By the end of this chapter, you will be able to:
Mastering these skills will help you work confidently and safely with electrical machines, making you a valuable professional in the electrical trade.
Electrical machines are fundamental components in refrigeration and air conditioning systems, enabling mechanical work from electrical energy or vice versa. Understanding the types and operation of single-phase electrical machines is essential for technicians working in Kenya’s HVAC industry, where reliable and efficient equipment is critical for maintaining cold storage, air handling, and climate control in commercial and institutional settings. This chapter introduces single-phase electrical machines, focusing on their types, construction, and application within refrigeration and air conditioning systems commonly used in hospitals, hotels, and manufacturing plants across Kenya.
Single-phase electrical machines are widely used in refrigeration and air conditioning equipment, particularly where three-phase power supply is not available or practical, such as in small-scale commercial buildings and rural installations. These machines convert electrical energy into mechanical energy, driving compressors, fans, and pumps essential for system operation. Familiarity with the types of single-phase machines allows technicians to select, maintain, and troubleshoot equipment effectively, ensuring optimal performance and energy efficiency.
Single-phase induction motors are the most common type of electrical machines found in refrigeration and air conditioning systems, powering components like condenser fans and evaporator blowers. Their simple design and robustness make them suitable for a wide range of applications, especially in Kenya’s urban and peri-urban areas where single-phase power supply predominates.
The construction of a single-phase induction motor includes two main parts: the stator and the rotor. The stator consists of laminated iron cores with windings connected to the single-phase AC supply, creating a pulsating magnetic field. The rotor, typically squirrel-cage type, is free to rotate within the stator’s magnetic field. The absence of a rotating magnetic field in single-phase motors necessitates additional mechanisms to start the motor.
The motor operates on the principle of electromagnetic induction, where the AC supply generates a magnetic field in the stator that induces current in the rotor, producing torque. However, the single-phase supply creates a pulsating rather than rotating magnetic field, which alone cannot start the rotor turning. To overcome this, auxiliary starting means such as starting windings or capacitors are incorporated.
At a retail cold storage facility, single-phase induction motors drive evaporator fans that circulate cold air uniformly. The motor’s reliability and ease of maintenance ensure minimal downtime, critical for preserving perishable goods.
Split-phase motors are a subtype of single-phase induction motors widely used in small refrigeration compressors and air handling units due to their moderate starting torque and simple design.
Split-phase motors have two stator windings: the main (run) winding and the starting winding. These windings are placed 90 electrical degrees apart to create a phase difference. The starting winding has a higher resistance and lower inductance compared to the run winding, which causes the current in the start winding to lead the current in the run winding, generating a weak rotating magnetic field.
When the motor starts, both windings are energized. The phase difference between the two currents generates a starting torque that rotates the rotor. Once the motor reaches about 70-80% of its rated speed, a centrifugal switch or relay disconnects the starting winding to improve efficiency and prevent overheating.
In a small hotel’s air conditioning system in Mombasa, split-phase motors are commonly used to drive condenser fans, where the load is light and the supply is single-phase.
Capacitor motors provide better starting torque and running efficiency than split-phase motors, making them ideal for refrigeration compressors and larger air conditioning fans in both urban and rural Kenyan settings.
These motors include a capacitor in series with the starting winding, creating a larger phase shift between the start and run windings. This results in higher starting torque, which is essential for starting heavy loads such as refrigeration compressors. The capacitor is disconnected by a centrifugal switch once the motor reaches running speed.
In contrast, capacitor-run motors keep the capacitor in the circuit during both start and run conditions. This improves running efficiency and power factor, reduces noise, and provides smoother operation. They are preferred in continuous duty applications like air handling units in hospitals.
Some motors combine both features, using a start capacitor for high starting torque and a run capacitor to maintain efficiency during operation. This type is commonly found in commercial refrigeration systems requiring reliable and energy-efficient motor operation.
At a county hospital’s central refrigeration plant, capacitor-start motors power the main compressors, ensuring reliable start-up under heavy load conditions and efficient running performance.
Universal motors are series-wound machines that can operate on both AC and DC supplies. Their high speed and compact size make them suitable for small fans and portable refrigeration units, although their use is less common in fixed HVAC installations.
