In previous post “SpeedControl of DC Motor” we discussed the method of speed control of DC Motor by ArmatureCircuit Resistance Control Method. In this post we will discuss the method of
speed control of DC Motor by varying the Field Flux which is known as Field
Weakening Method.

**By Field Weakening Method of speed control of DC Motor we can obtain speed above the base speed (Base speed means name plate speed or rated speed of DC Motor).**

We will discuss Field
Weakening Method of speed control of DC Motor for DC Shunt Motor and DC Series
Method separately.

**Field Weakening Method for Speed Control of DC Shunt Motor:**

The connection diagram for
the speed control of DC Shunt Motor by varying the Field Flux is given below.
As clear from the figure, a variable series resistance is added in the Field
Circuit of DC Shunt Motor.

Therefore by varying the
series resistance in the Field Circuit of DC Shunt Motor, current through the
Field winding of DC Shunt Motor can be varied and hence we can control i.e.
decrease the field flux.

Under steady running
condition, if the Field Circuit resistance is increased, the field current I

_{f }will decrease and which in turn will decrease the flux Ø. As the rotor speed W_{m }cannot change suddenly, hence because of decrease in field flux Ø, back emf**E**will decrease._{a}= K_{a}ØW_{m}
As, V

_{t}= E_{a}+I_{a}R_{a}_{}

**Hence, I**

_{a}= (V_{t}- E_{a}) / R_{a}will increase. Therefore DC Shunt Motor will draw more current from the supply mains when field circuit resistance is increased i.e. filed flux is decreased.

Now,

**T**will increase as the percentage increase in Armature current I_{e}= Torque of DC Shunt Motor = KaØI_{a}_{a}is more than the decrease in filed flux Ø. Therefore, Electromagnetic Torque of DC Shunt Motor will increase. As load torque is assumed constant, the Electromagnetic Torque produced by DC Shunt Motor is more than the load Torque, hence the load will accelerate. Because of acceleration of load the speed will increase which in turn will increase the back / counter emf E_{a}and hence I_{a}will decrease till electromagnetic Torque becomes equal to Load Torque.
Suppose,

I

_{a1}= Armature current of DC Shunt Motor when Field Flux = Ø_{1}and speed = W_{m1}_{}

I

_{a2}= Armature current of DC Shunt Motor when Field Flux = Ø_{2}and speed = W_{m2}_{}

For constant Load Torque T

_{L},
I

_{a1}= T_{L}/ K_{a}Ø_{1}_{}

I

_{a2}= T_{L}/ K_{a}Ø_{2}_{}

As Ø

_{1}> Ø_{2}, therefore I_{a1}<I_{a2}_{}

Now,

W

_{m1}= (V_{t}– I_{a1}R_{a}) / Ka Ø_{1}_{}

and, W

_{m2}= (V_{t}– I_{a2}R_{a}) / Ka Ø_{2}_{}

As discussed, percentage increase
in Armature current is more than the decrease in field flux, so

W

_{m1}< W_{m2}_{}

Thus we see that by Field
Weakening Method, speed more than base speed is obtained. This method of speed
control of DC Shunt Motor is very simple and economical and hence used
extensively.

**Field Weakening Method for Speed Control of DC Series Motor:**

Resistance of the field
circuit of DC Series Motor can be changed by three ways:

- By putting a resistor,
called
**diverter**, in parallel with the series field winding

- By tapping the series field winding

- By changing the field coil connection from series to parallel.

We will discuss each of
three methods one by one here.

**Diverter Field Control:**

The connection diagram of Diverter
Field Control Method is shown below.

As shown in figure, a
variable resistance called diverter is connected in parallel to the field
winding of DC Series Motor. As the resistance of Diverter is changed, the field
flux is changes which in turn cause the speed of DC Motor to change.

**Tapped Field Control:**

The connection diagram of
tapped filed control is shown in figure below.

When the field winding is
tapped, the number of series field turns N changes as tap is changed from one
position to another due to which Megnetomotive Force i.e. mmf = NI, where I is
the current flowing through the field winding, changes which cause the filed
flux to change. In this way by changing the Tap, field flux is changed which
cause speed of DC Motor to change.

**Series Parallel Field Control:**

In this method, series field
winding is divided into two equal halves which then connected in series or
parallel. Assume the two equal halves of series filed winding is connected in
series as shown in figure below.

Let the current flowing in
the Armature of DC Series Motor = I

_{a}and back / counter emf = E_{as}
Therefore, filed mmf F

_{s}= I_{a}(N_{s}/2 + N_{s}/2) where Ns = Series Field turns in each half
So, F

_{s}= I_{a}N_{s}…………………………………..(1)
and, E

_{as}= Vt – I_{a}(R_{a}+R_{s}) where R_{a}= Armature Resistance and R_{s}= Field Resistance
Now assume the two equal
halves of series filed winding is connected in parallel as shown in figure
below.

As both the halves are in
parallel therefore I

_{a}/2 current will flow in each half of the series field winding of DC Series Motor. Let the field mmf be F_{p}then
F

_{p}= (I_{a}/2)(N_{s}/2)×2 = I_{a}N_{s}/2
Therefore,

F

_{p}= I_{a}N_{s}/4 ……………………………………………(2)
If the resistance of field
winding is R

_{s}then resistance of each equal half = R_{s}/2 and because both halves are connected in parallel therefore equivalent field resistance = R_{s}/4.
Eap = Back emf

E

_{ap }= Vt – I_{a}(R_{s}/4+R_{a})
Thus we see that, E

_{ap}> E_{as}_{}

Now, as back emf is directly
proportional to Ø×W

_{m},
E

_{ap}/E_{as}= W_{m2}(I_{a}N_{s}/2)/W_{m2}(I_{a}N_{s}) ...............[From equation (1) and (2)]
Hence,

W_{m2} = 2W_{m1}(E_{ap}/E_{as}) |

**Thus, W**

_{m2}>W_{m1}as E_{ap}/E_{as}> 1

Thus for parallel connection
of series field coils results in higher operating speed of the DC Series Motor.

**Please write you review and suggestion. Thank you!**

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