Efficiency of a small electrical motor

[saddlestone-man]

TL;DR Summary

I've conducted an experiment to work out the efficiency of a small motor, and it comes out to be very low. Have I done my calculations correctly?

Hello All

I've conducted an experiment to work out the efficiency of a small motor, and it comes out to be very low. Have I done my calculations correctly?

I connected the motor to a gearbox (made from Meccano) and a winch and used it to lift a weight. I measured the DC voltage across the motor and the current through it. Here are my results:

Motor voltage=15V, current =0.9A
The motor/gearbox/winch lifted a 0.56kgm weight 0.2m in 25 secs.
Electrical energy used = 15 x 0.9 x 25 = 337.5W secs = 337.5J

Mechanical work done = 0.56 x 9.8 x 0.2 = 1.0976Nm = about 1J

If my calculations are correct, the arrangement had an efficiency of about 0.3%, which seems very low to me. I suspect the gearbox is not very efficient, but the results are surprisingly poor (to me anyway).

Have I made a mistake in the calculations?

best regards ... StefReference: https://www.physicsforums.com/forums/mechanical-engineering.101/post-thread

 

[malawi_glenn]

I do these kind of labs with my basic physics class (I have lego motors and some others) we usually get around 3-5% efficiency (without any gears or pulleys). Are you sure about that voltage?

Do you have the possiblity to add some pictures?

 

[saddlestone-man]

Many thanks, I'll add some photos tomorrow. Yes the voltage is correct.

Interesting about the efficiency of the Lego motors. Sound like my calculation could be correct.

The motor I'm testing is an old Meccano one and I'm guessing it was made more for low cost than efficiency. Also it doesn't have a permanent magnet, so maybe half the power is going to energise the stator.

 

[malawi_glenn]

You could also try with a lower weight, we most often get higher efficiencies for lighter weights.

 

[saddlestone-man]

Did you use the mass of the object in the mgh formula for calculating the work done? I mistakenly used the weight of the object and so was out by a factor of 9.8 for the work done. This makes the efficiency of the motors I tested even worse, by a factor of 9.8.

Here's a picture of my set-up.

best regards ... Stef

 

[russ_watters]

An erector set! I haven't seen one in a while! But what I rember from being a kid is the gears are neither lubricated nor on ball bearings, so there is a huge amount of loss just getting through the gear train.

 

[saddlestone-man]

This is Meccano, made in the UK. I think Erector is the US version?

Yes I would agree that the gear box is probably very lossy. Having just adjusted my test results by a factor or 9.8 (I originally used the weight, rather than the mass) of the weight being lifted, I now get efficiencies ranging between 0.03% and 0.17% (depending on the exact motor used and the weight being lifted), which seem very low to me.

 

[tech99]

I have often done the experiment with a good permanent magnet motor, a belt reduction drive onto a shaft and wrapping the string round this shaft, which is about 3 mm diameter. I have obtained very good efficiency from the motor, such as 80%. We also compare the energy using a Joule Meter and also by measuring V and I.

 

[jrmichler]

A good experiment is to remove the motor, and measure the power to spin the motor by itself. Then connect the motor to the gearbox and repeat with no load on the gearbox output shaft. Then tip the assembly on end so the string is being pulled straight, instead of being pulled over the "pulley".

I see a worm gear in there. A worm gear running in an oil bath can be only 50% efficient, so an unlubricated worm gear will be worse. Another good experiment is to repeat the test of the full setup, then add oil to every gear and bearing in the system including the motor output shaft.

You might surprised at the amount of friction in the total system.