Blower motor and resistor - current draw

[Cressida]

For automotive applications does the blower motor always draw the same amount of amperage regardless of the fan speed setting? I think it probably does because don't they use the resistor to just burn off part of the amps (produce heat) for the lower fan speed settings.

If that is correct then the interior cabin blower will be drawing the same amps at any speed setting. Therefore one would not be saving power at low fan speed since the same power is always used; it is either driving the motor or, for less than max setting, generating extra waste heat at the resistor?

 

[xxch4osxx]

I have always wondered about this as well.

 

[Lubener]

It takes additional voltage applied to a single winding to increase the motor speed,with the motor's resistance held constant, amperage draw increases. Usually there is a resistive voltage divider network which will give you your different speeds.The resistors don't burn off the amperage, they limit current flow, producing a voltage drop and heat is produced as a result.

 

[CR94]

Originally Posted By: Lubener
...The resistors don't burn off the amperage, they limit current flow, producing a voltage drop and heat is produced as a result.
That's correct. The resistor(s) is in series with the motor. That reduces both the voltage available at the motor and the current through the motor. Power is wasted at the resistor, but not as much as the additional power the motor would consume without the resistor. That's the simple, inefficient, conventional way of cheaply achieving multiple fan speeds. Some modern cars (including Prius) use electronics to do that.

 

[Anduril]

Current draw still goes down across the entire circuit. To illustrate, say you have 2 100 watt light bulbs in series. Each gets half the voltage, but because their resistance hasn't changed, the halved voltage can only drive half the usual amperage as well. Each bulb ends up drawing 25 watts, and the entire circuit draws 50.

The resistor lowers the voltage, consuming a small amount of power. The reduced voltage causes the blower motor to draw proportionally less amperage, and the whole circuit uses less power.

 

[Colt45ws]

You can see this if you turn it on in low mode vs in high. For me, turning it on in low has no noticable effect, while turning it on in high I can sometimes see the headlights dim momentarily from the inital jolt of current.

 

[i_hate_autofraud]

Years ago I had to fix a burned fan speed switch, low speed was low current, hi speed was hi current, about 15 Amps!

But a pal of mine was showing me how incandescent lamps and electric motors are 'non linear loads':

incandescent lamps resistance climbs with temperature and applied voltage

motors at a stand still or a stalled rotor, experience "locked rotor current" that is very high.

This has weird effects: an electric lawn mower may slow down in heavy grass, but add 3 extra extension cords and it'll almost stall in the same grass, even blow a breaker or fuse, turns out the extra voltage drop in the extra extension cords actually causes the current to climb way up when the mower bogs down in tall grass!

In the case of auto fan motors, the air flow, fan design and motor size are matched to each other, so the fan will never be overloaded. Older cars have resistors made of a spiral of nichrome resistance wire sitting in the air ducts to dissapate heat (not much, 30 watts or so). Newer cars have speed control modules (usually Pulse-Width-Modulation controls) with small heat sinks sitting in the ducts to cool off too. Guess which is more expensive !! lol

By contrast, a "resistor" by that I mean a resistor used in electronics from 1/8 watt to 100 watts, etc, is a linear load, resistance does not change with voltage or current until you push past their wattage rating.

 

[Anduril]

Originally Posted By: i_hate_autofraud
This has weird effects: an electric lawn mower may slow down in heavy grass, but add 3 extra extension cords and it'll almost stall in the same grass, even blow a breaker or fuse, turns out the extra voltage drop in the extra extension cords actually causes the current to climb way up when the mower bogs down in tall grass!

Yep, I have an air compressor that won't even start with more than 100' or so of cord.

 

[Johnny2Bad]

Originally Posted By: Anduril
Originally Posted By: i_hate_autofraud
This has weird effects: an electric lawn mower may slow down in heavy grass, but add 3 extra extension cords and it'll almost stall in the same grass, even blow a breaker or fuse, turns out the extra voltage drop in the extra extension cords actually causes the current to climb way up when the mower bogs down in tall grass!

Yep, I have an air compressor that won't even start with more than 100' or so of cord.


Try 50' of standard 14/3 extension cord, but left wound in a coil. Your compressor probably won't start.

Now, unwind the coil. Away she goes.

You should never run 100' of extension cord unless it's 12/3, by the way. 100' of 14/3 is potential fire hazard territory. With the right equipment, you will be able to see the wiring behind your wall from the outlet to the breaker heating up (glowing) under those conditions.

The thing to keep in mind is you may be aware that you have (typically) a 15 amp 120V circuit to plug into. But the wiring in your home is not limited to 15 amps, in the sense most people would think is happening. Ask for 20 amps, and it will try it's best to deliver it. Ask for 30 amps ... same thing.

You might think the breaker at the panel will trip at anything above a 15 amp draw. You would be wrong.

The breaker has no idea whatsoever how much current is flowing through the circuit. It senses heat only. In other words, when it trips, the fire was already on it's way.

And it's not a particularly precise device. It might trip at whatever heat is generated with a 15 amp load, or maybe it doesn't trip until 17 amps, or some other value. Rocket Science? Far from it. Are there brand-new breakers that have never cycled and are supposed to trip at 15 amps but instead don't trip at even 20 amp-equivalent heat loads? Yep.

Plus, every time a breaker trips, it's ability to sense the heat generated by your demand gets a little less reliable. After a while, it will not trip that next time, but instead just let the fire start. Just so you know.

 

[CourierDriver]

Electrician's know that if you increase the distance of the electrical unit from the power supply you must increase the wire size ,,,there are charts on this,,,bigger wire lets those electrons flow,,basic electricity 101,,,go to a trade school and make money...

 

[jjw6455]

"Try 50' of standard 14/3 extension cord, but left wound in a coil. Your compressor probably won't start.

Now, unwind the coil. Away she goes."

I hope you are kidding. Mechanical configuration has absolutely no affect on the flow of electricity. Measure the resistance of that wire coiled and straight, unless you have broken wires internal to the cord there will be no difference. V=I*R, no mechanical configuration in that equation.

 

[meep]

^^^ I was curious about this statement as well, only way this would work is if he's created inductance, though I can't see how with both hot and cold wound together...

 

[supton]

Originally Posted By: meep
^^^ I was curious about this statement as well, only way this would work is if he's created inductance, though I can't see how with both hot and cold wound together...


Ditto.

*

My dim memory of motors says that an electric motor can be modeled as a simple resistance and a voltage source. The resistance always exists (winding resistance), but the voltage source potential is a function of motor speed. At startup, the speed is zero and thus the supply sees the full motor resistance. As the motor spins up to speed, the voltage source increases in voltage, at the same polarity as the supply. Because this, the voltage across the resistance goes down; and thus does the current draw. With no load the voltage source goes quite high, and the apparent load seems low. However, as load is applied, the motor slows down and current goes up.

Most blower resistors appear to me to be simple series resistors, just meant to drop some voltage to the motor, thus limiting max speed.