Why motor protection is necessary? And types of fuses used in motor protection
Explanation about protection provided to induction and other types motor are given below:
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.
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 list of the most common fault conditions where motor damage can be avoided by some sort of motor protection:
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 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:
- Overvoltage
- Undervoltage
- 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.
Motor protection relays (overload relays)
Overload relays are used to protect electrical equipment, such as 3-phase AC motors and
transformers, against excessive temperature rise and measure the current to determine the
temperature-rise and danger to the object to be protected. Protective shutdown is performed via the motor switchgear – usually a contactor.
transformers, against excessive temperature rise and measure the current to determine the
temperature-rise and danger to the object to be protected. Protective shutdown is performed via the motor switchgear – usually a contactor.
1) Thermal motor protection relays
Principle of operation
Thermal motor protection relays contain three bimetal strips together with a trip mechanism in a housing made of insulating material. The bimetal strips are heated by the motor current, causing them to bend and activating the trip mechanism after a certain travel which depends on the current-setting of the relay. The release mechanism actuates an auxiliary switch that breaks the coil circuit of the motor contactor (Fig. 4.2-10). A switching position indicator signals the condition “tripped”.
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| Principle of operation of a three pole thermally delayed bimetal motor protection relay with temperature compensation |
A = Indirectly heated bimetal strips
B = Trip slide
C = Trip lever
D = Contact lever
E = Compensation bimetal strip
B = Trip slide
C = Trip lever
D = Contact lever
E = Compensation bimetal strip
The bimetal strips may be heated directly or indirectly. In the first case, the current flows directly through the bimetal, in the second through an insulated heating winding around the strip. The insulation causes some delay of the heat-flow so that the inertia of indirectly heated thermal relays is greater at higher currents than with their directly heated counterparts. Often both principles are combined. For motor rated currents over approx. 100 A, the motor current is conducted via current transformers. The thermal overload relay is then heated by the secondary
current of the current transformer. This means on one hand, that the dissipated power is
reduced and, on the other, that the short-circuit withstand capacity is increased.
The tripping current of bimetal relays can be set on a current scale – by displacement of the trip mechanism relative to the bimetal strips – so that the protection characteristic can be matched to the protected object in the key area of continuous duty.
reduced and, on the other, that the short-circuit withstand capacity is increased.
The tripping current of bimetal relays can be set on a current scale – by displacement of the trip mechanism relative to the bimetal strips – so that the protection characteristic can be matched to the protected object in the key area of continuous duty.
The simple, economical design can only approximate the transient thermal characteristic of the motor. For starting with subsequent continuous duty, the thermal motor protection relay provides perfect protection for the motor. With frequent start-ups in intermittent operation the significantly lower heating time constant of the bimetal strips compared to the motor results in early tripping in which the thermal capacity of the motor is not utilized.
The cooling time constant of thermal relays is shorter than that of normal motors. This also
contributes to an increasing difference between the actual temperature of the motor and that
simulated by the thermal relay in intermittent operation. For these reasons, the protection of motors in intermittent operation is insufficient.
Temperature compensation
The principle of operation of thermal motor protection relays is based on temperature rise.
Therefore the ambient temperature of the device affects the tripping specifications. As the
installation site and hence the ambient temperature of the motor to be protected usually is
different from that of the protective device it is an industry standard that the tripping characteristic of a bimetal relay is temperature-compensated, i.e. largely independent of its ambient temperature .This is achieved with a compensation bimetal strip that makes the
relative position of the trip mechanism independent of the temperature.
Therefore the ambient temperature of the device affects the tripping specifications. As the
installation site and hence the ambient temperature of the motor to be protected usually is
different from that of the protective device it is an industry standard that the tripping characteristic of a bimetal relay is temperature-compensated, i.e. largely independent of its ambient temperature .This is achieved with a compensation bimetal strip that makes the
relative position of the trip mechanism independent of the temperature.
