View Full Version : smallest cheapest motor than can burst up to 3kw

07-01-2011, 12:33 AM
I'm looking for a little motor to turn the crank on a shock dynometer. Ideally, I'd like to be able to reach 2m/s shock shaft speed while making 150kg force, so that comes out to 3kw for only a short period of time. The motor would drive a crank via a pulley or something. The plan would be to run the motor no longer than 10s so as not to heat up the shock.

So far, the easiest solution I have seen is to get a large RC plane motor which claims to be able to do a few kw, and then drive that with a cheapo electronic speed controller and LiPo battery pack. I would also have to buy a battery recharger.

Ideally I would like some kind of a controller that plugs into wall outlet, but my wiring here is only rated for 10A 120V, so I don't think I can get enough power out.

Is there a better solution involving a different motor or controller type?

07-01-2011, 12:44 AM
Brushless Treadmill motor? Just a larger version of a brushless R/C airplane motor, plus has speed control that plugs into your wall outlet...

07-01-2011, 01:13 AM
3Kw is 4 HP. You're going to run into the "Lies, Damn Lies, and Horsepower Ratings" effect.

Those RC airplane motors quote stall current, which is next to useless. Surplus treadmill motors are quoted the same way.

Your best bet might be a rare-earth servo motor, but of course with 120V mains @ 10A, you have a max of 1.2Kw...
The body of your post says 2Kw, that's more realistic on that circuit.

07-01-2011, 01:29 AM
Maybe a smaller motor drving a flywheel could generate the "shock" you need.

07-01-2011, 01:31 AM
Im with lazlo, brushless servo motor. They can put out about 3x their rated power for short periods.

07-01-2011, 01:34 AM
..but he says he's power source limited...

07-01-2011, 01:55 AM
I don't know if there's a brushless treadmill motor, but does it need to be brushless? A brush motor can have a very long lifetime in intermittent duty- but perhaps you want other characteristics that only a brushless motor can provide-

How much intermittent power can a 110vac circuit provide- more than 2000 watts usually.

Why is your circuit limited to 10 amps- it might be more cost-effective to change this instead of going with batteries/brushless motor/controller/control interface.

07-01-2011, 02:10 AM
How about a starter motor from a diesel engine, that should deliver some power for a short time.


07-01-2011, 02:23 AM

So how many watts peak can this motor do?

Model: HXT80-85-B
Input Voltage : max. 48V
Kv : 170 rpm/V
Weight: 1501g
Shaft: 10mm
Voltage Range: 20-48v
Non Load Current: 2.6A
Stator Size: 77x80mm
Power: 6000W
Turns: 8
Resistance: 37ohm
Idle Current: 2.6A

Equivalent: 50-70cc Gas Engine

07-01-2011, 06:49 AM
Um, what kind of shocks are you going to dyno. The shock dyno's i've seen for motox shocks are quite large and require something on the order of 10-50 horse power. An oil dampened shock, while some what easy to compress by hand will take tremendous amounts of force to move at high speeds.

Forrest Addy
07-01-2011, 08:26 AM
Want cheap? Get a used but usable truck starter. Run it from 24 volts if need be. Gobs of power. Have you worked the math? Strokes per minute?

07-01-2011, 09:50 AM
How about a starter motor from a diesel engine, that should deliver some power for a short time.


A starter motor may be a solution, they are series motors so have extremely high torque at low rpm, you have to ensure it is under load at all times otherwise you will run into dangerous runaway condition.
These are fairly compact motors for smaller cars or outboards and are suitable for short period operation.
The down side is the power supply, a large 4 diode bridge is all you need with a suitable 12v secondary transformer.

07-01-2011, 11:02 AM
as mentioned, starter motors will do it
if you need a heap of torque, use an old flywheel and ring gear
you can use 2 or more starters, as many as you can afford batteries to turn :D
oh, and a heavy duty charger of course
I take it this will be intermittent load

07-01-2011, 12:05 PM
How would I speed control a starter motor?

I'm not sure, but I may like the idea of a relatively higher rpm motor.
As a rough calculation, the crank would have a (adjustable) radius of about 3cm and so to make 2 m/s, it would have to turn about 10 times per second, or 600 rpm. However, the load would vary greatly on which part of the cycle the crank is in, from max force to almost zero. I figure with a higher rpm motor plus gearing, it would smooth out the load to the motor and thus give more even speeds.

07-01-2011, 12:35 PM
you speed control a starter the same way as any other motor
use PWM, it just takes more MOSFETS and heatsink
tho huge heatsinks are not really required if the transistors are not operating in their linear region

edit to say, decent sized flywheel would smooth things a lot and you could use another pinion off the ring gear to get any ratio you want

just a junkyard idea

Weston Bye
07-01-2011, 12:44 PM
smallest cheapest motor than can burst up to 3kw

Most small cheap motors do burst (explode) when subjected to 3kw. :D

Uh, sorry.

07-01-2011, 01:36 PM
If you want large power and speed control on a budget, get a used small electric forklift, or some similar type of device, large golf cart or invalid buggy etc.

If you want even cheaper and can suffer mechanical speed control how about going the truck starer route and control with a varispeed unit off a lathe - they go from 1:3 to 3:1.