What are the differences between 2-pole and 4-pole motors?

[James3849]

Hi everyone,

I'm a graduate mechanical engineer and I was wondering if I could open up the discussion of 2-pole versus 4-pole motors. Recently I have had to specify such motors for work and wanted to find out the advantages and disadvantages of each type.

Mechanically I realize that 2-pole motor being a higher speed (approx. 2900 rpm) are prone to wear more than its slower 4-pole counterpart. Also 2-pole's generally have a much higher NPSHR.

I am interested in other people's experience dealing with such motors and any other advantages and disadvantages that I havn't already stated. I am also interested in the electrical side of the matter, and the adv./disadv. there too.


Regards,


James3849

 

[Jwill]

Torque is the big thing with poles. Electrically it's the same series circuitry with more coils.

 

[Phrak]

Cost?

 

[Topher925]

Well first, tell us what kind of motors you are looking at. Synchronous, asynchronous, PM or not, etc. Usually the main reason for going with a higher pole is torque especially on the low end but there are many other factors.

Like you, I am a new ME and deal with motors along with their controls (SVM, SWM, DTC, FOC, etc) on a daily basis. The worst part about motors is they vary in so many ways its hard to determine what you want because there are so many ways of doing it. I found this website to be a great resource for information: http://www.electojects.com/motors/

 

[James3849]

I can't be too specific, because what I'm doing is putting together a standard specification for pumps for an entire mine redevelopment. For those who have worked for consulting companies, you will know just how much 'fun' specs are. Anyway its mainly just a contractual document which goes to a vendor or contractor stating what kind of pumps shall be purchased/used, and this is why I've raised the question because I don't have much experience with electric motors.

Phrak raises a good question, is cost a contributing factor? Not just the cost of purchase, but on going costs such as increased maintenance or electricity demand?


P.s. Looks like a great website.

 

[Jwill]

Phrak raises a good question, is cost a contributing factor? Not just the cost of purchase, but on going costs such as increased maintenance or electricity demand?

Cost? No, to my knowledge I don't think cost is going to be a big issue or maintenance and the efficiencey should be comparable at optimal speed. Though let me note that I am also a new ME and not the world's leading expert. I guess what I am saying is I don't think that the difference in a 2 or 4 pole motor is going to make or break your operation. For your situation with the mining pumps, I would guess that a 2-pole motor wouldn't have any problem as it is going to have a steady load.

 

[James3849]

Yeah most pumps will have steady loads, but some pumping applications will vary greatly, especially things like raw water which is used for washing down vehicles and stockpile sprinklers which are used to prevent ore from being picked up for the wind.

Anyway thanks guys for your help.

James3849

 

[Dunstrugglin]

I think the power factor comes into to it too, a two pole motor running on a higher power factor makes about the same torque as a four pole motor on a less PF and then we have to take into rotational mass too.and to top it of I would sugest a four pole motor is more expensive to manufacture

 

[LooseConn]

I love the topic, but how does one become a mechanical engineer grad without having to learn the difference between a two and four pole motor in and out?

 

[maimonides]

If you want to drive pumps with your motors, you´ll have to think about the pumps. Head and flow in a (centrifugal) pump are pretty sensitive to rpm, and AFAIK a pump can be built smaller (and cheaper) with a higher rpm (for a given flow and head ).
So I think you can´t specify the motors without specifying the pumps first.

 

[edwardo]

the concern of james is the wearing of 2 poles is faster than 4 poles. if you will compare their rpm, the 2 poles have higher rpm compared to a 4 poles motor. assuming that these motor have the same running time. of course the 2 poles motor will wear fast because it rotates more compared to 4 poles motor.
however jwill have a point. because lower rpm will affect the pump capacity. but i guess james tried already a 4 poles motor and he don't have problem with the pump efficiency or output.

 

[Travis_King]

If it's an underground mine the only things the mining companies usually care about are maintenance, noise created by the motors, and the heat the motors can take (these are standard ratings for electric motors). Many times they want them in explosion-proof casings as well.

If you're writing a spec, why not just find out what type of motors the mine likes and throw that in the spec?

As some others have said, if the main purpose for spec'ing the motors is the pumps, then you typically only have to be general (and list any specific requirements that the mining company demands) as the pump manufacturers will size and spec the motor for your application

Examples of typical mine requirements:
Do they have specific requirements for the baseplate patterns?
Do they dislike high rpm (like 3600 rpm) pump motors because they have a shorter working life/higher maintenance costs?
Are they very conscious of temperature rises (ie NEMA temperature ratings)?

