Improving Pumping Efficiency
Oversized pump motors are costing industry dear through increased capital costs and reduced efficiency. Andy Glover of WEG UK highlights a widespread problem and provides some pointers on how to achieve maximum pump/motor efficiencies
According to data from the British Pump Manufacturers Association, pumps represent the largest single use of motive power in industry and commerce, accounting for 31% of overall energy usage in UK industry alone. Pumps, like fans, compressors, conveyors etc, are motor-driven systems and, therefore, major energy users. A motor running at a typical commercial or industrial site for 4,000h a year has an annual electricity cost of about 10 times its capital cost. This is serious money, and effectively underlines why pump suppliers and pump users should ensure optimum efficiency from their systems by effectively matching drive motors to pumps. Unfortunately, this is not usually the case. A widespread culture of overrating motors has built up; this involves engineers at various stages of the pump system design process adding 10% or 15% to the motor capacity ‘just to be on the safe side’. This practice is so widespread that it is estimated that only 20% of the pump drive motors in operation are running at their full rated input. The implications for the end-user are, first, an exaggerated capital cost for the motor itself; secondly, a commensurate increase in associated equipment, such as motor starters, drives and cabling; and, finally, gross inefficiencies in the system operation.
While oversizing is a major concern, the problem that it seeks to avoid, namely undersizing, should not be ignored. Electric motors are supplied with a service factor that allows them to operate for short periods above their rated output. This is acceptable in systems where temporary overload conditions during pump starting are encountered. However, the downside of this operation is that the motor will run hotter, and if this persists damage to the insulation and bearings could occur, shortening the life of the motor.
With the pitfalls of oversizing and undersizing pump motors clearly understood, the process of motor selection is better placed to focus on the other major considerations that affect motor life and efficiency. These include: the power/torque required by the pump, pump speed, nature of starting (closed valve/open valve), operating conditions, temperature, and efficiency.
The torque-speed characteristics of the motor and pump should be matched to ensure the availability of starting as well as running torque for the pump. The starting torque is influenced by the method adopted to start the motor. Direct on-line (DOL) starting provides higher starting torque in comparison to Star Delta starting. In addition, the moment of inertia for the pump motor system has to be considered to determine the acceleration time for the motor to attain full speed.
If the method used to start a pump drive motor is DOL, the result will be high levels of torque that create mechanical stresses on the pump rotating components and fluid stresses in the hydraulic system. The same stresses can occur when stopping if the rate of deceleration of the motor is not controlled. The use of a variable-speed drive (VSD) or soft starter can easily overcome these problems and, in the case of the VSD, provide long-term energy-saving operation.
The speed of the motor should be sufficiently rated to ensure efficient delivery from the pump and to ensure external cooling of the motor. If the pump is operated at too slow a speed for extended periods, the motor’s cooling fan becomes ineffective, leading to temperature rise and motor insulation damage, or even motor failure. However, if the speed of the motor is too great, as can happen with DOL start-ups, uncontrolled acceleration can result in problems such as drawing a vacuum on the suction side of the pump, or surges on the discharge.
Nature of starting
A further consideration is the question of whether the start of the pump cycle is against a closed valve or the pumping action is required to too high a tank. In either case, the available torque of the motor can be exceeded, causing it to overload.
The presence of any vapour/gas/chemicals in the pump operating environment would necessitate the use of an explosion-proof motor. However, as a general consideration across all motor types, the voltage at the motor should be kept as close to the nameplate value as possible, with a maximum deviation of 5%. Although motors are designed to operate within 10% of nameplate voltage, large variations significantly reduce efficiency, power factor and service life. When operating at less than 95% of design voltage, motors typically lose 2 to 4 points of efficiency and experience service temperature increases that greatly reduce insulation life. Running a motor above its design voltage also reduces power factor and efficiency.
What also must be taken into consideration is that electric motors are sized according to the specific gravity of the liquid being pumped. If a low specific gravity pump is tested with water, or any higher specific gravity fluid, the increase in motor current could burn out the motor.
The motor must be supplied with an effective form of cooling (eg a fan) to ensure that internal losses are dissipated within the limits of the maximum temperature rise for the class of winding installation employed. If sufficient cooling is not supplied, damage to the motor insulation and to rotating bearings can occur, leading to premature motor failure.
Because most electric motors consume their capital cost each month to run, the question of energy efficiency is one of the most important for the pump user. It has been calculated that a single percentage point increase in efficiency will save lifetime energy costs generally equivalent to the purchase price of the motor.
This highlights the benefits of using high-efficiency motors, which attract cost offsets from European governments (Enhanced Capital Allowances in the UK) and provide lifetime cost savings in the order of 3–4 times the purchase cost.
As a general rule, high-efficiency motors garner the maximum savings when their operating regime is more than 4,000h a year and when the motors are loaded in excess of 75% of full load. The motor, or motors, selected should always be inverter rated, as this provides the gateway to far greater levels of energy saving. This is the result of centrifugal pumps presenting motors with what are known as variable torque loads. These are also sometimes referred to as ‘cube law’ or ‘square law’ loads.
In these cases, the torque required to turn the load decreases as a function of the square of the speed – ie, at 50% speed, the torque requirement will be 0.52 or 25%. Since the power required from the motor is a function of torque and speed, it follows that the load in terms of kilowatts increases or decreases as the cube of the load speed. In terms of the above example, driving the load at 50% speed requires only one eighth of the power needed to run at maximum speed, even though the flow rate will still be 50%.
Theoretically, all variable torque loads generate a flow, which is directly proportional to speed. It is this fact that makes it possible to realize very substantial energy savings on pump systems through the use of AC drives.
Rewind or replace?
Energy efficiency is also a major consideration when the question of repairing or replacing an existing motor arises. The repair-versus-replace decision is quite complicated and depends on such variables as the rewind cost, expected rewind loss, energy-efficient motor purchase price, motor size, original efficiency, load factor, annual operating hours, electricity price, availability of a government rebate, and simple payback criteria.
Among these variables ‘expected rewind loss’ is notable because when a motor is rewound its efficiency is reduced, and, according to many manufacturers, so is its reliability. The effect the expected rewind loss can have on system efficiency and, hence, long-term operating costs can be demonstrated by using the formula shown in box 1.
What must be borne in mind is that the saving of £886 a year is for just one pump motor (there may be many more in a plant), and although the cost of purchasing a high-efficiency motor is greater than that for rewinding, offsets in the form of Enhanced Capital Allowances on purchases of high-efficiency motors, plus long-term energy savings, which make a direct contribution to a company’s profit, make the high-efficiency motor the best long-term option.
WEG Electric Motors (UK) Ltd, 28/29 Walkers Road, Manorside Industrial Estate, North Moons Moat, Redditch, Worcs B98 9ND; tel: (01527) 596748; fax: (01527) 591133; email: [email protected]