Motor tech: Learn the Terms-Part 1
Most new E-bikers don’t care about the details inside the motors that we ride, as long as they just work. But, occasionally we use terms in our articles to describe some characteristic about electric motors, and we want our readers to understand why one choice is better than another for a given application.
This article is part-1 of a two-part series, with this first part covers the basic terms that are used to describe E-bike motors. Here is what part-1 covers:
Permanent Magnets and Electro-magnets
A permanent magnet (PM) is pretty easy to grasp. It a piece of metal that seems to be magnetic all the time, but it is only drawn towards ferrous metals, meaning metals that have iron in them (like steel). There are only three types of permanent magnets you are likely to hear about in E-bike motors: Ferric, Samarium-Cobalt, and Neodymium.
A few years ago some of the smaller and low-priced motors used the fairly weak “Ferric” magnets that are usually a dark grey color. The Kollmorgen motor is one example. These inexpensive magnets can still be occasionally found in childrens E-vehicles.
Samarium-Cobalt magnets were developed in the 1970’s, and they are VERY heat-tolerant. They retain their full magnetic power (measured in Gauss) when they are exposed to temperatures near their Curie temperature of 800C (1,400F). However, they are expensive due the need for using Cobalt, so…the desire for strong permanent magnets that were less expensive has brought us to the development of Neodymiums in 1982.
Neodymium magnets are the breakthrough that really allowed the boom in E-bikes to occur just about a decade ago in the early 2000’s. Neo’s have been around for a while, but they used to be very expensive. When the boom in computer hard-drives occurred, the computer industry needed a stronger magnet to package in the increasingly smaller drives that had to hold more information. The mass-production of neo magnets for computers dropped their price, and then the manufacturers of small permanent-magnet motors began to use neodymium magnets for mass-production.
To understand the heat characteristics of neo magnets, it might be useful to explain how permanent magnets are made. The base material is made and shaped, and then it must be heated up to a temperature that is above their “Curie Temperature”. This is the temp at which the individual atoms are at an energy state when they can spin and rotate freely without being affected by the magnetic orientation of their neighboring atoms.
At this point, the hot material is subjected to a very strong magnetic field that is produced by an electromagnet that is positioned near it. Then…while the electromagnetic field is still energized, the material is cooled. As a result, all of the atoms (each one being a tiny magnet itself), have all of their magnetic fields aligned in the same direction, and in effect they are all frozen in place, while still pointing in the same direction.
The Curie temperature of high-grade Neodymium (with added terbium and dysprosium) is 320C / 600F. However, common neo magnets are made from the cheapest grade, and can start to lose some of their magnetism at around 80C (170F).
E-bike motor magnets are a grade that is slightly higher than the cheapest variety, because they are often subjected to higher temps than they should be by unsuspecting customers. Years of posted experiments by real E-bikers on endless-sphere have produced a commonly held rule-of-thumb to avoid heating your E-bike motor to above 95C (200F).
If your motor feels like it has lost some power, and it also now has a slightly higher top-speed…you probably overheated your magnets…and there no way to fix that.
Of course, our experience has taught us that if a motor is completely cool under all conditions, it is probably somewhat larger, heavier, and more expensive than necessary for the job it’s given. And…on the other end of the scale, if you cannot hold your hand on a motor because it’s too hot, then you are probably converting too many battery watts into waste heat. So…any temps from room-temperature (during cruise-mode), up to 60C (140F) during your heaviest loads are a good optimum for an E-bike system design.
This brings us to electro-magnets
A basic experiment in electricity is to wrap a copper wire around a nail that’s made of steel (99% iron). When you apply Direct Current (DC) electricity through the wire, the nail temporarily becomes a magnet. Then…when you de-energize the wire, the nail stops being a magnet. Magnets have a north and south pole, and you can also reverse the poles of the magnet, by reversing the positive and negative posts of the battery you are using to make the nail a magnet.
