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What is the nominal power of a domestic universal motor?

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Viggo
#1
Mar4-14, 06:37 PM
P: 2
I have measured some properties of four sewing machine motors for domestic use. They are all universal motors (series motors) for the public grid voltage in Europe. The four motors are:

a) Singer 66K motor. Motor seems to be produced about 1930 in UK.
b) Singer 201K motor. Motor seems to be produced about 1950 in UK.
c) Sytrix BF40 motor. Made about 1970 in Denmark. Used on a Singer 237.
d) Chinese 180W motor made 2013. http://www.ebay.co.uk/itm/121137592820

The shaft power measurements are made by connecting the motor to a 6 mm diameter shaft, which also are a winch lifting weights from floor to ceiling. The weight passes some marks on a ruler on its way up. The winch thread diameter is 0.5 mm. A small video camera was used to record the lifting, and by a study of the video frames, the speed and power is calculated. A normal kitchen scale is used to measure the mass of the different used weights. The applied voltage is approx. 230 VAC, 50 Hz.



I have the following comments:

The Singer 66K and Sytrix motors have expected performance.

The Singer 201K motor seems to have an error in the design or a systematic production fault. Lower DC resistance and higher shaft power can be expected from a motor of this voltage and weight. The unexpected high DC resistance is both on stator and rotor windings.

The Chinese motor becomes uncomfortably hot when used in a few minutes. The design seems to have focus on using as little steel and copper as possible. The missing label and EMI filter makes it illegal to sell in most countries. I bought this motor 3 weeks ago, and knew that I could buy some performance issues it was inexpensive and the nominal power was high. Perhaps I should have asked for the weight of the motor beforehand, because it could give a clue on what to expect.

What is to be expected, when you buy a domestic electric motor with a specified nominal power rating? How can you determine, that such a motor is more a heating element than a motor?

On industrial AC motors, the norm specifies to write shaft power on the label. What can be expected from domestic electric motors?
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Windadct
#2
Mar7-14, 10:15 AM
P: 575
Hello Viggo - Motors in appliances are definite purpose - i.e. they are designed exclusively to run the appliance, how or how well they accomplish this is totally up in the air. The difference in the Singers could be due to age of design, cost of materials ( grade of the steel used) or single unit errors as you mentioned. As for the Chinese less steel and copper -- that cuts the cost, but makes for a lousy motor... to be expected?
I do not know what you mean by a "domestic motor" - I am assuming you mean use in a house - I can not think of any motors for use in a home that are not specifically for the application - as such each application then has different requirements, duty cycle (ratio of on / off or full load vs no load) is a big one, that changes with every application. Another critical design element is reliability - typically the cost of the appliance determines how hard the OEM builds for reliability -- AC units long time and many starts, blender 2-3 years of real use - as long as it blends well you will buy another one - and while they may not have motors don't get me started on toasters!
For industrial motors, general purpose, the nameplate and datasheet values should be referring to some standard as to how the data was generated, in the USA it is typically NEMA(Natl Electrical Manufacturers Assn), but there is also UL, or other standards that it may be required to meet. There are also many different grades, temperature ratings, efficiency - all affecting cost, weight and need to be accounted for in the application. A High efficiency induction motor may have 20X I nom inrush current on starting -if your supply can not provide that current then you may have a problem - or damage the motor - etc.
Motor selection, sizing, and application can be a full career.

Did you do the above analysis just for fun? For a casual analysis good work - I would identify which values are labeled and which were measured.....


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