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Motors and variable speed drives

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Electric motors have a range of industrial applications, including the operation of pumps, compressors and fans. Motors use substantial energy and can incur significant running costs, meaning great potential for savings.

Most motor energy consumption in Australia is mid-size motors with output power between 0.75kW to 375kW. These have been subject to minimum energy performance standards since 2002, and have become more efficient over time.

System configuration and optimisation

On average, motor systems lose over half their input energy before delivering end-use service. So, while energy efficiency of individual motors may be high, the efficiency of the system as a whole can be low. Improving the service delivery system can bring the greatest savings.

Once the system serviced by the motor is optimised, motors and drives can be selected to most efficiently meet final requirements.

Areas for system optimisation include:

  • rationalisation or separation of existing production lines or processes
  • arrangement of machinery to minimise distribution losses and facilitate energy recovery
  • ensuring pipes and ducts are of proper diameter to minimise friction
  • minimising pressure drops caused by flow obstructions, sharp bends, expansions and contractions
  • low-loss valves and fittings
  • ensuring driven machinery is operated at optimal efficiency points, and specified for task
  • ensuring all system components and filters are clean.

Motor speed control

Speed control allows motors to be oversized to meet extreme requirements without wasting energy during low demand. 

Pumps and fans typically have variable torque loads subject to the ‘cube law’, meaning that reducing motor speed by 20% can reduce power required by 50%.

Constant torque loads occur where torque is independent of speed.

This is often the case with:

  • hoists
  • conveyors
  • extruders
  • mixers
  • reciprocating air compressors
  • rotary screw air compressors.

In such cases, the speed/power relationship is proportional. This means that a 50% reduction in speed causes a 50% reduction in power.

Options for motor speed control vary from simple voltage-driven DC motors to fully functional, electronic AC motor systems.

AC motors can be designed with controls that switch between speed settings. Several smaller motors can be controlled with a switch for just enough motors to meet demand.

Variable speed drives (VSDs)

A VSD controls the speed and torque of an AC motor by converting fixed frequency and voltage input to a variable frequency and voltage output. System performance can be greatly improved by controlling speed to precisely match the load.

Along with system optimisation and motor efficiency, VSD motor control is one of the 3 main areas to achieve energy savings. Savings will depend on the nature and variability of the load and total operating hours. Where process output requirements vary by 30% or more, matching the load with a VSD can reduce energy use significantly.

Motor systems fitted with VSDs can bring other benefits, including:

  • reduced maximum power demand
  • reduced stress on system components
  • accurate control of pressure, flow and temperature
  • improved safety and amenity, through reduced heat and noise levels
  • integration of VSD control with building management systems (BMS).

In pumping systems, valve-throttling flow-control prevents pressure build-up. This is not efficient, because energy to the pump is not reduced. A VSD enables precise flow control without the energy losses of throttling. It ensures the system isn’t running at full-speed if not necessary.

To estimate energy savings when a VSD is applied to a variable or constant torque load, determine:

  • the lengths of time the equipment operates under various load conditions 
  • efficiency of the potential VSD and motor combination when operating in comparable situations.

Efficiency values for motor drive systems when connected to various loads can usually be obtained from equipment manufacturers.

Mechanical and hydraulic VSDs can suffer from inherent losses and are therefore not as energy-efficient as electronic controls.

Variable frequency drives

Modern electronic VSDs are also known as variable frequency drives (VFDs) as they work by varying the AC electrical input frequency to control drive speed.

VFD technology has widespread uptake with AC induction motors. VFDs are favoured due to their accurate speed variability from zero rpm to over 100% of the rated speed. VFDs also enable motor control in either direction.

VFDs may be of little benefit where precise motor speed control does not assist the production process or where hours of reduced demand are few. VFDs are also not recommended for applications in which slowing down the machine causes operating problems, such as insufficient torque or poor cooling.

Retrofitting VSD/VFD

VSDs can often be retrofitted to existing motors. Evaluation of motors and load requirements should be conducted to see where this is a feasible.

Combining an in-service AC motor with an electronic VSD provides effective speed-control technology without needing a different type of motor.

The performance of modern AC motors with VSDs now matches that of DC systems. ACs have significantly lower maintenance costs, making DC motor replacement cost-effective.

In some cases, single-speed motors can be retained to cover the variable baseload. These can be complemented with a VSD on an appropriately sized motor dedicated to providing the additional variable load.

Voltage and power factor

AC induction motors can cause a facility to have low-power factor. This results in additional electrical current to perform the required work.

Power factor can be improved by:

  • minimising oversized and inefficient motors
  • avoiding idling or lightly loaded motors
  • adding power factor correction devices.

Right-sizing motors

Motors should be correctly sized for the maximum intended duty.

Motor sizing must consider:

  • running load and power required to start the machine
  • speed and torque requirements of driven equipment
  • ability of the motor to respond to load changes.

