# Resistance of a fan calculation help

Thread Starter

#### Redson2

Joined Jun 27, 2021
5
Hi i try to measure the resistance of a fan using a ohm rule i connected the ground of the fan to a 3 volt(2 battaries) and the possitive to a measurement device and the other leg of the measurement device to the possitive of the fan i got some result and calculated the resistance but when i increased the volt and used ohm low to calculate the i , and measured the i i got different i in practice then in the calculation
can someone explain? thanks

#### KeithWalker

Joined Jul 10, 2017
2,095
The fan obviously has non-linear Voltage/Current characteristics. There are many different kinds of fan. Some use a simple brushed motor, Some use multi-phase brushless motors driven by electronics. Some contain electronic speed controllers but none of them will have electrical characteristics like a simple resistor.

Thread Starter

#### Redson2

Joined Jun 27, 2021
5
The fan obviously has non-linear Voltage/Current characteristics. There are many different kinds of fan. Some use a simple brushed motor, Some use multi-phase brushless motors driven by electronics. Some contain electronic speed controllers but none of them will have electrical characteristics like a simple resistor.
It happend also when i tried to measure the resistance of a motor

#### KeithWalker

Joined Jul 10, 2017
2,095
Yes, of course it did. I don't know what type of motor is in the fan you are trying to measure so I can not give you any specific comments about it. Do a little research and educate yourself:
https://en.wikipedia.org/wiki/Electric_motor

#### MisterBill2

Joined Jan 23, 2018
10,532
Why measure the resistance of a fan motor? If you are wanting to know the current it will draw at some voltage you need to measure the current at that voltage. A fan is a very non-linear load because as it spins faster it moves more air, and so the load increases. So with double the voltage you will get four times the current draw.

#### MaxHeadRoom

Joined Jul 18, 2013
24,218
If this is a PC type/version, they do not connect to the motor directly, but internal controller/commutation IC

#### BobaMosfet

Joined Jul 1, 2009
1,917
Hi i try to measure the resistance of a fan using a ohm rule i connected the ground of the fan to a 3 volt(2 battaries) and the possitive to a measurement device and the other leg of the measurement device to the possitive of the fan i got some result and calculated the resistance but when i increased the volt and used ohm low to calculate the i , and measured the i i got different i in practice then in the calculation
can someone explain? thanks
If you post an image of the fan, or at least enter the image on the sticker on the fan, it will tell us
1. Model
2. Voltage
3. Current
4. Wattage
5. Manufacturer

Which may help in obtaining the datasheet for it, and answering your specific question(s).

#### Tonyr1084

Joined Sep 24, 2015
6,611
Why measure the resistance of a fan motor? If you are wanting to know the current it will draw at some voltage you need to measure the current at that voltage. A fan is a very non-linear load because as it spins faster it moves more air, and so the load increases. So with double the voltage you will get four times the current draw.
Exactly! The fan powered at 1/2X will draw so much current because it's moving so much air. But when powered at X it will draw more current. Like @misterbill said, double the air flow requires four times the amperage at its rated voltage. Now, if you double the size of the fan blades you will not move more air, you will still move the same amount of air, but your current will skyrocket because of the extra weight it's trying to push. The motor will spin slower, the voltage will drop and the current will spike.

It's called the "Inverse Square" law. For instance, and this holds true in all manor of physics. If you have a half inch pipe it will carry so much water. But if you double the size of the pipe to 1 inch then it will carry four times as much water. Just calculate the area of a circle at half inch then do the same at one inch. You'll see the result is four times greater with the 1 inch than with the half inch.

Thread Starter

#### Redson2

Joined Jun 27, 2021
5
Why measure the resistance of a fan motor? If you are wanting to know the current it will draw at some voltage you need to measure the current at that voltage. A fan is a very non-linear load because as it spins faster it moves more air, and so the load increases. So with double the voltage you will get four times the current draw.
i try to build an irobot and want to know how mutch voltage i need so i want to calculate the resistance of the fan and motors so i could know how many battaries i need

Thread Starter

#### Redson2

Joined Jun 27, 2021
5
If you post an image of the fan, or at least enter the image on the sticker on the fan, it will tell us
1. Model
2. Voltage
3. Current
4. Wattage
5. Manufacturer

Which may help in obtaining the datasheet for it, and answering your specific question(s).
i tryed searching for its data sheet and found nothing if you succeed i will thank you

#### Papabravo

Joined Feb 24, 2006
17,580
i try to build an irobot and want to know how mutch voltage i need so i want to calculate the resistance of the fan and motors so i could know how many battaries i need
This is a fool's errand. Fans run on current not voltage and the static (not moving) resistance of the motor changes as soon as it begins moving and the motor begins to generate back-emf. What you need to do is figure out the relationship between current and fan speed. Then you can trade off fan speed for battery life.

#### crutschow

Joined Mar 14, 2008
28,537
A motor generates back-emf when it is running, which reduces the current through the motor, as compared to its measured DC resistance (which mainly determines the stall or starting current of the motor).

So, as others have stated, you need to measure the current the motor is drawing when operating with its normal load and voltage, to determine the battery size (Ah) you will need.

#### MaxHeadRoom

Joined Jul 18, 2013
24,218
AVC, Asia Vital Components make fan products both DC & EC So you would need to determine the type you have.
The simple DC type can be determined by shorting the supply and spinning the fan to detect resistance,

#### MisterBill2

Joined Jan 23, 2018
10,532
i tryed searching for its data sheet and found nothing if you succeed i will thank you
View attachment 243846
The fan innthe photo runs on 12 volts, so you will need 8 1.5 volt batteries or 2 6 volt batteries. It draws 700 mA and do the battery should have several amp-hours capacity.
Now for the rest of the project " i try to build an irobot " I do not even know what an Irobot is, but probably it has a lot of other things besides a fan that use power. Or possibly not.
The amount of information not provided tells me that Much more is required.

#### Alec_t

Joined Sep 17, 2013
12,466
If you have a half inch pipe it will carry so much water. But if you double the size of the pipe to 1 inch then it will carry four times as much water.
The analogy is good apart from the fact that, other factors being equal, the flow rate varies with the fourth power of the radius, because of viscocity effects. So doubling the pipe diameter would mean 16 times the flow rate.

Last edited:

#### MisterBill2

Joined Jan 23, 2018
10,532
Do you have a set of plans describing what the irobot will be constructed of, and from? Do you have other parts and pieces to build it from? What is it intended to do?? "Robot" describes an entire realm and there is so much more than just a fan required. So for anyone to provide any more useful advice, the Thread Starter, Redson2, needs to describe what sort of irobot is intended to create.

Thread Starter

#### Redson2

Joined Jun 27, 2021
5
Do you have a set of plans describing what the irobot will be constructed of, and from? Do you have other parts and pieces to build it from? What is it intended to do?? "Robot" describes an entire realm and there is so much more than just a fan required. So for anyone to provide any more useful advice, the Thread Starter, Redson2, needs to describe what sort of irobot is intended to create.
Irobot is an automated vacuum cleaner on wheels beside the fan there are 2 motors and driver ,ultrasonic sensor and servo motor . I am using an arduino for this

#### MisterBill2

Joined Jan 23, 2018
10,532
OK, now it is far more clear what the project involves. Thank You for the additional information. Interfacing the motors that drive the assembly will be a fairly big deal, as well as interfacing with the ultrasonic obstacle sensor. And the power requirements will be much more than just the vacuuming fan motor, although tha will probably be the biggest current draw. It will certainly be a challenging project.

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