How Do You Know How Many Poles a Motor Has

Past Lauren Nagel

Published:07-10-2021, Terminal updated: 07-10-2021

Introduction

If you are building a drone, RC vehicle, or any machine that uses a brushless DC (BLDC) motor, y'all have likely encountered the concepts of motor poles and motor Kv. Both of these parameters are useful for characterizing your motor and estimating its performance.

In this article we encompass what these metrics mean, how to calculate and measure them, and finally how they are related.

Table of Contents:

1. How to Determine the Number of Poles in a Brushless Motor
1. What are motor poles?
2. Motor poles vs. pole pairs
3. Motor poles and electric speed
4. How to calculate motor poles (equations)
5. How to measure out motor poles
2. How to Summate Brushless Motor Kv
1. What is motor Kv?
2. What does Kv rating hateful?
3. How to measure summate Kv (equations)
4. How to measure motor Kv
3. The Relationship Betwixt Motor Poles and Kv

Part 1: How to Decide the Number of Poles in a Brushless Motor

What are Motor Poles?

Within a brushless motor, there are a number of magnets lining the circumference of the rotor (figure 1). These magnets are also referred to as the 'poles' of the motor. When an electric current is delivered to the coils in the stator, the rotor starts to plough as its magnets repel the similar electromagnets.

In the diagram beneath, the blue rotor contains the permanent magnets in an inrunner motor (left) and an outrunner motor (right). The green stator holds the electromagnetic coils.

Figure 1: Motor magnets / poles in inrunner and outrunner motors

Motor Poles vs. Pole Pairs

The magnets lining the rotor each take their own north and south end, with just one of those ends facing and interacting with the stator. In that location are an equal number of north and south magnets facing out from the rotor, and each set of N and S magnets is referred to as a 'pole pair'.

For each pole pair in the motor there are two poles, so if the motor has 8 poles / magnets, there are 4 pole pairs. For this same reason, yous will almost ever have an even pole number (ii, iv, 6, 8, etc.), equally each magnet needs an opposite pole.

Ex.

• 2 pole motor → one n magnet and ane south magnet → 1 pole pair
• viii pole motor → 4 north magnets and 4 south magnets → 4 pole pairs

Relationship Between Motor Poles and Motor Electrical Speed

The speed of the motor is inversely related to the number of poles in the rotor. The human relationship is explained by the following formula:

Where:

N s = synchronous speed

f = frequency of iii-stage power supply

And #p = the number of poles

Alternatively, it can be written as:

Where:

N s = synchronous speed

f = frequency of three-stage power supply

And #pp = the number of pole pairs

How to Calculate Motor Poles

Since the number of poles is equal to the number of permanent magnets in the rotor, the simplest fashion to decide the number of poles is to count the magnets manually. This is possible if y'all can run across into your motor or if you lot are able to remove the outer casing. If you are unable to remove the casing this tin be a challenging chore as the magnets can be quite pocket-size and numerous.

Further reading: How BLDC Motors Work

Figure two: Counting the magnets / poles inside a brushless motor

Other simple procedures accept been suggested, similar waving a magnet around your rotor to see where allure and repulsion occurs. This method is most feasible for outrunner motors where the magnets are outside the stator nearly the surface. Be careful if you endeavour this technique as the magnets inside your rotor can be demagnetized by the influence of a stronger magnet.

Another option that requires a bit more effort and equipment involves finding the motor speed and power supply frequency experimentally, then rearranging the equation below to solve for the # of motor poles:

To measure your motor speed, you will need a piece of equipment to back-drive your motor (a drill, for instance), plus a rotation speed-measuring device to measure the number of rotations, such equally a tachometer. You volition also need a device to mensurate the frequency of the back EMF generated, such as an oscilloscope. Measure the speed and frequency simultaneously, then input the values into the equation to a higher place to decide the number of poles in your motor.

Measure the Number of Motor Poles

Perchance the simplest way to measure the number of poles in your motor is to have a test stand up determine it for you.

Several of our RCbenchmark examination stands come with automatic, pre-written scripts for computing the number of poles in your motor, including the Series 1520 and the Series 1580/1585 .

Simply enter the Kv rating of your motor in the script, and then run the automatic examination with your motor installed on the test stand (without a propeller, as the motor will run at full speed).

The software volition provide yous with the pole count for your motor every bit shortly as the examination is done, which takes just a couple of minutes.

Figure 3: The automated script that counts motor poles using the RCbenchmark software

Part 2: How to Calculate Brushless Motor Kv

What is Motor Kv for Brushless Motors?

Motor Kv provides a fashion to depict the relationship between the peak voltage and rotation speed in a brushless motor in a no-load condition. The unit for Kv is RPM/Five and it tin exist estimated past dividing the rotation speed of the unloaded motor with the applied voltage.

In truth, the voltage we should consider is in fact the back electromotive forcefulness (EMF) and not the applied voltage.

In a brushless motor, the back electromotive force (EMF) is a voltage that occurs in the contrary direction of the current provided past the power source, induced by the movement of the coils through the magnetic field in the motor. This can exist explained by Faraday's law, which shows that a coil moving inside a magnetic field creates a menstruation of electrons inside the ringlet, chosen a voltage or emf.

