# Compatable motors for picaxe 28?

Guys i got some motors as part of another kit, im just wondering will my picaxe 28 from the SHR be able to drive them ok with out burning out.

All i know about them is whats on this spec sheet

http://www.technobotsonline.com/model-motor-with-worm-drive-30510.html

Voltage Range: 3V to 6V
No Load Motor Speed (rpm): 9700 @ 3V, 17100 @ 6V
Gearbox Ratio 42:1 giving an output speed of 231 rpm off load at 6V
Stall Torque: 3V 122 g/cm, 6V 220 g/cm
Overall Size: 35W x 40H x 60L mm
Fixing Holes 4 x 4mm
Body Length: 32.6mm
Axle Length: 125mm
Axle Diameter: 3mm

What do you think??

Without knowing the current draw

it will be all but impossible to tell you what will or won’t work. I see they don’t offer the current draw for information. The problem is, if the motors pull too much current it will smoke/burn out the motor driver. The RPMs seem a good deal on the high side as well. I believe you would be better with something like the GM series motors like what solarbotics sells.

I agree with birdmun about

I agree with birdmun about the speed; those motors are probably way too fast.

To determine the current draw, you can connect a multimeter in series with the motor, and measure the current while you hold down the wheel to add some load.

You can also estimate the stall current by measuring the resistance across the motor contacts with nothing else connected. Then you can use this formula to determine the max current.

I (current in Amps) = V (voltage in Volts) / R (resistance in ohms)

For the value of V, use the voltage of your supply minus 1.5. The l293 motor driver drops about 1.5V. So if you use 4xAA alkaline batteries the voltage will be.

V = 6 - 1.5 = 4.5V

Let’s say you measure your motor resistance as 10 ohms. Now you can calculate the max current, which only happens if the wheel is kept from turning while the motor is on.

I = 4.5V / 10 ohms = 0.45 Amps or 450mA

That is well under the rated specifications for the l293d motor driver. Of course, I have no idea what the resistance of your motors actually is. Measure it and find out.

All that said, your motors are still way too fast.  ; j

Current Draw

Hi guys thanks for the tips, hooked it up to the amp meter today, just using x4 AA rechargables, under free running conditions they pull 180mA, under a real heavy load they are about 900mA (i couldnt actually get the motor to stall completely).

Thanks again

OK. So a motor driver that

OK. So a motor driver that can handle up to 1A per channel should be fine. The l293d used in the start here robot, would be fine, for example.

The speed may still be a problem. You might try to gear the motors down further, or use PWM to run them at a lower speed if you find that they are too fast.

Re: L293D

Most of the specs I have seen on the L293s say .6A with a peak of 1.2A. Wouldn’t a stalled motor pulling more than .6-.8A blow the L293?

Since 180mA is the free

Since 180mA is the free running current, maybe it will be double that under load. That’s still way under the 600mA per channel for the l293d. If the motor stalls, hopefully it will not be for an extended period. I’d say it is pretty safe to use.

However, the SN754410 motor driver is pin compatible with the l293d, and handles 1A per channel continous and 2A peak.

Sure the l293 may just be

Sure the l293 may just be able to handle it, what then about fitting an inline fast blow fuse rated for say 1A? Or is that just overkill?

Considering the SHR: It uses

Considering the SHR: It uses GM9’s that freewheel at ~80rpm and use 2 5/8" wheels. Some quick math with your quoted 231rpm spec says if you used little 1" wheels (many other things assumed to be equal here) your bot would have about the same speed.

Great point, JAX.I forgot to

Great point, JAX.

I forgot to consider the final gear reduction of the wheels!