Reversing the poles of the same electromagnet (back and forth) is useful to understand, because it’s a major component of how some modern E-bike motors work. One of the most basic laws of magnets is that: opposite poles are attracted to each other, and same-poles are repelled by each other.
If a PM has its north pole facing outwards, and it is located facing two electromagnets that are side-by-side….you can see that if the electromagnet on the left is energized so that its north pole is facing the PM, and the electromagnet on the right is energized so that its south pole is facing the PM, the left electromagnet will be pushing the PM to the right at the same time that the right electromagnet is pulling it to the right.
Direct Drive, Geared, or Mid Drive
Direct Drive hub motors (DD) are as simple as a motor can be. They are large enough that they are not stealthy, somewhat heavy, and when un-powered they have a little bit of magnetic drag (called cogging).
They remain relevant, and will likely survive because of several reasons. Their simplicity keeps their price down, so they will likely remain the most affordable beginners E-bike kit. Also, above 30-MPH they can handle more amps and heat than geared hub motors. And last…when combined with a sine-wave controller, they are virtually silent.
Direct-Drive hubs (DD), are about as simple as it gets. The axle and stator on the right are fixed, and the spinning part is on the left.
Geared hub motors have an internal gear-set that allows the motor to spin about 5 times for every time the wheel spins. This allows a relatively small and light motor to have as much torque as a larger direct-drive motor. One of the reasons that they are very popular is that they incorporate an internal freewheeling clutch, so when you are pedaling with no power on, it rolls easily with no resistance (no cogging).
These are the most popular type, for power levels between 250W-1,200W
Geared hub motors are the most popular for medium to low power levels.
The extra complexity and expense of a mid-drive has very specific benefits. The most common style allows the motor to use the bikes gears, and this is a huge help for two user-profiles. The European Union (EU) has a low 250W power limit, which makes it very difficult to climb steep hills. Allowing a small motor to downshift into a lower gear is a major improvement.
The other user who needs a mid drive is someone who hauls a heavy load on their bicycle up extra steep uphills. (a cargobike in San Francisco?). However, the most enthusiastic adopters of high-powered mid drives have been full-suspension off-roaders…
The most popular factory mid drive has been the Bosch drive, by a large margin. The most exciting mid-power mid drive kit has been the Bafang BBS02, and the best high-powered off-road mid-drive is the Lightning Rods 2800W kit (read our article on the LR kit here). Edit: as of 2016, the 1500W BBSHD has become quite popular.
A mid drive has a big advantage when climbing slow and steep uphills.
2-Speed Geared Hub
Here’s a late addition, the Xiongda company is producing a 2-speed geared hub which could have a big impact on the electric bike world. It has two sets of internal gears, and the motor reverses on-the-fly to engage a lower “hill climbing” gear.
They currently have only one model (as of 2014), which is a small 500W-max unit that has been designed for the European market. However, if they ever start making a larger hub that can provide 750W using 36V, I believe that will be a very exciting development for North American customers. (Read our article on the 2-speed Xiongda here)
Brushed and Brush-LESS
Industrial factories often use induction motors (as opposed to permanent magnets). In an induction motor, the stator (the stationary part) and the rotor (the part that rotates, sometimes called the armature), are both made from groups of electromagnets. Because of this, the power to both of them can be varied as desired.
Since the rotor spins, we have to add a method energizing and de-energizing the coils on the rotor while it’s running. For a brushed motor, this is accomplished by a type of switch-contact that rubs against one of the spinning parts (see video here).
The “brushes” are the contact that is typically on the stationary motor housing (so the amount of wear can be inspected even when the motor is running, as the brush gets shorter over time), and they are usually designed so that they are easy to clean and replace as needed. The part of the brushed contacts that is designed to last a long time (the surfaces that the brushes rub against) are typically attached to the rotor shaft, and it is called the commutator.
This is a graphic of a common tiny brushed motor. It has two curved magnets in the stator (attached to the outer shell). Since the rotor in the center has only three coils, the commutator at the end of the shaft has 6 contacts, which the two brushes rub against. One brush is connected to the positive wire coming from the controller, and the other brush is connected to the negative.
