The document discusses the universal motor, which can operate on both AC and DC power. It has a series-wound construction similar to a DC motor but with laminated magnetic components to reduce eddy currents from AC power. Universal motors are commonly used in appliances like drills, fans, and blenders because they can function using both AC household power and portable DC batteries. The document covers the construction, working principle, characteristics, and common defects of universal motors.

1. Page 1
Unit 04
Universal Motor

2. Page 2
Universal Motor (Mixy and Drill Machines)
• It is a series wound motor – Compensated or Non-Compensated
• It runs on same speed on D.C and A.C (Single Phase Supply)
• D.C. and A.C. Voltage must be same
• Frequency of A.C. must not exceed 60 Hz
• Suitable for sewing machines, table fans, vacuum cleaners, portable
drills, hair dryers, blowers and kitchen appliance etc
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3. Page 3
Construction of Universal Motor
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4. Page 4
Construction of Universal Motor
• Construction of a universal motor is similar to the construction of a DC
series motor. However, the whole magnetic path (armature, stator field
circuit etc) is laminated.
• Lamination is necessary to minimize the eddy currents which induce
while operating on AC.
• The armature is of wound type having straight or skewed slots and
commutator with brushes resting on it.
• The commutation on AC is poorer than that for DC, because of the
current induced in the armature coils. For that reason brushes used are
having high resistance
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5. Page 5
Construction of Universal Motor
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6. Page 6
Construction of Universal Motor
• Stator
• Made of thin silicon steen strips and laminated core.
• It has salient poles with one coil wound on each pole.
• Mostly it is a 2 pole machine
• Rotor
• It has slots with winding like armature
• It is also made of laminated core
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7. Page 7
Working Principle of Universal Motor
• A universal motor works on either DC or single phase AC supply.
• When the universal motor is fed with a DC supply, it works as an DC
series motor. That is, when flux due to field winding and flux due to
armature winding interact each other, a torque is produced which
causes rotor to rotate.
• When fed with AC supply, it still produces unidirectional torque.
Because, as armature winding and field winding is connected in series,
in same phase, direction of the current in armature as well as field
winding reverses at the same time.
• And hence, regardless of AC or DC supply, universal motor works on
the same principle that DC series motor works.
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8. Page 8
Characteristics of Universal Motor
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• Speed/load characteristics of a universal
motor is similar to that of DC series motor.
• The speed of a universal motor is low at
full load and very high at no load.
• Usually, gears trains are used to get the
required speed on required load.
• The speed/load characteristics are (for both
AC as well as DC supply) are shown in the
figure
• Power factor is very good (approx 0.95)

9. Page 9
Comparison of Characteristics – AC / DC / Universal
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10. Page 10
Characteristics of Universal Motor
• Speed is inversely proportional to Load
• No load speed is 2 ¼ times the full load speed of the motor
• Starting torque is 2 ½ times the full load torque
• Speed on A.C. is less than speed on D.C. for the same torque
• Starting torque is 3 times high on D.C. than A.C.
• Power factor on A.C. is very good (approx. 0.95)
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11. Page 11
Defects and Causes
Defects Causes
Motor does not start • Defective bearing
• Supply not available
• Open field or armature
• Short armature or field
Sparking on the Commutator • Defective Brush or Commutator
• Shorted Coil
• Overload or Opposite speed
• Earthed Coil
Low speed of motor • Defective bearing
• Shorted coil
• Open armature coil
• Wrong brush setting
• Dirty Commutator
• Fan unbalanced
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12. Page 12
Thank you!
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