Senior Project Design with DC Brushed Motor

[Loker]

Hi all,


I’m studying electrical engineering at university and I’m working on putting together an electric bike for my senior project . I have an old brushed DC hub motor and three new rechargeable 12V lead-acid batteries that have a capacity of 15Ah each. I also have a bike frame that fits my hub motor and an ebike throttle I inherited from one of my dad’s old projects. I’ve looked extensively but I can’t find any specs on my motor online.


The thing I’m struggling with is I have no idea how to build a control unit between the batteries and the motor. I think a buck converter whose output voltage is somehow regulated by the position of the throttle might work but I have a strong feeling that there’s more to it than that.


Does anyone know of any resources detailing the typical circuitry between batteries and a brushed DC hub motor in the context of electric bikes?


Any and all replies are appreciated!


-Sean

 

[Trail Cruiser]

Please attach some photos, maybe other members will recognize the motor. I have experience with old lead acid ebike and I even added a PWM controller to it. Maybe I can help you.

https://www.ebay.com/itm/12V-24V-48...111851538617?_trksid=p2349526.m4383.l4275.c10

 

[indianajo]

Brushed motors are obsolete because of the dropout problem. When they sit around the brushes lose contact with the commutator, which oxidizes. Then when you need it, it has to be disassembled and cleaned. Factories that used DC conveyor drive as late as the 00's had grim startup procedures on Sunday nights.
Modern bike motors resemble 3 phase induction motors ala Tesla, and controllers resemble variable frequency drives used on factory motors. Factory 3 phase motors run off DC rectified off the AC line, then the phase rotates around the field slightly ahead of the speed the motor is supposed to run.
The low cost of microprocessors, and the low cost of Fet transistors, has made this economic on bicycles. Bike motor controllers for 3 phase motors tend to go as low as $20, for ones of undefined reliability. See all the power wheels on amazon, ebay etc at the $210 price point.
Actual Dc motors, obsolete in industry, were run with chopper drives in locomotives fairly late. The width of the on pulse was controlled, low for slow and wide for fast. DC motors do have higher stall torque than AC 3 phase. Not an important characteristic in bicycles. The old way of controlling DC motors was with resistors in series with the field coils, but this was never very satisfactory because of the high power levels, and the sliding taps which burned and arced.
For some practical details, I learned a lot from Audel's motor book.

 

[Trail Cruiser]

As a student studying electrical engineering course, I presume you want this to be your project ebike to make it run so you can have applied knowledge and hands on experience. And of course, improvise it to your own personal taste and gain self confidence doing it. Ebikes has gone a long long way in technological advances to the motors, controllers, and batteries. However, having hands on experience at the very basic level help you have a better feel and confidence on ebikes, both old and the current technologies.

Regarding the arcing and burning out of contact points on brushed motors, you can apply a simple method used by car engines in the past (1970's and older), before the advent of electronic ignitions. In the past, cars used contact points to power the spark plugs. They used a "condenser" (a paper capacitor) to mitigate arcing (and burning out) of the contact points. You can also put paper capacitor linked between the positive and negative terminals of the motor to minimize arcing at the brushes.

 

[Ann M.]

Depending upon how many watts your brushed motor is, @Loker, you can find many decent 36V controllers online from suppliers like electricscooterparts.com or partsforscooters.com or others without reinventing an old wheel. With high or rough use brushed motors will need to have the brushes replaced periodically as they wear and the carbon buildup blown out of the motor along with possibly resurfacing the commutator. Something our shop has to do occasionally on the 1500 watt eGO scooters. On the eGO scooters, there are 2 wires going to the motor with the electricity and another small wire connected to a temperature sensor coming from the motor to prevent overheat damage. Here's a link to a wiring diagram for the last version of their SC controller which had 2 speed settings, variable throttle, lights, turn signals & regenerative braking.

 

[Loker]

Thanks for all the replies! See images below for my motor.

Regarding my project: I appreciate the suggestions for controllers but I want to design and build a controller from scratch. I'm also a broke college student, so I'd like to avoid buying the 3-phase motor that @indianajo mentioned, if possible. Earlier today I hooked up my motor to a power supply in one of our labs at my uni. It started to turn at around 3V, but it didn't sound good - lots of noisy scraping on the inside. I brought the voltage up to about 17V and got it spinning decently fast. My plan right now is to take it apart, clean it, and replace the brushes (if they need it). I'll also invest some time into researching the condensers/paper capacitors that @Mark Peralta mentioned (thanks for that suggestion Mark). Then, I'll research the crap out of PWM controllers and start on a design of my own.

Anyone have any suggestions on guides to clean brushed DC motors and/or good resources on building PMW controllers in the context of Ebikes?

Thanks again!

-Sean

 

[Trail Cruiser]

Simple DIY PWM controller.

 

[indianajo]

Pulse width controllers are a simple timer circuit (555), one with a fixed off time and another with a variable on time controlled by putting a pot as the R in the RC timer circuit. See 555 timer datasheet on datasheetcatalog.com . Look up 555 oscillators. The 555 drives a transistor output to actually switch the current to the motor. Don't forget the flywheel diode to dump the kickback voltage at transistor collector to +PS when the motor shuts off. Needs to be a several amps diode. Junk switcher supplies are the best source of these. Also the heat sink and sometimes the transistor.
Noise could be the brush springs rubbing on the commutator, cutting grooves in it. Else the bearings are bad. Maybe you can lubricate them maybe not.
The commutator capacitor solves a problem I didn't mention. Either your motor has one built in or it won't fit. The problem I highlighted is commutators oxidizing while sitting around, refusing to pass current (operate) when brought out of storage. One weekend is storage to a used DC brushed motor.
Lead acid batteries also sulphate up when not used, I'd be surprised if your old batteries will supply actual useable current. Maybe if gel cells. Test them with a load resistor after charging. Voltage collapses when you try to draw the proper current with a 10 ohm or lower resistor. Probably the betteries won't take significant charge current either. Really old lead acid batteries from before the no-maintenance battery was invented, can be desulfated by pouring out the acid, installing water, charging, then putting the acid back in. Use safety glasses. Wash your hands afterwards. Once they took the antimony out of the battery plates, there is nothing I've found that will bring back a sulphated lead acid battery.
No-maintenance Lead acid battery is such a curse in cars that aren't driven much, I'm considering buying lithium to start my hobby cars. I don't drive enough to keep a battery from sulphating. I ride a bike everywhere, which keeps my heart arteries from sludging up. Also my lawn tractor - needs a new battery every other spring. I