Electric Bike Project - Open Ended Questions

<...approximately 37.5 A when running unloaded....>

No. When running fully loaded. When it is running light the current will be less than this.

<....limit the maximum output amperage of a PWM controller circuit without losing a lot of energy to heat dissipation in a resistor....30 amp circuit breaker in front of the motor.....don't want it to kick the circuit off every time....full throttle....>

How about wiring an ammeter into the circuit and looking at that when the motor is running?

Can you define what it means for a motor to be "fully loaded?"

If your PWM controller is just a chopper ( i.e. the output is a 24Vpp square wave with a variable duty cycle) and the motor is rated at 24V, you couldn't fry the motor (unless it was mechanically overloaded).

And it can be mechanically overloaded, but it should be able to handle short periods of overload, if the time that it isn't fully loaded averages out to O. K. levels.

John, this is just some more information following, so I'm not inferring that you don't know this: Current is directly related to torque, and torque = moment of inertia X angular acceleration = force X distance = K_t X current (where K_t is the torque constant of the particular motor you have).

What is the maximum slope you hope to be able to ride up? How much do you and the bike weigh? Will you have human-powered assistance similar to old Mopeds?

The bike I'm building this platform on is a Novara Corsa...bought used and in need of some part upgrades. I plan to replace the standard 1 wall rim with a double wall and a heavy-duty rear hub. It will have an 8 speed cassette with an FFS (front freewheel system) crankset.

I just received that controller today and apparently they sent me the wrong one because its only rated at 25 amps! I ordered a 40 amp but I was thinking about what that meant wrong...I was thinking that if I ordered a controller that had too high an amp rating that it would burn out the motor. That is, that the maximum amperage the controller would deliver was 40 amps and that it would not go higher. That whole electricity and magnetism class was really difficult that way. Needless to say I will be ordering a more heavy duty controller.

I have 2 12v 18Ah SLA batteries on the way I plan to use for my primary bank, and will be wiring the bike with the option of adding a second removable bank of batteries.

The motor RPM will be reduced by a set of chain sprockets and a jackshaft: 11 teeth at 2600 RPM to 40 teeth at 750 RPM, then on the crankset drive it will go from 16 tooth to a 44tooth crank, which has an adjacent pulley that is 30 tooth. That will drive the rear cassette/cluster. I'm going to replace the cluster i have with an 8 speed setup...probably ranging from 13-26 tooth. With that I've calculated (assuming an outter wheel diameter of 28") a top speed range from 26.24-52.54 MPH. SCARY.

I will most likely end up running this bike at the lower throttle levels...

OH and I weigh about 250, the bike and gear I'm estimating will weigh a good 70-80 lbs.

I know thats a lot...and I'm on a diet :) Since the bike has a front freewheel (or clutch bearing), I can pedal or not at will while the motor is turning. I've designed it so that the jackshaft drive has a clutch bearing as well so that while I am pedaling the chain can spin freely over the still jackshaft. I know there will be energy lost in the chain drag but I've chosen to accept that as a negligable problem in the design concept. Ideally I'd like to have used cog-belt drives but that would cost me dearly!

Don't worry about the current rating of the controller -- the higher, the better -- unless, of course, the controller is a constant current controller, which I'm sure it isn't. Typical PWM controllers will deliver current as needed. A 40 amp rating means that it is capable of delivering 40 amps, but it may well only need 10 amps (for example) for the speed and power. It may well deliver 50 amps for a brief period without damage -- many things of that sort do. Were I to do the same thing, I think I would put an ammeter or indicator of some sort on the bike to show current, and an alarm that sounds after several seconds of overload so that one can know when to start pedalling to help offset the load on the motor and controller.

Hello dequinox,

I hope the bicycle you intend to use is really a tricycle because you will need about 60 to 100 lbs of batteries to get much use out of this motor. I wonder if you have examined any golf carts to see how they work. They use lower RPM or geared down motors and they have substantial battery chargers. So, I think you will want to investigate existing solutions before you start to assemble.

Also keep in mind that 1 horsepower = 746 Watts and that means that what ever mechanical framework you use to support your motor or transfer energy to a shaft will need to be able to resist bending against the power of about 1.2 horses. You will also need to consider the ratio of motor RPM to your vehicle wheel RPM to determine your maximum speed. This also means you should consider what the maximum safe speed of your vehicle is going to be and how the assembly will react to an unpleasant stop. After all, you don't want to be injured by your own invention.

If this sounds complicated, then perhaps you are beginning to appreciate the complexity of your endeavour. I'm not trying to discourage you, but perhaps that motor is more powerful than you realize. You should be very careful. And if you later decide that a different motor suits your design or that you need speed reducing gearing, then consider it as progress. Usually the design work begins from the other end, designing the vehicle first and then selecting a suitable motor.

Actually I don't think I'm going to need 60 lbs of batteries unless I am mistaken in that 2 12v 18Ah batteries hooked up in series will provide 18ah at 24v. I have done a little research and it indicates that Ah capacities are additive when batteries are hooked in parallel and remain constant when hooked up in series. The two batteries I ordered will total to about 26 lb.

I calculated my motor would draw a maximum of 37.5A, and 18Ah/37.5A=.48 hr ...so about a half hour if the motor is at FULL draw during that whole time...which it won't be. The fact that I am able to shift will assist in making the system a bit more efficient so that the motor can run at lower rpm at my top speeds.

