# How can i find the RPM/torque of this electric fan with all of this information?

• thebluser
In summary, an ideal electric fan with a constant supply of 0.49 HP (367.8 watt), 3 motor poles, 8 blades, and a radius of 36 cm, would have a torque of 1.46 Nm and a speed of 2400 rpm. However, without knowing more information such as the type of motor and its efficiency, it is impossible to accurately determine the torque or RPM.
thebluser
an ideal electric fan has: a constant supply of 0.49 HP (367.8 watt), 3 motor poles, 8 blades, radius of 36 cm. based on all of that info. how can i find the torque/RPM or both?
if you know the answer tell me it with the method to find the torque/RPM.
p.s. i just threw in all of the information that i know about so if one of the information pieces is not necessary, that's why. also, i don't have an electric fan that has all of this information so i can't test it, nor build it.

Power is torque times angular frequency

so you need to pick a speed to get the torque or vice versa. Motors with the same power will differ in actual torque, depending on their speed.
Angular frequency, ω is 2pi s/60
where s is the RPM figure

You'd have to make a lot of assumptions here, such as at the motor running at peak power rpm when driving the fan. Then assuming an efficiency of say 80%, you'd need to find at what rpm the fan consumes .8 x 0.49 hp = .392 hp. An ideal dc motor would have to be designed so that it's peak no load rpm would be double the rpm the fan needs to consume .392 hp. An ideal dc motor produces zero torque at peak rpm, maximum torque when stalled, following a linear formula based on torque = peak torque x (1 - rpm/(peak rpm)). Maximum power occurs at 1/2 peak rpm, 1/2 peak torque x 1/2 peak rpm.

an ideal electric fan has: a constant supply of 0.49 HP (367.8 watt), 3 motor poles, 8 blades, radius of 36 cm. based on all of that info. how can i find the torque/RPM or both?

Unfortunately that's not enough information.

If it was a 60Hz AC motor the rpm might be roughly

Ns = 120 F/p

where

Ns = Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles per phase winding

Ns = 120 * 60/3 = 2400 rpm

power = torque * Angular velocity

so

torque = power/angular velocity

= 367.8/(2*pi*2400/60)

= 1.46 Nm

Last edited:

## 1. How do I calculate the RPM of an electric fan?

To calculate the RPM (revolutions per minute) of an electric fan, you will need to know the number of poles on the motor, the frequency of the power supply (in hertz), and the number of phases. Use the formula RPM = (120 * frequency) / (number of poles * number of phases).

## 2. What information do I need to find the torque of an electric fan?

To find the torque of an electric fan, you will need to know the power input (in watts), the RPM, and the efficiency of the motor. Use the formula torque = (power input * 60) / (2 * π * RPM * efficiency).

## 3. Can I find the RPM/torque of an electric fan without knowing the efficiency?

Yes, you can still calculate the RPM/torque of an electric fan without knowing the efficiency. However, the result will not be as accurate as it would be with the efficiency included in the calculation.

## 4. What is the significance of the number of poles on an electric fan motor?

The number of poles on an electric fan motor refers to the number of magnetic poles present in the motor. This number determines the speed at which the motor will run and is essential in calculating the RPM of the fan.

## 5. Is there a simpler way to find the RPM/torque of an electric fan?

Yes, you can use a tachometer to directly measure the RPM of the electric fan. However, for calculating the torque, you will still need to use the formula mentioned above.

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