EET 306 ELECTRICAL MACHINE Single-Phase Induction Motor Baharuddin Ismail Syafruddin

Types of Single Phase Motor The main types of single-phase motor : a. Single-phase induction motors - Split-phase type - Shaded pole type - Reluctance start type b. Single-phase synchronous motors - Reluctance motor - Hysteresis motor Syafruddin

c. Single-phase commutator type motors - Repulsion motor - repulsion induction motor - ac series motor - Universal motor Syafruddin

Single Phase Induction Motor The single-phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and so forth. The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly. One is the main and The other is the auxiliary winding or starting winding Syafruddin

Single Phase Induction Motor Stator Syafruddin

Single Phase Induction Motor This “single-phase” motors are truly two- phase machines. The motor uses a squirrel cage rotor, which has a laminated iron core with slots. Aluminum bars are molded on the slots and short-circuited at both ends with a ring. Cross-sectional view of a single-phase induction motor Syafruddin

Cross-section view of a single phase motor 4-pole run winding 4-pole start winding Rotor (squirel cage) Syafruddin

Single Phase Induction Motor Figure 3 Squirrel cage rotor Syafruddin

Single Phase Induction Motor The single-phase induction motor operation can be described by two methods: Double revolving field theory; and Cross-field theory. Double revolving theory is perhaps the easier of the two explanations to understand Learn the double revolving theory only Syafruddin

360 Chapter 7 Induction Motor Double revolving field theory A single-phase ac current supplies the main winding that produces a pulsating magnetic field. Mathematically, the pulsating field could be divided into two fields, which are rotating in opposite directions. The interaction between the fields and the current induced in the rotor bars generates opposing torque Under these conditions, with only the main field energized the motor will not start 5/18/2019 360 Chapter 7 Induction Motor

360 Chapter 7 Induction Motor Single-phase motor main winding generates two rotating fields, which oppose and counter-balance one another. However, if an external torque moves the motor in any direction, the motor will begin to rotate. 5/18/2019 360 Chapter 7 Induction Motor

Single Phase Induction Motor Double revolving field theory : The pulsating filed is divided a forward and reverse rotating field Motor is started in the direction of forward rotating field this generates small (5%) positive slip Reverse rotating field generates a larger (1.95%) negative slip cont... Syafruddin

Each of the rotating fields induces a voltage in cont... Each of the rotating fields induces a voltage in the rotor, which drives current and produces torque. An equivalent circuit, similar to the equivalent circuit of a three phase motor, can represent each field The parameters of the two circuits are the same with the exception of the slip. The two equivalent circuits are connected in series. cont... Syafruddin

Figure 4 shows the equivalent circuit of a single cont... Figure 4 shows the equivalent circuit of a single phase motor in running condition. The current, power and torque can be calculated from the combined equivalent circuit using the Ohm Law The calculations are demonstrated on a numeri- cal example Syafruddin

Single Phase Induction Motor Figure 4 Equivalent circuit of a single-phase motor in running condition. Syafruddin

The results of the calculations are: Single-phase IM The results of the calculations are: - Input power: - Developed or output power: Syafruddin

Single Phase Induction Motor Figure 5 Single-phase motor mechanical output power and electrically developed power versus speed. Syafruddin

The single-phase motor starting torque is zero because of the pulsating single-phase magnetic flux. The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three phase motor. Two perpendicular coils that have currents 90° out-of- phase can generate the necessary rotating magnetic fields which start the motor. Therefore, single-phase motors are built with two perpendicular windings. Syafruddin

The axes of the two windings are 90o with respect to each other. Split-Phase Motor This machine is constructed with two separate stator windings, called main (run) and auxiliary (start) windings The axes of the two windings are 90o with respect to each other. cont… Syafruddin

cont… Vsource Irun The start winding is out of phase with the main (run) winding, as a result of the different reactances of the two windings. Once the motor started, centrifugal switch (CS) is used to disconnected the start winding Istart Isource Syafruddin

Starting torque The single-phase motor starting torque is zero because of the pulsating single-phase magnetic flux. The starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in a three-phase motor. Two perpendicular coils that have currents 90° out-of-phase can generate the necessary rotating magnetic fields which start the motor. Therefore, single-phase motors are built with two perpendicular windings. Syafruddin

why a single-phase single-winding IM produces no starting torque? The single winding of stator of a single phase IM is distributed in space so that space fundamental of mmf is the most dominant component of the actual mmf distribution. When the winding carries a sinusoidal current, it produces a sinusoidally space-distributed mmf whose peak value pulsates with time. The mmf at any time is F = Fpeak cos θ where θ is the angle measured from the winding axis Now Fpeak = Fmax cos ωt so that the mmf has both space and time distribution expressed as F = Fmax cos θ cos ωt F = ½ Fmax cos (θ-ωt) + ½ Fmax cos (θ+ωt) This equation tell us that pulsating single-phase field can be considered as superposition of two rotating field rotates at synchronous speed in opposite directions (Fig.2), the forward rotating field, Ff = ½ Fmax cos (θ- ωt) the backward rotating field, Fb = ½ Fmax cos (θ+ωt) Syafruddin

phase shift The phase shift is achieved by connecting a resistance, an inductance, or a capacitance in series with the starting winding. Most frequently used is a capacitor to generate the starting torque. Syafruddin