The reason brushLESS motors are popular with E-bike DIY builders is that…even though you may want to run 36V on a motor when you first install it, using a brushless motor allows you to run that same motor at many different voltages. The 9C Direct Drive (DD) hubmotor is advertised to run at 36V/48V, but…the Pikes Peak winner used 111V on a 9C, with no problems.
I have read that brushes are messy, they have to be replaced on occasion, and are less efficient, but…brushes are designed for a specific voltage range (higher voltages require a wider spacing) and DIY builders want the freedom to experiment and upgrade. To be fair, brushed motors (like the large Agni) use controllers that are MUCH LESS expensive than brushless controllers, and for some builders, that remains a significant benefit.
But how do the controllers for permanent-magnet brushless motors know when to turn the electro-magnets (in the stator) on-and-off with perfect timing? Since these are 3-phase motors (all the coils in the stator are assigned into three groups of coils), there are three hall sensors in a common brushless Ebike motor.
Each of the three hall sensors has three legs (nine legs total), and a 3-phase motor has 5 hall wires. This is because one wire provides a positive to all three of the sensors, one wire provides the ground/negative to all three sensors, and then the three remaining wires are for the on/off signal…one signal wire for each sensor. The signal is a low-amp 5V pulse. So…5V is supplied to each positive leg on the three hall sensors, and when a magnet is next to the sensor, the 5V passes on through the hall sensor into the signal leg back to the controller.
Here are three Hall sensors in the normal configuration. The left leg on each Hall sensor is the positive, the middle leg is the negative, and the right leg is the signal wire. In this pic, the middle Hall sensor is bad (no 5V signal when you put a magnet next to it) and it is about to be dug out, and a new one epoxied back in. Three red wires joined into one wire (positive), three black wires joined into one wire (negative), and a blue/yellow/green for the three 5V signal wires.
When repairing damaged hall wires, the 5 small wires for hall sensors should have high-temp Teflon insulation (instead of the common PVC) , but they can be as small as 30-AWG if you want. Due to the low-amp current of the Hall signal, a 24-AWG is plenty fat. Most controllers have only sensored or sensorless operation, but…some controllers have both: the option to start-out as sensored (using the Hall sensors), which provides very good slow-speed control…and then switching to sensor-less control. Having an integrated sensorless control option means you always have a limp-home mode if you overheat one of the Hall sensors.
The upscale Tidalforce motor (no longer in business) used optical sensors to inform the brushless controller of the position of the rotor. Optical triggers can operate at ultra-high RPMs, but Tidalforce decided to use them because they had seen how Hall sensors are occasionally damaged by heat. The halls must be located near the rotor magnets, and the rotor magnets must be very close to the stator poles, and…it is the stator that gets hot in the motor. This means Hall sensors will always be near the hot parts.
Optical triggers can be located far away from the heat. Of course, it is best to avoid generating excessive heat in the first place.
When presented with a choice of getting a motor with or without Hall sensors…always get Hall sensors. A motor with Halls can still be run with a sensorless controller, but the Halls are there if you ever want to start using them.
Brushed motors: PMG-132, Agni-95R, Etek (brushed motors don’t need rotor position sensors, such as Halls/optical)
Brushless: Mars, Also…about 5 years ago, just about every E-bike hub motor discontinued making any brushed hubs due to low sales. Brushless became popular about the time neodymium magnets started to take over in E-bike motors, roughly the year 2000.
The permanent magnets in a Permanent Magnet Direct Current motor (PMDC) are always mounted side-by-side with alternating pole-faces pointed towards the stator-poles. Because of this, they will always have an even number of magnets.
The electromagnets (the on/off coils in the stator) are typically arranged in groups of three, called “3-phase”. If you imagine that the 12 numbers on the face of a clock are the stator-teeth, then 1, 4, 7, 10 would all be combined into one of the phase groups, and those four would all be energized and de-energized at the same time. Following this pattern, 2, 5, 8, 11 would be the second phase-group, and 3, 6, 9, 12 would be the third phase-group.
