Blower Motor Question

Hi,

I have question about DC blower motor in typical automobile air conditioner. Some circuit have variable resistor that used to adjust air flow from low to hi. I'm wonder if voltage across blower motor changed or current through motor changed to make air flow more or less when you change fan speed on panel control. My guess voltage stay same and current adjusted to make more or less air out. Thanks for help.

 

[jbartos]

Suggestion: There are various dc motor types and principles. Essentially, if there is a need for the higher output (shaft) power the DC motor, no matter of which type, will consume more power. If the motor input voltage stays constant, then the only variable left to increase is the motor input current. The specific dc motor control scheme may vary. However, wherever possible, it is more convenient to control the dc motor voltage, e.g. the dc motor field voltage.

 

Thank you for reply jbartos. I look at circuit diagram and it look like voltage being controlled. Current depend on resistance of variable resistor when slide switch in truck. Resistance of motor constant. Thank for your time. Have nice weekend.

R Chung

 

[electricpete]

For speed control application typically a shunt-wound motor is used (vs series wound). That means the field is wound in parallel with the armature.
In that motor,
Eg=NKPhi
Ia = (Vt – Eg)/R = (Vt – NKPhi)/R
T = K Phi Ia = K Phi(Vt – NKPhi)/R
The T vs N curve is Straight Decreasing line with:
* Slope is – K^2 Phi^2 / Ra
* T intercept (max T) is KPhi Vt/ Ra
* N intercept (max speed) is Vt/ K Phi
There resistor may be in either the field or the armature circuit. Increasing resistance in either circuit should increase speed (of course assuming constant voltage applied to the winding/resistor combination). Although if there are resistors in both circuits than increasing both would increase speed until the point that motor stalled. If the field current is reduced too far the armature current will become excessive and burn up the armature.

 

[BJC]

GM cars, The older ones like I drive use resistors in the armature circuit to control motor speed. Turning the speed control switch from low to high reduces the resistance in the armature circuit. The last position "high" has no resistance at all in the circuit.
When the resistors go bad ( they are wire wound and in the air flow of the blower to keep them cool. You can pull them out and see if they are bad ) you have high speed only on the fan.
The resistors are fairly easy to replace on most cars. GM has also designed the control switch marginally and that is much harder to get at.

 

[electricpete]

I should correct my response to state that DEcreasing the resistance of either resistor increases the speed.

The symbols in my equations are defined as
N= speed
T = Torque
Eg = Generated EMG (internal voltage)
Vt= terminal voltage
Phi = flux
K = conversion/constant
R=Ra = armature resistance
I = armature current
Rf = field resistance - I didn't mention this one but hopefully it's apparent that decreaseing the field resistance increases the field current which increases the flux proportionally.