Does it matter to anyone at the mine if the motors are 2 pole vs 4 pole? The electricians shouldn't care because electrically they don't have anything different to do (as long as they can figure out what wiring pattern the factory used: star, delta, wye). If nobody cares, then why spec it?

If you don't have to spec something, say it's up to the vendor. If you're really worried, say its up to the vendor with your company's approval required.

edit: Both have their advantages and disadvantages. 2 poles are lighter and cheaper, but they require much higher NPSH and wear much quicker. 4 poles are more reliable and easy to maintain/repair, but they are generally bigger and bulkier. If your main concern is that these are miners and are likely to beat up on these pumps (which is very likely), then you're probably safer with the 4 pole flavor.

Another point, slower pumps with their big impellers are generally less susceptible to water hammer effects in a loss of power event.

 

[tygerdawg]

You may just settle on 4-pole because those are more readily available, IMHO. Joe The Maintenance Guy can get replacement 4-pole motors to meet a voltage/HP spec from almost anywhere to return a down unit back to service quickly. That would make the manager happy. In my experience, 2-pole motors are typically used for high RPM / low torque applications like 15,000 RPM milling spindles (with a VFD). Could a low-torque 2-pole motor survive the cyclic torque load of a pump?

 

[rollingstein]

One big parameter is whether the application requires variable speed I think.

 

[jma19312]

Motor issues and pump issues are different matters

The 2-pole / 4-pole issue with motors is more important if there is a starting torque to overcome since the rotor radius required and the stator mass increase with additional poles so therefore does the torque available. Regarding motor wear due to higher speed, AC motors have no brushes so bearings are the only contact between rotating and stationary points and L-10 bearing life standards have no rating difference based on rotational speed. If proper bearing lubrication procedures are maintained the higher speed does not change the L-10 rating. 2-pole motors have lower total mass and often cost less per HP. The smaller diameter can be a benefit in tight spaces.

With centrifugal pumps there is no starting torque. To generate the centrifugal force needed to produce the dynamic head required to move the liquid rotational speed and impeller radius work together so higher rotational speed requires a smaller impeller & bowl therefore they often cost less. If solids are being pumped the higher speed increases wear and maintenance. Respecting NPSHr, impeller design and geometry are far more critical than rotational speed. The pump manufacturer's curves are based on actual tests and these are authoritative.

 

[LAZYELECT]

From an electrical engineering standpoint 2 pole vs 4 pole is neither here nor there. Seems more important to match the water quality to the flow so clean water high flow smaller bore, 2 pole or suspended solids, low speed larger bore 2 pole.
With the reduced costs and massive increases in efficiency of VSD's, it is quite often more economical to run a faster motor slowed down with a VSD. I did the calcs on Yaskawa Matrix drive (awesome if you haven't seen them yet - no DC BUS 100% torque at 0 rpm) and saved 50% on energy costs running at 80% speed. This is because flow (based on the Laws of Affinity for pumps) is proportional to speed, head pressure is proportional to speed^2 (squared) and Power is proportional to Speed^3 (cubed). So for a 15KW pump running at 80% speed, the power drawn was 7.68KW. {P1/P2=(N1/N2)^3}. This also increased the power factor to .98 from .85, although this did not change the running cost (as they were charged on KW'S) it did save 1800KG of CO2 emissions and if it had been an original install it could have been spec'd with 29A infrastructure instead of 34A full speed current (smaller cables breakers transformers etc etc). This also allows you to slow down the process during low demand times as apposed to stop - starting (inrush current jolting mechanical bits) and the slower it goes the more you save (remember the head pressure drops by the ^2 as well) to around 15% KW's at half speed. Add to that the ability to ramp up and ramp down, an intelligent system integrator and designer could make a far better solution for less cost (setup and running). This is just the Gist however it can be quote complex with multiple pumps in series/parallel and dynamic load changes.

 

[notcreativenow]

The force on the rotor in a 2 pole motor is not balanced as they are in a 4 pole motor. Consider the force on the rotor. This alone is enough to make a 2 pole motor thoroughly perceptibly noiser and vibration excelled. 4 pole motors are preferrable until their speed is insufficient. If the 4 pole speed is inadequate then use a 2 pole. Remember, these days, variable frequency drives give you some additional freedom as i regularly operate 4 pole motors at 100Hz - beyond that torque starts to roll off to the extent that the gains of 4 pole are marginalized in favor of 2 pole application.