Permanent Star and Energy Savings

It is PEAK torque that varies by the square of the voltage reduction, running torque is more or less linear with a slight drop in torque as the voltage is reduced. But you really cannot factor that in anyway unless by some miracle, your load never needs to accelerate. Think of motor torque as having two components; steady state torque and accelerating torque. You ALWAYS need steady state torque, but you only need accelerating torque when you first start the load, and if you have to RE-accelerate the load. Accelerating torque then is ADDITIONAL torque needed above and beyond the steady state torque, and that is the PEAK torque that we are talking about, the torque that gets drastically reduced when voltage is reduced.

When you have a PERFECTLY steady load, all you need is steady state torque. But if there is EVER a step CHANGE increase in the load, you need MORE torque, that PEAK torque. It works kind of like this: Load is increased, the motor slows down, fewer lines of magnetic force are cut in the rotor and stator relationship, mutual induction decreases, impedance decreases so current increases, higher current then creates stronger magnetic fields and more torque, the ACCELERATING torque mentioned earlier. The motor then re-accelerates the load back to the steady state, but draws more current as long as the load remains. If, in that process, you lose 67% of the available peak torque, it is going to be the ACCELERATING torque that was needed to react properly to that step change in loading.

So real world...

If you know EVERYTHING that could possibly EVER happen with regard to that motor and the load it is driving and predict EXACTLY the amount of torque that you need, the scheme of changing to Star can possibly offer a very SLIGHT reduction in the magnetic LOSSES that take place in a motor that make it a motor. But follow me closely here, its easy to lose this conceptual train and people jump off this cliff too easily because of it.

So those magnetic losses comprised 25% of the total losses, and if you reduced that loss by 10% by reducing the voltage by connecting in Star, then your losses are only 22.5% and you have reduced the losses BY 2.5%. But that is a percentage of LOSSES, not a percentage of POWER and that's where the mistake often happens. If a motor nameplate says it is 90% efficient, that means 10% of the power is losses. Of that 10% of POWER that is comprised of losses, only 25% is from MAGNETIC losses. So if you have saved 10% of your MAGNETIC losses, you have really only saved 2.5% of the 10% of total power. So that is only .0025, or .25% (1/4 of 1%) of the total power the motor is consuming.

But it is something and something is better than nothing, right? Well, maybe not. Because if there is any increase in load at all, those savings go right out the window if your motor stalls or overloads.

So where can this miraculous stead state highly predictable reduced load happen? In a very SELECT set of circumstances. A pump system that for some reason is now severely over sized compared to the capacity it needs to deliver can be put into Star as opposed to buying a new pump. For some reason, several states in India seem to think that a lot of motors used there on agricultural pumps are grossly over sized, I heard it's because the farmers are poor and the pumps were obtained as surplus from other applications. But because they are poor, they are unable to afford new pumps so they must continue to use them in a highly inefficient manner. So I have seen, from things that friends over there have sent, actual government pamphlets promoting this concept of connecting these motors permanently in Star as a way to reduce some of this gross inefficiency.

Unfortunately, what is being promoted as a specific response to a specific situation, get misinterpreted by other people who are totally unfamiliar with the original issue and over-generalize the information to think, and unfortunately promote, the idea that this will ALWAYS save energy. It does not, and in fact has lead to many many motor failures as a result.