Electrical motors are powerful machines which are the backbone of todays modern world. Motors are designed for particular rated power with particular rated RPM.
Since voltage remains the same which is been applied across the motor. It is the current which varies according to the power rating. In proportion to power rating; motors have current rating.
Under the rated current, motor runs smoothly with maximum efficiency. Exceed current, more than the rated current, produces an excessive stress on motor. Vibration, noise and temperature all increases when increased current. It further decreases the efficiency and also the overall life of the motor. Further more, Much more higher current; can damage the motor permanently by damaging its insulation.
For the safety of system and safety of motors, Current must be monitored. In fault situation; such as overloading, When current exceeds the limit there must be provision of tripping .
There are multiple type of safeties are available for motors such as “thermal overloading relay“, “motor drives” and “MPCB”.
MPCB
MPCB – A “Motor protection circuit breaker“. MPCB are specially designed circuit breakers for motors which are having a current range like wise in “Thermal overload relay“. Like “MCB“, MPCB have a operating handle for switching.
Working of MPCB
MPCB has two working functions :
- Function of isolation as does circuit breaker.
- Function of Thermal-magnetic tripping as does by overload relay.
For isolation function, Contacts of circuit breakers are under tension with mechanical spring mechanism. Operating handle in MPCB, triggers the trip mechanism of the circuit.
In “overload condition“, Bi-metallic thermal strips works. When current exceeds the limit over the time, Which further rises the temperature of bi-metallic thermal strips. Due to high temperature Bi-metallic thermal strips get bent down. The bent further triggers the trip mechanism. Trip mechanism further break off the contact.
In “short circuit condition“, Solenoid coil works. When short circuit appear; heavy current flows rapidly through the solenoid coil. Solenoid becomes electromagnet at that time and intensity depend upon the level of current. Magnetism further triggers the trip mechanism. Trip mechanism further break off the contact.
- Some MPCB operating handle has three selections : ON, OFF and TRIP
- Some MPCB operating handle has only two selections : ON,OFF(TRIP).
Current Setting dial of MPCB
Motors are designed for particular rated power and rated speed. In proportion to power, Voltage remains constant while it is the current which flows in proportion to the output power.
That means current actually describes the running power rating. MPCB monitors this current for the protection, When current exceeds the limit, Circuit get trip.
MPCB have fixed current range. According to the motor rating and required safety parameters, This range is selected.
Selection of MPCB
Motors are designed with rated voltage, rated current, rated frequency. Further more, Motors have particular duty type. Whether it is continuous duty or intermittent duty or variable discrete load.
- According to motors rating, MPCB has selected. Like its voltage and frequency.
- Current and class type can be selected according to motor specifications and need of application.
Selection of current rating of MPCB
Since motors have rated current. Rated current is the maximum safe limit up to which motor precisely and efficiently. Under the limit, motor achieve its maximum useful life.
“Than, Why do we want to exceeds the rated current of the motor ?“
MPCB current setting must not be higher than the rated motor current. Parallelly with MPCB current setting, Instantaneous electronic release stays in proportion. Instantaneous electronic release is instant trip in short circuit condition.
Short circuit breaking capacity must also be selected wisely. It must not be lower, As Lower short circuit breaking capacity can leads to complete damage of MPCB after short circuit.
| Rate 3 phase power rating of motor | Rated current | Setting range for thermal overload release | Instantaneous electronic release | Short-circuit breaking capacity at 400 V AC, Icu |
| 0.75 | 2.5 | 1.8 to 2.5 | 33 | 100 |
| 2.2 | 6.3 | 4.5 to 6.3 | 82 | 100 |
| 4 | 10 | 7 to 10 | 130 | 100 |
| 5.5 | 12.5 | 9 to 12.5 | 163 | 100 |
| 7.5 | 16 | 10 to 16 | 208 | 55 |
| 11 | 22 | 16 to 22 | 286 | 55 |
| 15 | 32 | 27 to 32 | 400 | 55 |
Wiring of MPCB
When current exceeds the set limit, As per according to the level as it follows inverse time characteristics(higher the current, lower will the tripping time) it trips the main power circuit like what MCB do.
Along with main power circuitry, MPCB have provision of contacts “NO “and “NC“. These auxiliary contacts moves parallelly with positioning of MPCB. These contacts are used as a signal or alarm in control circuit.
We can say’ MPCB has two circuits. “Power circuit” and “Control circuit”.
Next, MPCB and contactor wiring are discussed through an example.
MPCB+ Contactor wiring
This is circuit of DOL starter. Let say; for an Induction motor of 4 kW, 400V AC 3 phase, rated current 8 Amps. Both power circuit and control circuit are discussed next.
Power circuit consist 3 phase power as an input to MPCB and MPCB feed power at input side of contactor. Another side of contactor, Motor is connected. Here in this arrangement contactor act as a switching agent. While MPCB provides the protection in “overload “and “short circuit” conditions.
In fault situation, MPCB performs 2 functions. 1st; It disconnect the power to contactor. 2nd, its disconnected the control circuit signal.
Control circuit consist MCB and than both NC contacts (NC MPCB and NC Stop button). Than NO contact of start button and than a solenoid coil is connected. Neutral is directly connected to coil of contactor. For latch or hold circuit, NO contact of contactor is connected parallelly across the NO contact (start button).
Control circuit break off in 3 conditions : NC tripping of MPCB, NC break off of Stop button and 3rd when power goes off.
This circuit is a basic example of used of MPCB. In PLC circuit MPCB contacts are used to feed the signal to PLC.
