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Single-Phase Induction Motors

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Single-Phase Induction Motors EE311 MODULE 5 MLMCE Single-Phase Induction Motors A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has (i) a squirrel-cage rotor identical to a 3-phase motor and (ii) a single-phase winding on the stator. The single-phase stator winding produces a magnetic field that alternating in the air gap not rotating. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce any torque in a stationary squirrel-cage rotor. (Net torque is zero). However, if the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors. Nor can it be employed fur a motor located at some inaccessible spot. A single-phase induction motor is not self starting one but requires some starting means. KTUNOTES.IN Making Single-Phase Induction Motor Self-Starting To make a single-phase induction motor self-starting, we should somehow produce a revolving stator magnetic field. This may be achieved by converting a single-phase supply into two-phase supply through the use of an additional winding (starting winding). When the motor attains sufficient speed, the starting winding may be removed depending upon the type of the motor. As a matter of fact, single- phase induction motors are classified and named according to the method employed to make them self-starting. 1 Downloaded from Ktunotes.in EE311 MODULE 5 MLMCE Types of Single-Phase Motors Single-phase motors are generally built in the fractional-horsepower range and may be classified into the following four basic types: 1. Single-phase induction motors (i) split-phase type (ii) capacitor type (iii) shaded-pole type 2. A.C. series motor or universal motor 3. Repulsion motors (i) Repulsion-start induction-run motor (ii) Repulsion-induction motor 4. Synchronous motors (i) Reluctance motor (ii) Hysteresis motor Split-Phase Induction Motor The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M. The starting winding is located 90° electrical from the main winding and operates only during the brief period when the motor starts up. KTUNOTES.IN The two windings are so resigned that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance as shown in the schematic connections in Fig. Consequently, the currents flowing in the two windings have reasonable phase difference c (25° to 30°) as shown in the phasor diagram in Fig. 2 Downloaded from Ktunotes.in EE311 MODULE 5 MLMCE Operation (i) When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is. (ii) Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25° to 30°) between them as shown in above Fig. Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor. The starting torque is given by; Ts kIm Is sin where k is a constant whose magnitude depends upon the design of the motor. (iii) When the motor reaches about 75% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends uponKTUNOTES.IN the load on the motor. These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to drive: (a) fans (b) washing machines (c) oil burners (d) small machine tools etc. Capacitor-Start Motor The capacitor-start motor is identical to a split-phase motor except that the starting winding has as many turns as the main winding. Moreover, a capacitor C is connected in series with the starting winding as shown in Fig. 3 Downloaded from Ktunotes.in EE311 MODULE 5 MLMCE The value of capacitor is so chosen that Is leads Im by about 80° (i.e., ~ 80°) which is considerably greater than 25° found in split-phase motor. Consequently, starting torque (Ts = k Im Is sin ) is much more than that of a split-phase motor Again, the starting winding is opened by the centrifugal switch when the motor attains about 75% of synchronous speed. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. Capacitor-start motors are used where high starting torque is required and where the starting period may be long e.g., to drive: (a) compressors (b) large fans (c) pumps (d) high inertia loads. Capacitor-Start Capacitor-Run Motor This motor is identical to a capacitor-start motor except that starting winding is not opened after starting so that both the windings remain connected to the supply whenKTUNOTES.IN running as well as at starting. Two designs are generally used. (i) In one design, a single capacitor C is used for both starting and running as shown in Fig.(i). This design eliminates the need of a centrifugal switch and at the same time improves the power factor and efficiency of the motor. (ii) In the other design, two capacitors C1 and C2 are used in the starting winding as shown in Fig. (ii). The smaller capacitor C1 required for optimum running conditions is permanently connected in series with the starting winding. The much larger capacitor C2 is connected in parallel with C1 for optimum starting and remains in the circuit during starting. The starting capacitor C1 is 4 Downloaded from Ktunotes.in EE311 MODULE 5 MLMCE disconnected when the motor approaches about 75% of synchronous speed. The motor then runs as a single-phase induction motor. Because of constant torque, the motor is vibration free and can be used in: (a) hospitals (b) studios (c) ceiling fan and (d) other places where silence is important. A.C. Series Motor or Universal Motor A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply. These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque (See characteristics of DC motors here). That is why, universal motors generally comes built into the device they are meant to drive. Most of the universal motors are designed to operate at higher speeds, exceeding 3500 RPM. They run at lower speed on AC supply than they run on DC supply of same voltage. Construction of UniversalKTUNOTES.IN Motor 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. Field 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 induce while operating on AC. The rotary armature is of wound type having straight or skewed slots and commutator with brushes resting on it. The commutation on AC is poorer than 5 Downloaded from Ktunotes.in EE311 MODULE 5 MLMCE that for DC. because of the current induced in the armature coils. For that reason brushes used are having high resistance. Working 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 a DC series motor. (see working of a DC series motor here). 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. The direction of this 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 and field winding reverses at the same time. Thus, direction of magnetic field and the direction of armature current reverses in such a way that the direction of force experienced by armature conductors remains same. Thus, regardless of AC or DC supply, universal motor works on the same principle that DC series motor works. Applications of Universal Motor . Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers, domestic sewing machine etc. The higher rating universalKTUNOTES.IN motors are used in portable drills, blenders etc. Alternator The machine which produces 3-phase AC power from mechanical power is called an alternator or synchronous generator. Alternators are the primary source of all the electrical energy we consume. An alternator operates on the same fundamental principle of electromagnetic induction as a d.c. generator i.e., when the flux linking a conductor changes, an e.m.f.
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