Energy Savers for Induction Motors

By Marke

The energy saving technology originally developed by Nola of NASA in the 70’s frequently rears it’s head. There are many claims and counter-claims, but there are some basic facts to consider:

  • The technology operates by reducing the voltage applied to an induction motor when the efficiency of the motor has dropped significantly. The efficiency of three phase motors typically only falls significantly at less than 25 - 30% rated load. Not to many machines actually operate for periods at loads less than this except clutch driven loads. The power factor of the motor falls in approximately the same manner as the efficiency and this is used in the Nola algorithm.

  • The losses in the motor are primarily iron loss and copper loss and at full load, these are of a similar magnitude in many designs.

  • The reduction in voltage reduces the flux in the iron, and thereby reduces the iron loss of the motor.

  • The current flowing to the motor comprises two major components, the magnetising current and the load current.

  • As you reduce the voltage, the magnetising current reduces, and the load current increases. If the load current is significantly less than the magnetising current, you will see a reduction in current and copper loss. If the load current is higher than the magnetising current, you will see an increase in current and copper loss.

  • Once you have installed a Nola type device, you can not “calculate” the energy consumed. A common trap of the sponsors of this technology is to multiply the current to the motor by the voltage out of the energy saver and by a nominal power factor. This is totally incorrect and can be proven with a rotating disk KWHR meter. - gives impressive (fictional) results in some installations though!!

  • The major war cry with this technology is that all oversized motors will save energy. There is a trend today to reduce the claimed magnitude of energy saved, but in reality, motors are very efficient at full load and remain at a high efficiency until the load is very low. Some machines have a higher efficiency at 75% load than at full load. Additionally, as the motor size is increased, the efficiency tends to improve, so you can find that running a motor at 75% load is often more efficient than running a smaller motor at full load.

  • The greatest energy saving (KW) is under open shaft conditions, and the savings reduce as the load is increased and can actually become negative (waste more power) at half load and above.
    There are machines that will save energy with this technology, but these are generally small machines where the design flux density is higher than large machines and the consequent iron loss is significantly higher relative to the motor loading.

The technology is not necessarily snake oil, but many of the claims certainly are.

You can only save a portion of the energy that is being wasted.