Get your motor running
Pumps account for nearly one third of energy use in UK industry. Andy Glover explains why over-sizing pump motors is costing industry dear, and outlines the key factors in motor selection.
Pumps, similar to fans, compressors and conveyors, are motor-driven systems and, therefore, major energy users. A motor running at a typical commercial or industrial site for 4,000 hours a year has an annual electricity cost of about ten 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 is so widespread 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, firstly, an exaggerated capital cost for the motor itself. Secondly, there is an increase in associated equipment such as motor starters, drives and cabling. And, finally, there are gross inefficiencies in the system operation.
While over-sizing is a major concern, the problem that it seeks to avoid - under-sizing - should not be ignored. Electric motors are supplied with a service factor that enables 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 motor insulation and bearings could occur, shortening the life of the motor.
With the pitfalls of over-sizing and under-sizing 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 following:
Power/torque required by pump
The torque-speed characteristics of the motor and pump should be matched to ensure availability of starting as well as running torque for the pump. The starting torque of the motor is influenced by the method adopted to start the motor. Direct on line (DOL) starting provides higher starting torque in comparison with Star Delta starting. In addition, the moment of inertia for the pump motor system has also 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 savings.
The speed of the motor should be rated sufficient 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, then the cooling fan of the motor becomes ineffective, leading to temperature rise and insulation damage, or even motor failure. However, if the speed of the motor is too great, as can happen with DOL start-ups, then 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 - closed valve / open valve
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 overload.
The presence of any vapour, gas or 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 two to four 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 considering 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 to ensure 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 failure.
Because most electric motors consume their capital cost each month to run, the question of energy is one of the most important for the pump user.
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 (ECA in the UK) from European governments and provide lifetime cost savings of three to four times purchase costs.
As a general rule, high-efficiency motors garner the maximum savings when their operating regime is more than 4,000 hours 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 is 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. In other words, at 50% speed, the torque requirement will be 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. Hence, driving the load at 50% speed only requires an 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 realise 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, and original efficiency, load factor, annual operating hours, electricity price, availability of a government rebate, and simple payback criteria. Among these, expected rewind loss is notable because, when a motor is rewound, its efficiency is reduced, as is, according to manufacturers, its reliability.
The cost of purchasing a high-efficiency motor is greater than that for rewinding. However, offsets in the form of enhanced capital allowances on purchases of high efficiency motors, plus long-term energy savings make the high-efficiency motor the best long-term alternative.
Andrew Glover, WEG Electric Motors. T: 01527 596748.