A universal motor can be defined as a motor that can be operated on both AC and DC supply. The internal design of the motor is such that the nature of the voltage supply has no significant effect on the working of the motor except for the losses, which are different with different types of supplies.
Universal motors are well known for their high starting torque and high-speed operation. This is one of the reasons why these motors are preferred for traction and loading purposes.
Construction of Universal Motor
A universal motor, in general, is nothing but a DC Series Motor. Thus, having a thorough understanding of the design and working of a DC series motor is of great importance to understanding the overall aspect of a universal motor.
Like any other DC motor, a universal or DC Series motor has two windings – the field winding on the stator and the rotatory armature winding. As the name says, the field and armature winding are connected in series, thus allowing the same current to flow through both the armature and the field winding.
As a universal motor is also expected to work on AC, it is designed to handle power losses due to AC, like the eddy current losses for which laminated core or the magnetic path is used. Similarly, the Hysterisis loss is also taken care of.
The rotating armature is wound type with skewed or straight slots and a commutator with high resistance brushes resting on it. The brushes have higher resistance as the commutation for AC supply is poorer than that of DC because of the current induced in the armature coils.
Working of a universal motor
A DC series motor is a universal motor because it can work on AC and DC supplies, even though the name suggests it to be a DC motor. This is because, unlike other DC motors that vibrate or fail to work on AC supply, a DC series motor can generate positive torque for both positive and negative voltage supply.
To understand how that happens, we need to learn about the working of the internal circuit of a DC series motor, as shown above.
A series motor has both armature and field winding connected in series. Because of this, the armature current is equal to the field current, which is nothing but the source current.
The following expression gives the torque generated by a universal motor.
Where
Tα =T Torque of the motor in Newton-metres.
P = No. of poles
Z= Number of armature conductors
A= The Number of parallel paths
Ia= Total current through the armature conductors
ϕ=Magnetic field flux per pole
The number of parallel paths, the number of poles, and the number of armature conductors in a given machine are constant. Therefore, the torque generated by a universal motor is directly proportional to the magnetic field flux per pole and the armature current. Thus,
The magnetic field flux per pole is generated due to the current passing through the field coil, which can be denoted by If. Hence, we can say that.
Therefore, the torque generated by a DC motor can be said to be directly proportional to the product of the armature and the field current.
In the case of a DC Series motor, the field and the armature windings are connected in series. Thus, the current flowing through both windings is equal to the source current, as evident from the circuit diagram above.
So, the source current is,
Therefore, the torque generated by the series motor is
Therefore, in a DC series motor, the generated torque is directly proportional to the square of the armature current (or the field current). This is the primary reason why this motor can be operated on both DC and AC supply.
In the case of a DC supply, the voltage has no polarity, while in the case of an AC supply, the voltage is positive for one-half cycle and negative for another half cycle. As the torque of the DC series motor is proportional to the square of the armature current, the torque generated by the motor is always positive. This is what makes a DC series motor eligible to be called a universal motor.
Torque and Speed Characteristics
As the motor’s torque is directly proportional to the square of the armature current, the universal motor is known for its high starting torque. This can be better understood by considering the voltage equation of the motor given as
Where V is the source voltage, E is the back-emf of the motor, and Ia and Ra are the armature current and armature resistance, respectively.
As evident from the equation, the motor’s back EMF reduces the armature current. This back EMF is directly proportional to the speed (rpm) of the universal motor. Therefore, when the motor speed is almost zero, the motor’s back EMF remains zero at the starting point. Thus, the current flowing through the armature is maximum at the starting point given as
As the torque generated by the DC series motor is directly proportional to the square of the armature current, the starting torque of the DC series motor is extremely high.
Beyond a certain value of the source current, the field coil saturates. Therefore, the torque of the universal motor is no longer directly proportional to the square of the armature current. There is a linear relation between the torque and the armature current henceforth.
The speed of the motor is given by
As evident, the speed of the motor is inversely related to the field flux per pole. In a series motor, as discussed earlier, the field flux is directly proportional to the field current, which is equal to the armature current. This armature current rises as the load on the motor rises.
Therefore, at no load condition, the motor attains a dangerously high speed (ideally infinite, as the armature current is zero. That’s why a universal motor shouldn’t be operated without load.
The universal motor is known for its high-speed performance. The following curve gives the torque-speed characteristics of a universal motor.
The following curve gives the speed-torque characteristics of a universal motor for both AC and DC supply.
Applications of Universal motors
- Universal motors are used in Vacuum cleaners, drink and food mixers, electric sewing machines, etc.
- The higher-rating universal motors are used in portable drills, blenders, etc.
