Oversizing pump motors costs industry dear

Andy Glover of WEG UK highlights a widespread problem and provides selection pointers on how to achieve maximum pump/motor efficiencies.

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.

Pumps like fans, compressors conveyors, et al, are motor driven systems and, therefore, major energy users.

A motor running at a typical commercial or industrial site for 4000 hours 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.

Although 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.

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.

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.

DOL starting provides higher starting torque in comparison to 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 direct on line, 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 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 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 motor insulation damage, or even motor failure.

However, if the speed of the motor is too great - as can happen with DOL startups - then uncontrolled acceleration can result in problems such as drawing a vacuum on the suction side of the pump, or surges on the discharge.

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 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 2 to 4 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%age 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 the order of 3-4x purchase cost.

As a general rule, high efficiency motors garner the maximum savings when their operating regime is more than 4000 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%.

As 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 the 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 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 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.

The energy cost per annum of any electric motor is calculated as the product of hours used per year by the kilowatt-hour tariff by the operating point power, divided by the operating efficiency.

Applying this formula to a typical pumping system using a high efficiency motor rated 95% leads to an annual cost of GBP 27,190.

Using the same formula for a rewound motor with a 92% efficiency rating the result would be GBP 28,076 per year.

What must be borne in mind is that the saving of GBP 886 per year is for just one pump motor - there may be many more on 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 are a direct contribution to a company's profit - make the high efficiency motor the best long term alternative.

Back to top