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 also have a operating handle for switching. In short, MPCB is combination of “MCB and Thermal overload relay“.
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 our need.
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.
| 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 a motor 4 kW. Both power circuit and control circuit are discussed.
Power circuit consist power to MPCB and MPCB feed power at input side of contactor. Another side of contactor, Motor is connected.
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.
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
- As an isolator switch for motors
- As a signal provider in fault situation
- Short circuit protection
- Phase failure protection
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.