Protecting Your Pump: Smarter Motor Control Pays Off

Pumping systems may be tucked away in basements, plant rooms, factories and lift stations, but they loom large on the energy bill across every sector that depends on them. From mining and manufacturing to food processing, commercial buildings and municipal waterworks, pumps remain some of the biggest - and most often overlooked - energy users on site. In an environment where reliability, performance and cost control are under growing pressure, delivering the most efficient pumping solution has never been more critical. Pipes, Pumps & Valves Africa finds out how smarter motor control is reshaping pump performance across industries.

Mitsubishi Electric Automation estimates that pumping systems in the United States alone account for around 25% of industrial energy use, while water, wastewater and irrigation together consume 55 billion kilowatt-hours per year. Around 90% of the energy used in the water and wastewater sector comes directly from pumps and blowers. “It’s a pretty big piece of the pie,” says Wes Gulik, Industry Marketing Manager (Water–Wastewater) at Mitsubishi Electric Automation, speaking during a recent online event.

The stakes, however, are not only financial. The US Environmental Protection Agency reports that 7% to 8% of community water systems experience a Safe Drinking Water Act violation each year, underscoring the operational and compliance risks tied to pump performance. As a result, improving efficiency, enhancing performance and reducing lifecycle costs have become top priorities. How one starts and controls pump motors has emerged as a frontline issue and the industry is steadily shifting from simple contactors to soft starters and, increasingly, to variable frequency drives (VFDs).

Direct-on-line starters: simple, but brutal

According to Ben Strong, Industry Marketing Manager (Drive Systems) at Mitsubishi Electric Automation, the most basic method is direct-on-line (DOL) starting using a contactor, breaker and thermal overload. It is entirely binary: on or off, 0% or 100%. “From a contactor standpoint, it’s very cost-effective,” says Strong. “It’s really simple to see the status - it’s closed or it’s open. There really isn’t an in-between.”

But that simplicity comes at a cost. When Mitsubishi Electric instrumented a 1 hp motor on a self-priming pump and started it across the line, the results were stark. “When we started that pump, we put eight times the motor’s full-load amps into that mechanical system. That’s a lot of energy over a brief period of time – and not all of that’s doing useful work.” Those high inrush currents translate into: mechanical stress on belts, shafts and couplings; hydraulic shocks, including check-valve slam and pressure transients and reduced equipment life DOL starters also offer no speed control, no ramping and minimal diagnostic capability beyond basic auxiliary contacts. In more complex control systems, Strong notes, they can even become a liability.

Soft starters: gentler starts, limited control

Solid-state soft starters are often the next step up, says Strong. They use SCRs (thyristors) to ramp voltage from around 40%, up to full voltage, smoothing the acceleration of the motor.

“Soft starters do reduce the amount of current the motor uses when it starts,” he explains. “But it’s still a lot more than the motor full-load amps.” On the same 1 hp pump, a soft starter reduced the inrush to about 3.5 times full-load amps – better than DOL, but still substantial.

The advantages include the ability to ramp starts and stops, reduced mechanical and hydraulic shock, built-in overload protection and high efficiency in steady-state operation (due to bypass contactors in many designs). However, the limitations are equally clear. “Soft starters are not variable speed drives,” says Strong. “You can’t control the speed. There are reasons why you’d want to - variable demand, variable level, coordinating flow and the pump affinity laws where maybe just slowing it down a little bit saves a lot of energy. You can’t really do that with a soft starter alone.”

VFDs: speed, savings and smarter control

For Strong, it is clear where the industry is heading. Variable frequency drives (VFDs) convert incoming AC to DC and then back to AC using IGBTs and pulse-width modulation (PWM) to produce a controlled output frequency and voltage. The key advantage is speed control.

“With a VFD, you can vary the speed, you can ramp, you can do whatever you need to do with regards to motor speed to make sure the system operates properly at the highest energy efficiency without causing issues,” he explains.

In Mitsubishi Electric’s tests on the same 1 hp pump, the VFD ramped the motor up smoothly and held current at around full-load amps – with no violent inrush and no hydraulic shock.The energy-saving potential is where VFDs stand out most, says Strong. “If you slow a pump down to a 10% decrease in flow and a 19% decrease in head, we can see a 27% reduction in power usage. That’s why we say VFDs are energy efficient. But we have to be careful. Pumps have an operating range and we do not want to operate pumps outside that range.”

Recalling one particular incident where a user operating turbine pumps too slowly led to severe cavitation damage and roughly a million dollars in repairs, Strong says the lesson is clear: VFDs unlock savings only when applied with a proper understanding of system hydraulics. While VFDs are often perceived as expensive, Strong stresses the importance of looking at total cost of ownership. In one success story, Mitsubishi Electric compared a soft starter solution with a VFD panel on a 100 hp pump in a US utility where electricity costs ranged from $0.42 to $0.50 per kWh. Using the affinity laws and local tariffs, the VFD delivered an estimated annual saving of around $143 000 compared with a soft starter.