Electric motors are generally inefficient when operated at loads below 40% of their rated output. They’re often most efficient at between 70% and 80% of rated output.

Oversizing motors affects efficiency and power factor, and raises installation and operating costs. Undersizing a motor means it will have to work harder, leading to higher temperatures, reduced efficiency and shortened life.

When sizing motors, assess the required load to avoid replacing like-with-like. Sizing should be based on actual loads rather than rated motor capacity.

High-efficiency motors

High-efficiency motors can cost up to 40% more than older, standard-efficiency motors, but the payback period can be less than 2 years from energy saved.

The Australian Government’s Energy Rating website has an exhaustive database of mid-sized electric motors sold in Australia since 2002. The database includes the efficiency of each motor at low, medium and high loads.

Energy star rating labels are required on motors. However, all motors subject to regulation must display labels stating whether the motor is standard or high efficiency.

Motor management and maintenance

A properly maintained motor can perform up to 15% more efficiently. It’s worth implementing a motor management and maintenance program.

A large industrial facility can contain thousands of motors, with most energy consumed by a few essential systems. Maintaining motors under a schedule is likely to deliver the most benefits with the fastest payback.

Motor energy use can be monitored using electricity metering equipment. Sensors can flag potential problems before they cause trouble (see the Industry 4.0 guide for more information).

Assessments of motors should factor in run time, environmental conditions and the consequences of failure. Inspections can be done using an infrared thermal scanner to identify motors that are running hot.

There are a number of maintenance tasks to ensure motors are functioning optimally:

  • Keep motors and fans clean.
  • Check for excessive vibration which may be a sign of motor misalignment.
  • Check for connections or wires that might be loose or damaged.
  • Keep motors cool through adequate ventilation
  • Lubricate motors, bearings, gearboxes and chain drives according to the manufacturer’s recommended intervals and lubricant specifications.
  • Ensure belt drives are correctly tensioned and clean. Ensure multiple belts are evenly matched. 
  • Re-assign motors within a factory according to actual measured loads.
  • Replace old motors before they break down.

Innovations

Permanent magnet motors

Permanent magnet motors (PMMs) use magnets instead of traditional metallic motor components, resulting in improved torque and efficiency.

PMMs will generally only operate when connected to a dedicated VSD which has been optimised for their control. Hybrid PMMs are available as direct replacements for conventional AC induction motors in most applications.

Synchronous reluctance motors

Synchronous reluctance motors (SRMs) feature sophisticated geometry that keeps internal resistance consistently low in all motor shaft positions. This increases the available flux and motor output.

SRMs are inherently efficient as the current does not need to flow to the rotor, minimising energy losses. Compared with an AC induction motor, SRMs produce more power at a smaller size and can deliver superior low-speed torque and efficiency.

Magnetic bearings

A recent innovation is the use of magnetic bearings to support spinning shafts in motors and driven equipment. The technology uses a system of active electromagnets that can adjust for any wobble and keep the shaft perfectly supported without physical contact.

Magnetic bearings allow faster motor speed and improved reliability. They also reduce friction, heat and vibration, and don’t need lubrication.

Alternative materials

While manufacturers offer high-efficiency motor drive combinations, there are concerns about the availability of required rare-earth metals such as neodymium. Research is identifying rare-earth replacements in combination with alternative motor configurations that retain the efficiency of PMMs.

Software controls

VSD software includes settings to deliver maximum energy savings. It also provides self-learning and modelling capabilities that can minimise the need for sensors as inputs into the control system.

Internet-enabled systems

Next-generation VSDs are designed for internet connectivity, offering a single point of connection for a multitude of sensors and data points. This allows for rapid analysis of system performance and predictive scheduling of equipment maintenance and repairs. It can notify operators by smartphone before a failure occurs.

Regenerative capability

Regenerative drives recover motor-braking energy rather than it being lost to heat, using inverters to convert the resulting DC power into AC power. The extra cost of a regeneration is worthwhile in VSDs only where the system requires frequent braking and starting.

Integrated packages

VSDs are usually purchased as standalone devices, but are available as part of integrated packages. Integrated VSD and drive packages have a number of advantages, including:

  • eliminating separate enclosures and reducing required floor space
  • decreasing costs
  • eliminating long cable runs between motor and drives
  • opening many more motor systems to the potential of variable-speed control.

Case studies

NSW Environment, Energy and Science Using an operating lease to finance more efficient motors.

NSW Environment, Energy and Science Financing more efficient motor fitted with variable speed drives.

Victorian Energy Saver Variable speed drives on fans and pumps.

Read more

Adjustable Speed Drive Part-Load Efficiency Guide (PDF 485KB) US Department of Energy

Electric motors efficiency regulations Energy Rating

Improving Motor and Drive System Performance (PDF 2036KB) US Department of Energy

Measurement and Verification Operational Guide - Motor, Pump and Fan Applications (PDF 730KB) NSW Government

Reduce motor running costs in your business Sustainability Victoria