Place these in the weird science catagory

[Roswell Bob]

Most switched mode power supplies (smps) exhibit what is called negative resistance.

Many motors pull less current when loaded than when unloaded.

Any more?

 

[Rednecktek]

Many motors pull less current when loaded than when unloaded.

The ammeters on my compressors and pumps would disagree with that observation.

Evidently ice is technically not a solid, despite what my butt says when I slip.

 

[Roswell Bob]

The ammeters on my compressors and pumps would disagree with that observation.

Evidently ice is technically not a solid, despite what my butt says when I slip.

Ice feels very solid.

Small inexpensive motors are typically designed with minimal amounts of iron and copper. This causes large reactive out of phase currents. Basically the motor inductance becomes saturated and currents are actually non-linear. The current becomes a little bit non-sinusoidal. When the motors are loaded the voltage that the magnatizing inductance sees goes down and motor leaves the saturated region and currents decrease quite a bit. It probably isn't easy to see on a compressor or pump. It is very evident on a dyno where the motor load is controlled.

 

[robby]

I know motors are typically more efficient when run at 75% of the rated load but I have never heard of an unloaded motor actually using less power than a loaded one.

Err meant that the other way around

 

[Roswell Bob]

I know motors are typically more efficient when run at 75% of the rated load but I have never heard of an unloaded motor actually using less power than a loaded one.

An unloaded motor will always use less power than a loaded one. I assume you meant to day that you have never heard of an unloaded motor using more power than a loaded motor. A small three phase induction motor can have a larger current at no load than it does at full load. This current is magnatizing current which is out of phase with the voltage by nearly 90 degrees. So the power is very small. As the motor is loaded up the phase angle between the current and voltage changes and the power increases. I know it sounds crazy that the current can actually go down as torque is increased, but again, it is very easy to see when you load up motor on a dyno. This mode of operation is limited to small horsepower motors. The current will go down with torque. Eventually as torque is increased further the current will begin to go up. I was a technician working on motor drives when I first witnessed this. I took a motor class at UW Madison. The professor told us about manufacturers cutting corners on small motor construction and took us through the math.

 

[robby]

An unloaded motor will always use less power than a loaded one. I assume you meant to day that you have never heard of an unloaded motor using more power than a loaded motor. A small three phase induction motor can have a larger current at no load than it does at full load. This current is magnatizing current which is out of phase with the voltage by nearly 90 degrees. So the power is very small. As the motor is loaded up the phase angle between the current and voltage changes and the power increases. I know it sounds crazy that the current can actually go down as torque is increased, but again, it is very easy to see when you load up motor on a dyno. This mode of operation is limited to small horsepower motors. The current will go down with torque. Eventually as torque is increased further the current will begin to go up. I was a technician working on motor drives when I first witnessed this. I took a motor class at UW Madison. The professor told us about manufacturers cutting corners on small motor construction and took us through the math.

Yep I meant it the other way around.
I believe what your saying, but I got to tell you that it really sounds impossible, it verges on feeling like a free energy idea. I do believe you, it's just hard to conceive any situation in which more torque is produced with less energy.

 

[Roswell Bob]

Yep I meant it the other way around.
I believe what your saying, but I got to tell you that it really sounds impossible, it verges on feeling like a free energy idea. I do believe you, it's just hard to conceive any situation in which more torque is produced with less energy.

More torque requires more power. Horsepower is the product of RPM and Torque. The power is current x voltage x cosine. The cosine of the angle between current and voltage. If the current lags voltage by 90degrees the average power will be zero. (Cos 90=0) As the motor is loaded up the phase angle moves from near 90degrees (all magnatizing current) towards zero degrees (all torque producing current) (Cos 0=1). The key to grasping the concept is understanding that the phase angle between current and voltage is changing.