Blower Motor Current v Blower Motor Speed

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Blower Motor Current v Blower Motor Speed

Postby Steve Smith » Fri Jan 19, 2018 3:13 pm

I hope this may be of interest as I often thought the restriction of airflow across a typical blower motor (Heater Fan etc.) would result in excessive current draw and hence a blown fuse.
This was misunderstanding on my behalf (now clarified) thanks to a recent experiment proving otherwise.

Long story short……….
A HV Battery Cooling Fan Motor had been replaced on an Electric Vehicle for a High Current Draw fault code (Customer reported an Instrument panel warning light set to “On” whilst driving)

As luck would have it, I was able to recover the replaced Cooling Fan Motor for brief testing.
The waveform below monitors the current consumption of the Cooling Fan Motor, the speed of the Motor Armature and the supply voltage whilst I momentarily blocked the Fan Motor inlet. (Closed the air intake)

Blockage of inlet.jpg
Blocked Inlet


As you can see the results above indicate the complete opposite of what I thought would occur!
With the intake blocked, the Cooling Fan Motor current consumption decreased and speed increased!

Now look at the Cooling Fan Motor when I momentarily blocked the Fan Motor outlet

Blockage of outlet.jpg
Blocked Outlet


Once again the same result, with the outlet blocked, the Cooling Fan Motor current consumption decreased and speed increased!

After a little research here:
http://hvacrfundamentals.blogspot.co.uk ... motor.html
I discovered the truth in this scenario is the fact that reducing the airflow (restricting intake or outlet) results in the fan blades transporting less air (volume) and so reduces the loading on the fan motor, hence the decrease in current and increase in speed (which is expected for reduced load)
The Link above gives a far better explanation and the included comments give a wonderful analogy of speed and load using a Wheel Barrow

I am not sure what the current draw specifications should be for the HV Battery Cooling Fan Motor given the captures above indicate approx. 25 amps continuous which seems high. (Remember the fan was not installed in the vehicle during the tests above)
What I do know is the replacement motor cured the warning light concern.

Thinking now of cabin ventilation systems and complaints surrounding low air flow/volume we still need to be mindful of the Blower Motor inlet and outlet vents (E.g. blocked Pollen Filters etc.)

However, where motor speed or repeated motor fuse failure is suspected as the cause of low airflow/volume, the motor assembly and control circuit require investigation

I hope this helps, take care…….Steve
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Re: Blower Motor Current v Blower Motor Speed

Postby steveashcroft » Fri Jan 19, 2018 8:17 pm

How do you get the speed to show as a waveform on the bottom IE 63.9 & 83.4?
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Re: Blower Motor Current v Blower Motor Speed

Postby liviu2004 » Fri Jan 19, 2018 9:19 pm

High load for such fan would be caused by bearing failures.
Restricted flow it is known to reduce efficiency, load and increase speed. Never played with a vacuum cleaner? :lol:

I will have a nice case study soon, you will see the same for the fuel pump, I need some time to make it.
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Re: Blower Motor Current v Blower Motor Speed

Postby Steve Smith » Mon Jan 22, 2018 2:42 pm

Hello and thank you for the feedback

The speed signal of the Fan Motor was captured using the Pico Optical Sensor kit
https://www.picoauto.com/products/nvh-a ... sensor-kit

Here we place a piece of reflective tape anywhere about the motor armature.
The Optical Sensor light beam is then aimed at the motor armature where the reflective tape will momentarily interrupt the light every revolution, so generating a pulse from the optical sensor (0-5 V square wave signal)

The pulse signal is then connected to Channel C and used to to denote one revolution of the armature.

Using a Math Channel we then graph the frequency of the signal to derive the speed

Using the Tools tab select Math Channel > Create > Next > Advanced and click on freq and the letter C. This will appear in the formula box, then click Next. Now enter a Name & specify a Colour and click Next, select a Unit & Range and finally click Finish. To apply and view your Math Channel tick the box adjacent to your created Math Channel “freq(C)” in the Library section of the Math Channel selection table.

FREQ C.jpg
Math Formula


I will post a video soon on this technique and use RPM (Not Hz) as this will assist with interpretation when reviewing data

I hope this helps, take care......Steve
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Re: Blower Motor Current v Blower Motor Speed

Postby Steve Smith » Tue Jan 23, 2018 1:59 pm

Here is a brief video on the method for graphing the Fan Motor Speed.

Motor Speed Math Channel Creation.mp4
RPM Math Channel
(6.08 MiB) Downloaded 76 times


I have opted for RPM as this is easier to interpret.

I hope this helps, take care.......Steve
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