Energy saving device for freezers

[omega1]

What he is saying though actually appears to defy ohms law. Surely if a a motor is drawing more current then the power consumption is going to go up which inturn means the motor will heat up more and more energy will be lost as heat. If they are doing this with large motors then ir will not be beneficial to the life of the motor.

I still do not understand what he saying about controlling the speed of the motor if you reduce the voltage the motor will not run slower if it is an induction motor otherwise speed control would be easily done by varying the voltage but this makes no difference, so thyristor soft start units and inverters have been designed to do it. At the power station i work at when the reactor trips the supplies to the gas circulators are lost, the supply is then switched back onto them and the only way that can be used to control their speed is to use inverter drive units to vary the frequency.
these motors are a lot larger than what is found on compressor though!

cheers karl

 

[CHJ]

...snip...if you reduce the voltage the motor will not run slower if it is an induction motor otherwise speed control would be easily done by varying the voltage ...snip...

If my understanding is correct they are not talking about reducing the voltage, that just generates heat from waste power.

I believe they are referring to switching off the power for a fraction of a second at the phase crossover point (0volts) which wastes little power in the switching devices but has a small effect on the overall power delivered to the device, and therefore allows a small amount of rotational slippage to occur in the motor under load reducing the effective rpm..

As far as Induction motors are concerned in non critical speed situations, like a wood lathe, it is possible to reduce its rotational speed by loading it, if taken too far to the point where it is going so slow that the load overcomes the electrical torque it stalls. You will also see this on an induction motor table saw if cutting too vigorously. Taking things this far will obviously be detrimental to the motor eventually because of increased non effective current draw heating it up.

But I can see that under controlled conditions on something like a large compressor that is not speed critical that you can manage power supplied, against rotational speed loss due to load, to effect a small percentage saving.

I am no expert on High Energy power control but the above is what I understand of the subject from my time in an Aircraft electrical design office where I had to interface 60kw alternators with various aircraft systems.

 

[Anonymous]

Yep

What is happening is that the peak voltage is the same, however, the time period where voltage is applied (1/2 cycle of mains voltage) is reduced by a small period of time and so the power delivered to the motor is effectively reduced by a small amount - as is torque

 

[omega1]

cheers for the reply Tony at least you know what you are saying, the other guy was saying that if you adjust the voltage then you would adjust the speed which is not true. Adjusting the time period though will vary the speed because you are adjusting the frequency of the supply which is what i was trying to say.

If he believes that adjust the voltage make a difference to the speed then I suggest he goes back to school/college as this is basic electrical principles that I learned on my onc in the 1st year! :shock:

cheers karl

 

[CHJ]

...snip.... Adjusting the time period though will vary the speed because you are adjusting the frequency of the supply which is what i was trying to say.

They are not adjusting the frequency, Just the period of time the power is switched on. They are interrupting the supply for a very short period in every cycle.

The power during the dotted line period is switched off. An induction motor under no load will still run at its design speed.

 

[omega1]

Hi Chas
What you have described is correct for constant speed induction motors that are used continuously but the load is either on or off and during the off period the motor power can be limited thus saving energy.
Andy has said that the motor will draw more current but the power will be lower as you are basically reducing the voltage and hence the motor would run slower. This statement is not true because P= IsqR so if the resistance windings is constant and the current increase then so will the power as the equation shows. Also varying the voltage will not adjust the motors speed.
Regarding the fridge motor then that would assume that the motor is either vastly overrated or runs continuously 24Hrs a day.

Regards karl

 

[CHJ]

...snip...This statement is not true because P= IsqR so if the resistance windings is constant and the current increase then so will the power as the equation shows. Also varying the voltage will not adjust the motors speed.

Looks as though we are talking at cross purposes, and wandering away from the original topic.

I do know that an induction motor windings as the name implies are as much inductive as resistive so current draw can be quite a way out of step with the voltage peak causing considerable (paid for via a watt meter) energy losses and makes any calculations theoretical and in the real world only controlled monitoring will show the effect of all factors.
The Power factors in aircraft supplies are always checked in real time for new installations and not just signed off on the calcs.

The efforts that were put into balancing the inductive-capacitive factors involved in the 3+ megawatts that my last shop consumed resulted in 'charged power' savings equivalent to running several lathes and milling machines.

 

[trevtheturner]

"......... and wandering away from the original topic" ............

which was Dave_G asking where he could find a Savaplug!! :lol: :lol:

Cheers,

Trev.

 

[CHJ]

"......... and wandering away from the original topic" ............

which was Dave_G asking where he could find a Savaplug!! :lol: :lol:

Cheers,

Trev.

I don't know either Trevor, :lol: