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Variable speed PM compressor is `a huge leap`

01 April, 2002

Variable speed PM compressor is `a huge leap`

Ingersoll-Rand claims it has made "a huge leap in technology" with an air compressor that it says is the first to be driven by a hybrid permanent magnet synchronous motor. The motor is linked to a standard inverter, resulting in what IR calls "the first true variable speed drive compressor". The company promises that over the compressor`s operating life, its energy and running costs will be at least 28% lower than a conventional system.

Existing variable speed compressors use an inverter to control a conventional induction motor drive train. According to IR, these compressors use the same power to produce 10-15% less air.

The new compressor design, called Nirvana, is said to produce more air, over a wider operating range with no increase in power consumption. It delivers air at a constant pressure, regardless of the demand, and can operate at maximum efficiency from 25% loading to full load, according to IR.

Unlike conventional compressors, the new design does not blow down. Instead it simply reduces speed and air volume to reduce demand. Rather than running unloaded, it shuts down completely and can perform an unlimited number of starts without decreasing the motor life.

Another claimed attraction of the new design is its low in-rush current. When conventional compressors are started, they can draw currents up to eight times larger than their normal full-load running current. With the Nirvana compressor, the in-rush is less than the full-load current, reducing peak demand charges.

The compressor motor, developed by Leroy Somer, is about one third of the size of an equivalent induction motor. It has a salient-pole stator and a permanent magnet rotor which can spin at speeds of up to 6,500 rpm. There are no bearings to fail, and instead of conventional windings, the motor uses independent coil modules that can be removed and replaced on site if they fail. Ingersoll-Rand says that this means that any repairs can be carried out in a few hours at a low cost, compared to the days or weeks normally needed to rewind a conventional motor.

Reliability is also said to enhanced because the Nirvana compressor has fewer rotating parts than any other compressor in its class. And because the motor drives the compressor directly, there are no gears, pulleys, belts, couplings or motor shaft seals to wear out, leak or need replacing. There is also nothing to get out of alignment.

Ingersoll-Rand contends that the new compressor could help to cut the 80TWh of electricity that the EU consumes every year to drive compressors. This figure represents about 10% of all electricity consumed by industry and in some sectors, such as the glass industry, the figure can reach 30%.

• Ingersoll-Rand is recalling about 458,000 portable air compressors in the US, because internal corrosion could cause them to burst open, allowing pressurised air to escape. There have been 11 reported incidents of the air tanks failing, but no injuries. The compressors, which were sold under various brand names between 1983 and 1991, were not marketed in the UK.

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