That is, they're great for EV hacking and conversion. They're compact and modular, require no support of rotating axles from the parent vehicle, and can be designed around the vehicle to be propelled. Pure DC electric hub motors, in fact, were used in some of the first electric (and hybrid electric) cars.
They are also not as complex and mystical as one might think. The advent of my project RazEr, a stock Razor scooter with a custom built electric conversion, has raised many questions from amateur EV builder looking to construct their own brushless hub motors. Until now, I have not had a single collective resource to point anyone towards, nor have I been confident enough to understand what I actually built to write about it for other hackers.
Hence, I will attempt to show that a brushless DC permanent magnet hub motor is actually relatively easy to design and build for the hobbyist, resource access considerations aside. I will first exposit some of the details of brushless DC motor theory as applied to hub motors. I will provide some thoughts and pointers about the mechanical construction of the motor itself and how to source major components. Finally, I will briefly glean over ways to control your newfound source of motion. The arrangement of this Instructable is designed for a readthrough first - because it relays theory and advice more than specific instructions on how to create one particular motor.
This is intended as a basic primer on DC brushless hub motors. Many assumptions, shortcuts, and "R/C Hobby Industry Rules of Thumb and Hand Waves" will be used. The information is purposefully not academic in nature unless there is no way to avoid it. The intention is not to design a motor that maintains above 95% efficiency across a thousand-RPM powerband, nor win the next electric flight competition, nor design a prime mover that will run at constant power for the next 10 years in an industrial process. Motor theoreticians avert thine eyes.
I will assume some familiarity with basic electromagnetics concepts in order to explain the motor physics.
Below is an exploded parts diagram of a prototype motor that I am in the process of designing and building. Let's clear up some of the vocabulary and nomenclature immediately. The can (or casing) hold a circular arrangement of magnets (electrically called poles) and is supported on one or both ends by endcaps. This whole rotating assembly is the rotor. Internally, the stator is a specially shaped piece of laminated iron pieces (the stack) which holds windings (or coils) made of turns of magnet wire on its projections (teeth). It is stiffly mounted to the shaft (a nonrotating axle) which also seats the bearings for the rotor assembly.
Step 1: Hub motor design considerations
I mean, read these few pointers which highlights some design tradeoffs and considerations involved in the use of hub motors! They are not perfect solutions to every drive problem, and some of the shortcomings are dictated by the laws of physics.
Hub motors are inherently heavier and bulkier than driven wheels.
Until we make magic carbon nanotube superconductors en masse, motors are essentially chunks of steel and copper, both very heavy elements. What happens when you increase the weight of a wheel two- or -threefold is a drastic increase in the unsprung weight of a vehicle, or weight that is not held up by a suspension. For those of you in the know about vehicle suspension engineering, unsprung weight negatively affects the ride and comfort of a vehicle. If you just drop hub motors into a vehicle previously endowed with indirectly driven wheels, expect a change in ride performance.
This is more of a concern for passenger cars and sport vehicles than anything else, as most small EVs such as bikes and scooter won't have suspensions at all. However, the keyword here is small. You might have gathered from my other instructable that some times it's all but impossible to simply fit a larger motor in an enclosed space. A hub motor will inevitably take up more space in the vehicle wheel. This matters less for larger wheels and vehicles. The MINI QED and Mitsubishi MIEV are example of car-sized hub motors that have been well-integrated into the vehicle design through some pretty serious re-engineering of how the wheels attach to the car frame. You might have to do the same for your scooter, bike, or couch.
A hub motor powertrain will generally produce less torque than an indirect-drive system
Don't expect any tire smoke from your hub motors. An indirect drive motor, such as one geared to the wheels through a transmission, has the advantage of torque multiplication. This is how a 400 horsepower diesel engine in a semi truck can haul itself and 80,000 more pounds up a mountain road, but a 400hp Corvette could not do the same - the semi engine goes through a painstakingly complex arrangement of gears to transmit many thousands of foot-pounds of torque at the drive wheels. A Corvette is light and fast, and hence the 400 horsepower in its engine is mostly speed.
From physical mechanics, power output is a product of both torque and speed. Due to curiosities in the laws of nature, it is much easier to make a fast but low torque motor than a slow and high-torque one, power output levels being equal.
As it relates to motors, this is why your typical drill motor spins at upwards of 30,000 RPM, but you only get a few hundred RPM out at the screwdriver bit. The drill motor has been engineered to produce maximum power at very high rotational speeds, which is sent through a gear reduction to crank your drill bits hard enough to do this.
But your hub motor is direct drive. There's no bundle of pointy steel things to convert its rotational velocity into torque. A hub motor can only lose mechanical advantage because the wheel essentially must be larger in diameter than the motor. Comparatively few in-wheel motors have internal gearing - these are most often found on bicycles, since they have a large diameter, and hence loads of space, to work with. It is not that much more difficult to incorporate a gearset into your hub motor, but it is beyond the scope of this Instructable.
The bottom line is, while a 750 watt DC motor on your Go-Ped might let you perform a wheel-spinning launch, a 750 watt hub motor will probably not.
Hub motor drivetrains will generally be less electrically efficient than an indirect drive system
It is certainly true that hub motors bypass practically all the mechanical losses associated with a clutch, transmission, axles, and gears that you typically find in a vehicle powertrain. In fact, drive components alone can eat up 15 to 20% of the power produced by the engine. Imagine if that were gone - what could you do with 15 to 20% more power?
A hub motor will typically have a torque-produced to force-on-the-ground transmission of almost 1. The torque of the motor only has to go through the tire, with its rolling friction and deformation forces. But what hurts the hub motor is electrical efficiency.
A motor is a transducer. Input electrical power and out comes mechanical power - usually. Electrical power is defined as
Pe = V * I
where V is the voltage across the motor and I is the current flowing into the motor. V has unit volts and I has unit Amperes. Mechanical power is
Pm = T * ω
where T is the torque output in Newton-Meters and ω is rotational velocity in radians per second (units 1 / time, because radians are unitless!)
It is perfectly within reason to be inputting electrical power to the motor but get no rotation out. This is called stall or locked rotor condition, and it kills motors. This occurs when T is not enough to overcome the forces pushing back against a motor - think of driving up a really steep hill.
In this case, your efficiency is precisely zero. Zilch, nada, nihil, nothing. Mechanical power out is zero, but electrical power in is nonzero.
While it is true that both motors must start the vehicle from standstill, and thus have zero efficiency for a split second, the fact that hub motors must operate continuously at high T and low ω is the distinguishing factor. Other laws of physics dictate limits of torque output, which I will get to shortly. A\ hub motor has to draw a higher current for the same torque output, and current is what causes heating in wires (not voltage). The more current there is, the more heat is generated.
This is called Joule heating and is governed by the power law Pj = I² * R. It is a square law: double the current, quadruple the heat.
Now you see why hub motors are less efficient electrically than indirect drive motors. Hub motors are low speed creatures, and will inevitably spend much of their lives at or near stall condition. This occurs whenever the vehicle is moving at low speed or accelerating. A hub motor will see more moments of low or zero efficiency than an indirectly driven, geared motor.
The bottom line is, prepared to see a decrement in battery life if you swap your existing drive system with a hub motor.
Now that I have told you the reasons to not build and use hub motors, let's get on to how you can build and use hub motors.