Overloading an electric motor

[Daox]

I guess the idea of this thread is to learn more about how much you can overload a motor to get more power out of it.

For instance, I was poking around ebay just checking things out and came across this motor.

It says it is a 4.5 hp motor. What is a rough guess at what I could get out of it at a higher voltage? What limits the amount of voltage I can use? What else could I do to get more power out of it? I know you can only run them like this for short periods of time because heat will be an issue. I'm thinking the brushes will be a limiting factor, but also the windings. Is there any good 'general rule' as far as this goes?

[Gone4]

Overloading a motor is done often and is quite safe. However, they are rarely ran at overloaded voltages and current for long periods of time. Usually the resistance of the components start to expend too much energy into heat and things start to break down.

[Twilly]

You need to look at the weight of the motor, This will tell you alot about the windings and such. For a small car, you could get by with a motor 80- 100 lbs, this motor at 24 lbs will smoke in a short while

[Daox]

Well, I'm really trying to move an entire car alone with that motor. Maybe electric assist of some sort. I'm just trying to get a some info into the electric motor world to see what would be needed to do different things.

So, any ideas how much I could pump through that little motor?

[Christopher Jordan]

Originally Posted by Twilly

For a small car, you could get by with a motor 80- 100 lbs, this motor at 24 lbs will smoke in a short while

Uh oh!! Overloading the heck out of mine.......

countersTrike

[frodus]

for most cars/small trucks, you need AT LEAST a 100lb motor in order to have the current carrying capability as well as the heat dissipation capability. Even at 100lbs, it may not move a car very well if the car is heavy. You never said what type of car it was.

a 24V 25lb motor will smoke in 5 minutes if you try to drive ANYTHING over about 300lbs.

No load amps is 45A, FLA is 150... at 17,000RPM!!!!!... so even if you COULD use it, 24V results in 17000 rpm. I bet this motor has almost NO torque. Plus, the gearbox you'd need to convert 17000 down to ~3-4000 to match the transmission would cost more than it would to get the right motor...

The main thing I'd be concerned with, is the fact that it says intermittent duty. Even if you got a torque curve for this, its just not suitable for a car.

EV's are not cheap, there's a reason. You can't just use a cheap motor and a cheap controller, you need current carrying capability, and many times, thats going to cost you.

Start looking at advanced DC motors and curtis or alltrax controllers. You need to start at the bottom before buying parts. You need to get rough drag/weight its got, and what speed you want to go. Then you can calculate the motor torque and HP needed to get you there. THEN you choose a motor, and a matching controller. And parallel to this, you have to decide what voltage you want to run (72, 96, 120, 144)... and how much battery weight you'll have, and how much distance you want (more lead = more range, voltage = speed, torque = amps).... Sounds like you haven't done alot of background research.

go here to see what I mean:
http://www.geocities.com/CapeCanaver...79/evcalc.html

[Daox]

I typoed my previous response. I'm not making an ev. I am just thinking about some sort of electric assist for acceleration (think simplified hybrid retrofit). This means intermittent duty is probably fine. It also allows me to get away with a much smaller motor (depending on design and intent of use). However, if I don't know how much I can overload, I don't know how small I can go.

Anyway, I was just using that motor as an example because thats what I found browsing one day. It wasn't meant to be taken as "can I use this to move a vehicle", it was simply an example piece. I do see your point about the rpms and torque.

[MetroMPG]

Daox - I don't think there's a simple rule of thumb for how much you can overvolt/overload a motor.

The question comes up pretty frequently on the EVDL, and if there were a rule of thumb, it'd get trotted out each time.

Consider that Mike D originally field tested his DIY EV assist in his Insight with a 24v electric wheelchair motor running @ 48v. Probably weighed approximately what this one does - maybe less. But he found it got too hot, and ultimately switched to an ETEK permanent magnet (8hp continuous, if I'm not mistaken).

It's all about the heat. You can stuff as much V & A through a motor as you like, provided you can keep it from getting too hot.

[WaxyChicken]

Interesting toppic.
Does the heat transfer through the motor's metal casing / housing?
If so, then maybe I can add heat sinks to my motor to make it tolerate heat better.
(same concept as a computer CPU heat sink)

That is a problem I'm concerned about when i do my build because in Arizona the summer heat can reach 120F / 49C and Zero humidity.

[frodus]

does the motor sink heat to the case? yes.

Can adding heatsinks cool it down, yes, but only a few degrees. The heat you need to get rid of, is in the copper windings. By the time it gets to the outside of the motor, it can be too late.

Blowing air on motors helps, many people do it with eteks, but they wear quick, and some people have problems with them coming loose inside the housing because of the heat.

Best thing to do is get the RIGHT motor for the job, don't halfass it. Don't spend 1/4 of the money just to get a motor that could be overpowered for 5 min to move the car. Get the motor that can move the car for 45min or an hour without melting into a pool of metal.

Its all about duty cycle. Sure, it may handle 600 amps.... for like 5 seconds.