How to save energy with fixed speed motors

Fixed-speed motors comprise the largest number of motors in use in industry, but for many factories their potential for savings is largely ignored, says James Bowler.

The potential for energy saving using AC drives in variable speed systems is well known and the successes using the technology, well documented.

However, by far the largest number of AC motors in use in industry today run at fixed speeds and the potential for energy saving with these units has been largely ignored.

This policy is short sighted as worthwhile savings can be achieved for a relatively small investment using energy optimising soft starters.

Although induction motors are relatively efficient machines - motors larger than 11kW, are rarely less than 90% efficient at full load, it is well known that a motor connected to a normal fixed-voltage supply network will experience a worsening power-factor and efficiency as the motor load reduces.

Both of these problems can be addressed using energy-optimising soft starters such as Fairford's QFE series.

In theory, the potential for energy savings can arise in all aspects of a motor's operation; in practice, however, it is during the motor run phase that the soft starter's optimising function will act to improve the overall efficiency of a motor, giving rise to an opportunity for energy cost reduction.

The energy-optimising mode is the normal operating condition for a Fairford QFE soft starter once the motor has reached full speed and is driving the load at the torque required by the application.

The energy optimising process is continuous and remains in effect until it is inhibited or a stop command given.

It is also active while accelerating the motor in the ramp-up phase: here, the QFE system software computes a reference value for the power-factor of the motor being controlled.

During optimising this value is constantly compared with the running power-factor, and from this comparison the software continuously computes, adjusts and updates the firing points of the power thyristors so that the total energy delivered to the motor corresponds to that required by the load.

By eliminating the waste of energy in over-fluxing the air gap the efficiency is improved and simultaneously, the power factor of the motor is maintained at the best value possible for all load conditions.

The management of power factor by the Fairford System of control does not detract in anyway from the capability of the motor to respond to a fluctuating load demand and the performance of the motor is unaffected in every respect.

The optimising feature is a purely electrical function, which has the effect of ensuring that motor delivers the torque demanded at all times, but only drawing the precise amount of magnetising current necessary to support the motor output.

Without this feature, the motor would draw a constant magnetising current based on the supply voltage regardless of load.

* How much energy saving? - This is quite a complex question to answer since there are so many variables involved, ranging from the motor characteristics to the voltage of the supply network.

As far as the power-factor is concerned, the lighter the load, the greater the improvement.

For example, a motor running at 10% of rated output would normally have a low power factor, perhaps in the region of 0.3.

With the Fairford optimising active, this could rise to the region of 0.4 or so.

Much also depends on the efficiency of the motor being controlled.

As a rule of thumb, larger motors tend to be better designed with more copper and iron, relatively smaller air-gaps etc., making for higher efficiency machines, whereas their mass-produced low kilowatt counterparts can exhibit quite low efficiencies.

The following tables can be used to give an approximation of the potential savings to be achieved from motors connected to power supplies whose voltage supply is in the region of 5% above nominal for the motor design and whose average load is 30% or less than rated output.

Motor size (kW, HP) - estimated savings (% rated kW).

Less than 55 - 2222 - 5555 - 110 - More than 110 - Less than 7.5 - 106.53.52.51.5.

The estimated savings must be modified as follows: Motor Poles percent slip: Number of Poles - Change to est.

savings - % Slip Change to estimated savings 2468 -0.5% - No change+0.5%+1.0% - 0.5235 -0.5% - No change +0.5%+1.0%.

To estimate the potential annual cash value of the optimising function the following formula should be used: Savings (GBP) = (Unit cost of electricity/kWh) x (motor rated kW) x (hours run/year) x (% estimated savings for motor size +/- motor poles modifier +/- percent slip modifier).

For example, an injection-moulding machine, due to the cyclic nature of the load, typically runs with a load factor of 10% for 90% of the time.

So, assuming it is fitted with a 20HP (15kW) 1480 rev/min motor and operated on a 7 days a week, 50 weeks/year, 3-shift working pattern at a unit cost of power of 5p/kWh, the estimated saving would be: (GBP 0.05) x (15) x (50 x 7 x 24 x 0.9)h x (6.5 + 0 + 0)% = GBP 368.55 per year.

Depending on the tariff applied, an additional amount in the region of GBP 40/year could arise from a reduction in maximum demand due to the optimising feature.

Even allowing for an error factor of 50% (e g, motor efficiency is higher or load factor is greater), the savings would still represent an attractive rate of return on capital employed in most circumstances.

* About the author - James Bowler is with Fairford Electronics.

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