I have spent 2 months planning and making different calculations on this project so I definitely understand the complexity of it. I've gone through dozens of different drive-mechanism designs and considered motors of different power... I wanted the 900 watt motor and found a good deal on one so I bought it.

Nice project! I always thought bikes would make a great and cheap transportation system if the human power factor was eliminated!

I have worked with electrical and electronics for all of my life along with mechanical drive systems too. So here are some points to consider.

First. your 900 watt motor is capable of putting out 900 watts continuous but typical DC electric motors have a good fudge factor ratio of inverse proportionality of power to time. That is they can put out double the power at 50% duty cycle or put out 4x the power at 25% duty cycle! Assume 1 minute as a safe duty cycle time period.

Second. A 40 amp controller is likely going to burn up right away. A good rule of thumb is the controller should be rated at least 2x the peak amps draw your motor can take. I would recommend a 100 amp or higher capacity one myself. And properly sized wiring to match.

Third. 900 watts on a bike is about 10 times the continuous power a young healthy human can produce!

Fourth. You will need a good way to gear down your motor. At 30 MPH a 26 inch wheel is going 388 RPM. So calculate accordingly. Big sprocket on the drive wheel small sprocket on the motor.

Fifth. A rolling bike on flat smooth ground is fairly efficient. Consider a rough rule of power to distance to be about 25 watts per mile per 100 pounds of weight at 10 MPH. 4x that at 20 MPH. Faster speed takes more power so do hills and rough ground. These are only approximates. Total system efficiency and other factors can greatly increase or decrease your distance per charge. If your not already aware but your batteries Ah (Amp hour) rating times the voltge gives you the watt hour capacity. A 24 volt 10 Ah battery has about 240 usable watts of power when its new. And the safe peak amp draw is typicaly 10x the Ah rating but that number can vary greatly from one type of battery to another.

I am looking forward to what you come up with!

Non of this is set in stone but it should at least give you some reasonable numbers to work with while in the design stages.

Thanks for the input! The motor I have was used on eScooters ...but I don't have any detail about it other than the ratings I have mentioned. I don't know what the "Peak safety amperage" is for it...you know how some engineers will build in a safety margin for peak amperage.

If you look above you'll see my calculations for top speed...27ish to 52 mph! I think I will try to stay on the lower rpms. I don't have many hills that are more than like 15% grade for any significant distance between my home and where I really want to commute so thats good.

So for the 18Ah batteries that I have coming ...~300lbs of weight if I am lucky...and I'd like to commute at about 20mph most of the time...

100w(300lb)/mi at 20mph? thats a lot of power... let me know where I screwed my understanding of that up because I'm sure I have. I am still amateur at the physics behind all this, I squeezed by with Bs and Cs in the general physics courses...

Also here are the components I am using:

The controller I have...probably will be returned.

http://www.monsterscooterparts.com/s500controller.html

And this is the motor (with kit...and the controller I'm supposed to have had arrive today is the one listed here):

http://www.monsterscooterparts.com/24-volt-900-watt-motor-controller-throttle-kit-premium.html

It seems they have paired the 40amp controller with the very motor I ordered. Do they rate the controller by how much current it will provide rather than how much it can take running through it?

This is the 40 amp controller: http://www.monsterscooterparts.com/scmogti24v40.html and they say

"Add more ZIP to your Schwinn, GT, Mongoose, or IZIP electric scooter! Our most powerful 24 volt controller. Can replace 25A and 30A voltage controllers with the 3-pin connector to add more power to your electric scooter. BEWARE: The price paid for that increased speed will be reduced electric motor life."

Do you think the motor's life will be reduce that much from a stronger controller?

"A 40 amp controller is likely going to burn up right away ... "

Don't the controllers have current limits where you come from? Any controller I'd consider buying would be self limiting when overloaded. Confess I haven't checked the spec of the proposed controller(s) beyond what was given in the links, but they'd have to be seriously deficient if they couldn't deliver their rated current - and protect themselves and the load if it attempted to draw more.

...and couldn't I conceivably put a circuit breaker or perhaps a small capacitor bank in between the batteries and the controller to protect it even if it wasn't self-protecting?

Up front sorry for spelling. The spell check here regularily fails to launch for me for some odd reason.

I am not that familiar with the small bike power controlers actualy. I work more with the larger comercial and industrial stuff and typicaly its common place to use oversized controlers for motors that will be subjected to regular short term overloading.

If the manufactuerer rates it to work with the motor just ignor me untill it burns up!

I was on the asumption that the whole setup was a collection of misc parts and pieces and not a matched set. My mistake then.

As far as current limiting I have no idea what your control box has or does. I have seen far to many DC motor controlers that had absolutly no practical current limiting capability designed into them. The controlers fail when ever the motor gets stalled out if the the fuse or circuit breaker doesnt do its job fast enough. Unless specificaly stated by the manufacturer I just asume no current limiting is built in when working with DC motor controlers.

<Unless specificaly stated by the manufacturer I just asume no current limiting is built in when working with DC motor controlers.>

Probably a safer bet...I think I will stick with that. Perhaps putting a fuse on the battery side and a circuit breaker on the motor side will be relatively safe enough for what I'm doing...

Traler the batteries and run a cable to the control panel. Keep it as simple as possible. Not really too many options for batteries if you want any useful distance without pedaling home or to a special warm and happy place to recharge.

JL Mealer
Mealer Companies LLC
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