Time Constant and RPM of a DC Motor?

I'm not sure but I think different motors have different characteristics. Two 24Vdc motors will not necessarily perform in exactly the same way.

There may be others more qualified to help you but allow me to offer a suggestion. If you have a handheld tachometer, your could test the rpm vs voltage and make a graph or multiplier. Then, you can take your voltage multiply it by your factor or formula and get the rpm.

It might not be 100% accurate because the speed may be affected by the load on the motor. Even if you hold the voltage steady, the speed may vary as the load changes.

If you don't have access to a tachometer,then you probably don't have access to a stroboscope or oscilloscope either. Get in contact with your nearest high school or college electronics department, or perhaps a surplus electronics store. There should be someone there willing to help.

Attach a very small rare earth (powerful) magnet to one side of the shaft. Mount a very small coil of wire or a Hall-effect detector to the frame of the motor and very near the magnet. Observation of the timing of the pulses generated on the oscilloscope can let you easily calculate the speed of the motor. Some oscilloscopes would actually read out the frequency in Hz; multiply by 60 and you have RPM.

It would probably require a storage 'scope (much harder to find) to let you time several sequential revolutions during startup to calculate your tau.

The motor's generator constant K, in Volts/radian per second is equal to the torque constant in Newton-meters/amp.

Put an arm on the shaft, with a weight at the end and then applying a current through the motor (use a variable power supply and an ammeter in series) so that the arm, which you have arranged to be horizontal, just starts to lift. Then:

T= torque = m.g.L

m = mass in kilograms, g = acceleration due to gravity = 9.8 metres/second^2, L = length of arm in metres.

K = T/A

K = motor constant in Newton-metre/amp = generator constant in Volts/radian per second.

Now to convert to Volts/RPM:

1 Volt/radian per second = pi/60 Volts/RPM

Regards, Greg

Use the torque formula provided to yield torque.

Then to find speed (RPM)

Speed= (5252.1)(Hp)/(T)

Hope this helps

A couple things,

You have the rated voltage (24VDC), do you have the rated speed?

Speed/Volts= the ratio your after.

Try spinning the motor at a known RPM (a 600 RPM drill motor), measure the voltage.

This won't be 100, but it will be close.

The ratio will be affected by losses in the motor (current squared multiplied by resistance). The more current, the more losses.

Not sure what you've got available (or, what you're "allowed" to use). To measure the RPM you could try winding some thin nylon onto a long thin dowel.

How about gearing the revs right down: band around a small pulley on the shaft and a tyre-less bicycle wheel: you should be able to count the revs.

Now by mounting a pen or pencil near the centre of the wheel and pulling a piece of paper underneath at a constant speed as you start the motor you might get close to the tau. You can work out the speed of the paper from the previously measured revs and the constant frequency of the "wave" when the motor reaches full speed.

Just ignore me if I'm being silly: not sure what the context of the question is.

I only can suggest a method to measure the rpm. It is possible to use the light pulse frequecy of not incandescent bulbs/lamps which is either 50 or 60 Hz depending whereyou are. This frequency of the main is maintained within about 0.2 Hz constant.

If you make a disk with a number of white and black sectors and measure its apparent rpm you can deduct knowing the number of sectors the speed of the shaft. This approach was used many years ago to check and set the rpm of a plate player. Since there are no more such sound carryers the technique went lost. To have an accurate result use so many sectors that for a cycle the angle will be less one sector or slightly above. The accuracy can be very high.

Thanks to all who had given their suggestion. i will work on ur suggestions

but no body had given solution to my second problem that is

" How time constant (tau) of a small 24V D.C motor current whose rating is about 1 A

can be found practically???"

as time constant of D.C motor is very small as compared to Gear motor and i have the facility of Oscilloscope.

Time constant for what in a DC Motor?

Not a practical calculation. Time constant for charge/discharge? That is for Capacitors and Inductors. Motors work on EMF.

I think your question of Tau is confusiing in its relation, hence no answer.

Are you looking for voltage decay below a certain percent?

Are you looking for time to accelerate to Full load RPM, NO Load RPM etc?

Evo,

I copied this from the Megaflux catalog at www.emoteq.com

Motor Constant (KM)

is the ratio of the peak torque to the

square root of the input power at stall with 20°C ambient

temperature. This ratio is useful during the initial selection

of a motor because it indicates the ability of the motor to

convert electrical power into torque.

KM = TP (Peak Torque)/√PP (Peak Input Power)

or

KM = KT (Torque Constant)/√RM (Terminal Resistance)

Electrical Time Constant (tE)

is the ratio of inductance

LM in Henries, to the resistance RM in Ohms. This is the

inductance and resistance as measured across any two

phases in a delta or wye configuration.

TE = LM/RM

Mechanical Time Constant (tM)

is the time required to

reach 63.2% of the motors maximum speed after the

application of constant DC voltage through the commutation

electronics, ignoring friction, windage, and core

losses.

TM = JM * RM/ KT*KB

i am looking for

Mechanical Time Constant (tM)

is the time required to

reach 63.2% of the motors maximum speed after the

application of constant DC voltage

I got it now. I thought it was something like that.

I will see what I can come up with for ya.

Hope you get what you need.