In order to avoid unexpected breakdowns, costly repairs and subsequent losses due to motor downtime, it is important that the motor is fitted with some sort of protective device. Generally speaking, motor protection can be divided into the following 3 levels:
• External protection against short circuit in the whole installation. External protection device is normally different types of fuses or short circuit relays. This kind of protection device is compulsory and legal and placed under safety regulations.
• External protection against overload of specific equipment; i.e. to avoid overload of pump motor and thereby prevent damage and breakdown of the motor. This type of protection reacts on current.
• Built-in motor protection with thermal overload protection to avoid damage and breakdown of motor. The built-in protector always require an external circuit breaker while some built-in motor protection types even require an overload relay.
What are the fault conditions ?
A wide range of faults can occur different places in the application. Therefore, it is important to anticipate the cause of events, and protect the motor against obstacles in the best possible way. What follows is a list of the most common fault conditions where motor damage can be avoided by some sort of motor protection.
• Problems with the power supply quality:
– Imbalanced voltages/currents
– Frequency variation
• Installation, supply & motor failures
• Slowly developing temperature rise:
– Insufficient cooling
– High ambient temperature
– High altitude operation
– High liquid temperature
– Too high viscosity of the pumping liquid
– Frequent starts
– Too big load inertia
– (not common for pumps)
• Quickly developing temperature rises:
– Locked rotor
– Phase breakage
To protect a circuit against overloads and short circuits, a circuit protective device must determine when one of these fault conditions occurs. It must then automatically disconnect the circuit from the power source. A fuse is the simplest device for accomplishing these two functions. Normally fuses are built together by means of a safety switch, which can switch off the circuit. On the following pages, we will present three types of fuses as to their function and to where they are used: Fusible safety switch, “quick-acting” fuse and “time-lag” fuse
1.Fusible safety switch
A fusible safety switch is a safety switch, which is combined with a fuse in a single enclosure. The switch manually opens and closes the circuit, while the fuse protect against overcurrent protection. Switches are generally used in connection with service when it is necessary to cut off the current, or in connection with fault situations. The safety switch is a switch, which is placed in a separate enclosure. The enclosure protects personnel against accidental exposure to electrical connections and against exposure to weather conditions. Some safety switches come with
a built-in function for fuses, and some safety switches come without built-in fuses containing only a switch. The overcurrent protection device (fuse) has to recognise the difference between overcurrent and short circuit. Slight overcurrents for example, can be allowed to continue for a short period of time. But as the current magnitude increases, the protection device has to react quickly. It is important to interrupt short circuits immediately. The fusible disconnect switch is an example of a device which is used for overcurrent protection. Properly sized fuses in the switch open the circuit when an overcurrent condition occurs.
A circuit breaker is an overcurrent protection device. It opens and closes a circuit automatically at a predetermined overcurrent. When the circuit breaker is applied correctly within its rating, opening and closing the circuit breaker
does not damage it. It is easy to reactivate the circuit breaker immediately after a overload has occurred. The circuit
breaker is simply reset after the fault is corrected. We distinguish between two kinds of circuit breakers: Thermal and magnetic circuit breakers.
Thermal circuit breakers
Thermal circuit breakers are the most reliable and cost-effective type of protection device that exists and are well-suited for motors. They can withstand high-level current waves, which arise from motor starts and they protect the motor against failure e.g. locked rotor.
Magnetic circuit breakers
Magnetic circuit breakers are precise, reliable and cost-effective. The magnetic circuit breaker is stable temperature-wise, meaning that it is rarely affected by changes in the ambient temperature. Compared to thermal circuit breakers, magnetic circuit breakers offer a more precise trip time. The illustration on your right-hand side shows the characteristics of the two types of circuit breakers.
• Make it possible for the motor to handle harmless temporary overloads without interrupting the circuit, i.e. motor starting.
• Trip and open a motor circuit, if the current exceeds its limits and might damage the motor.
• Are reset either automatically or manually once the overload situation has passed.
Advanced external motor protection relays
More advanced external motor protection systems can also protect against overvoltage, phase imbalance, too many starts/stops, vibrations, PT100 temperature monitoring of stator and bearings, insulation resistance and monitor ambient temperature. Further, advanced external motor protection systems are able to handle the signal from
built-in thermal protection.