PUMP HANDBOOK
EFFICIENT OPERATIONS OF
PUMP SYSTEMS (PART 5)
The PIA’s Australian Pump Technical Handbook is a cornerstone text for the Australian pump industry and, in our opinion, a must have for anyone who deals with pumps on a regular basis. In this ongoing series, we feature abridged chapters from the classic book to showcase the various areas covered and to reacquaint readers with the technical aspects of pumps. In this issue, we continue exploring energy efficiency in pumping systems, focusing on drives and controls. Drives and controls
There are several major classes of driver that are commonly used in pump applications: • Electric motors • Internal combustion engines • Steam turbines • Compressed air • Hydraulic motors
Electric motors
Electric motors are probably the most economical and common driver for pumps, and it is generally not complicated to select a motor as a driver. To select the most energy efficient electric motor, the following should to be considered: • The motor should not be oversized – electric motors meeting International Standards come in specific ratings (i.e. 1.1kW, 2.2kW, 3kW, etc). Normally a motor would be selected in a standard-size up from the maximum kW absorbed by the pump, for example, for 13.4kW absorbed, a 15kW motor would be selected. However, if the pump kW absorbed is less than five per cent below a standard motor size, the next size up should be selected • The motor efficiency from several manufacturers should be checked in order to maximise the motor efficiency relative to the capital cost. This is particularly important for larger motors where a one per cent efficiency improvement can reduce the lifecycle cost of the pumpset by an amount that could make a cheaper but less efficient motor look expensive • The motor should be MEPS Hi Efficiency certified Electric motors come in all shapes and sizes, and can be categorised into two main types: • Standard motors • Special motors Standard motors are available as DC motors in low power sizes and AC induction motors. In Australia, the most common type of standard motor found in pumps are AC induction motors.
AC induction motors
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Some fundamental points that should be considered when applying a squirrel cage induction motor to a pumping applications, include: pump industry | Summer 2022 | Issue 38
• Rating voltage – the motor winding should be chosen to suit the power supply available • Standard voltages – these include 240V, 415V, 690V, 1,000V, 3.3kV, 6.6kV, 11kV and 33kV. It should be remembered that the higher the voltage, the lower the current for the same power rating • Rated speed – the motor windings should be configured to suit the rotational speed of the motor. Standard speeds include: 2-pole (nominally 3,000rpm); 4-pole (nominally 1,500rpm); 6-pole (nominall 1,000rpm); and 8-pole (nominally 750rpm). Pump performance curves are often based on these speeds • Rated torque – it should be established how much torque an application needs, and a motor with a rated power that suits the starting and running requirements of the pump should be selected. This is particularly important for many positive displacement pump applications, but less so generally for rotodynamic pumps which don’t have high starting torque requirements
Motor starting
Most pump applications simply need the pump to start and stop as required to meet the demand from the user. There are a variety of commercially available motor starting methods, which are summarised in Table 1. Advantages
Disadvantages
Direct online
Reduced cost High starting torque
High in-rush current
Start - delta
Reduced in-rush current
Reduced starting torque
Autotransformer
Reduced in-rush current
High weight
Soft starter
Reduced in-rush current Controlled start and stop
Reduced starting torque
Variable speed drive
Controlled speed Energy saving at reduced speed
Higher cost
Table 1.
The preferred starting method would be determined by the conditions of service for the pump and local restrictions on
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