DC MOTOR

SERIES MOTOR
KD SIR
DC SERIES MOTOR:

DEFINITION
 DC Series Motor
 A DC series motor is a type of DC motor where the field
winding is connected in series with the armature winding.
Characteristics:
1. High Starting Torque: The series connection of
the field and armature windings results in a strong
magnetic field at low speeds, producing a very high
starting torque.
2. Variable Speed: The speed of a series motor is
highly dependent on the load. As the load increases,
the speed decreases.

3. High Speed at No Load: Without a load, a

series motor can reach dangerously high speeds,
potentially damaging the motor. Therefore, it's
crucial to always operate a series motor with a load
connected. KD SIR
 CURRENT OF THE MOTOR
DEPEND UPON THE “LOAD”
 DC Series Motor
 Current of the motor depend upon the “Load”
At Light Load:
Magnetic Flux: Weak
Speed: High
Armature Current: Need small for produce the require torque.

At Heavy Load:
Magnetic Flux: Strong
Speed: Low
Armature Current: Need Large for produce the require torque.

Load and Armature Current: As the load on the motor increases, the
armature current also increases to provide the necessary torque.
KD SIR
TORQUE VS. ARMATURE
CURRENT (Ta-Ia)
 DC Series Motor
 The torque of a DC series motor is directly proportional to the square
of the armature current (Ia²)
 Torque Equation: The torque (T) developed by a DC motor is
given by:
 T ∝ Φ * Ia
 Series Moto (Series Connection):
** Field current (If) = Armature current (Ia).
 Flux-Current Relationship:
The flux per pole (Φ) is directly proportional to the field current (If).
** Φ ∝ Ia
•T ∝ Ia * Ia •Ia = √T
❑ Torque-Current Relationship:
•T ∝ Ia² •N ∝ 1/√T
•N ∝ 1/Ia
KD SIR
 SPEED
CHARACTERISTIC
 DC Series Motor
 At Light Load: Speed vs. armature current (N-Ia)
Magnetic Flux: Weak
Speed: High N ∝ Eb/ɸ
Armature Current: Need small for produce the require torque.
 At Increase Load:
Magnetic Flux & Armature Current : increase
Speed: Decrease
 Speed-Flux Relationship:
•The speed of a DC motor is inversely proportional to the field
flux.
***Speed (N) ∝ 1 / Flux (Φ)

***Speed (N) ∝ 1 / Ia
✓ Speed (N) is almost inversely proportional to the Armature CurrentKD
(Ia)
SIR
 DC Series Motor
 Why DC-Series Motor Run With Load
Speed of DC series Motor= N ∝ Eb/ Φ
 Flux-Current Relationship:
The flux per pole (Φ) is directly proportional to the field current (If).
** Φ ∝ Ia

When series Motor Run at “No Load”

Ia = Ise = IL = 0
Armature Current = Series Field current = Load Current = ZERO

NOW THE SPEED (N) ∝ ∞ (Infinity) KD SIR

 SPEED-TORQUE
CHARACTERISTIC
 DC Series Motor
 The speed-torque characteristic of a DC series motor is inversely
proportional to the load torque.
Speed-Torque characteristics of DC series motor:
The relationship between speed and torque is given by:
𝑉 −𝐼𝑎 𝑅𝑎
N=
series motor:
𝑘∅

ϕ ∝ Ia ∝ T
 Speed-Torque Relationship:
Speed and torque are inversely proportional to each
other. *** Speed (N) ∝ 1 / Torque
KD SIR
 DC Series Motor

Rectangular Hyperbola
Ta-Ia curve is parabola for smaller values of Ia.

T ∝ Ia.There fore, after magnetic saturation, Ta-Ia curve

becomes a straight line.

