Energy Saving

[bob14]

Hi Mark,

Have you come across del star energy controller? The controller apparently monitors the current consumption of an electric motor and automatically switches the motor from delta to star to save energy. Is it worth trying? Thanks to give your expert opinion.

Best Regards.

Bob

[marke]

Hi Bob

I am certainly aware of the concept, but I have not seen this promoted for many years.

The first question that I always ask, is how much energy are you wasting, and then how much of this energy are you likely to save.

The concept of reducing the voltage on partially loaded motors, is not new and was the foundation of the energy saving algorithm initially promoted by Frank Nola of NASA back around 1978.

The original soft start technology developed by AuCom was based on a derivative of this concept also.

The induction motor, when operating, has two major components of current : the magnetizing current and the load current.

The magnetizing current is inductive and is 90 degrees out of phase with the applied voltage. The load current is in phase with the applied voltage.

Included in the "load current" there is also a small component of loss current that is in phase with the applied voltage.

Reducing the voltage on the stator of the motor, reduces the magnetizing current and the iron loss, but also reduces the flux in the gap. Reducing the flux gap, effectively reduces the torque capacity of the motor at a given slip, so in order to develop the same level of torque, the motor slip must increase.

If we consider the shaft power of the motor and as the speed does not alter significantly (apart from a small increase in slip) then the shaft power is constant. This means that the KW into the motor to produce shaft power does not alter. At reduced voltage, the "work current" component increases and this increases the i²R losses in the stator.

So reducing the voltage on the stator :

• reduces the iron losses in the motor
• increases the slip losses in the motor
• increases the copper losses in the motor.

The reduction in iron loss will only add value IF the iron loss reduction is greater than the increase in slip loss and copper loss.
- This is generally true only at very light load (almost open shaft) or with motors that are operating with excess flux due to a) over voltage operation at rated frequency, b) under frequency operation at rated voltage, c) very small motors that tend to be saturated by design.

Then we get down to the real question, how much energy and money do we really same? in most cases, very little.
Modern high efficiency motors will have iron losses of only a few percent of the power rating of the motor, and the proportion of the iron loss that will be saved and not offset by increased copper and slip losses may be as low as 30%. So if we look at say a 100KW motor with an iron loss of say 3% and under open shaft conditions, we can save 30% of this, then the actual saving could be less than 1KW.  Typically, a small saving relative to the cost of the control equipment.

If we now look at the Star/Delta switching, this is open transition and generates significant current and torque transients at every transition and will lead to significant mechanical damage over a period of time.

 

[bob14]

Great post Mark. Thank you very much.

Best Regards.

Bob