A DIY one-phase motor can be an educational beginners project. They are simple to construct and understand. However they do not run smoothly, since every pole in the motor is energized and de-energized at the same time. I have seen 5-phase and even 7-phase motors (Falco and Fisher & Paykel)…and they advertise themselves as being smoother and more efficient.
Although the 5-phase/7-phase motors may be slightly better, they are also more complex and more expensive. There is no mystery to the popularity of 3-phase motors. They are reasonably smooth and efficient in operation (much smoother than a one-phase), and they are simpler and less expensive than 5-phase motors.
Inrunner or Outrunner
An inrunner is the common style of motor that most people see on a regular basis. The outer shell is stationary, and the spinning rotor has the permanent magnets attached to it in the center of the motor. An outrunner is quite an odd configuration. The stator with its copper coils are in the center, and they are attached to one of the end plates. The shaft, the outer shell, and the permanent magnets (inside the shell), are the spinning parts. Outrunners are common on a Radio Controlled (RC) model planes.
Here are the major components of a common RC outrunner. The assembly on the right contains all the spinning parts.
The 80-100 RC outrunner (80mm diameter and 100mm long), and the 80-85 have both been used many times on custom electric bike projects. The controllers for them are very tiny (but expensive!). The 63mm diameter RC outrunners have been used on E-bike friction drives, and also powered skateboards.
If two radial motors have the same diameter, an outrunner will put the airgap farther away from the axle, and that increases its leverage, providing more torque per watt.
In the pic above, a GNG on the left has been scaled to match the diameter of the MAC on the right. Because the coils are taller than the thin magnets, and outrunner configuration places the magnetic flux that is the air-gap between the permanent magnets and the coils closer to the edge of the motor…this puts the power application as far away from the shaft as possible.
This improves the torque-per-watt applied to the motor due to the extra leverage, called the “air-gap radius”. This is a huge benefit to small motors. However, this also means that the hot coils in an outrunner will shed its heat to the core, which typically does not shed the motors heat very well to the outside air.
The 4-inch Transmagnetics motor is an inrunner. The stator is mounted around the outer edge, and the spinning rotor operates in the center. This configuration sheds heat well.
With an inrunner, the relatively small diameter of the rotor (in the center) must spin many more RPMs in order to provide the same power (compared to an outrunner of equal diameter). This means that an inrunner must sometimes rely on a large external reduction (belts and chains) to get the output speed down to the wheel RPMs. Remember, a 26-inch wheel at 26-MPH spins only 333-RPM, while…electric motors usually perform very efficiently near 3,000-RPMs.
One benefit of an inrunner is that since the hot coils are attached to the stationary outer shell, they typically shed heat well to the outside air.
Inrunners: Astro, GNG, Transmagnetics, mid drives such as the Bosch, Panasonic, and BBS02/BBSHD.
Outrunners: Most E-bike hub motors (whether geared or Direct-Drive), Most RC motors such as Turnigy.
Kv and “Turn Count”
There is a certain amount of airspace around each steel stator “tooth” in a motor of a given size. You could fill that space with either many turns of thin wire, or…fewer turns of fatter wire. A low “turn count” motor will spin faster per each volt that you apply, and a high turn-count motor will spin slower.
This is called the motors “Kv”, from “Konstant velocity” (I don’t know why, but it involves German researchers in the late 1800’s). The powerful Cromotor’s Kv is 9.3 RPMs per volt. So…10V would spin a Cro at 93-RPMs and…100V would spin it 930-RPMs. You can change a motors Kv by removing the stock wires and re-winding it with a different diameter of wire, or by re-terminating the phases from Delta to Wye/Star (which is described in part-2)
Here are two examples of the same stator from an 80-85 RC outrunner. One of them has a high turn-count (low Kv) winding, and the other has a low turn-count (high Kv)
Most E-bike kits will stock only the two most popular Kv’s of a given motor, and they are sometimes mis-named “speed” and “torque” to make it simpler for the customers to decide. The QS company stocks many different Kv’s of their popular hub motors. The Bafang factory produces their popular BPM geared hub in nine different Kv’s, but most retailers only stock the two most popular models.
