Motor Efficiency: Understand Why 0.85 Matters

[TSN79]

In an assigment I'm using the general formula P=F*v, where P is the effect of a motor. The motor drives something through a series of sprockets, who's efficiency is 85 %, and so it seems intuitive to me to multiply the resulting P with 0,85 to get true effect, but apparently the solution divides by 0,85 which results in a higher P than to begin with. I don't get the logic in this, so if someone can explain why this is correct I would like to hear it.

 

[Päällikkö]

What's the question?
Could it be that you're given the final output power (that is, after the sprockets), and asked the power of the motor?

 

[kyle8921]

Thank you for sharing your assignment and question about motor efficiency. The formula you have used, P=F*v, is commonly used to calculate the power or effect of a motor. However, in this case, the motor is driving something through a series of sprockets, which means that there are additional components that can affect the overall efficiency of the system.

The 85% efficiency you mentioned refers to the efficiency of the sprockets, not the motor itself. This means that for every unit of power (P) produced by the motor, only 85% of it is actually transferred to the driven object. The remaining 15% is lost due to friction, heat, and other factors.

In order to calculate the true effect of the motor, we need to take into account the efficiency of the sprockets. This is why we divide the resulting P with 0.85. This will give us the actual power output of the motor, taking into account the losses due to the sprockets.

Think of it this way: if you have a 100-watt light bulb and you want to know how much electricity is needed to power it, you would divide 100 by the efficiency of the light bulb, which is typically around 90%. This means that you would need approximately 111 watts of electricity to power the 100-watt light bulb.

Similarly, in your assignment, dividing by 0.85 takes into account the efficiency of the sprockets and gives you the true power output of the motor. I hope this explanation helps clarify the logic behind it. If you have any further questions, please don't hesitate to ask. Keep up the good work in your studies!