8/3/2019 Technical Notes TM022 en RevA 2008 Comparison 2pole vs 4pole[1]

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Comparison of 4-pole and 2-pole designs forlarge machines (typically rated over 7 MW)

TM022 EN Rev A 2008

Large motors and generators (typically ratedover 7 MW) are available in both 4-pole and2-pole designs. 4-pole synchronous machines

have solid rotors with salient poles while 4-poleinduction machines have squirrel cage rotors. In2-pole machines the rotors are cylindrical.

4-pole machines are smaller and lighter thantheir 2-pole counterparts, which can be animportant actor in applications where space islimited, such as the marine sector.

Cooling arrangements

The compact design o ABBs 4-pole machinesmeans straightorward, symmetrical cooling withgood control o hotspots. In 4-pole synchronous

machines the solid rotors are designed tomaximize the surace o the feld coils exposedto the cooling air, resulting in highly efcientcooling.

The design o 2-pole rotors with their slotwedges and retaining rings - means that accessto cooling air is more restricted.

Critical speed

4-pole machines always run below their frstlateral critical speed. This means that they canoperate across their entire speed range without

orbidden speed windows a actor whichprovides increased versatility, especially inmotors that are used with VSDs.

ABBs 4-pole rotor design with salient poles (i.e.synchronous machine rotor) has a large diameterwith plenty o inertia, providing excellent speedstability.

Vibration and noise performance

Subcritical operation and sturdy rotor design

mean that ABBs 4-pole machines have goodvibration perormance. As the frst lateralcritical speed is well separated rom theoperating speed, vibration conditions remainconsistent throughout the entire operatingrange. 2-pole rotors, by contrast, have to passthrough their critical speed, which will requiremore advanced rotor balancing control andprocessing during manuacturing.

4-pole machines also have signifcantly betternoise perormance than 2-pole machines dueto the lower rotor speed.

Servicing and maintenance

ABBs 4-pole machines have long serviceintervals thanks to their robust rotor designwith a limited number o components. Unlike2-pole rotors, 4-pole rotors have a boltedpole tip and no retaining rings which meansmaintenance o 4-pole rotors is ast and easy.

The lower speed o 4-pole machines, comparedto 2-pole machines, means that they are subjectto less general wear and tear.

When considering servicing requirements,

however, it is important to take the entiresystem into account. Gearboxes have suchhigh maintenance requirements that a systemconsisting o a 4 pole machine with a gearboxgenerally needs more maintenance than a2-pole machine operating without a gearbox.

8/3/2019 Technical Notes TM022 en RevA 2008 Comparison 2pole vs 4pole[1]

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www.abb.com/motors&generators

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Application summary

Applications

Generators driven by gas or steam turbines

Many units operating at 3000 rpm are actuallydriven by a 3600 rpm turbine. The turbineis thereore not used at its optimal operatingpoint, which reduces the utilization actor othe system. A 2-pole machine is oten usedbecause this avoids the need or a gearbox, butin many cases a 4-pole machine with gearbox

would have a lower investment cost andprovide a better utilization actor.

For systems running at 3600 rpm, 2-polemachines are the avored option becausegearboxes are not needed.

Above or below 3000/3600 rpm, 4-polemachines are the preerred option.

Motors for turbo compressors

For turbo compressors operating at 3000/3600rpm, 2-pole machines are avored as nogearbox is required. In systems operating atfxed speeds above 3000/3600 rpm, 4-polemachines are the preerred option

Speed 4-pole 2-pole

Fixed

below 3000/3600 rpm ++

Gearbox often needed

Gearbox always needed

3000/3600 rpm Gearbox always needed +

Gearbox not needed

above 3000/3600 rpm ++

Gearbox always needed

Gearbox always needed

Variable (using VSD)

++

Subcritical operation means wider

speed range

Super critical operation: machine

has to pass through critical

speed

Operating range can be increased by exceeding 50/60 Hz frequency

++ favored option

+ slightly favored option