Circuit Breakers
Circuit breakers are the first line of defense against motor short circuits. When the short circuit happens, the current suddenly increases, possibly to several tens of times or even higher than the normal working current. Such high currents can immediately burn out the motor windings or seriously damage other electric equipment. A suitable circuit breaker installed in time can cut off the power in time when a short circuit occurs, thus protecting the motor and electrical system.
Three-phase circuit breakers should be able to sense the short circuit current. Since the short circuit current rises very rapidly, the breaker needs to act fast. Many modern breakers would have instantaneous short circuit protection, which trips at and cuts the power when the current reaches some pre-set value. In addition to this, circuit breakers also should have overload protection. When this current surpasses the rated current of the motor over a long period, it may not cause immediate damage to the equipment. However, the high-temperature rise of the motor may lead to winding insulation aging, damage, and even a short circuit.
Be chosen according to the values of rated current and short circuit current, in addition to the power of a motor. The rated current of the breaker is always about 20% higher than that of the motor, so it can protect the motor without any tripping when there is normal fluctuation of loads in the motor.
Regular checks and tests should also be performed on the circuit breaker to ensure that the performance is normal. In practical applications, the trip time of the circuit breaker is usually controlled within the range of between 0.02 seconds and 0.1 seconds, effectively preventing the hazards of short circuits.
Overload Relays
Apart from the possibility of short circuits, motors are also in danger of overload. Some overload relays may monitor the current of a motor; once the current goes over the safe range, the relay will automatically disconnect the circuit for the protection of the motors.
Overload relays can be divided into thermal relays and electronic relays. A thermal relay generally works on the principle of thermal expansion of a bimetallic strip. When there is too much current through the motor, due to the heat generated by the current, it bends the bimetal plate, which acts to trip the relay. Although relatively cheap, thermal overload relays have slow response speed and are heavily affected by the ambient temperature.
On the contrary, the operation of an electronic relay is faster and more precise. It can track the current real-time change and automatically trip with the set current value and time curve. Usually, more demanding industrial equipment adopts an electronic relay in order to provide more reliable protection.
In motor protection, the former application field of thermal relays is for small motors, and electronic relays are more suitable for large motors. Particularly in the process of motor starting, since the current surges for a moment, an electronic relay may automatically regulate the trip time according to the rising curve of the current in order to avoid spurious operation.
Fuses
Fuses are another type of important short circuit protection device applied commonly as secondary protection for motors. The role of a fuse is to melt its internal wire when, due to heat produced in the current exceeding its rated value in the circuit, the circuit gets cut off, and further damage from the short circuit current is prevented.
In the selection of the fuses, the rated current is chosen according to the rated current of the motor, usually a little bigger than the maximum working current of the motor. Besides, different fuses have different speeds for melting; slow-blow fuses are suitable for handling such large instantaneous current at motor startup, while fast-blow fuses have good performance in protecting the motor windings and electronic components against short circuit impacts.
The ambient temperature affects the melting characteristic of the fuse. The fuses will also be more prone to blowing at higher temperatures; therefore, for high-temperature environments, higher-rated fuses may be applied. For high-risk environments, fuses and circuit breakers together can implement dual protection to enhance motor safety further.
Ground Fault Protection
One of the most common causes of motor short circuits is indeed ground faults, especially under harsh environmental conditions, such as damp or dusty locations, where insulation tends to readily age or get damaged and hence often causes a shortage of motor windings or wiring to the ground.
This generally employs ground fault relays or ground leakage protection switches that can detect ground current and disconnect the circuit upon detection of an abnormality. Generally speaking, national standards have called for the tripping time of ground fault protection to be within 0.3 seconds in order to minimize damage to electrical equipment and reduce personal injuries.
In practical applications, a sensitivity adjustment function should also be added to this type of ground fault protection device in order to cater to various kinds of motors and different environments.
Surge Protectors
Short circuit problems could be caused by voltage surges or instantaneous voltage spikes in the power grid. Surge protectors effectively absorb and dissipate voltage spikes, thereby avoiding surge voltage breakdown or short motor windings.
The main function of surge protectors is voltage spike absorption. In case of surge voltage appearance in power electric networks, a varistor of the surge protector quickly conducts and releases excessive energy of voltage to the ground, saving motor windings from destruction. Surge protectors in their modern version are based on composite materials and can bear multi-impact influences of surges without destruction.
The installation location is very important, usually, it is installed at the power inlet or parallel to motor control equipment. In use, surge protectors must be inspected and changed out regularly to ensure effectiveness, especially in areas where the electrical grid is commonly disrupted by thunderstorms in which the surge protector plays an especially vital role.
Proper Grounding
Basic precautions against motor short circuits include proper grounding. A well-designed grounding system efficiently spreads fault currents, hence preventing the occurrence of short-circuiting or electric shock to motors and electrical equipment due to current accumulation.
In the design of the grounding system, the consideration of low-resistance grounding is very important. The grounding resistance shall be controlled to be within the range formulated by the national standard, normally no more than 4 ohms, in order to guarantee that the fault current can be quickly and smoothly discharged to the ground. The cross-section and material of the grounding wire shall be selected according to the power of the motor and the environment in which it is used, ensuring that it can bear short circuit currents without overheating or melting.
The grounding systems also require periodic testing, especially for areas with high humidity or during the rainy seasons, where the grounding resistance increases, thereby reducing the effectiveness of grounding and increasing the chances of motor short circuits.
Soft Starters and Frequency Inverters
Modern industries use soft starters and frequency inverters, which protect at a higher level. During the startup, a soft starter can effectively control the motor’s current so that the instantaneous high current does not damage the windings in the motor. Frequency inverters monitor and protect current and voltage much better.
The soft starter, during motor startup, gradually ramps up the voltage in a soft way. This makes the motor start its running easily and hence avoids electrical surges that may further cause a short circuit or winding damage. In addition, under heavy-load startup conditions, the importance of a soft starter is very vital due to the fact that it reduces fluctuation in current and prolongs the life span of the motor.
Frequency inverters are capable of not only controlling the speed of a motor but also carrying out complete protection functions for such motors: short circuits, overload, overvoltage, and undervoltage protections. Once a short circuit or a current abnormality has been detected, the frequency inverter immediately stops the motor and sends a fault alarm to prevent further damage.
Soft starters and frequency inverters provide protection functions that are usually more sensitive and accurate when compared to those that conventional circuit breakers and relays may provide. For this reason, they are particularly suited for the most demanding industrial applications. Their protection functions, however, are effective only to the extent that the soft starters and frequency inverters are serviced regularly and their parameters adjusted to the actual conditions of the motor load.
