E-bike Motors

One of the first things I wanted to find regarding e-bikes, was more details about the motor that an e-bike uses and the controller that drives it. Most electric golf carts use “brushed DC motors. Both the field and armature are wound with coils of wire and provide their energy to spin the motors armature (rotor) through electro-magnet fields. The energy is supplied from the DC battery pack and is delivered through the motor speed controller through a technique called Pulse Width Modulation (PWM). Some of the motors used are “series motors” where the field and rotor are wired in series with each other and are controlled by “pulsing” (through PWM) the electromagnets to speed the cart’s travel up with wider pulses or to slow the cart down by supplying smaller (less wide) pulses. Other carts use “shunt” motors that use a separate “excitation” for the field than is used by the rotor. However, the energy is supplied in either case (series or shunt) to the armature through brushes that “ride on” a commentator that turns with the armature.

Most e-bikes, however, use a “Brushless DC motor (BLDC Motors). There are lots of advantages to using the BLDC’s in the e-bike situation. Due to the compact design of them, the entire motor is often placed inside of the hub of the e-bike. Obviously, there are no brushes to “wear” out so there is hardly any maintenance to the motor. The stator of the motor contains permanent magnets and actually remains stationary (bolted to the forks) while the field is made of electromagnets and spins around the rotor (with the wheel). They do however, require a very different controller that the series and shunt golf cart motors do.

In any DC motor, to get the rotor to spin, a synchronized reversal of direction of current flow through the electromagnets of either the stator or the field must occur in order to keep the rotor moving. After starting to move, the rotor will hit a place where that without the reversal of one magnet field or the other (field or rotor) the motor would stop spinning. The reversal must be repeated time after time as the rotor spins. In the series and shunt motors used in electric golf carts, the reversal is accomplished by the use of a commentator that spins with the rotor (armature). The commentator has brushes “touching” it and they are connected to the armature’s energy source (battery pack and controller). Every time the commentator rotates just a little bit, the brushes “reconnect” to another piece of the armature that has its electrical magnetic wiring reversed compared to the previous one, so the “push and pull” action of the magnetic fields perpetuate the spinning of the rotor. The process is called “commutated”.

An e-bike’s brushless motor has to have some other way to accomplish reversal of polarities of magnetic fields. First off, the rotor of the motor, unlike our golf cart series or shunt motors, has permanent magnets built into it instead of electromagnets. That saves a bunch of space and allows the motor to be much smaller. After all, we are probably going to try to fit this motor inside the hub of a bicycle, so every little bit of space is going to count. There are e-bike motors that are mounted toward the center of the e-bike, but they are much less common. They are called “mid driven” instead of “hub driven” e-bikes.

So, with permanent magnets mounted to the rotor, the polarity of the electromagnets of the stator is what will have to be commutated. In order to do so, the controller takes over and does the reversal of current through those electromagnets its self. It is done (interestingly enough) in three different “phases”. That means as the magnetic fields are reversed, it happens in three different electrical circuits that are designed to maximize the power of the spinning rotor. Three different “groups” of electromagnets are energized at a time through the PWM process. So, our commutation occurs in a three-phase fashion by the controller, not by an actual mechanical commutator. Devises called “hall sensors” are used within the motor to detect the position of the rotor and report it to the controller, so that the energy to the electromagnets of the stator can be applied at precisely the correct time to keep the motion of the rotor occurring with the correct synchronization. The controller becomes a VERY important part of the operation. Well, the next thing I want to do some research on, regarding e-bikes, is the issue of hub drive versus mid drive, so stay tuned, Ron.            

For information about books written by Ron Staley about both electric and gas driven golf carts and their repair, visit the following links.

Electric Golf Cart Repair, both as an eBook and in Hardcopy:         

Book: Ronald L Staley: 9780578560557: Amazon.com: Books

Gas Golf Cart Repair, both as an eBook and in Hardcopy:Gas Golf Cart Repair Book: Ron Staley: 9798987911303: Amazon.com: Books