Oversizing costs

According to data from the BPMA, pumps represent the largest single use of motive power in industry and commerce, accounting for 31% of overall energy usage in UK industry alone, writes Andrew Glover.

Pumps like fans, compressors conveyors, et al, are motor-driven systems and 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.

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 widespread - an estimated 20% of pump drive motors in operation are running at their full rated input.

The implications for the end-user are an exaggerated capital cost for the motor itself; a commensurate increase in associated equipment, such as, motor starters, drives and cabling; and gross inefficiencies in the system operation.

Whilst oversizing is a major concern, the problem that it seeks to avoid - undersizing - 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.

The process of motor selection is better placed to focus on the other major considerations that affect motor life and efficiency, including power / torque required by the pump; pump speed; and 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 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 two to four points of efficiency, and experience service temperatures 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 (i.e. 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.

Energy efficiency
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 in the order of three to four times purchase cost.

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 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 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 very substantial energy savings on pump systems through the use of AC drives.

Energy efficiency is also a consideration when repairing or replacing an existing motor arises. The repair-versus-replace decision is 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 variables, "expected rewind loss" is notable because when a motor is rewound its efficiency is reduced, and, according to many manufacturers, its reliability also.

The effect the expected rewind loss can have on system efficiency and, hence, long term operating costs can be demonstrated by using the formula below.

Although the cost of buying a high efficiency motor is greater than that for rewinding, offsets in the form of ECA on purchases of high efficiency motors, plus long term energy make the high efficiency motor the best long-term alternative.

Andrew Glover is with WEG Electric Motors (UK).
T: 01527 596748.

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