Sensitivity to phase failure
The tripping characteristic of three-pole motor protection relays applies subject to the condition That all three bimetal strips are loaded with the same current at the same time. If, when one pole conductor is interrupted, only two bimetal strips are heated then these two strips must alone produce the force required to actuate the trip mechanism. This requires a higher current or results in a longer tripping time.
If larger motors (≥ 10 kW) are subjected to these higher currents for a longer time, damage
should be expected. In order to also ensure the thermal overload protection of the motor in the cases of supply asymmetry and loss of a phase, high quality motor protection relays have mechanisms with phase failure sensitivity (differential release).
If larger motors (≥ 10 kW) are subjected to these higher currents for a longer time, damage
should be expected. In order to also ensure the thermal overload protection of the motor in the cases of supply asymmetry and loss of a phase, high quality motor protection relays have mechanisms with phase failure sensitivity (differential release).
2) Single-phase prevention operation
For protection of single phase AC current- or direct current loads, all poles should be connected in series to ensure the force required for tripping the switch mechanism and to prevent tripping by the phase failure protection
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| Series connection of the poles of the motor protection relay for single-phase operation |
3)Thermistor protection relays
PTC sensors (Positive Temperature Coefficient) are most frequently used in low-voltage motors for sensing the windings temperatures. Their resistance increases steeply) at the
rated operating temperature TNF which makes it possible to provide simple and economical
tripping devices. The sensors – normally 1 per phase – are imbedded by the motor manufacturer in the windings and are connected in series to terminals. The rated operation temperature TNF is selected in accordance with the insulation class. If early warning before tripping is required it is possible to install a second set of temperature sensors with lower operation temperature that are connected to a separate tripping device
rated operating temperature TNF which makes it possible to provide simple and economical
tripping devices. The sensors – normally 1 per phase – are imbedded by the motor manufacturer in the windings and are connected in series to terminals. The rated operation temperature TNF is selected in accordance with the insulation class. If early warning before tripping is required it is possible to install a second set of temperature sensors with lower operation temperature that are connected to a separate tripping device
In order to ensure the proper functioning of the protection system modern trip devices monitor their measuring loops for short-circuits and interruption.
4) Relays for NTC sensors
The characteristics of NTC sensors (Negative Temperature Coefficient) show decreasing
resistance with increasing temperature. They are used in special cases and require tripping
devices with an adjustable response threshold. With one set of sensors, early warning and
tripping can be realized.
resistance with increasing temperature. They are used in special cases and require tripping
devices with an adjustable response threshold. With one set of sensors, early warning and
tripping can be realized.
5) Metal resistance sensors
These sensors – for example Pt 100, Ni 100, Ni 120, Cu 10 – are especially suitable for medium and high voltage motors. They are used for measuring temperatures of windings and other machine parts, such as bearings. Tripping devices have a correspondingly larger number of inputs. The operation and warning levels can be adjusted. Frequently important temperature values are also shown on displays.
we will discuss three types of fuses as per their function and to where they are used:
Fusible safety switch,
“quick-acting” fuse and
“time-lag” fuse
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 withservice 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
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
Quick-acting:
"Quick-acting” fuses Nontime-delay fuses provide excellent short circuit protection. However, brief overloads, such as motor starting currents, may cause problems for this kind of fuse. Therefore, nontime-delay fuses are best used in circuits, which are not subject to large transient currents. Normally, nontime-delay fuses hold some 500% of their rated current for one-fourth of a second. After this time, the current-carrying element melts, and opens the fuse.Thus, in motor circuits, where the starting current often exceeds 500% of the fuse’s rated current, nontime-delay fuses are not recommended.
“Time-lag” fuses:
This kind of fuse provides both overload and short-circuit protection. Typically, they allow up to 5 times the rated current for up to 10 seconds and for shorter periods even higher currents. Usually, this is sufficient to allow a motor to start without opening the fuse. On the other hand, if an overload condition occurs and persists for a longer period of time, the fuse will eventually open.
Reference:Motor Book Grundfos and Low-Voltage Switchgear and Controlgear
Technical Document Allen Bradley
Technical Document Allen Bradley
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