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iPM compressor motor ‘exceeds IE5’ and slashes losses

05 March, 2020

The compressor-maker Atlas Copco has developed and patented an interior permanent magnet (iPM) motor which, it claims, has a higher efficiency than the IE5 “ultra premium efficiency” level. It is using the motor to drive the rotors in some of its compressors directly, resulting in energy savings of up to 30%.

In an iPM motor, the magnets are embedded into the rotor, avoiding the risk of them becoming detached as can happen if PM motors using surface-mounted magnets run too fast. The iPM machines can therefore run at higher speeds, enabling direct coupling to the compression element with a 1:1 ratio.

Coupling the motor directly to the compressor rotor avoids the losses associated with gears, belts and shaft seals. The motor is cooled by the compressor oil, which is effective at all running speeds, avoiding the need for a cooling fan that reduces efficiency. This is especially true when the motor is running at low speeds. The oil that cools the motor is heated before it reaches the compressor element, helping to prevent condensation under low-load conditions.

The iPM motor has a high torque capability, allowing the compressor to start while pressurised, and avoiding the need to blow down the compressor when it stops in standby mode. As a result, air that has already been compressed is not wasted.

Conventional compressor motors have two bearings, one for the element drive end, the other for the non-drive end. In Atlas Copco’s design, the drive end is supported on the compression element’s bearings, so the motor needs only one bearing, cutting friction losses further.

Atlas Copco’s interior permanent magnet motor couples directly to the compression element

Unlike motor applications such as cranes or conveyors that operate with variable torque and at constant speeds, air compressors ideally require constant-torque, variable-speed operation. Whether the compressor is running at 20% or 100% of its top speed, the torque is the same. Atlas Copco has therefore developed its own converter technology specifically to drive the iPM motors, rather than relying on general-purpose motors and VSDs.

The company is using the technologies in its GA VSD+ rotary screw stationary compressors which, it says, achieve IES2 efficiencies – the highest class for a combined motor-and-drive as defined in IEC 61800-9-2.

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