When the motor turns, the dorsum EMF generated is proportional to the speed of the rotor: equally RPM increases, dorsum EMF does as well. At full throttle with no load (ignoring the motor'south own inertia), we can describe the relationship with this formula:

What does the Kv Rating Mean?

The Kv rating provides an estimate of how many rotations a motor will undergo for every volt applied to it. This rating can be helpful for comparing motors that are of the same size physically, but have different functioning characteristics due to their inner workings.

As a general rule, as the number of windings in the coils increases, the Kv of the motor decreases. Mechanically speaking, low Kv motors take a higher number of windings of a thinner wire and the thin wire carries more volts at lower electric current. Loftier Kv motors take fewer windings but with a thicker wire that tin can carry college electric current with fewer volts. The equations in the adjacent section provide a mathematical demonstration of this concept.

Farther reading: BLDC Motor Power and Efficiency Assay

Practical to drones, low Kv motors tend to operate at lower RPMs and produce more torque, platonic for larger propellers and large drones. High Kv motors operate at higher RPMs and are platonic for low torque, small, and fast spinning propellers. Therefore, lighter, fast moving drones such as racing quads are all-time served by loftier Kv motors and heavier, slower moving drones are better served past low Kv motors.

Effigy four: The EMAX Militarist Pro racing drone uses a 2400 Kv motor whereas the Foxtech Gaia 190MP heavy elevator drone uses a 100 Kv motor

Some other applied application is for inrunner and outrunner motors. An inrunner typically has a college Kv value than an outrunner of the same size. The larger diameter of the rotor in the outrunner allows for more permanent magnets. More than magnets (poles) → lower speed → lower Kv. The pocket-sized bore of the inrunner besides ways it has a smaller circumference to cover for one rotation, thus more rotations for the same voltage.

It'southward important to note that Kv is not a definitive way to evaluate a motor's performance as the make of motor can also bear on its efficiency. For the same Kv value, a motor from one make may perform better or worse than another, which volition only exist revealed through testing.

How to Summate Motor Kv

If we go back to our before formula:

We can rearrange it to evidence that:

To approximate Kv, you lot can swap 'Back EMF' with input voltage in the equation, which will give you a pretty good judge of motor Kv.

For a more than accurate estimation, you can measure the voltage between two leads in the circuit to obtain your root mean squared (RMS) voltage, and then multiply information technology by 1/sqrt( 2) or 0.7071 to obtain peak voltage, which you tin can plug into this equation:

0.95 is a value that accounts for deviations from the theoretical model to provide a value closer to that which is experimentally observed. This gene is commonly accepted for Kv calculations.

How to Measure BLDC Kv

Measuring Kv experimentally is possibly the best mode to get an authentic idea of your motor's true Kv. Nosotros have regularly observed differences between the stated and measured Kv of the motors we've tested, which yous will probable observe also.

Nosotros provide a pre-written script in our RCbenchmark software that allows you to measure out Kv experimentally by running your motor through a no-load test.

All you accept to do is claw up your motor to an RCbenchmark test stand, input the number of motor poles in the Setup tab, then run the test (without a propeller). The software volition calculate your motor'south Kv automatically in one case the exam is finished.

Fifty-fifty if your motor is labelled with a Kv rating, information technology tin be interesting to test it to see if the experimental Kv matches what is advertised.

Figure 5: The automated script that calculates Kv using the RCbenchmark software

Part 3: The Human relationship Betwixt Motor Poles and Kv

As shown in the previous sections, motor Kv and the number of poles are both related to the speed of the motor / RPM. We know that as rotation speed increases, Kv increases equally well. Alternatively, a higher number of poles corresponds to motors that operate at lower RPMs.

The relationship between the number of motor poles and Kv is thus inversely proportional. This makes sense when we think nearly it practically.

A larger motor with a greater number of poles will crave loftier torque and operate at a low operating speed. The motor used would thus accept a high number of poles and a depression Kv.

A smaller motor with fewer poles will operate at high RPM and produce relatively low torque. The motor will therefore have a low pole count and a high Kv.

Conclusion

The concepts covered in this article, brushless motor poles and motor Kv, are two concepts that are key for agreement the properties and operation of BLDC motors. Knowing how to translate these numbers can help you cull the best motor for your drone or electric aircraft and meliorate your operation and efficiency. Efficiency is directly related to flight fourth dimension, payload and range, so this data is very valuable.

If you would like to be able to measure out the number of poles in your motor and your motor's Kv automatically, we propose checking out the Series 1520 Thrust Stand and the Series 1580/1585 Test Stands. As mentioned in the article, their software comes with pre-written scripts for automatically calculating both of these values. Saving you valuable time while offering more opportunities to narrate your motor.

If you have any questions about the contents of this article, don't hesitate to leave usa a comment and nosotros will be sure to answer.

Further reading: Automatic Brushless Motor and Propeller Testing

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Source: https://www.tytorobotics.com/blogs/articles/how-to-calculate-motor-poles-and-brushless-motor-kv