About selection of MPCB
- Motor’s rated voltage : 400V AC 50Hz
- Motor’s rated current : 8 Amps
- Motor’s rated power : 4KW
- MPCB voltage standard : 400V AC; 50Hz
- MPCB class : Class 10. (MPCB trips under 10 Sec. or less when current reaches 6 times the selected current)
- MPCB short circuit current rating : 50KA
- MPCB current range : 7 to 10 Amps
( Since motors rated current is 8 Amps, It should not be more than that motor current must be selected after observing the current on full load. Let suppose, At full load motor’s current is 6.5. MPCB range can be selected at 7,7.5 or 8 Max. for more precautionary point of view, Ampere can be selected at 7 Amps)
MPCB vs thermal overload relay
| PARTICULARS | MPCB | THERMAL OVERLOAD RELAY |
| Operating handle | Available | Not available |
| Current setting dial | Available | Available |
| Din rail fitting | Available | Not available |
| Add on block provision | Available | Not available; generally |
| Power circuit isolation provision | Available | Not available, Main circuit can not be isolated directly |
| Direct mounting with contactor | Possible in some ranges | Possible |
MPCB vs MCB
| PARTICULARS | MPCB | MCB |
| Operating handle | Available | Available |
| Current setting dial | Available | Not available, Fixed rating |
| Din rail fitting | Available | Available |
| Add on block provision | Available | Not available generally, Special MCB have provision. |
| Power circuit isolation provision | Available | Available |
| Direct mounting with contactor | Possible in some ranges | Not possible, conductors or wires required. |
| Precise current range | Available | Not available, fixed rated current |
Applications of MPCB
- Motor protection from overloading
- Motor protection from single phasing (Some specified MPCB)
- As an isolator switch for motors
- As a signal provider in fault situation
- Short circuit protection
- Phase failure protection
Frequently Asked Questions About MPCB
Frequently Asked Questions About MPCB
1. What is an MPCB?
MPCB stands for Motor Protection Circuit Breaker. It is a protective device specially designed for electrical motors. An MPCB can provide protection against conditions such as motor overload and short circuit, while also providing a means of switching and isolating the motor circuit.
2. What does an MPCB protect against?
An MPCB primarily protects a motor against overload and short-circuit conditions. Depending on the MPCB design and manufacturer, it may also provide protection against phase failure or phase imbalance. The exact protection features should always be checked in the manufacturer’s specifications.
3. How does an MPCB work?
An MPCB generally uses thermal and magnetic/electromagnetic protection. During an overload, the thermal mechanism responds to excessive current over time and trips the circuit. During a short circuit, the magnetic or instantaneous release responds rapidly to the high fault current and disconnects the circuit.
4. What is the difference between an MPCB and an MCB?
An MCB is a general-purpose circuit protection device, while an MPCB is specifically designed for motor protection. An MPCB normally provides an adjustable motor-current setting and is designed to work according to the characteristics and requirements of motor circuits. An MCB generally has a fixed current rating and is not a replacement for a properly selected MPCB.
5. What is the difference between an MPCB and an overload relay?
Both can provide overload protection for motors, but they work differently in a motor starter arrangement. An overload relay normally works together with a contactor and trips the control circuit when an overload occurs. An MPCB combines motor overload protection with short-circuit protection and circuit switching/isolation in a single device.
6. Can an MPCB protect a motor from single phasing?
Many MPCBs are designed with phase-failure or phase-loss protection, but this feature depends on the specific model. Therefore, the manufacturer’s datasheet should be checked before relying on an MPCB for single-phasing protection. For critical motor applications, the complete protection scheme should be properly designed.
7. Can an MPCB protect a motor against short circuit?
Yes. An MPCB generally includes an instantaneous magnetic or electromagnetic release that operates during a short-circuit condition and rapidly disconnects the motor circuit. However, the MPCB must have a suitable short-circuit breaking capacity for the available fault level at the installation point.
8. How do you select the correct MPCB for a motor?
The MPCB should be selected according to the motor’s rated current, voltage, frequency, motor type, application and starting conditions. The adjustable current range of the MPCB should cover the motor’s rated current, and the device’s short-circuit breaking capacity must also be suitable for the installation.
9. How is an MPCB connected with a contactor?
In a typical DOL motor starter, the MPCB is installed in the motor power circuit, generally upstream of the contactor. The contactor then switches the motor supply. The MPCB’s auxiliary contact can also be incorporated into the contactor control circuit so that an MPCB trip prevents or stops the contactor from energizing the motor.
10. What happens if the MPCB current setting is incorrect?
If the MPCB current setting is set too low, the motor may trip unnecessarily during normal operation or starting. If it is set too high, the MPCB may not provide the intended overload protection for the motor. Therefore, the setting should be selected according to the motor’s rated current and the manufacturer’s instructions.
SUMMARY
- Electrical motors are essential for modern operations, functioning under specific rated power and RPM.
- Current varies with power ratings, and when exceeded, it stresses the motor, increasing vibration, noise, and temperature, which may shorten its lifespan or cause irreversible damage.
- To ensure safety and efficiency, monitoring devices like Motor Protection Circuit Breakers (MPCB) are essential.
- MPCBs provide both isolation and thermal-magnetic tripping functions, protecting motors from overload, short circuits, and phase failures.
- Proper selection of MPCBs is crucial, aligning with the motor’s specifications. Additionally, understanding wiring and applications of MPCBs enhances their effectiveness in various motor protection scenarios.
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