Switches/Motors Flashcards
What is the main type of fault the HRC fuses are designed to provide protection for?
Short-circuit current
You have interchanged two of the supply phase connections to a motor, so the motor direction can be reversed. What type of motor are you reversing?
Three-phase induction motor
You have interchanged the connection to one of the windings so the motor direction can be reversed. What type of motor are you reversing?
Single-phase, split-phase motor
Single-phase, capacitor-start motor
A 400V three-phase motor is supplied from a final sub circuit protected by MCBs. The motor is required to be isolated but the isolater is not located adjacent to the motor.
Isolation has been confirmed by using the prove-test-prove method. State TWO actions either of which can be taken to ensure that the motor remains isolated.
Lock the isolater in the off position.
Open and lock the MCBs
Disconnect the cables at the load end of the conductor.
Disconnect the cables at the switchboard.
What is the main type of fault the thermal overload is designed to provide protection for?
Overload
A switch controls two heating elements of the same resistance of an electric blanket. The switch has three positions - low, medium and high.
a) What type of fault would cause the blanket to draw current only in the high and medium positions? The switch is not faulty.
b) What type of fault would cause the blanket to draw current only in the medium and low positions?
a) One element is open circuited.
b) There is no fault.
A three-phase electric motor with a flexible cord and plug attached is to be tested before being returned to service.
a) State the minimum accepted value for the insulation resistance between the windings of the motor.
b) State the maximum acceptable value for the resistance of the protective earthing conductor of the motor.
a) 1 Mega Ohm
b) 1 Ohm
You have interchanged two of the supply phase connections to a motor, so the motor can be reversed. What type of motor are you reversing?
Three phase induction motor.
A motor consists of two windings.
You have interchanged the connection to one of the windings so the motor direction can be reversed. What type of motor are you reversing?
Single-phase, split phase motor.
Or, single phase, capacitor-start motor.
A three-phase induction motor is protected by a thermal overload in a DOL starter.
State the reason why HRC fuses are installed at the origin of the final subcircuit supplying the motor.
To protect the final sub circuit and the motor against excessive (fault) current conditions.
A three-phase motor controlled by a DOL starter has been operating normally.
The motor is re-started and when the start button is pushed on the DOL starter the contactor closes but as soon as the start button is released again the contactor drops out of the circuit.
State the cause of this fault.
The maintaining circuit is open circuited.
A three-phase motor controlled by a DOL starter drives a wood planer. The protective device in the motor starter has tripped due to an overload. State TWO conditions that could cause an overload.
Note: the wood planer is not jammed.
Loss of one phase.
Imbalance in the winding resistance.
State the most common method used to vary the speed of a universal motor.
By varying the supply voltage.
A three-phase induction motor, controlled by a star/delta starter, is running.
State TWO types of protection devices, either of which will detect loss of one phase and open the motor circuit.
Either;
Phase failure relay.
Thermal overload.
Thermistor relay.
The connections to the FIELD winding in a motor are changed. What type of motor is being reversed?
A universal motor.
