June 10, 2008 - 11:18pm — Webbot

First of all let me preface all of this by saying that I
pretty much bought a motor at random (very cheap on eBay) so I’m not suggesting
that this is a suitable motor for **your** project.

So here is what I purchased (at around $11 for two)

This page gives links to the following two datasheets:-

http://docs-europe.electrocomponents.com/webdocs/0030/0900766b800305e5.pdf

http://docs-europe.electrocomponents.com/webdocs/001b/0900766b8001b258.pdf

which are less than informative. My motors arrived with no wiring/plugs etc so the colour charts were less than useful – at which I point I wondered if I would ever make them work!!

However:- this is what I could learn from the datasheets:-

- moves 7.5 degrees per step (ie 48 steps per revolution on the motor shaft)
- has 6 wires
- is
a
**uni-polar**motor - has a 50:1 gearbox (so 50x48 or 2400 steps per revolution on the output shaft)
- holding torque = 100 Ncm
- working torque = 100 Ncm
- typical working torque = 65 Ncm
- Step rate at typical working torque=300Hz
- requires a 5V supply
- Resistance = 9.1 ohms per phase
- 550 mA per phase

So long as you have two of the last three parameters then you can calculate the other using the equation Volts = Amps x Ohms

There is one potentially important statistic missing from
these datasheets that can be calculated from the data given. If we can issue a
maximum of 300 steps per second, (300Hz), and – due to the gearbox – we need to
issue 2400 steps for a full revolution then the **fastest** we can turn the
output shaft is (2400/300) ie once
every 8 seconds. This may be rather
slow for an output shaft if it is connected to wheels but could be fine if we
are rotating the turret of a tank. So this is an important fact to ascertain
before purchasing your motor.

*So what have we learned?* The gearing, steps per revolution and the maximum frequency
produce trade-offs between torque and speed. Whether the motor is uni-polar or
bi-polar can also influence the required current draw, and torque, as discussed
earlier.

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