Universal Motor

UNIVERSAL MOTOR

Introduction

A universal motor is a special type of motor which is designed to run on either DC or phase AC supply. These motors are generally series wound (armature and field winding are in series), so it produce high starting torque. Most of universal motors are designed to operate at higher speeds exceeding 3500 rpm. They run at lower sped on AC supply than they run on DC supply of same voltage, due to the reactance voltage drop which is present in AC and not in DC.

Construction of Unviersal Motors

Construction of a universal motor is very similar to the construction of a DC machine. It consists of a stator on which field poles are mounted. Filed coils are wound on the field poles. However, the whole magnetic path (stator field circuit and also armature) is laminated. Lamination is necessary to minimize the eddy currents which induced while operating on AC. Unviersal motor is shown in Figure 2.44

The rotary armature is of wound type having straight or skewed slots and commutator with brusher resting on it. The commutation on AC is poorer than DC because of the current induced in coils. So that we are using brushes with high resistance.

Working of Universal Motor

It will works either in AC or DC supply. When the Universal motor is fed with a DC supply, it works as a DC series motor. When current flows in the field winding it produces an electromagnetic field. The same current also flows from the armature conductors. When a current carrying conductor is placed in an electromagnetic field, it experiences a mechanical force. Due to this mechanical force or torque, the rotor starts to rotate. Its direction of force is given by Fleming's left hand rule.

When fed with AC supply, it still produces unidirectional torque. Because, armature winding and field winding are connected in series, they are in same phase. Hence, as polarity of AC changes periodically, the direction of current in armature field winding reverses at the same time. Thus, direction of magnetic field and the armature current reverses in such a way that the direction of force experienced by armature conductors remains same. Irrespective of the supply (AC or DC), universal motor works as a series motor work.

Speed/Load Characteristics

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. Characteristics curve are shown in Figure 2.45.

Applications

1. Home Applicance like vacuum cleaners, food mixers, domestic sewing machines.

2. Higher rating universal motors are used in portable drills, blenders etc.

Speed Control of Universal Motor:

Speed Control of Universal Motors can be possible by following ways

1. Phase Angle Control

2. PWM Chopper Control

In Phase Control Method, Speed Control is achieved by varying the firing angle for the TRIAC. Phase angle control is very cost effective solution but not very efficient. In PWM method rectified AC line voltage is switched at a high frequency by a Power MOFSET 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 by means of phase angled partialization. This method consists of changing the RMS voltage applied to the motor. In case, the voltage is a function of the firing angle of the Tirac. 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 thristor 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 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 changes. With the variations of the PWM duty cycle, the effective voltage seen by the motor can be changed. The advantage of PWM modulation with respect to phase-angle partialization is higher efficiency, less acoustic noise and better ECM behaviour, but it can have an impact on brush life duration.

In 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 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 input current. It transforms useful input signal into light, sends it across the dieclectric 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 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 MOCS 3020 series consists of gallium arsenide infrared emitting diodes, optically coupled to a silicon bilateral switch. They are designed for applications requiring isolated triac triggering.

Types of Universal Motor

There are two types of Universal Motor

i. Non-Compensated Type with Concentrated Poles

ii. Compensated Type with Distributed Field

iii. Non-Compensated Universal Motor

The Non-Compensated Motor has two salient poles and it is laminated as shown in figure 2.46.

The armature is of wound type and the laminated core is either straight or skewed slots. The leads of the armature winding are connected to the commutator. High resistant brushed are used along with this type of motor to help better commutation. An equivalent Non-Compensated Type Universal Motor is shown in figure 2.47

Compensated Type with Distributed Field

The compensated type Universal Motor consists of distributed field winding and the stator core is similar to that of split-phase motor. We know that split phase motors consists of an auxiliary winding in addition to main winding. Similar to the split phase motors, the compensated type also consists of an additional winding. The compensating winding helps in reducing the reactance voltage which is caused due to alternating flux, when the motor runs with the AC supply.

An equivalent compensated type Universal Motor is shown in figure 2.48.