Th e capacitor-start-and-run motor is sometimes diffi cult to troubleshoot because of the number of components that must be added to a regular motor to produce it. Th e windings, bearings, potential relays, starting capacitor and running capacitor must all be checked.
Th e windings of a CSR motor can be checked easily using an insulation tester (megger) to determine if the windings are shorted, open, or grounded. In most cases, the windings will be enclosed in a casing and the terminals will be on the outside of the casing. However, the type of motor makes little diff erence in checking the winding, as long as the technician uses the correct terminals.
To ensure that the motor is in good condition, there are two basic inspections (tests) that must be done. Th e one is a visual inspection (mechanical test) and the other is an electrical inspection (test). Th e visual inspection is done by looking at the motor and inspecting its overall condition, whereas an electrical inspection involves the use of an insulation tester (megger) to do some electrical tests.
• Visual inspection test
Th e fi rst and very important test is the visual inspection. Th ere are two types of visual inspection tests that can be done: fi rstly, the electrical inspection and secondly, the mechanical inspection.
Visual electrical inspection:
– Check if electrical cables are isolated and the cover plates holding the terminals are sealed.
Visual mechanical inspection:
– Check the mounting of the motor. Make sure all bolts are tight and there is no play in belts or pulleys.
– Check condition of the rotor and shaft : key way, front bearing and back bearing.
– Check condition of motor frame and condition of the terminal box.
– Check front/back-end shield, stator/fi eld housing, condition of cooling fan and fan cover and cooling fi ns
Once the motor has been disconnected and removed from its position, the following three electrical tests can be done: continuity test, insulation test and the earth continuity test.
• Th e continuity test
Th is test is performed using an insulation tester or continuity tester. It determines the continuity of the conductors as well as the condition of the capacitors. Th e value of the test should be low. Th e number of windings and the size of the motor will infl uence the value of the reading.
• Insulation test
Th is test has TWO components. One must fi rst determine the value of insulation between the conductors. One then determines the insulation between the conductors and the capacitors. Th is test is performed using an insulation tester.
Th e values should be high to infi nity (1 MΩ or higher).
• Earth continuity test
Th is test is performed using an insulation tester or continuity tester. It determines the continuity of the earth wire to the distribution board or earth spike. Th is value should be high to infi nity (1 MΩ or higher). It also tests if there are any leakages/shorts between the windings and earth.
Activity 1
1. Name 4 types of single-phase induction motors.
2. Name 4 methods used to generate a rotational magnetic fi eld in a phase motor.
3. Describe the principle of operation of a CSM motor.
4. Name 4 tests to be performed on a single-phase motor.
5. Give an application for a split-phase motor.
6. Draw a neatly labelled diagram of a CSR motor.
7. Describe how you will go about faultfi nding on a single-phase induction motor.
8. Explain in your own words how you will change the direction of rotation of a capacitor-start induction motor.
9. Why is it important to do the electrical testing of a motor only aft er it has been disconnected ?
10. What is the importance of the earth insulation test?
Take note Th ese three electrical tests must be done in the correct order as given in the text.
Practical 1
Testing of a capacitor-start motor Form of activity: Testing
Material and Equipment:
• Insulation tester/megger
• Capacitor-start motor
• Single-phase supply Instructions:
1. Teacher will provide learner with the single-phase motor to be tested.
2. Learners to ensure they select the correct settings on the insulation tester (megger).
3. Learners to use the insulation tester to take the necessary readings and record these in the table provided.
A1 – A2 A2 – B2 B1 – Cb
A1 – B1 A2 – Ca B2 – Ca
A1 – Ca A2 – Cb B2 – Cb
Ar – Cb B1 – B2 Ca – Cb
A2 – B2 B1 – Ca
Continuity readings (take A as main winding and B as start winding) Component Reading Conclusion
A1 – A2 B1 – B2 Cap a – Cap b
Insulation readings between components Component Reading Conclusion A1 – B1
A1 – Cap a B1 – Cap a
Insulation readings to earth
Component Reading Conclusion A1 – Earth
B – Earth
Mechanical/Visual inspection of motor
Condition of rotor and shaft
Component Report on condition
Key way Front bearing Back bearing
Condition of motor frame Condition of terminal box
Flange/foot mount Front/back-end shield Stator/fi eld housing
Mounting bolts and nuts/screws Condition of cooling fan Fan cover and cooling fi ns
4. Describe how the rotational direction of this motor can be changed.
Practical 2
Wire a direct-on–line motor control circuit Form of activity: Simulation
Material and equipment:
• 1 × motor control board
• Connecting wires
• 1 × contactors with auxiliary contacts and overloads (or separate overloads)
• 1 × stop button
• 1 × emergency stop button (press type)
• 1 × start button
• 1 × single-phase motor Instructions: Wiring of circuits
Wire the equipment according to the given diagrams. Do NOT switch on the power supply before your TEACHER has checked the circuits. First build the control circuit and then ask your teacher to check and assess. Only when the control circuit is correct, may you start with the main circuit.
Control circuit:
• Correct the fi rst try, 10 marks
• Correct the second try, 5 marks
• Correct the third try, 2 marks
Main circuit:
• Correct the fi rst try, 10 marks
• Correct the second try, 5 marks
• Correct the third try, 2 marks
Control circuit for DOL motor
Main (power) circuit for DOL motor
Live
Stop button
Start button NO
MC 1
Main contactor MC 1
Overload Neutral
L N
MC 2 Overload
Single phase motor Single-phase motor
RCL
Resonance Capacitive
reactance
A B
Phasor diagrams Series RLC circuit
A B
Introduction
Up to now the focus has been on resistors connected in series, parallel and combination networks. Ohm’s law was investigated and it was proven that the current is always directly proportional to voltage, but inversely proportional to resistance. And although that is completely true for pure resistive circuits, it is not always true for circuits containing coils (inductors) and capacitors. Th e focus now shift s to the eff ect that these components will have on a circuit when connected to an AC supply. Th e components that will be dealt with are the resistor, the coil and the capacitor.