What can I use to count the number of rotations that an DC motor makes accurately?

Answer 1

Strobe light - motor will appear stationary when the light flashes at the same frequency as the motor

Answer 2

Hmm...Either you need an optoelectronic counter, which is pricey but available from most electronics catalogs, or you can do the following:

Put a gear or pulley assembly on the motor that reduces it by a significant number, say 50:1. On the other gear or pulley, make a highly visible mark. Next, start a stopwatch and count the revolutions that the mark makes for the next 15 seconds. Multiply that by your ratio (which in this example would be 50), and then multiply that result by 4 (4 x 15 secs=1 minute), and that should closely approximate the motor's RPM.

There are two possible backdraws to this method: Firstly, whatever pulley or gear assembly will load the motor slightly, so your results will be slightly lower than the motor's true unloaded RPM. Most good gear and pulley sets will have an efficiency rating (usually around 95%), which you can use to tell how much the gears or pulley system is affecting the outcome. Secondly, if you are using a very high RPM motor, you may need a two-stage gear or pulley assembly to reduce the rotations enough that you cna accurately count them. The calculations work the same way, except you need to multiply the reductions together. For example, if you have a 50:1 reduction going into a second reduction of 10:1, your final reduction is 500:1.

Answer 3

There are encoders. Some are wheels with holes. An led shines through the holes as it rotates. A detector counts the light flashes. DC motors controlled by a computer uses these. Some motors are sold with the encoder already attached. One can control, count, 1/10 of a rotation.

Answer 4

strobe light should never be used. motor will look stationary at MANY MANY frequencies if the strobe is significantly slower than the motor.

if you marked the motor shaft at one point and it rotated at 1000rpm,you would see it stationary at any frequency that is a factor of 1000. imagine if rpm is 26000.

i suggest using a photo tachometer designed for such purposes that you require. you attach a marked disc to the motor and the devices counts how many marks it sees in a certain period of time.

Answer 5

P= C W = C 2 pi N=(3)^(1/3) U I cos phi
N=((3)^(1/3) U I cos(phi))/(C 2 pi )

Answer 6

a non-contact tachometer