chengny
Full Access Member
- Joined
- Feb 22, 2012
- Posts
- 4,086
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- Location
- NH
- First Name
- Jerry
- Truck Year
- 1986
- Truck Model
- K3500
- Engine Size
- 350/5.7
There are other factors, but the load on a blower/fan motor is mostly related to the design and sizing of the blades, shaft speed & available air flow capacity.
Assuming that the speeds of these two types of fans is the same, that leaves the available air flow and more importantly the blade design/total surface area of the blading as factors on total motor load.
The surface area affected by curvature and width of the blades is wicked complicated so disregard that.
The number of blades is usually not a major factor concerning load calculations - generally more blades are specified on a fan hub just to reduce turbulence and associated noise produced by the flow across the fan blades as they pump the air. Think of it this way; a lot of blades move more little slices of air, while fewer blades move fewer but bigger slices of air. The delivered quantity is the same but since the frequency of the pulses is higher (in the fan with many blades), the noise level drops.
So when doing a theoretical calculation of electrical load on the fan motor, that pretty much leaves us uneducated types with only one thing to consider - blade length.
Looking at the images would it be fairly accurate to say the A/C fan blades are about 3.75" in length, and the heat only fan blades are about 2.75" in length?
If so, we get a ratio of 3.75/2.75, which equals 1.366 and it follows that the surface area of the A/C blading is about 40% greater than the non A/C fan.
It would also follow that the electrical load on the A/C blower motor would be 40% greater at the highest speeds.
Thats the theoretical approach. Here is the real world way to determine the actual difference in motor load - expressed as amperage draw:
Use a battery charger that has a ammeter to show charge rate (it has to be an old type that will supply current even if the battery is dead - not one of those new ones).
Take a good heat only fan and attach the positive clip to the fan terminal and the negative to a good clean ground on the frame/motor housing. Turn the battery charger on. This will run the blower as if the control switch was on the highest setting (i.e. with the resistor stack by-passed). Note the amp draw as indicated by the meter on the charger. Record that and disco the charger.
Do the same test with a fan used in a A/C equipped system.
Compare the two readings.
This is best conducted with both fans mounted in their native blower housings and also with the ventilation controls set to the maximum flow position.
But to do all that is a giant pain in the ssa. So if this test was conducted with the fans clamped gently in a vise (i.e. with unlimited available air flow capacity) it would still provide very accurate data on the amp draw of the motor - it will be higher for both than if they were installed but the ratios should be valid.
I was going to do this test today for you dude, But I couldn't find a heat only fan and the Pats game was on. I'll see if I can find a heat only fan - I might have junked my last one a couple of weeks ago. My wife has been cranking up the old "this house looks like a gas station junkyard (whatever the hell that is)" song. So I have been reducing inventory.
Assuming that the speeds of these two types of fans is the same, that leaves the available air flow and more importantly the blade design/total surface area of the blading as factors on total motor load.
The surface area affected by curvature and width of the blades is wicked complicated so disregard that.
The number of blades is usually not a major factor concerning load calculations - generally more blades are specified on a fan hub just to reduce turbulence and associated noise produced by the flow across the fan blades as they pump the air. Think of it this way; a lot of blades move more little slices of air, while fewer blades move fewer but bigger slices of air. The delivered quantity is the same but since the frequency of the pulses is higher (in the fan with many blades), the noise level drops.
So when doing a theoretical calculation of electrical load on the fan motor, that pretty much leaves us uneducated types with only one thing to consider - blade length.
Looking at the images would it be fairly accurate to say the A/C fan blades are about 3.75" in length, and the heat only fan blades are about 2.75" in length?
If so, we get a ratio of 3.75/2.75, which equals 1.366 and it follows that the surface area of the A/C blading is about 40% greater than the non A/C fan.
It would also follow that the electrical load on the A/C blower motor would be 40% greater at the highest speeds.
Thats the theoretical approach. Here is the real world way to determine the actual difference in motor load - expressed as amperage draw:
Use a battery charger that has a ammeter to show charge rate (it has to be an old type that will supply current even if the battery is dead - not one of those new ones).
Take a good heat only fan and attach the positive clip to the fan terminal and the negative to a good clean ground on the frame/motor housing. Turn the battery charger on. This will run the blower as if the control switch was on the highest setting (i.e. with the resistor stack by-passed). Note the amp draw as indicated by the meter on the charger. Record that and disco the charger.
Do the same test with a fan used in a A/C equipped system.
Compare the two readings.
This is best conducted with both fans mounted in their native blower housings and also with the ventilation controls set to the maximum flow position.
But to do all that is a giant pain in the ssa. So if this test was conducted with the fans clamped gently in a vise (i.e. with unlimited available air flow capacity) it would still provide very accurate data on the amp draw of the motor - it will be higher for both than if they were installed but the ratios should be valid.
I was going to do this test today for you dude, But I couldn't find a heat only fan and the Pats game was on. I'll see if I can find a heat only fan - I might have junked my last one a couple of weeks ago. My wife has been cranking up the old "this house looks like a gas station junkyard (whatever the hell that is)" song. So I have been reducing inventory.
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