Universal Motor Definition
A Universal motor is a special type of motor especially designed to work on AC and DC supply. Construction and working of a universal series motor is the same as the construction of a DC series motor because the armature winding and the field winding of the Universal motor are connected in series.
The starting torque of the universal series motor is very high same as the DC series motor.
The universal motor is more popular among all the motors not due to the reason that it can work both on AC as well as DC but due to its high starting torque.
Why Universal motor is preferred over the DC series motor in various applications?
The Dc series motor and the universal motor both have high starting torque but the Universal motor is preferred over Dc series motor in various applications because Dc series motors are large in size and heavy weight whereas the universal motor is compact in size and lightweight.
For this reason, the universal motor is preferred in applications like hand-operated tools, and portable condition motor applications for example drilling machines, mixer grinders, vacuum cleaners, etc.
Construction of Universal Motor
The construction of a Universal motor is the same as a DC series motor. Just like all other motors, it has two main parts, a stator and a rotor.
The stator is the stationary part of the motor. At the inner portion of the stator poles are made and around this pole, copper winding is done which is called field winding.
The rotor is the rotating part of the motor which is also called armature. A commutator is attached to the rotor and to that commutator carbon brushes are attached.
Winding around the outer core slots is present which is called armature winding.
Working of Universal motor
Case (1): When DC supply is given to the Motor
When a DC supply is given to the field winding of the motor then a constant magnetic field is produced due to DC current and we know that the frequency of DC current is zero so the polarity of the field does not change with time and remains the same.
Here field winding is connected in series with the armature winding as a result same DC current will flow in the armature winding.
We know that when a current-carrying conductor is placed in any magnetic field, then the conductor experiences a force in the tangential direction.
Here also current flows in the armature winding and the armature is placed in the magnetic field due to this effect torque will act on the armature and the armature starts to rotate and the direction of rotation of the armature is the same in the direction of flow of the current in the armature winding.
Case (2): When Single single-phase AC supply is given
When a single phase as supply is given to the Universal motor then a variable magnetic field is produced around the field winding. As the frequency of the AC current is not zero, the polarity of the magnetic field produced changes with time
During the positive half cycle of AC current north pole will develop on the left side and the South pole will develop on the right side. Then in this condition, the direction of Magnetic flux will be from the north pole to the south pole, and the current in the armature winding is determined according to the direction of the field flux.
As shown in the given fig. cross (X) indicates the current flowing in the inward direction and the dot (.) indicates the flow of current in the outward direction. So in this case, the north pole of the field is along the cross and the south pole of the field is along the dot side. Due to this torque will act on the armature in an anticlockwise direction.
During the negative half cycle of AC current polarity of the field winding changes and the left side becomes the south pole and the right side becomes the north pole due to this direction of the current in the armature winding will change which means after reversing the direction of the current direction of magnetic field will changes.
Also in this case north pole is along the cross side and the south pole is along the dot side same as in the previous condition that’s why the direction of the force on the armature remains the same. Due to this armature will rotate in an anticlockwise direction just like it rotates in a positive half-cycle case.
Here by using Fleming’s left-hand rule, we can also check that after changing the direction of the current, the direction of force remains the same or not.
The main concept behind the operation of the universal motor on a 1-phase AC supply is that when the given supply gets reversed then the field flux polarity will also change as well as the direction of the armature current changes but the direction of the torque on the armature remains the same.
That’s why the armature of the universal motor always rotates in the same direction either or DC or AC current supply.
Universal motor on AC supply vs DC supply
The speed of the universal motor on the dc supply is more than the speed of the Ac supply as speed is more for the dc supply than Ac supply then torque in case the dc supply is also more than the Ac supply and as a result efficiency of the Universal motor on Dc supply is more in comparison to efficiency of universal motor on Ac supply.
But the sparking produced by this motor on DC supply is less than the sparking produced by this motor on Ac supply.
Points to Remember about Universal motor
- Field winding and armature winding are connected in series.
- The speed of this motor generally lies between 3000 rpm to 12000 rpm.
- In this motor speed is inversely proportional to the load.
- This motor is preferred for high speed and high starting torque.
- It has the highest power-to-weight ratio.
- The direction of rotation can be changed either by changing the field or armature winding.
- At no load, the speed of the motor is dangerously high.
- The speed of the universal motor is generally controlled by “Gear trains”.
- Pulsating torque is minimized by both rotor and load inertia.
Advantages of Universal motor
- Simple and robust structure.
- Portable motor.
- It can be operated on ac and dc.
- High starting torque.
- It rotates at a very high speed (more than 3000 to 20000 rpm).
- This motor is easy to control.
Disadvantages of Universal motor
- Very noisy motor.
- This motor has a shorter life span.
- The universal motor gets heated quickly during operation.
- Higher maintenance is required.
Application of universal motor
- Universal motors have various household applications like drink and food mixers, vacuum cleaners, domestic sewing machines, etc.
- Used in hair dryers, grinders, and table fans.
- Used in polishers and blowers
- The high-rating motor is used in portable drilling machines and blenders.
Questions related to Universal Motor
Q1. If a particular application requires high speed and high starting torque then which of the following motor will be preferred?
(a) Shaded pole motor (b) Capacitor run motor
(c) Universal motor (d) Capacitor start motor
Ans. (c) Universal motor
Q2. Choose the correct statement about the universal motor.
(a) Speed in the case of AC supply is equal to speed in the case of DC supply.
(b) Speed in the case of AC supply is less than speed in the case of DC supply.
(c) Speed in the case of AC supply is more than speed in the case of Dc supply
(d) None of the above
Ans. (c) speed in the case of AC supply is less than the speed in the case of DC supply.
Q3. The speed of the universal motor can be reduced by using which of the following.
(a) V-belt
(b) Brakes
(c) Gear train
(d) Chain
Ans.(c) Gear train
Q4. What is a Universal motor?
(a) Compensated Dc motor
(b) Homo polar Dc motor
(c) Commutated Dc series motor
(d) Shunt Dc motor
Ans. (c) Commutated Dc series motor
Q5. Which of the following motors is also known as a single-phase series motor?
(a) Reluctance motors
(b) Repulsion motors
(c) Stepper motors
(d) Universal motors
Ans. (d) Universal motors