hello

im new at this so please bear with me. im trying to figure out a couple of things i look after a large refrig plant with 6 large screws and 4 piston machines im try to get things to run better on power and starting.my boss has been talking to powerboss he is gunho about putting in these machines but im not sure. here is a couple of questions.

1/a screw compressor has a varying load the machine will load and unload on pressure but once my machine gets to about 70%load they become inefficient .but with a drive a can slow my machine down and keep it loaded.
but powerboss is telling me that is wrong and iwould be better of with a powerboss but i disagree with them.

they are saying that they can save around 12-20% on the power but they will not put one in so we can try it

these motors are around 220-265kw.

2/ i have a condensor fan turning on and off on pressure i want to install a vsd drive to these motors with a pid lood so then i can maintain a constant pressure .but once again powerboss is telling me that this is the wrong way to do it.but iknow it is right my boss is leaving this one in my hands

please help me.



thanks

fridgie100

dear sir,
first i would like to say welcom,but
basiclly , i don't undearstand clear what is the application of your qustions , so i think that you were talks about a hydrolic power station or power pack process

please replay if possible

regards

Hello fridgie100

Welcome to the forum.

There are basically two different techniques that are applied to induction motors to save energy.
One is a variable voltage constant frequency system based on patent taken out by Frank Nola of NASA back in the seventies, and the other is variable voltage variable frequency.

The variable voltage fixed frequency method reduces the iron loss in the motor and provided that the motor is operating at light load, this can result in energy savings. Reducing the voltage on a loaded motor, will often increase the losses resulting in no savings.
The amount of energy that can be saved using this method is limited by the amount of iron loss in the motor. Small single phase motors have a high iron loss per KW of motor power, but large three phase motors have a low iron loss per KW of motor power.
This can be seen by looking at the rated efficiency of the motor. The iron loss is typically 30 - 60% of the full load losses of the motor. If the motor efficiency is 95%, then the iron loss would be in the order of 2 - 3% of the motor rating and you can only save a portion of that if the motor is operating very lightly loaded, so true savings may be in the order of 0.5 - 1% of the motor rating if the motor is operating open shaft, however, if the motor is running open shaft, then the power consumed will be primarily the iron loss and windage loss with some copper loss so the power saved could be as high as 50% of the no load losses. (Still very small related to the motor rating because the no load losses are very small related to the motor rating.)

The variable voltage variable frequency method slows the machine down and reduces the mechanical losses in the machine. This can result in appreciable energy savings because the losses in a partially loaded machine can be quite high relative to the total power input to the machine. This can only work on machines that can be slowed down and still do the required work.
For example, a pump can be slowed down if the required flow is less than the rated flow. A compressor can be slowed down if the less than full output is required.

With the variable voltage method, you are trying to improve the efficiency of an already efficient part of the system. With the variable frequency method, you are affecting the overall efficiency.

The Powerboss uses the variable voltage method only, so can not improve the efficiency of a compressor except by stopping and starting it.

Best regards,
Mark.

Hi Fridgie100, the screw application is a very standard use of VSDs and it is well proven, you run the screws loaded and vary the speed. This maximises the mechanical efficiency of your machines.
The condensor fan application again is a standard one and works well, it will save considerable energy on the fans but also can save a lot of energy by controlling the compressor discharge pressure much more closely.
I have seen many systems up to 300kW running on drives.
Marke has covered off the powerboss angle nicely, they will not offer anywhere near the claimed savings, the physics simply do not allow it.

Ken

I recently got into another war of words with a Powerboss distributor. Loads of fun. He was trying to explain to my customer how the Powerboss still saves energy when its bypass contactor is closed! It just goes to show how well they actually train their distributors.