Universal Motor

A universal motor is a series-wound electric motor that can operate on both AC and DC power. These are a lot compared to those of DC series motors but the series motor develops less torque when working from an AC supply than when working from an equivalent DC supply. The direction of rotation can be changed by interchanging connections to the field concerning the armature as in DC series motor.

The working of a universal motor is similar to a series dc motor. On the other hand, the universal motor is designed for AC operation. It is competent to work at either AC or DC. In this manner, its development is a little distinctive. The field winding and armature winding are connected in series; both windings are energized when voltage is applied to the motor. The field and armature windings produce a magnetic field which causes the armature to rotate. Modest universal motors usually have no remuneration and replacement winding; they have two salient poles with excitation winding. The response between magnetic fields is caused by either AC or DC power.

The universal motor produces the electric torque proportional to the quadrate of the supply current. Since the same current flows through the field winding and the armature, it follows that ac reversals from positive to negative, or from negative to positive, will simultaneously affect both the field flux polarity and the current direction through the armature. This means that the direction of the developed torque will remain positive, and rotation will continue in the same direction. Thus, a universal motor can run both on dc and ac. So the electric torque has the same torque direction at any current polarity and also at AC power. The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings, the AC starting current will always be less than the DC starting current. Consequently, the starting torque on AC power will be lower than the starting torque on DC power. The characteristics of the universal motor are very much similar to those of D.C. series motors, but the series motor develops less torque when operating from an A.C. supply than when working from an equivalent D.C. supply.

There are various applications where universal motors are used, such as power drills, wash machines, blowers and kitchen appliances, etc. And they are also used for numerous different purposes where speed control and high qualities of speed are necessary. We can found universal motors in less than 1000watts also. Universal motors of a given horsepower rating are significantly smaller than other kinds of AC motors operating at the same frequency.

Speed control of Universal Motors can be possible by following ways

Phase angle Control
PWM Chopper control

In phase angle control method Speed control is achieved by varying the firing angle for the TRIAC. Phase angle control is a very cost-effective solution but not very efficient. In the PWM method rectified AC line voltage is switched at a high frequency by a Power MOSFET or IGBT device to generate time-varying voltage for the motor. In this method to control the motors by providing stable speed control, preventing large currents and drawing minimum harmonic current from ac mains supply are required. To meet these requirements using AC chopper with current and speed feedback is preferred.

The AC universal motor drive controls the rotation speed using phase-angle partialization. This method consists of changing the RMS voltage applied to the motor. In this case, the voltage is a function of the firing angle of the Triac. Continuous speed control of a universal motor running on DC is very easily accomplished using a thyristor circuit. A thyristor supplies the motor during the positive mains half cycle. Both the thyristor and its control are connected in such a way that the motor back-EMF compensates the motor load variations to adjust the speed. The pulse width modulation (PWM) technique, also known as the chopper drive is used to adjust the voltage applied to the motor. With the variation of the PWM duty cycle, the effective voltage seen by the motor can be changed. The advantage of PWM modulation concerning phase-angle partialization is higher efficiency, less acoustic noise, and better EMC behavior, but it can have an impact on brush life duration.

In the below application, the field and armature windings of the motor are connected in series through the armature commutator. Therefore the universal motor is also known as an AC series motor or an AC commutator motor. The universal motor can be controlled either as a phase-angle drive. In this application, we used the phase-angle control technique to control the voltage given to the motor. A phase shift of the gates pulses allows the effective voltage, seen by the motor to be varied. The phase-angle drive requires just a TRIAC. These are part of the thyristor family and are closely related to silicon-controlled rectifiers. However, unlike SCRs, which are unidirectional devices that can conduct current only in one direction, TRIACs are bidirectional and so current can flow in either direction, these are more commonly seen in circuits like motor drives. TRIACs are usually seen in simple, low-power applications like household dimmer switches.

MOC3021 is an Optocouplers. An optocoupler connects input and output sides with a beam of light modulated by the input current. It transforms useful input signal into light, sends it across the dielectric channel, captures light on the output side and transforms it back into electric signal These are typically come in a small 6-pin or 8-pin IC package, but are essentially a combination of two distinct devices an optical transmitter, typically a gallium arsenide LED and an optical receiver such as a phototransistor or light-triggered diac. The two are separated by a transparent barrier which blocks any electrical current flow between the two but does allow the passage of light. The MOC3020 Series consists of gallium arsenide infrared emitting diodes, optically coupled to a silicon bilateral switch. They are designed for applications requiring isolated Triac triggering.

Now you have an idea about the universal motors if you have any further more queries on this topic or the electrical and electronic projects leave the comments section below.