CrazyEngineers Archive
Old, but evergreen and popular discussions on CrazyEngineers, presented to you in read-only mode.
@mibawad • 27 Jan, 2011
I am using DC permanent magnet motor as a DC generator. I would like to calculate the estimated output voltage and current from the generator at known speeds. I already know the armature resistance and inductance for the motor. Do you know where can I find the equation?
Many thanks
@narayana murthy • 28 Jan, 2011 Electrical Engineering Tutorials: D.C Generator E.M.F Equation,Terminal Voltage and Ratings
i think this may help you
@freak16 • 28 Jan, 2011 DC genarator are of various connections:
1) Separately exicted
2) DC shunt excitation
3) DC series excitation
4) DC componund excitation
DC permagnet is considered as equivalent to dc separately excited motor
considering the magnetism of magnet does not deteriorate with passage of time ( but it do go down with time) you can apply the equations for separately excited.
As NM mentioned..
@Arp • 29 Jan, 2011 freak is right
@mibawad • 01 Feb, 2011 Many thanks for your reply. I would to know if I know the generator speed and the resistance and inductance values for the generator, and the back emf constant. Could I calculate the current and output voltage. I can't find this relation because I found that I have two unknown (I and V) in one equation. Is there two equations to calculate them.
@narayana murthy • 03 Feb, 2011 generator speed can be known from name plate details as i know
resistance and inductance can be calculated from v and I equations
I can be known from ammeter which is connected at output side
@freak16 • 03 Feb, 2011 DC machine does not have inductance as far as i know.
We use the equation for generator
E (induced emf) = Vt(terminal voltage) + Ia * ra
Where E is directly proportional to speed
E = K w
W= speed of rotor
K can be found out from no of conductors parallel path and other parameters
Let me know if problem persists
@narayana murthy • 03 Feb, 2011 @freak: thanks freak iam forgotten as iam seeping about DC machines so i mentioned that
@mibawad: yes freak is right there is no concept of inductance in DC machines
inductance and capacitance only appears in AC machines because there is no alternating current in DC machines
inductance only appears at only there is a existence of alternating current
@mibawad • 07 Feb, 2011 @freak
What I want to say that for DC motor or generator:

If I know K, w, Ra and La, and I want to calculate V and I according to the speed w. I couldn't from this equation because the number of unknown is more than the number of equations.
@freak16 • 07 Feb, 2011 Will dce machine is having inductance i strongly doubt..
And you need to know amout the load somehow..
you cant absolutely..
@narayana murthy • 07 Feb, 2011 • 1 like freak is right we have no inductance in DC machines
there is no inductance
@mibawad • 07 Feb, 2011 @freak
Therefore, I couldn't calculate the current and voltage generated from the DC generator without knowing the load as well. Is that what do you want to say?
@freak16 • 08 Feb, 2011 @mibawad
Yes buddy.. i am saying the same thing..
And without knowing supply voltage as well as load no engineer of world can do it..
@mibawad • 08 Feb, 2011 But, there is no supply voltage for generator. It is only in case of DC motor
@IES • 12 Feb, 2011 @freak: Yes there is inductance associated with the coils present in parallel paths,
which produces Reactance Voltage.
@lal • 14 Feb, 2011 • 1 like What about this eqn : E = (fi)*Z*N*P/(60*A)

Fi - flux
Z - no. Of conductors
P - parallel paths
A - its value depends on winding. Eg: 2 for a simplex winding

This is the induced EMF. To get the terminal voltage the armature copper loss is subtracted. (Assuming its seperately excited)
@Akshay Singh • 17 Jan, 2016 I am thinking to build a generator with both rotor and stator rotating in this way relative speed between rotor and stator I.e N increase hence more electrical power
@Akshay Singh • 17 Jan, 2016 Anyone online please help
@Akshay Singh • 17 Jan, 2016 What no one answer here
@Vinay Mishra • 30 Jan, 2020 • 3 likes

An equation which popped up in my mind is:

EMF equation of Generator-

    Eg = PΦNZ / 60A


    P = No. of field poles

    Φ = Flux produced per mole

    Z = No. of armature conductors

    A = No. of parallel paths in the armature

    N = Rotational speed of the armature (rpm)

Torque equation of Generator-

    1) Armature Torque(Ta)-

        Ta = (PZ / 2πA) × Φ.Ia (N-m) 

        In this equation, the term (PZ / 2πA) is constant. That means Torque of the armature is directly proportional to the product of magnetic flux(Φ ) and the current flowing in the armature(Ia).

    2) Shaft Torque(Tsh)-

        Tsh = output in watts / (2πN/60) 

@Akshay, You can go through the EMF equation of Generator to increase the number of rotations(rpm) in generator.


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