Kv = Konstant velocity
kV = kilo-Volt (1,000-volts)
High Pole count, low pole count
The BMC geared hub motor is on the left, and the low pole-count Bafang-BPM is on the right. The green circuit-board at the top left is what anchors the three Hall sensors, The BPM’s three Halls are on the opposite side.
In the picture above, you can immediately see the visual difference between a high pole-count motor and a low pole count. The benefit of the (more expensive) high pole-count motor is that if you want the option to use a sensorless controller, having many small poles will provide a smoother take-off from a stop.
However, if you think you might want to try a higher voltage later (which will make the motor spin faster), you may find the high-pole-count motor to be limited in the electrical switching frequency (how fast each phase has to be turned on and off) when using common controllers.
And then there is the issue of eddy-current losses. Eddy currents are too complex to fully explain here, just know that for very high RPMs, you need thinner laminations and a lower pole-count, or…you will reach a certain RPM (different for each motor design) where there is a sudden and seemingly unexplainable increase of heat.
One example of this is the popular BMC geared hubmotor. The standard model performs well at 36V and 48V. But when builders began using it at 60V and 72V (which makes it spin faster), some of them would encounter heat spikes when they were at their highest RPMs (caused by eddy currents and a high switching frequency). Ilia at ebikessf.com upgraded the BMC geared hubmotors with thinner laminations for builders who want to run higher RPMs.
Here is the stator of a tiny high-RPM RC outrunner. You can see that the stator is made from a stack of 22 very thin 0.35mm thick sheet-metal laminations.
A stator core must be made from some type of steel, but they are not solid blocks. The stator is made from many thin slices (called laminations) that are stacked together to make the final shape. Laminations that are 0.50mm thick are common and very affordable (due to the high volume of production).
The thinner (and slightly more expensive) 0.35mm thick lams are the next common upgrade, but I have recently seen some motors being produced wit 0.27mm lams. The High-efficiency Joby motor (no longer made) is capable of 10,000-RPM’s, and it uses 0.20mm thick lams.
High-RPM laminations also have a higher content of silicon in their steel. I don’t know why that helps them run cooler…but it does. A typical silicon alloy for laminations would be around 3% Si. An alloy of 4%-6% Si-steel can be found, but…it also makes the steel very brittle and difficult to form, so…3% Silicon in steel motor lams is common.
Axial or Radial
Axial motors are rare, almost all of the motors that you can find in the electric bike world will be radial-flux. The Agni motor is one of the few axial motors around, and it performs well (designed by Cedric Lynch).
I believe that the reason so many motors are radial flux is because a factory can make several motor sizes and power levels by simply making the stator-stack longer or shorter. There have been examples of axial-flux motors that added more stators and rotors onto the same shaft, but that doubles the size…rather than allowing a motor family to have many small steps in the size-range.
Here is a video of a small axial-flux of a very similar construction and operation to the brushed Agni, PMG, and Etek (the Mars is almost identical, but it is brushless). The video only shows one permanent-magnet stator, but these motors listed have two stators with a single spinning rotor coil-set in the center. Make note of the two spring-loaded brushes at the beginning (made of carbon).
Since the center area inside a common radial-flux motor does not have any magnets or electro-magnets there, an axial-flux of the same diameter and width would have more magnetic interactions. That makes an axial-flux more power-dense per volume, which also means…you could get the same output power from the same input watts with a slightly smaller Axial, compared to a larger radial.
Axial = Agni-95R, PMG-132, Mars, Etek
Radial = Almost all E-bike hub motors, RC motors, Astro, GNG, Transmagnetics
Radial-flux motors are common, and axials are rare, but…both can perform well if designed from the beginning for a specific task.
Written by Ron/Spinningmagnets, May 2014