Speed control of A D. C. Machine/Motor

Hello Friends,
In our current article we will discuss how speed of a D. C. Motor can be controlled i.e. Which factors are controlled to control the speed of the motor ?
From the back e.m.f. Equation of a D. C. Motor we have : E_b=PΦNZ/60 A
Where,
P = Number of poles in the machine
Φ = Flux enclosed to a pole
N = Speed of Machine in r. p. m.
Z = Number of Armature Conductors
A = Number of Parallel Paths in D. C. Motor
Also from equivalent circuit of a D. C. Motor we have
V = E_b+I_aR_a
Here V is applied voltage to the D. C. Motor
E_b is induced back e.m.f. in motor Armature.
I_a is Armature current and
R_a is Armature resistance.

Normally Armature resistance of a D. C. Motor is of the order of 0.5 ohm.

From above two equations we have
E_b=V-I_aR=NPΦZ/60A
Now we have to control N by changing values of other variables we have
N=(V-I_aR_a)(60A)/PZΦ This equation decide what should be changed to change the speed of a D. C. Motor. We know that for shunt motor flux is constant i.e. Independent of Armature current while in series motor the flux induced in D. C. Motor depends upon Armature current, so speed control method of both motors will be different.

Speed Control of D. C. Shunt Motor

We know that in equation
N=(V-I_aR_a)(60A)/PZΦ The following are constants i.e. Can't be changed

• 60 A
• P
• Z
• Φ

Note : Φ can be changed in D. C. Shunt motor by adding external variable resistance in field winding.

Speed Control of D. C. Series Motor

Similarly in D. C. Series Motor, We have the same equation
N=(V-I_aR_a)(60A)/PZΦ The following are constants i.e. Can't be changed

• 60 A
• P
• Z

Here Φ can be changed by simply increasing or decreasing Armature resistance.

Speed Control of D. C. Motor ( Series and Shunt Both )

From above discussion it is clear that following can be changed in both series and shunt motors

• Applied Voltage V
• Armature Current I_a
• Armature Resistance R_a
• Flux Φ

Based on the quantity to be controlled the method of speed controlled are named as

• Voltage Control Method
• Armature Resistance Control.
• Field Flux Control

Lets discuss one by one in detail.

Voltage Control Method

From the back e.m.f. Equation E=PNZΦ/60A=V-I_aR
We have N=(V-I_aR_a)*(60A/PZΦ)
If Flux kept constant and P, A, I_a, R_a and Z are already constant so
N=(V-K)*K₁
here K and K₁ are constant.
Now N is directly proportional to applied voltage. So if applied voltage is increased then the speed of the DC machine will increased and vice versa.

Armature Resistance Control.

If we increase armature resistance then Speed of the motor will be reduced and vice versa.

Field Flux Control

Increasing flux in a DC motor will reduce speed of the machine and vice versa.