Starter for ten03 December 2014

We're all aware of the importance of choosing higher-efficiency electric motors, pumps, fans, etc, to cut costs and emissions, but are we missing a trick? Brian Tinham reports

With the relentless pressure to cut the cost of motive power in factories and plants – not just capital investment but, increasingly, also energy consumption – it's no surprise that enlightened engineers are slowly turning to higher-efficiency electric motors. The EuP (Energy using Products) regulations for electric motors (EC 640/2009, which deliver on the Eco-design directive EC 2005/32) are also driving their choice. Indeed, from 1 January next year, all motors in the range 0.75–375kW sold in Europe must be certified to at least IE3 premium efficiency, or IE2 (high efficiency) if equipped with a variable speed drive (VSD).

So much is, or should be, common knowledge. Generally, it's good news for businesses – which will see energy bills decline – and the planet, as carbon emissions reduce in lock step. However, the regulations say little about preferred electric motor starter techniques, and that's where engineers who choose cheaper kit may still be missing tricks. On the one hand, plant reliability, downtime and hence maintenance may well be poor, due to mechanical stresses (to motors and driven loads) and power network disruption caused by low-tech starters. On the other, they may also be wasting vast amounts of energy.

As Andrew Preston, drives product manager at automation giant ABB, puts it: "People are keen to purchase efficient equipment, but sometimes they don't think about the efficiency of the whole system. So they buy efficient motors and pumps, for example, but then specify soft starts to save on the cost of VSDs. But that means they could be wasting huge amounts of money by failing to optimise motor speeds."

And while engineers might argue that running pumps below BEP (best efficiency point) incurs energy losses of perhaps 2 to 3%, reducing motor speed might save 50% on electrical energy. That's surely a no-brainer.

So what are the electric motor starter choices? And which should you go for? The simplest and cheapest method is DOL (direct on line), which requires only a contactor and overload relay. Moving up the cost and sophistication range, we come next to star-delta starters, followed by soft starts and ultimately VSDs (otherwise known as inverters or frequency converters). For the sake of completeness, historically there are also auto-transformers and resistance starters, but these are rarely now offered or implemented.

Very high starting torque
Considering the pros and cons, while DOL starting is cheap, the most obvious disadvantage is that initial energising of the motor's internal magnets draws typically six to eight times the rated motor current. That means very high starting torque and, in turn, potentially slipping belts, shock loads on bearings and mechanical drive components, as well as possible product damage.

Jerry Hodek, of electric motors manufacturer Marathon Electric, says that DOL starting can also cause voltage problems in the mains – and not just on your own site. "Voltage spikes can burn out fuses, contactors and switchgear; cause lights to flicker, cause problems with other equipment, such as control systems and computers; cause false sensor readings; and disrupt radio and wi-fi signals," he advises.

Hence, his recommendation that DOL only be used where the driven load can withstand the shock of uncontrolled start-ups and where voltage drop and other disturbances to the mains supply can be tolerated. "In general, only small- and medium-sized motors should be started so abruptly. Larger motors and motors in sensitive applications need reduced voltage starting," states Hodek.

Moving on to star-delta starters, at their most basic these consist of three contactors, an overload relay and a timer designed to switch the contactors from 'star' to 'delta' configuration after a preset period. With star-delta, both motor current and torque are two-thirds lower in star than in delta, meaning that shock loads are considerably reduced, compared to DOL.

Since star-delta is by far the cheapest option for voltage reduction, it is a popular choice. However, while they theoretically solve much of the inrush current issue, starting torque can be a problem, with the potential for motors simply to sit stationary in the star phase before being catapulted up to full speed in the transition to delta. And there is no facility for managing ramp-up speed.

Looking at soft starters, these effectively throttle power input digitally by reducing and controlling the voltage at start-up. They are more expensive, but offer much greater protection for the motor as well as programmable ramp-up rates and torque, which can be valuable. Preston explains that tailoring the starting speed and current not only means the electrical network doesn't take a hit, but that the mechanics of the motor and driven load are also treated sympathetically.

"Also, being electronic, they offer more sophisticated overload protection – certainly compared to DOL equipment, which uses a crude overload relay that has to be capable of withstanding the huge inrush currents," he comments. "Soft starters also measure actual current, using internal CT [current transformer] devices, with programmable limits."

"Soft starts are more expensive, but offer many advantages in terms of finesse and reliability," agrees Hodek. "They are particularly popular on water and waste-water pumping installations, where motors are large and water hammer must be avoided at all costs."

However, at the top of the pyramid are VSDs, which not only control start-up speed digitally – by frequency switching under PWM (pulse width modulation) voltage or current control, as per soft starts – but also offer full ultimate motor speed control. Hence their huge energy-saving potential – and not only for applications that require variable speed control.

Energy saving could be massive
As Preston says, drives can also operate motors at fixed speed, but reduced from maximum to whatever the application can tolerate. "You can use them as frequency converters to drop from 50Hz to, say, 45Hz. On a simple quadratic load, such as a pump or fan, that can equate to 27% energy saving, which over the long term could be massive. Even reducing to 48Hz yields 12%, and dropping the frequency to 40Hz can almost halve energy consumption."

But there's more. Preston also points to the issue of safely starting loads that are already 'wind milling', potentially backwards. "The only device capable of starting these smoothly is a drive, because it first captures the motor spin frequency. Drives have various starting methods, but they can all output a scan frequency at a significantly reduced voltage. At the point where drive output matches the frequency of the motor, they detect zero slip. Then they can fully flux the motor and take it up to the desired speed."

He concedes that some soft starts offer a braking function, but this is another huge advantage for drives, and it could be applicable to anything from air conditioning plant and factory air extraction fans to water pumps, many of which are rarely at rest. "Other methods would result in significant mechanical jolts to the system, with all the implications in terms of reduced plant lifecycle and maintenance costs."

Starter choice considerations
? Is the mains electrical supply robust and reliable? If not, a controlled starting method is advisable.
? What are the start-up torque, load, inertia, acceleration and speed range requirements for the motor application?
? Assess the motor characteristics in terms of its response to initial inrush currents, potential for locked rotor currents and their impact on torque output.
? Can the driven load tolerate shock loads that may lead to excessive vibration on machinery, or water hammer and pressure surges in pumping applications?
? How frequently is the motor system likely to be started? If it's an emergency pump start, then costly starters can't be justified.
? Is the motor system oversized or has the application changed such that reducing motor speed would be viable?
? How long does the motor run? Long running hour applications should consider drives in either full VSD mode, or frequency converter mode to take advantage of energy.

Brian Tinham

Related Companies
ABB Ltd
Marathon Technologies UK Ltd

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