Universal motors consist of a stator and rotor with series-connected windings. The magnetic fields of the stator and rotor interact to produce torque. Because the current direction reverses simultaneously in both windings on AC supply, the motor maintains unidirectional torque, allowing it to operate efficiently on AC or DC.
Universal motors are found in portable refrigeration units used by field technicians or in small evaporator fan motors in certain compact air conditioning units within retail shops.
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Create a free accountThis chapter explored the fundamental types of single-phase electrical machines, highlighting their construction and operational principles. It examined DC machines, focusing on their role as both motors and generators, and the characteristics that make them suitable for specific applications. The discussion then shifted to AC single-phase motors and generators, explaining their working mechanisms and typical uses in various electrical systems. Transformers were analyzed in detail, emphasizing their function in voltage transformation and electrical energy distribution. The chapter concluded by illustrating practical applications of both AC and DC machines across different sectors, demonstrating how their unique properties are harnessed to meet diverse industrial and commercial needs.
Type: Individual
| Tools & Equipment | Materials |
|---|---|
| Digital multimeter | Single-phase induction motor (capacitor start) |
| Phase tester | Single-phase shaded pole motor |
| Insulation resistance tester (Megger) | Single-phase universal motor |
| Screwdrivers (flat and Phillips) | Single-phase synchronous motor |
| Insulated pliers | Note book and pen |
| Identification tags and markers |
| S/N | Item | Quantity |
|---|---|---|
| 1 | Single-phase induction motor (capacitor start) | 1 Pc per Candidate |
| 2 | Single-phase shaded pole motor | 1 Pc per Candidate |
| 3 | Single-phase universal motor | 1 Pc per Candidate |
| 4 | Single-phase synchronous motor | 1 Pc per Candidate |
| 5 | Digital multimeter | 1 Pc per Candidate |
| 6 | Phase tester | 1 Pc per Candidate |
| 7 | Insulation resistance tester (Megger) | 1 Pc per Candidate |
| 8 | Screwdrivers (flat and Phillips) | 1 set per Candidate |
| 9 | Insulated pliers | 1 Pc per Candidate |
| 10 | Safety gloves (electrical rated) | 1 pair per Candidate |
| 11 | Safety boots | 1 pair per Candidate |
| 12 | Safety goggles | 1 Pc per Candidate |
| 13 | Dust coat/overall | 1 Pc per Candidate |
| 14 | Work bench | 1 Pc per Candidate |
| 15 | Identification tags and markers | 1 set per Candidate |
| 16 | Note book and pen | 1 set per Candidate |
| Items to be Evaluated | Marks Available | Marks Obtained | Comments |
|---|---|---|---|
| TASK 1: Safety and Preparation | |||
| Wore appropriate PPE including dust coat, safety boots, gloves, and goggles (Award 1 mark for wearing all PPE correctly or zero) | 1 | ||
| Prepared work area and ensured all tools and machines were available and properly arranged (Award 1 mark for proper setup or zero) | 1 | ||
| Sub-Total | 2 | ||
| TASK 2: Identification of Electrical Machines | |||
| Physically inspected each single-phase machine for nameplate data and construction features (Award 1 mark for each correctly inspected machine or zero) | 2 | ||
| Used digital multimeter and phase tester to verify winding continuity and phase presence (Award 1 mark for each correct test or zero) | 2 | ||
| Performed insulation resistance test on each machine using Megger (Award 1 mark for each correct test or zero) | 2 | ||
| Sub-Total | 6 | ||
| TASK 3: Classification and Reporting | |||
| Classified each machine correctly as capacitor start, shaded pole, universal, or synchronous motor (Award 1 mark for each correct classification or zero) | 3 | ||
| Explained operational characteristics and typical applications for each machine (Award 1 mark for each explanation or zero) | 3 | ||
