- 29 November 2005 -

Oversizing pump motors - the problems

A widespread culture of over-rating pumps has lead to gross inefficiencies in system operation. Andrew Glover of WEG UK suggests that oversizing pump motors can decrease efficiency and cost effectiveness and shows how manufacturers can achieve maximum pump/motor efficiencies by ensuring that motors match pumps.

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, etc 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 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 over-rating 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, firstly, 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.

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.

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:

Power/torque

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 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 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.

Operating conditions

Whilst the presence of any vapour, gas or chemicals in the pump operating environment would necessitate the use of a specific hazardous area motor (i.e. non-sparking or explosion Proof), 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 can invariably affect 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 can experience increases in winding temperature which greatly reduce insulation life.

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.

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