Page 55 - Energize September 2021 HR

TECHNICAL

Overall efficiency improvement with the subsequent energy provide a lower energy consumption in pump units with variable fluid
savings in EMDS can only be achieved with an integrated flow. The study further shows that IE4 class motors may not provide
system consisting of well-matched and highly efficient individual significant advantages over IE3 class motors when they are used in
components, operated according to the required load in an variable flow fixed-speed applications.
efficient way. Complete system optimisation falls outside the Thus, the selection of an electric motor based on its IE energy
scope of this article, and the focus will be on the use of efficient efficiency class will not necessarily result in minimum energy
motors and VFDs. consumption in a number of applications, such as variable flow
pumps. It should be noted that for frequency-controlled electric
Electric motor efficiency motors, the IEC 60034-30-2 standard (SATS 60034-30-2:2020)
Many countries have adopted minimum energy performance establishes the efficiencies in seven load modes different from the
standards (MEPS) for electric motors based on IEC standard IEC rated mode. IEC 60034-2-3, 2020 specifies a test method for motors
60034-30-1 (SATS 60034-30-1). There is currently no MEPS for driven by converters, while IEC TS 60034-30-2, 2016, covers an
electric motors in South Africa, and it is possible to buy motors of efficiency classification for motors driven by converters.
any efficiency rating, provided they comply with safety standards. With the prevalence of offerings of motor and motor drive
Initiated by the DMRE, the South African National Energy as a combination unit, some industries have adopted the power
Development Institute (SANEDI) is expanding its Energy Efficiency drive system (PDS) concept and specify PDS efficiency instead of
Standards and Labelling (S&L) programme to study the energy motor efficiency. Some countries have adopted a system of PDS
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use of motors available in South Africa and suggest new minimum efficiency labelling similar to motor efficiency labelling. Because
energy performance standards. 1 of the increasing popularity of matched drive, motor and pump
combination units, certain industries are considering an EMP
The IEC standard identifies four efficiency classes: efficiency labelling system. 4
• IE1-Standard efficiency
• IE2- High efficiency Variable speed control or VFD
• IE3-Premium efficiency Rotary machines such as pumps, fans and compressors
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• IE4-Super premium efficiency comprise 70% of motor applications. For the purpose of this
article, we shall consider the application of VFD in a centrifugal
The efficiency rating varies with motor size and type. Figure 3 shows pumping system. Centrifugal pumps comprise 80% of all
an efficiency chart for 4-pole motors. industrial pumps. Pumps are one of the most common variable
The IE5 ultra-premium efficiency class has not been specified speed drive (VSD) system applications and special interest has
in standards yet, but some manufacturers have already developed focused on improving their energy efficiency by using variable
motors which will comply to that standard. speed control instead of throttling or other less efficient flow
control methods.

Traditional pump control methods
The most common parameter to be controlled in a pumping system
is the flow rate. The conventional control method is to run the
pump at full speed and control the flow by mechanical means. The
parameters of the pump and system that control operation are:
• Head: The net work done on a unit weight of water by the pump
impeller. It is the amount of energy added to the water between
the suction and discharge sides of the pump. Pump head is
measured as the pressure difference between the discharge and
suction sides of the pump.
• Static head: The vertical distance from the water level at
the source to the highest point where the water must be
Figure 3 Efficiency classes for 4 Pole motors (IEC)
delivered. It is the sum of static lift and static discharge.
Static head is independent of the system discharge and is
Energy efficiency ratings constant for all values of discharge. However, it is possible
While the energy efficiency of an electric motor is measured at full that the static head may vary over time due to the changes in
load, the efficiency varies with load. Many electric drives will be the system.
operated at less than the maximum load for much of the operating • System curve: This defines the characteristics of the system
cycle, and this needs to be considered when selecting a motor. that the pump feeds; it quantifies the resistance in a system due
In pump units, electric motors often run at half or even a quarter to friction and elevation change over the range of flows.
of their rated power value for a significant part of the time, and as • Operating point: A centrifugal pump can operate at a
a result, exhibit a reduced efficiency. Existing standards do not combination of head and discharge points given by its pump
establish the minimum efficiencies at loads below the rated value for curve. The particular combination of head and discharge at
electric motors powered directly from the mains. which a pump is operating is called the pump’s operating point.
Studies have shown that that the use of an electric motor with Once this point is determined, brake power, efficiency and net
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a higher efficiency at the rated load (high energy efficiency class, positive suction head required for the pump can be obtained
according to IEC 60034-30-1 (SATS 60034-30-1) does not always from the set of pump curves.

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