KD SIR
 APPLICATION
ADV. & DIS-ADV.
 DC Series Motor
Applications:
•Traction Motors: In electric vehicles, trains, and trolleys.
•Hoists and Cranes: For lifting heavy loads.
•Electric Fans: To provide variable speed control.
•Mixers and Blenders: For high-torque applications.
•Vacuum Cleaners: To handle heavy loads.
Advantages:
•High starting torque
•Simple construction
•Relatively low cost
Disadvantages:
•High speed at no load (requires load for safe operation)
•Poor speed regulation
KD SIR
 DC MOTOR
ALL FORMULA
Torque load characteristics  Important Formula
❖ DC motor Torque (T) ∝ Φ x Ia series motor:
❖ For series motor= Φ ∝ Ia ϕ ∝ Ia ∝ T
❑ Torque of series motor: -

❑ Torque-Current Relationship: •Ia = √T

•N ∝ 1/√T
•N ∝ 1/Ia
Speed Vs load characteristics:
 Speed-Torque Relationship:
*** Speed (N) ∝ 1 / Torque
Speed

N ∝ Eb/ɸ
KD SIR
DC SERIES MOTOR
STARTER
 DC Series Motor
 A starter is necessary to start a DC motor because :

Armature current in D.C. motor-

At the time of starting back emf of D.C. motor is zero i.e. Eb = 0 So, that at
the time of starting, armature current.

So, armature current is very large at the time of starting.

To prevent this high current, we use starter.
It helps in restricting the initial high armature curran that is
present on account of starting back emf being zero

KD SIR
 DC Series Motor
DC Series Motor Starter Working Principle:
 Holding coil protects the motor against
“no volt” and “no load”
HOLDING
COIL
 Overload release protects the
motor against the flow of excessive
current.
OVERLOAD
Starters are used with DC motors to: RELEASE
•Protect the motor:
•Starters limit the starting current and torque to
prevent damage from high currents and torque.
•Restrict armature current:

•Starters restrict the armature current because there

is no back emf while starting.
KD SIR
 DC Series Motor
 DC Series Motor Efficiency
 Electrical motor efficiency is defined that the ratio of mechanical power
developed to the total electrical power input.
In the case of DC motor, total electrical power input = VIL
Where V is the supply voltage and IL is load current.

Mechanical power developed = EbIa

Where Eb is back emf and Ia is armature current.

So, efficiency is given by =

Application:
In dc series motor, armature current = load current.

Therefore, the ratio of back emf (Eb) to supply voltage (V) indicates efficiency.
KD SIR
MCQ FOR DC
SERIES MOTOR
 DC Series Motor
 DC SERIES MOTOR-MCQ
1. Which D.C. motor has got maximum self-relieving property?
>> Series motor
2. The direction of rotation of a D.C. series motor can be changed by___

>> interchanging field terminals and interchanging Supply

3. Why the D.C series motor field winding is wound with thick wire?
>> To carry the load current
Speed and Speed Control
4. Why series motor produces high torque and speed initially without load?
>> Absence of back emf
5. The speed of a D.C. series motor is____(respect to Armature Current)
>> inversely proportional to the armature current
 DC Series Motor
 DC SERIES MOTOR-MCQ
6. No-load speed of which of the following motor will be highest?
>> Series motor
7. Which of the motor has the poorest speed regulation?
>> Series motor
8. In a D.C. series motor, if the armature current is reduced by 50%, the
torque of the motor will be equal to______
>> 25% of the previous value
9. A dc series motor is best suited for driving
(a) lathes
(b) heavy machine tools
(c) cranes & hoists
(d) shear & punches
(c) cranes & hoists KD SIR
 DC Series Motor
 DC SERIES MOTOR-MCQ
10. A d.c. series motor develops a torque of 20Nm at 3A of load current. If
the current is increased to 6A, the torque developed will be:
(a) 80 Nm
(b) 20 Nm
(c) 40 Nm
(d) 10 Nm

11. The current drawn by a 220 V dc motor of armature resistance

0.5 ohm and back emf 200 V is……
(a) 40 A
(b) 400 A
(c) 44 A
(d) 440 A
KD SIR
 DC SERIES MOTOR-MCQ DC Series Motor
12. A d.c. series motor is accidentally connected to single-phase a.c. supply.
The torque produced will be
(a) Of zero average value
(b) Oscillating (d) Pulsating and unidirectional
(c) Steady and unidirectional
(d) Pulsating and unidirectional

