Ahorro Motores De Inducción

© T.M.Empson 1998

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Energy Saving systems for Induction motors.

Are they a sham? Do they work?

There seems to be a resurgence in interest in the Nola energy saving algorithm for

induction motors, with a number of manufacturers beginning to market “new” and

“improved” versions of this technology.

The technology was originally proposed and developed by Frank Nola of NASA in the mid to late 70s

as a means of reducing energy wastage on small single phase induction motors. From the initial NASA

developments, we saw a number of manufacturers world wide gaining manufacturing rights and

marketing the technology in various forms. Difficulties were experienced in the early days in applying

this technology to three phase motors in a fashion that it would perform with stability and reliability.

Many patent applications were made in the early 80s covering variations on the technology as it could

be applied to the three phase applications. An early application by Rutherford and Empson was

successful in both operating as specified and in being granted letters patent. Many of the other early

three phase patent applications were purely speculative and could not possibly achieve the desired

results.

The concept of energy saving has always been an attention grabber, especially when the promised

savings are high and the potential for a reduction in running costs appears high. The initial introduction

of this technology was a marketing person’s dream, and some very extensive marketing plans were

implemented in the early 80’s. Unfortunately, the marketing was based on the results achieved with

very small machines, and expectations were high because of the results so achieved. There were

many promises made to prospective users based on extrapolated data which was not field verified at

an early stage, and could not be realized in real applications. I recently reviewed some promotional

material which disturbed me in that exactly the same misrepresentation as was common from some

suppliers of the technology in the early 80’s was again the foundation for a major promotion of this

concept.

As we experienced in the early eighties when we were manufacturing similar products, there appear

to be many misconceptions about the performance of induction motors and many claims are based on

the presumption that the induction motor at less than full load, is an inherently inefficient device. We

withdrew from promoting this type of device as a result of expectations in the market place that

resulted from overzealous marketing with totally unrealistic claims which could not be achieved without

inventing perpetual motion. - I recall a refrigeration engineer who had been promised a 50% energy

saving on his 50KW refrigeration units which were constantly running at about 50% load. He had tried

numerous units to no avail, and eventually approached me on a recommendation. I asked him for the

efficiency of the motors at this load and he found that it was about 87%. As he immediately began to

see, there was no way that he could save 50% of the energy consumed by the motor. To do so would

require a motor efficiency of over 100% which is just not possible with an induction motor and today's

technology.

There is no doubt that under the right conditions, the technology as proposed by Frank Nola and the

many variants thereof, can reduce the energy drawn by an induction motor, and thereby achieve some

benefit. The problem is that as the result of limited technical understanding of the induction motor and

it’s characteristics, erroneous claims are being made by extrapolation of results achieved with small

motors. Worked examples often show flawed methodology in making power measurements in three

phase three wire installations.

1. The Technology.

The basic algorithm is proposed by Frank Nola 20 years ago is to monitor the power factor of the

motor, and to reduce the voltage when the power factor is dropping in a manner as to increase the

power factor. There is a correlation between the power factor of the motor, and the motor efficiency

such that the power factor will begin to fall when the efficiency of the motor falls. As such, the energy

saving algorithm will act to improve the motor efficiency by reducing the iron loss in the motor. In some

cases of very lightly loaded motors, it will also reduce the magnetizing current and where this is much

© T.M.Empson 1998

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greater than the work current, the copper loss may also be reduced. Although there may be some

slight differences in the way the modern algorithm is implemented, I do not believe that there can be

any significant improvement in the energy savings experienced in the early days when we were

experimenting with this type of product. We found that the limitation was not the controller, but the

inherent efficiency curves and characteristics of induction motors.

2. Induction Motors

Large induction motors are inherently very efficient with efficiency figures as high as 95% at full load

being quoted. The efficiency will fall at reducing load, however the efficiency only falls by a small

margin between full load and half load. For example, a Brook Crompton 110Kw motor type 7GUD315S

is rated at 92% efficient at full load, 91% efficient at three quarter load and 89% efficient at

half load. As the shaft load is reduced, the current reduces, reaching a minimum of the magnetizing

current of the motor. The magnetizing current for an induction motor can vary between 20% of the

rated full load current and 60% of the rated full load current of the motor depending on the motor

design. As the load is reduced, the power factor of the motor also reduces by a small margin. With the

Brook Crompton 110Kw motor, the power factor at full load is quoted at 0.92, three quarter load is 0.91

and at half load is 0.88. In this example, the potential energy saving at half load is very small. It is

probably not unreasonable to expect that the maximum efficiency of a given induction motor is not

going to be much in excess of its rated full load efficiency, so a half load efficiency gain of 4% may be

achievable under ideal conditions, but with the non sinusoidal currents created by the use of an energy

saver, this is not achievable in practice.

Induction motors have five major components of loss; Iron

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