| Prepared a clear and concise report summarizing identification, classification, and characteristics (Award up to 4 marks based on clarity, completeness, and accuracy) | 4 | ||
| Sub-Total | 10 | ||
| PRODUCT CHECKLIST | |||
| Report includes correct identification and classification of four single-phase machines (Award 1 mark per correctly identified and classified machine or zero) | 4 | ||
| Report contains accurate descriptions of operational principles and typical applications (Award 4 marks for accuracy and completeness or zero) | 4 | ||
| Sub-Total | 8 | ||
| GRAND TOTAL | 26 | ||
Type: Individual
| Tools & Equipment | Materials |
|---|---|
| Screwdrivers (flat and Phillips) | DC motor stator core |
| Allen keys set | Armature with commutator |
| Pliers | Brush holder assembly with carbon brushes |
| Wire brush | End shields (front and rear) |
| Multimeter | Shaft with key |
| Bearing set | |
| Machine screws and nuts (M5 x 20mm) | |
| Insulation tape |
| S/N | Item | Quantity |
|---|---|---|
| 1 | DC motor stator core | 1 Pc per Candidate |
| 2 | Armature with commutator | 1 Pc per Candidate |
| 3 | Brush holder assembly with carbon brushes | 1 Pc per Candidate |
| 4 | End shields (front and rear) | 1 Set per Candidate |
| 5 | Shaft with key | 1 Pc per Candidate |
| 6 | Bearing set | 1 Set per Candidate |
| 7 | Machine screws and nuts (M5 x 20mm) | 6 Pcs per Candidate |
| 8 | DC power supply 12V 5A | 1 Pc per Work Station |
| 9 | Multimeter | 1 Pc per Candidate |
| 10 | Screwdrivers (flat and Phillips) | 1 Set per Candidate |
| 11 | Allen keys set | 1 Set per Candidate |
| 12 | Pliers | 1 Pc per Candidate |
| 13 | Wire brush | 1 Pc per Candidate |
| 14 | Insulation tape | 1 Roll per Candidate |
| 15 | Work bench | 1 Pc per Work Station |
| 16 | Protective gloves | 1 Pair per Candidate |
| 17 | Safety goggles | 1 Pair per Candidate |
| 18 | Dust coat/overall | 1 Pc per Candidate |
| Items to be Evaluated | Marks Available | Marks Obtained | Comments |
|---|---|---|---|
| TASK 1: Assembly and Testing of DC Motor | |||
| Wore appropriate PPE (dust coat, gloves, safety goggles) (Award 1 mark for correct PPE or zero) | 1 | ||
| Handled tools and equipment safely and correctly (Award 1 mark each for correct tool use and safety observed) | 2 | ||
| Assembled stator, armature, bearings and shaft as per specifications (Award 1 mark each for correct assembly of stator, armature, bearings, shaft) | 4 | ||
| Fitted brush holder assembly and secured carbon brushes properly (Award 1 mark each for fitting brush holder and brush positioning) | 2 | ||
| Mounted end shields correctly and tightened machine screws securely (Award 1 mark each for correct mounting and tightening of screws) | 3 | ||
| Inspected commutator and cleaned with wire brush (Award 1 mark for proper cleaning or zero) | 1 | ||
| Connected motor terminals to DC power supply correctly (Award 1 mark each for correct polarity and secure connections) | 2 | ||
| Performed operational test checking motor rotation and current draw (Award 1 mark each for motor rotation, correct direction, and current within rated limits) | 3 | ||
| Isolated power supply and performed safe disassembly (Award 1 mark for correct isolation and safe handling) | 1 | ||
| Performed housekeeping: cleaned tools, workspace and stored equipment (Award 1 mark for proper housekeeping or zero) | 1 | ||
| Sub-Total | 20 | ||
| PRODUCT CHECKLIST | |||
| Assembled DC motor matches drawing dimensions: length 150mm, shaft diameter 12mm (Award 3 marks if all dimensions conform or zero) | 3 | ||
| Motor runs smoothly without unusual noise or vibration (Award 3 marks for smooth operation or zero) | 3 | ||
| Brushes make firm contact with commutator with no sparking observed (Award 2 marks for proper brush contact or zero) | 2 | ||
| Electrical connections are secure and insulated properly (Award 2 marks for secure and insulated connections or zero) | 2 | ||
| Sub-Total | 10 | ||
| GRAND TOTAL | 30 | ||
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