When one refers to How many poles an electric motor has, they are commonly referring to how many magnetic poles there are. Many often wonder the differences between motors having more or less poles. Here, we will focus on a brief discussion concerning Brushless motor magnetic poles. Main focus will be placed on In Runner Motors, where the rotor spins inside of the stator.
Common in runner motors have 2 or 4 magnetic poles. There are some motors containing 6 poles as well. Outrunner motors may have in excess of 6 magnetic poles. You may want to read more on outrunners vs inrunners and what the differences are.
Motor Poles vs Motor Characteristics
The largest characteristic that a motor with more poles displays when all else is equal is a drop in the KV value. The KV value as we all know is the amount of RPM per volt. When all else is equal between 2 brushless motors, the one containing a higher pole count will have a lower KV value. This is the number one important characteristic of differences in a motors pole count. All else that we will talk about is based off of this.
How Cogging is effected by Pole Count
If you haven’t already read the article on cogging, I’d recommend following the link to read on this further. It may not be what you thought it was. Pole counts do influence cogging torque on slotted motors. Generally on inrunner motors, cogging torque increases as you increase in magnetic pole count.
A drop in KV, what does that do? – Brushless Motor Poles
When a motors KV value drops when we keep the motor the same physical size and the number of motor electrical winds (wind / turn) the same, we gain motor torque. The torque is gained as a result of lower KV. Depending on the exact application this may be an advantage or disadvantage. More torque may mean greater acceleration but a lower KV value will reduce maximum RPM achievable.
How Does a Manufacture maintain a usable KV? – Brushless Motor Poles
There is one route a Motor Manufacture may take in order to bring the KV back up to a usable amount. The best option is to decrease the amount of winds in the motor. A 2 Pole motor for example will typically have many more winds than a 4 Pole motor. When a 4 Pole motor has the amount of windings reduced, KV will increase back to the amount needed. When a winding from the motor is removed, the physical size of the motor remains the same resulting in a void. The best solution to this created void is to add more copper. More Copper is added to the winding. This is done in such a way similar to moving down in the Gauge type. In other words the wire used as a wind is now increased in diamater.
As we know from electrical theory, when the cross section of a wire is increased the resistance decreases. This decreased resistance would allow a greater current load to pass through the motor.
Advantages and Disadvantages – Brushless Motor Poles
When comparing a 4 Pole motor that has the same physical size as a 2 Pole motor and the same KV as a 2 Pole motor, the 4 Pole motor in theory would be able to handle greater current loads. The greater current loads will result in greater power. This is similar to saying a 4 pole motor has better overall efficiency when compared to a 2 pole motor.
Although it sounds like there are many advantages, there are also some down falls to this as well. It is very common for 4 pole motors to not have as many motor options as a similar 2 pole motor. Less motor options may make it difficult to get the exact KV value you need. This is even with 4 pole motors using both wind types. A wind type is the difference in how a wind is terminated before it gets to the 3 wires hanging out the brushless motor that you typically see. These wind types are commonly known as the Delta wind and the Wye wind. Using one wind type over another does provide different motor characteristics yet again. We will note this as a possible disadvantage as well.
Which Pole Count Works best for me – Brushless Motor Poles
Overall, it will come down to testing, testing, and testing. These differences explained above may be very subtle to the point where you don’t even note any differences. There may be many reasons to this including your driving habits. In order to determine which works best for you, it is recommended to give both types a try. You may prefer either one of them.