According to ABB Drives, there are some 10 million electric motors driving production within the UK. They are by far the biggest users of electricity in industry, so controlling them for optimal production is a key target. Variable speed AC drives (VSDs) play an important role in delivering energy savings.
The original sizing of motors is often based on an operation under the worst conditions, often resulting in unnecessary oversizing. Often when selecting components from catalogues, the correct size is not available, leading to the purchase of larger units. This oversizing is a source of waste and inefficiency.
Industrial load types
In general, all motor-driven loads fall into two groups, either variable-torque or constant-torque. Applying VSDs to variable-torque and constant-torque loads can result in energy savings and productivity improvements. However, energy savings are most noticeable when applying drives to variable-torque loads.
Variable torque
Variable-torque loads include centrifugal pumps and fans, employed in a variety of applications such as:
- Air handling units
- Industrial process cooling pumps
- Chilled water pumps
- Hot water circulation pumps
- Cooling tower fans
- Return air fans
- Circulation and supply pumps
- Combustion blower fans
How energy savings works
The requirement for torque, and hence current, increases with the square of the speed. The voltage varies in proportion to the speed, so power actually varies in proportion to the cube of the speed. Hence, by reducing the speed, the power consumption falls by the cube of the speed change. Engineers refer to this relationship as the Affinity Law or the Cube Law.
The savings derived from the Affinity Laws are theoretical. In real applications, the particular characteristics of the system will determine the actual savings achieved. To determine realistic savings, therefore, it is important to have experienced appraisal engineers inspecting and analysing the system. Short payback times are often achievable, but they must be realistic.
Benefits of using VSDs with variable torque motors loads
Reducing the speed of a pump or fan with a VSD will often save between 30 and 50 per cent of the energy, while savings of up to 80 per cent are possible in some cases. In reality, very small speed changes can yield large energy savings. These speed changes will often be almost unnoticeable in the overall process. As an example, a ten per cent speed reduction [5Hz] can result in a 27 per cent energy saving.
The VSD can also match the exact needs for cooling or lubrication or airflow or pressure, thus allowing optimal process performance, increasing product quality, and reducing wear and tear.
Constant-torque load settings
With such applications, the torque requirement throughout the speed range is the same. The power change is proportional to the speed of change.
- Typical constant-torque load applications include:
- Air compressors
- Hydraulic power packs
- Conveyors
- Mixers
- Positive displacement pumps
- Rotary kilns
- Agitators
- Crushers
- Surface winders
Benefits of using variable-speed drives
While there are still savings available, they are not as dramatic as those seen in variable-torque applications. However certain applications such as air compressors and hydraulic power packs can achieve significant savings depending upon the on-load and off-load cycle times.
The main benefits of VSDs in these applications are precise process speed control and starting/stopping with controlled acceleration/deceleration using modern motor algorithms. Providing only the required air or hydraulic pressure reduces waste and running costs as opposed to on/off with fixed speed motors.
As well as reducing the effects of oversizing usually inherent in the design of the system, improved control reduces mechanical stresses and, therefore, maintenance requirements, giving higher production availability and lower lifetime costs.
On an extruder, for example, improved accuracy of motor control and speed regulation could result in higher quality output, while reducing mechanical stresses and improving machinery lifespan.
Whilst most new machines are optimised for energy savings, many existing machines and motors have the potential for energy savings. Importantly, modern VSDs are suitable for use with most electric motors without modification and we can help.
