Can someone help me? Equation for finding speed in an DC motor?

Your question is like asking "What is the speed of a car?". There are many types of DC motors and conditions to consider to determine the speed. The only upper limit of speed for a series wound DC motor is the load, at no load this would be friction of the air, brushes, and bearings. The maximum torque is developed at startup.

hi

you want to measure speed of dc motor i.e. RPM of your motor

just connect 60 teeth wheal on your motor shaft and measure a fruequency of that rotation. in simple way measure number of pulsess in one second for one rotation i.e your RPM

regards

Use strobo-Scope for speed- measurements.

If you have a specific motor in mind,you should be able to look on the data tag on the motor to find that info. or go to the manufacturer and find the spec. sheet on that motor for that information. Generally they can tell you how many rpm per volt.

I assume you are after some equations. The equations for a DC motor are:

V = r.i+L.di/dt+K.w

and T = K.i where:

T = torque in N.m (in a more complete analysis we would include a term equal to J.dw/dt for the load imposed accelerating the rotational inertia of the motor plus load, where J = mass moment of inertia in kg.m^2, but we can ignore this in a steady speed situation)

w = motor speed in radians per second (w = 2.pi.RPM/60

K = motor constant either in N.m/amp or in Volts/(radian per second) [note that the torque constant and generator constants are identical in SI units]

r = armature resistance in ohms (including resistance in brushes)

L = armature inductance in Henries. In steady state conditions the inductive term can be ignored (di/dt = 0)

i = motor current in amps

V = voltage across motor terminals.

Note the equations may be rearanged slightly to get, in the steady state case:

V = r.T/K + K.w which can be rearranged to get

w = (V-r.T/K)/K

If you use any other units the equations will have to be modified to include conversion constants (not recommended).

Regards,

Greg

Is it a speed regulated motor?

Is the load on motor constant?

Is the voltage and current applied to the motor constant?

If So speed may be constant.

Use Taco meter to read the speed at different input currents. Draw a plot for currents and speeds read by taco meter.

Equation wont help here if your system specifications are not known.

Most of the people derive an empirical formula for such kind of things.

If you don't want accurate reading take two speed readings at lowest current and highest current inputs.

draw a plot of by taking different currents on x-axis at equal intervals between the lowest and highest limits that you used to read speeds. On Y-axis Plot speeds and mark those two points that you obtained on the plot. Join those to points and find the slope of joining line.

now speed is equal to slope multiplied by current. This is a crude method, I know.

See post #5. There is no other way to calculate speed of a DC motor. The speed constant of a motor is rarely given in manufacturers' specifications.

If you're taking a test, then the K_v of the motor must be given as well as the applied voltage. Keep in mind that DC brush motors have a fairly fixed "brush voltage drop," which is also not generally given in mfgs' specs. Also, the armature windings will have a voltage drop as a function of DC resistance and current.

speed=(120*induced emf)/(flux per pole*pole*no.of conductors*no.of parallel paths)