Capacitor-Start Motor Figure 6 shows the connection diagram of a motor using a capacitor to generate the starting torque. When the motor reaches the operating speed, a centrifugal switch turns off the starting winding. The centrifugal switch is necessary because most motors use a cheap electrolytic capacitor that can only carry ac current for a short period. Figure 6 Capacitor- start motor A properly selected capacitor produces around 90° phase shift and large starting torque. Syafruddin

Capacitor-Start Motor (cont..) By addition of a Capacitor in series with the start winding, Istart to lead Vsource . A properly selected capacitor produces around 90° phase shift between Irun and Istart. This results in a large starting torque and less source current. Syafruddin

Torque–speed characteristic of a small single-phase induction motor. Syafruddin

Permanent Split-Capacitor Lrun Rrun Lstart Rstart Syafruddin

Capacitor-Start Capacitor-Run Motor (CSCR) or Two-Value-Capacitor Motor Start capacitor Switch Run capacitor 120 V 50 Hz Rotor Run winding Start winding Syafruddin

Capacitor-Start Capacitor-Run Motor (cont…) The run capacitor is an oil-filled, low-value capacitor that gives the motor the run characteristics of the permanent-spilt capacitor motor. The start capacitor, being a high-value, ac electrolytic, provides high starting torque when the start switch is closed by a centrifugal switch actuator. Syafruddin

The advantage of a capacitor run over a capacitor start motor are: It has a better power factor (ii) It has more efficiency (iii) It has quieter and smoother operation (iv) It used for both easy and hard to start loads Syafruddin

Single-Phase shaded pole Motor The motor has two salient poles excited by ac current. Each pole includes a small portion that has a short-circuited winding. This part of the pole is called the shaded pole Figure Concept of single-phase shaded pole motor. cont… Syafruddin

Figure Shaded pole motor for household fan. cont… Figure Shaded pole motor for household fan. cont… Syafruddin

A less effective but more economical method using shaded pole motors cont… A less effective but more economical method using shaded pole motors The motor has two salient poles excited by ac current. Each pole includes a small portion that has a short-circuited winding. This part of the pole is called the shaded pole. The main winding produces a pulsating flux that links with the squirrel cage rotor. This flux induces a voltage in the shorted winding cont… Syafruddin

The induced voltage produces a current in the shorted winding. cont… The induced voltage produces a current in the shorted winding. This current generates a flux that opposes the main flux in the shaded pole (the part of the pole that carries the shorted winding). The result is that the flux in the unshaded and shaded parts of the pole will be unequal. Both the amplitude and the phase angle will be different. cont… Syafruddin

The motor efficiency is poor but it is cheap cont… These two fluxes generate an unbalanced rotating field. The field amplitude changes as it rotates. Nevertheless this rotating field produces a torque, which starts the motor in the direction of the shaded pole. The starting torque is small but sufficient for fans and other household equipment requiring small starting torque. The motor efficiency is poor but it is cheap Syafruddin

Universal Motor The universal motor is a rotating machine similar to a DC motor but designed to operate either from DC or single phase AC. The stator and rotor windings of the motor are connected in series through the rotor 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 or as a chopper drive The universal motor has a sharply drooping torque-speed characteristics of a DC series motor Typical applications is in vacuum cleaners, drills, and kitchen aplliances Syafruddin

Stepper Motors A Stepper motor is special type of synchronous motor which is designed to rotate a specific number of degrees for every electric pulse received by its control unit. Typical steps are 7.5 to 15o per pulse. It moves in accurate angular increments known as steps, in response to the application of digital pulses to the electric drive circuit. Two basic types of stepper motor (differing only in rotor construction) : - permanent-magnet type - reluctance-type (has a ferromagnetic rotor). Syafruddin

Motor Selection Criteria Available power (DC or AC) Operating condition Starting characteristics (torque and current) Operating speed Forward/reverse operation Acceleration characteristics (depending on load) Efficiency at rated load Syafruddin

Motor Selection Criteria (cont…) Overload capability Electrical and thermal safety Life span and maitenance Mechanical aspects(size, weight, noise level, environment) EMC and EMI Control complexity and cost Syafruddin