Power to the AC people

In the world of the fork lift, DC power has always been predominant. But, gradually, this is changing.

A ccording to Richard Baxter of Hamech, UK distributor for Mitsubishi Forklift Trucks, the most significant development in warehouse trucks in recent years has been the addition of AC motors.

Historically, DC power was the preferred option to power lift trucks because their low voltage systems – typically up to 80V – would have required expensive power semiconductors for conversion from DC to AC. The conversion also uses up energy which, considering the limited amount of energy stored in a battery, is very important to control.

But times have changed and AC power is gradually being accepted as the preferred power supply. The reason is twofold: Advances in electronics have made the semiconductor technology cheaper and conversion from DC to AC has become more efficient.

And AC motors have fewer parts than DC ones, which cuts down on maintenance. Some would argue that maintenance rather than outright performance is the key issue.

Manufacturers such as BT and Mitsubishi claim their latest designs exploit the benefits of AC to achieve improved performance and efficiency at lower overall cost.

But not everyone within the industry is convinced. Yale, one of the biggest lift truckmanufacturers, does not yet produce an AC-driven truck. The reason, according to one source, is that Yale is not convinced that the technology has any real benefits.

Detractors say that though AC motors have theoretical performance benefits over both conventional and more modern ‘separately excited’ DC motors (see sidebar), under operational conditions the performance of AC and separately excited DC-powered trucks are much the same.

‘Keen discussions still continue in the industry regarding the merits, or otherwise, of AC versus modern DC electric motor technology,’ says Andrew Morely, marketing manager of the UK arm of German fork lift manufacturer Lansing Linde. ‘But in reality, in performance terms, there is little to choose between the two systems.’

Nevertheless, fork truck manufacturers continue to add AC models to their ranges. Mitsubishi recently launched what it claims is ‘the reach truck of the future’. The company says it is the first reach truck to employ AC technology on all functions – drive, hydraulics for operating the lifting forks and mast, and power steering.

Swedish truck maker BT Rolatruc launched its Reflex range of AC electric warehouse fork lifts in 1997. The company was quick to point out the advantages, especially on productivity, although at the time, many people dismissed the claims as hype.BT commissioned Dutch journal Transport + Opslag (Transport & Storage) to assess its claims. The manufacturer provided the trucks and a driver and the magazine took up the challenge.

While BT’s own test results showed the AC truck had a performance margin over itsDC equivalent of 8% and an energy efficiency improvement of about 5%, the T+O tests were even better.

The reason for this was this was due to differing ratios between distance lifted to distance driven in the test from the standard 1:10. Where the amount of driving was greater in proportion to the amount of lifting, the AC-powered trucks showed a greater increase in overall productivity. With a ratio of 1:12 route a performance improvement of 10.8% was achieved.

David Rostance of BT Rolatruc says: ‘AC power has almost universally been accepted by the major manufacturers and I foresee it remaining in prominence for many years – or until the next technological leap.’

But what of Mitsubishi’s claim that its truck is the first with all-AC technology? BT argues that, in reality, there is little benefit to overall performance in using AC technology to drive the power steering.

Mitsubishi accepts this, but counters that with AC control on all three motors the truck can be more easily maintained, and that this alone justifies its use.

On balance, while separately excited DC motors will almost certainly maintain a presence within the materials handling industry, there is no doubt that the trend towards AC motors will continue.

Technology: AC versus DC

Conventional series wound DC motors have their armature and field windings connected in series, through which the same current flows. As the load increases, the current in the armature and field also increases, with a corresponding reduction in motor speed.

Torque also tails off proportionally as motor speed increases.

AC motors, on the other hand, provide high acceleration and speed, laden or unladen, coupled with variable torque/speed ratios for total flexibility in any application.

However, the modern, separately excited (shunt wound) DC motor is a different matter. When combined with high frequency digital controllers, the shunt wound DC motor achieves virtually the same performance as the AC version, including variable torque/speed ratios.

On separately excited DC motors, armature and field windings are driven separately, enabling different currents to be fed to the two windings. Thus the motor field can vary independently of the armature, resulting in the ability to maintain motor speed under a wide variation of load conditions and input voltage. The result is greater versatility.

The maximum theoretical performance of a DC motor is constrained by the ability of the carbon brushes and commutator to convey the current to the armature. The AC motor has no such constraint since it does not have these components.

But in practice operational realities such as the available space for battery, gearbox and bearings constrain the theoretical performance of AC.