Efficiency of a small electrical motor

In summary: I often get efficiency as high as 95%, with no load on the output shaft.I would guess that the efficiency would be lower if the gearbox was more efficient, but it's good to have a lower limit to ensure that the efficiency is not falsely claimed.In summary, Stef has conducted an experiment to work out the efficiency of a small motor, and it comes out to be very low. He connected the motor to a gearbox (made from Meccano) and a winch and used it to lift a weight. He measured the DC voltage across the motor and the current through it. Here are his results: Motor voltage=15V, current =0.9A The motor/gearbox/winch lifted a
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?

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?

Last edited:
hutchphd
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.

Tom.G
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

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.

Lnewqban
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.

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.

russ_watters and Lnewqban
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.

russ_watters and Lnewqban

What is the definition of efficiency in a small electrical motor?

Efficiency in a small electrical motor refers to the ratio of output power to input power. In simpler terms, it is the measure of how well a motor converts electrical energy into mechanical energy.

How is efficiency calculated in a small electrical motor?

Efficiency in a small electrical motor can be calculated by dividing the output power by the input power and multiplying by 100 to get a percentage. The output power can be determined by measuring the motor's mechanical power, while the input power is the electrical power supplied to the motor.

What factors affect the efficiency of a small electrical motor?

There are several factors that can affect the efficiency of a small electrical motor, including the design and quality of the motor, the type and condition of the materials used, the load on the motor, and the operating temperature. Additionally, the efficiency can also be influenced by the type of motor and the type of power supply.

How can the efficiency of a small electrical motor be improved?

To improve the efficiency of a small electrical motor, it is important to select a high-quality motor with an efficient design and use high-quality materials. Proper maintenance and regular lubrication can also help improve efficiency. Additionally, reducing the load on the motor and operating it at a lower temperature can also increase efficiency.

Why is it important to consider the efficiency of a small electrical motor?

The efficiency of a small electrical motor is important because it directly affects the motor's performance and energy consumption. A more efficient motor will require less energy to operate, resulting in cost savings and a reduced environmental impact. It also contributes to the overall reliability and longevity of the motor.

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