Saving energy with two-speed motors

Nigel Evenett contends that there is plenty of life left in two-speed motors.

With the on-going development of AC frequency inverters, you could be forgiven for assuming that the multispeed AC induction motor or, more specifically, the two-speed motor has seen better days.

Providing the possibility of infinite speed control, together with other functionality, the inverter certainly brings benefits, particularly when used within closed loop applications.

Yet this sophisticated motor control is in reality an expensive "overkill" for simple applications, particularly for fan and pump duties.

If all that is required is a high and a low output, a two-speed motor is a simple, efficient and cost-effective alternative solution, particularly in lower power ratings below 15kW.

By their application, two-speed motors reduce energy usage by giving the user the lower speed and hence lower power option, more closely matching actual demand.

This is probably why they attract Enhanced Capital Allowances from the Carbon Trust in the same way as Eff 1 motors from manufacturers listed on the Energy Technology List.

The principal obstacle to opting for two-speed motor solution is availability.

This is because the inverter supplier only needs to stock one product to cater all speed ranges.

However, by stocking several speed options the motor supplier can still compete and, lets face it, you have to buy a motor anyway - the inverter is always optional.

Although to the uninitiated the world of the two-speed motor might be considered complex and confusing, in reality it is quite simple.

Here is a brief overview.

First, it is worth mentioning that two-speed motors are only available as three-phase machines; designing them in single-phase would be too cumbersome and not cost effective.

Secondly, there are two categories, each with its own specific drive characteristics appropriate for particular duties.

One offers variable torque which is ideal for fan and pump applications.

With this motor, output power at low speed is less than one quarter of that at high speed.

The second delivers constant torque, which is particularly appropriate for crane and machine tool applications.

This motor delivers more than half the output power at low speed that is provided when it is operating at high speed.

There are a number of design choices.

The simplest design uses two separate windings (or dual wound) because the motor has two distinctly different windings of different polarities within the same lamination pack with each having its own power rating.

With each winding totally separated it's like having two motors within one.

This enables motors to be designed with star/delta starting for one or both speeds.

This design is also used for those applications where the polarity ratios of pole change are inappropriate.

Dahlander, pole changing or tap wound design comprises a single winding with two stator winding polarities which gives the ability to change motor speeds by varying the connections.

Polarity speed ratios of 2:1 are obtained by bringing out centre tapped leads from the winding and double starring the high speed configuration.

This means that to engage the low speed a simple three-phase feed is required.

However, when switching to high speed, control gear must put the low speed terminals into the star configuration.

Because the motor only has a single winding, this is achieved simply and cost-effectively by using three simple contactors.

If you are getting ahead of me here, you would have realised that it is possible to have a combination of both the above systems and obtain a three- or four-speed motor.

Such designs can exist but they have all but died out in new selection because when more than two speed settings are required the inverter comes into it's own and is usually more cost effective.

The third kind of two-speed motor is the PAM (pole amplitude modulation).

Developed from the pole change winding, it enables the provision of polarity ratios other than 2:1.

This again is a single tapped winding with internal connections.

These connections are all internal to the motor.

In operation it is similar to the Dahlander motor type except it gives scope beyond the 2:1 ratio.

With strong links to the ventilation industry, Lafert carries European stock of variable-torque motors ideal for fans, in the two separate winding types: 4/8-pole, 4/6-pole and 6/8-pole.

In the Dahlander type, 2/4-pole, 4/8-pole and 6/12-pole are available.

Power ratings available vary from 0.25/0.07 to 17/4kW in motor frame sises 71-180 all rated at 400V three-phase 50Hz.

The multimount design of the Lafert AM range ensures short delivery times for any mounting configuration B3, B5 or B35.

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