Troubleshooting
44 Common Issues
This chapter will be focusing on troubleshooting common problems found in the power and control circuits. Troubleshooting motors and the mechanical systems they drive is an extensive topic beyond the scope of this text.
A motor may not start for a variety of reasons. Knowing what to check for and where to begin is a valuable skill that can reduce downtime when a fault occurs.
Usually, there are two fundamental steps in troubleshooting. The first is a cursory visual inspection, followed by a more in-depth examination with testing equipment and referencing several diagrams
Motor control circuits have several protective devices built into them to protect wires and equipment. It is always a good idea to start your visual inspection by checking overload and overcurrent devices. A visual inspection sometimes quickly locates a fault or identifies situations requiring maintenance. For example, you might see a tripped indicator on a circuit breaker, switch or overload relay. Resetting the device might be all the circuit needs to resume working.
If an overload or overcurrent situation has occurred, it is advisable to find the source of this fault. A visual inspection can identify burnt or heat-discoloured components, loose wires in terminal blocks, or mechanical parts that bind together or cause excessive friction.
When there is no obvious cause for the problem(s), it is often necessary to analyze circuit diagrams and take measurements of voltage and resistance at key points of the circuit. To do this, we use voltmeters and ohmmeters. A firm understanding of their use and limitations is necessary for their safe operation in circuits.
In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
An automatic device that is designed to safely disconnect circuits under fault conditions. Most circuit breakers provide Overload and Overcurrent protection, and are rated in Volts, Amps and Horsepower.
A device for making or breaking the connection in an electric circuit.
A heater element paired with normally-closed contacts that open once the heater gets too hot. Two types of relays are the bimetallic strip and the melting solder pot.
A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.
A sharp and fast rise in current over a short period of time (fractions of a second) where the value of current is far greater than the nominal line current.
A diagram that uses lines and symbols to show the electrical continuity and connections of a circuit. Can be used either for troubleshooting (schematic diagrams) or connection purposes (wiring diagrams).
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).
The opposition to the flow of current in an electric circuit, measured in ohms (Ω).
A device testing and measuring the potential difference (voltage) between two points. Leads are connected in parallel with the circuit, and the meters very high internal resistance will draw a small current which can be used to determine the level of voltage.
Can be digital or analogue and measure either AC or DC.
A device used to measure the resistance of a circuit. Ohmmeters must not be used on live circuits. Ohmmeters connect a small internal voltage source to the circuit that is being measured or tested, and determine the value of resistance or continuity by measuring what value of current flows through the meter.
Can be either digital or analogue.