AC synchrounous motor - why operating temperature is lower when load increased

[goliantan]

I am recently doing some experiment about the AC synchronous motor (single phase). I am thinking of the operating temperature to be increase when load increased. But the result from my experiment is not as what I was thought. The operating temperature reduced when load increased.
Can anyone explain why the operating temperature become lower when I increase the load applied to the motor.

 

[RCinFLA]

For a synchronous motor the power factor improves as load is applied. That by itself should not be the cause. It is probably due to reduction in core losses.

Same thing happens with a power transformer at moderate loads. At higher loads the IR loss heating of wire adds to heat.

 

[goliantan]

For a synchronous motor the power factor improves as load is applied. That by itself should not be the cause. It is probably due to reduction in core losses.


During my experiment, I did monitored the power factor. Actually the power factor reduced when load increase. Input voltage remain same all the time.
Can you elabarate what actually happen that cause the operating temperature reduce when load increase? Could it due to the back emf?

 

[Pommie]

As the load increases so does the current and so the motor will produce more heat. More heat means it has to have a cooler environment so it doesn't overheat. I think you are confusing the temperature it WILL run AT with the temperature it CAN run IN.

Mike.

 

[goliantan]

As the load increases so does the current and so the motor will produce more heat. More heat means it has to have a cooler environment so it doesn't overheat. I think you are confusing the temperature it WILL run AT with the temperature it CAN run IN.

Mike.

I also think like what you thought. But the problem is I use thermocouple to measure the motor temperature when motor operating. I found that the temperature is actually lower when load is higher....that is why i am confusing...why this happen.!?

 

[Roff]

Had the motor been run, or moved from a different location, before you started your experiment? Are you certain that the motor temperature had settled to ambient before your experiment?

 

[crutschow]

The motor current, and thus its power dissipation, should always increase with an increase in load.

Was the PF lagging or leading? Was the motor set with a high field current to provide a leading PF?

 

[RCinFLA]

PF improves with loading because the stator magnetizing inductance current become a smaller percentage of motor current as it is loaded. (same thing that happens with a power transformer primary magnetizing inductance)

You may have a shaded pole or permanent-split capacitor motor.

 

[Roff]

For a given VA, doesn't a higher power factor result in higher dissipation? And, if more loading results in more current being drawn, won't the I²R losses (and temperature) still go up, regardless of power factor?

 

[goliantan]

in my experiment on the ac synchronous motor,i can concluded few finding:
when i increase the load :
1.power factor increase.
2.current measured reduce.
3.phase between input voltage and current become smaller (close to 180 degree).
4.the steady state temperature reduce.

I have did a few times to confirm my result. The results are same as above. Now I would like to know how the increasing of load to the ac synchronous motor could affect the steady state temperature? Is there any formula I can refer to so that i can calculate and estimate how much steady state temperature will differ when I change the load.

Anyone can help me up???