AC/DC Motor values measured against labeled.

[Marco_F]

Hi All,

New in the forum.
Apologies in advance for my lack of knowledge in matter, some things will be easy to understand for me, others, not.
I would like to learn and understand about motors, especially because work-related.

I have a small motor. The label says (and feel free to correct me):
- 24 V
- 1.2 A
- 2 W Max
- 0 to 60 Hz
- The motor has no permanent magnets but electromagnets as stator.

Other values are not relevant to me.
My questions are:

1. Why I cannot make sense of the 2 W? What I mean is: 24 V x 1.2 A should give me 28.8 W. Why the label states 2 W?
2. Why the labels states 1.2 A? When I run the motor at 24 V DC, I can barely measure 0.55 A.
3. When I measure the motor internal resistance, I read 12 Ohms on my multimeter. But when I run the motor at 24 V DC, I measure about 0.55 A. If I apply Ohm's Law, the resistance should be 24 / 0.55 = 43.64 Ohms. But the measured value directly with multimeter gives me only 12 Ohms.

In all furnace, I feel like I'm failing to understand the equations

V = I x R
and
P = V x I (or P = V² / R )

None of the formulas I apply seem to make sense.
What am I doing wrong?
Thank you guys,

Marco

 

[waross]

2 W: Mechanical Watts, not electrical Watts. 2W/746 W per HP = .0027 HP
Why 1.2 Amps: The current increases with the load. At full load the motor will draw 1.2 Amps.
And
Resistance: That is an AC motor and the current is limited by the impedance, not the resistance on an AC inductive load.
The impedance increase with increasing frequency.
Most motors act as induction generators. When the motor is turning the motor will be generating a back Electro-Motive Force.
The EMF opposes the applied voltage so that the effective voltage driving the current through the motor windings will be much less than the applied voltage.
That motor is probably highly resistive (low X/R ratio).
A very low X/R ratio will explain the motor usability from zero Hertz (DC) to 60 Hz.
Ans also will explain the wide difference between the electrical Watts (1.2 x 24 = 28.8 Watts) and the low mechanical Watts (2 Watts).
If you compare the motor speed when supplied with 24 Volts DC versus 24 Volts RMS AC, you may find that the motor runs slightly slower on AC and draws slightly less current on AC. (Less current means less torque and less HP, less torque means less HP.)

In grade 10 science class and in grade 12 honours physics class, you are taught very basic concepts.
When you get out in the real world you learn that things are a little more complex.
Current equals Volts divided by Resistance: No; Current equals Volts divided by Impedance.
Power (Watts) equals Amps times Volts: No; Power equals Amps times Volts times Power Factor times Efficiency.
Electrical power in Watts equals Volts times Amps: mechanical power in Watts equals ( Volts times Amps) minus losses.
Conversion between American Gallons and Imperial Gallons (Used in Canada).
According to any Canadian text book that I have ever seen, Five American Gallons equals four Imperial Gallons. 5/4
In the real world: Six American Gallons equals five Imperial Gallons. 6/5 (Feel free to challenge me on this.)


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Ohm's law
Not just a good idea;
It's the LAW!