3.7 Types of Single-Phase Electrical Machines

Single-phase electrical machines are widely used in refrigeration and air conditioning equipment due to their compatibility with the common single-phase power supply available in most Kenyan commercial and residential settings. These machines convert electrical energy into mechanical energy or vice versa, enabling the operation of critical components like compressors and fans. Their selection depends on factors such as starting torque, efficiency, and load characteristics.

3.7.1 Construction and Working Principles of Single-Phase Motors

Single-phase motors have a relatively simple construction but require specific arrangements to start and run efficiently in refrigeration and air conditioning applications. The fundamental components include the stator, rotor, and auxiliary windings, each designed to create a rotating magnetic field from a single-phase supply. The working principle revolves around the interaction between magnetic fields generated by the stator windings and the rotor, producing torque.

Stator and Rotor Components

• Stator Core and Windings: The stator consists of laminated steel sheets to reduce eddy current losses and carries the main and auxiliary windings. The main winding is connected directly to the power supply, while the auxiliary winding provides the phase difference needed for starting torque.
• Rotor Types: Common rotor types include squirrel cage and wound rotors. The squirrel cage rotor is robust, simple, and widely used in refrigeration compressors, providing reliable operation.
• Starting Mechanisms: Since a single-phase supply does not create a rotating magnetic field naturally, auxiliary windings with capacitors or shaded poles are used to generate a phase shift, enabling the motor to start.
• Enclosure: Motors are typically enclosed in dust and moisture-resistant casings to protect against harsh environments often encountered in refrigeration plants.
• Bearings and Shaft: Bearings support the rotor shaft, ensuring smooth rotation, and the shaft transmits mechanical power to the compressor or fan.

Working Principle

• The single-phase supply energizes the main winding, producing an alternating magnetic field.
• Auxiliary winding, often connected through a capacitor, creates a phase shift, producing a second magnetic field out of phase with the main field.
• The interaction of these fields produces a rotating magnetic field, inducing current in the rotor.
• The rotor current generates its own magnetic field, which interacts with the stator field, producing torque and causing the rotor to turn.
• Once the motor reaches near synchronous speed, the auxiliary winding is often disconnected by a centrifugal switch or relay to improve efficiency.

3.7.2 Capacitor Start and Capacitor Run Motors in Refrigeration Systems

Capacitor motors are prevalent in refrigeration and air conditioning for their high starting torque and efficient running characteristics. These motors use capacitors to improve phase displacement between the main and auxiliary windings, enhancing performance during start-up and continuous operation.

Capacitor Start Motors

• High Starting Torque: The start capacitor creates a large phase shift, generating a strong starting torque necessary for compressors in refrigeration units that require considerable force to overcome initial inertia.
• Start Capacitor Characteristics: These capacitors are electrolytic, designed for short-term use during motor start-up, and disconnected after the motor reaches about 75% of its rated speed.
• Centrifugal Switch Operation: A mechanical switch disconnects the start capacitor and auxiliary winding from the circuit once the motor accelerates, preventing capacitor damage and improving efficiency.
• Applications: Widely used in small to medium refrigeration compressors and fan motors, especially in commercial refrigeration units.
• Limitations: The start capacitor must be properly rated and maintained to prevent failure, which would result in motor starting difficulties or overheating.

Capacitor Run Motors

• Continuous Capacitor Usage: Unlike capacitor start motors, capacitor run motors use a run capacitor connected in series with the auxiliary winding during the entire operation, improving power factor and efficiency.
• Improved Running Performance: The run capacitor provides a continuous phase shift, reducing current draw and electrical noise, which is beneficial for sensitive refrigeration control systems.
• Durability: Run capacitors are oil-filled or film-type capacitors designed for continuous duty, making these motors more reliable in harsh refrigeration environments.
• Applications: Common in high-efficiency fans and blower motors within air conditioning units and heat pumps.
• Energy Efficiency: These motors consume less power during operation, contributing to lower electricity bills in commercial establishments like hotels and hospitals.

3.7.3 Split-Phase Motors and Their Role in Air Conditioning Equipment

Split-phase motors are among the simplest single-phase motors, providing moderate starting torque and suitable for applications where starting load is not excessively high. Their design and operation principles make them cost-effective choices for certain refrigeration and air conditioning components.

Construction Features

• Main and Auxiliary Windings: The motor contains two windings positioned 90 electrical degrees apart on the stator; the auxiliary winding is designed to have higher resistance and lower inductance.
• No Capacitor Required: Unlike capacitor motors, split-phase motors rely on the inherent resistance difference to create a phase shift between the windings.
• Centrifugal Switch: This switch disconnects the auxiliary winding once the motor reaches about 70-80% of its rated speed, reducing power loss.
• Rotor: Typically a squirrel cage type, ensuring robustness and minimal maintenance.
• Enclosure and Cooling: Motors are housed in ventilated enclosures suitable for the ambient temperatures found in refrigeration rooms and air handling units.

Operational Characteristics

• Starting Torque: Provides moderate starting torque, sufficient for fan blades and small compressor motors in air conditioning units.
• Starting Current: Higher starting current compared to capacitor motors, which may cause voltage dips in weak electrical installations.
• Efficiency: Lower running efficiency than capacitor run motors, leading to higher operational costs in continuous duty applications.
• Applications: Commonly used in smaller refrigeration systems, such as domestic refrigerators and air conditioning fan motors where starting load is manageable.
• Maintenance Considerations: The centrifugal switch is a mechanical component that requires periodic inspection to prevent failure, which can lead to motor overheating.

3.7.4 Shaded Pole Motors and Their Use in Auxiliary Refrigeration Devices

Shaded pole motors are simple single-phase motors characterized by low starting torque and simple construction, making them suitable for auxiliary applications in refrigeration and air conditioning systems where load demands are minimal.

Construction Specifics

• Shaded Pole Design: Each stator pole has a copper ring (shading coil) that delays the magnetic flux in part of the pole, creating a weak rotating magnetic field.
• Rotor: Squirrel cage type, simple and robust, with low inertia suitable for low-power applications.
• Enclosure: Usually open or semi-enclosed, designed for low power dissipation environments.
• Capacitors and Starting Aids: Not required, simplifying the motor design and reducing cost.

Operational Attributes

• Low Starting Torque: Typically 25-30% of full load torque, restricting their use to low-load fans and auxiliary pumps in refrigeration plants.
• Reliability: Very reliable due to absence of starting switches or capacitors, suitable for continuous duty in stable conditions.
• Efficiency: Low efficiency compared to other single-phase motors, but acceptable where power consumption is not a critical factor.
• Applications: Used in evaporator fans, small condensate pumps, and ventilation fans in air conditioning systems for hospitals and office buildings.
• Cost-effectiveness: Very economical for light-duty applications, reducing capital expenditure in auxiliary device installations.

Chapter Summary

This chapter explored various types of single-phase electrical machines, highlighting their construction and operational principles. It then examined DC machines, focusing on their components and modes of operation. The study progressed to AC single-phase motors and generators, explaining their working mechanisms and practical uses. Transformers were discussed in terms of their function in voltage regulation and power distribution. The application of both AC and DC machines was analyzed, emphasizing their roles in different electrical systems. The chapter also covered types of earthing, detailing their importance in safety and system protection. Identification of earthing points on electrical installations was explained to ensure proper grounding practices. Finally, methods for testing earthing systems were presented to verify their effectiveness and compliance with safety standards.