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Old Dec 29, 2009, 07:31 PM
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what is the difference between a 2 pole and 4 pole brushless motor ?

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Old Dec 30, 2009, 07:00 PM
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United States, CA, Santa Barbara
Joined Jun 2009
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Two pole has two electromagnets, 1 north, 1 south.
Four pole has four electromagnets, 2 north, 2 south.
Four pole motors have a north-south-north-south configuration.

Standard brushed motors have 3 poles, some up to 9 poles. Brushless motors go from 2 poles all the way up to 8 poles.
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Old Dec 30, 2009, 07:55 PM
NeverAgainVolunteerYourse lf
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Australia, QLD, Regents Park
Joined Mar 2007
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Hi

"Poles are the number of sets of three-way electromagnetic windings that a motor has.

In the simplest three-phase motor, there are 3 separate electromagnets formed by the single set of three-way windings. Thus, there is a set of North-South electromagnetic poles formed. This motor is said to have "2 poles" (although strictly speaking, the motor has 6 electromagnetic poles).

As the three voltage phases, each 120 degrees off from one another, gradually rise and fall, the strength of each electromagnetic winding set rises and falls in relation to the frequency of the voltage changes. This causes the rotor to rotate once per voltage cycle. In a 60 Hertz system, this results in a base speed of 60 Hertz, or 3600 rpm. A 50 Hertz system has a base speed of 3000 rpm.

The next most complex motor has two sets of three phase windings, and is called a "4-pole motor". It is the most common motor produced, and has a 60 Hertz base speed of 1800 rpm (the 50 Hertz speed is 1500 rpm).

6-pole motors operate at 1200 rpm (1000 rpm at 50 Hertz). 8-pole motors operate at 900 rpm (750 rpm at 50 Hertz). "

OK I copied this from somewhere else but its in simple terms, discusses AC motors and brushless motors operate on a similar principle.
The hi lighted section is relevant because brushless ESCs are variable frequency and along with PWM are able to vary the speed of the motor.
As can be seen more poles means slower motor given voltage and frequency remain the same.
As brushless motors are Permanent magnet machines some of the quoted material doesn't apply so its possible to have a 2 pole motor faster than a 4 pole motor due to different winding configuration {more turns}

Nick
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Old Dec 31, 2009, 11:02 AM
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which is better for an rc speedboat ?
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Old Dec 31, 2009, 04:11 PM
NeverAgainVolunteerYourse lf
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Australia, QLD, Regents Park
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Number of poles is probably only a minor consideration, better to look at Kv and torque needed (determined by size of boat).
I'm not an FE guy but I believe most of the motors used are 2 pole motors, they calculate on spinning the prop at 30 000 to 35 000 Rpm and match voltage to Kv to achieve that figure.
If you post details/ pic's of the boat in the Fast Electric boats section you will get some knowledgeable guys helping you out

Nick
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