke up before finished chewing on the thought. I'd give myself a "wrong" on my inital post.

thanks again for catching it .

Cheers also !

old jim