13. A 220 V DC machine has armature resistance of 0.5 Ω. If full load

armature current is 15 A, find induced EMF when the machine acts as a
motor.
(a) 212.5 V
(b) 235 v
(c) 220 v
(d) 227.5 V

KD SIR
 DC SERIES MOTOR-MCQ DC Series Motor
14. The function of commutator in the dc motor :
(a) To collect current from the conductors
(b) To change ac into dc
(c) To conduct the current to the brushes
(d) To change dc into ac
Ans. (d) : The function of commutator is to change DC into AC and vice-versa.

15. The air-gap between the yoke and armature in dc motor is

kept small
(a) To achieve a stronger magnetic field.
(b) To avoid overheating of the machine.
(c) To avoid locking of the armature.
(d) To avoid transverse motion.
(a) To achieve a stronger magnetic field.
KD SIR
 DC SERIES MOTOR-MCQ DC Series Motor
15. In DC series motor the starting torque is very high, up to ____ the full - load
torque.
A. three times
B. five times B. five times
C. two times
D. four times
16. In a DC series motor, what is the relationship between speed and
armature current?
A. Speed is independent of armature current.
B. Speed is directly proportional to armature current.
C. Speed is directly proportional to the square of armature current.
D. Speed is inversely proportional to armature current.
D. Speed is inversely proportional to armature current.

17. For which type of motor are both the given characteristics true?
1. No-load condition is dangerous.
2. Variable speed. 2 : Series motors KD SIR
 DC SERIES MOTOR-MCQ DC Series Motor
18. A d.c. series motor should always be started with load because
A. at no-load it will rotate at a dangerously high speed
B. at no-load it will not develop high starting torque
C. it cannot start without load
D. it draws a small amount of current at no-load
A. at no-load it will rotate at a dangerously high speed
19. The Speed-Torque characteristic of a series motor is a/an
A. Circle
B. Parabola
C. Ellipse
D. Hyperbola
D. Hyperbola
Speed-Torque characteristic also known Mechanical characteristic.
20. The series motors have a relatively
(a) Zero starting torque
(b) Low starting torque
(c) High starting torque
(d) Medium starting torque
(c) High starting torque T ∝ Φse x Ia =>> T ∝ Ia^2 KD SIR
 DC SERIES MOTOR-MCQ DC Series Motor
21. For a given torque, reducing the diverter resistance of DC Series motor…
A. Increasing its speed but armature current remains the same
B. Increasing its speed demanding more armature current
C. Decreasing its speed demanding less armature current
D. Decreasing its speed but armature current remains the same
B. Increasing its speed demanding more armature current N ∝ 1/ Φse
22. Which motor should not be used in centrifugal pump?
(a) Shunt
(b) Series
(c) Cumulatively compound
(d) Differential compound

(b) Series
23. When the load is removed, _________ motor will run at the highest speed.
(a) shunt
(b) series (b) Series
(c) cumulative compound
(d) differential compound
KD SIR
Applications of different DC motors are given below.
DC Series wound motor:
•It has a very high starting torque.
•No-load condition is dangerous.
•It has Variable speed
•Hence it is used for heavy-duty applications such as electric railways, mine hoists, continuous
conveyors cranes, rolling mills, metallurgical works, etc.
DC Shunt-wound motor:
•This is an almost constant speed motor and its torque varies nearly as the current.
•Hence, it is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps,
fans, woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders,
small printing presses, paper making machines, metal cutting machines, etc.

DC Cumulative compound motor:

•It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing
the load.
•Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators,
freight elevators, etc.

DC Differentially compound motor:

It has low starting torque and has a constant speed, but the dangerously high speed at no load under the
circumstances it is seldom used.
KD SIR