MOTOR CONTROL CENTRE (MCC)

Power : 10 kWRated Voltage : 400 VRated Current : 19.7 AFrequency: 50 HzConnection: YPower Factor : 0.86RPM: 990Efficiency: 85.2 %Insulation Class: FIP: 55

NEMA CLASS B

Motor runs in Industrial application having part load of 70 to 87 % (14 to 17.1 A)

NEMA Design A: Maximum 5% slip, high to medium starting current, normal locked rotor torque, normal breakdown torque, and suited for a broad variety of applications - as fans and pumps.

NEMA Design B: Maximum 5% slip, low starting current, high locked rotor torque, normal breakdown torque, suited for a broad variety of applications, normal starting torque - common in HVAC application with fans, blowers and pumps.

NEMA Design C: Has maximum 5% slip, low starting current, high locked rotor torque, normal breakdown torque, and suited for equipment with high inertia starts - as positive displacement pumps.

NEMA Design D: Maximum 5% slip, low starting current, very high locked rotor torque, and suited for equipment with very high inertia starts - as cranes, hoists etc.

NEMA (National Electrical Manufacturers Association) Designs

NEMA Code letter designation to classify motors by the ratio of locked rotor KVA per horsepower.

NEMACode Letter

KVA/HPWith Locked rotor

Approximate Mid-Range Value

A 0 - 3.14 1.6

B 3.15 - 3.55 3.3

C 3.55 - 3.99 3.8

D 4.0 - 4.49 4.3

E 4.5 - 4.99 4.7

F 5.0 - 5.59 5.3

G 5.6 - 6.29 5.9

H 6.3 - 7.09 6.7

J 7.1 - 7.99 7.5

K 8.0 - 8.99 8.5

Locked Rotor Current:It is current drawn by the motor when the motor rotor is locked (stopped) but full voltage is applied to the motor terminal.

Starting Current: It is the current drawn by the motor at the time of motor starting period.

Example of Locked rotor current is Motor Stall.

It is 5 to 7 times the Full Load Current.

Induction Motor Takes approximately 10 second to reach its rated speed depending upon the size.

When motor reaches to its rated speed, starting current comes down to the normal valu.

In some starting methods, lower voltage is applied to the motor terminal and hence the starting current is reduced

Power Rating in HP and Locked Roror CodesLess Than 1 HP L

1 to 2 HP L or M

3 HP K

5 HP J

7.5 to 10 HP H

More than 15 HP

G

Code Letter : G ( for 10 kW Motor )Minimum Locked Rortor Current : 5.6 x FLC = 110.3 AmpMaximum Locked Rotor Current : 6.3 x FLC = 124.1 Amp

FUSE

CIRCUIT BREAKER

CONTACTOR

OVERLOAD RELAY

MOTOR CONTROL CENTRE MAIN EQUIPMENTS

Motor Control Centre Schematics

What is the difference between overcurrent protection and overload protection?

Overcurrent protection is protection against excessive currents or current beyond the acceptable current rating of equipment. It generally operates instantly. Short circuit is a type of overcurrent. Magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection.

Overload protection is a protection against a running overcurrent that would cause overheating of the protected equipment. Hence, an overload is also type of overcurrent. Overload protection typically operates on an inverse time curve where the tripping time becomes less as the current increases. Overload relays as well as “slow blow” fuses are commonly used to provide overload protection.

Some devices provide both overcurrent and overload protection. A thermal-magnetic circuit breaker has both thermal (overload) and magnetic (overcurrent) elements. Both elements operate as described above. Likewise the dual element fuse has both instantaneous and inverse time characteristics in the same fuse providing both overcurrent and overload protection

FUSE RATING Based on NEC 430-152

No TYPE OF MOTOR Time Delay Fuse Non Time Delay Fuse

1 Single Phase 300 % 175 %

2 3 Phase 300 % 175 %

3 Synchronous 300 % 175 %

4 Wound Rotor 150 % 150 %

5 Direct Current 150 % 150 %

Maximum Size of Time Delay Fuse : 300 % x FLC : 3 x 19.7 = 59 Amp

Maximum size of Non Time Delay Fuse: 175 % x FLC : 1.75 x 19.7 = 34.5 Amp

Dual Element Fuses (Time Delay Fuses)with both thermal and instantaneous trip features that allow the motor starting current to flow for a short time without blowing the fuse.

Time delay (Dual Element) LV HRC Fuse Links

3 NA 3014 35 A, 500 VAC

Make: SIEMENS

Time delay fuses can also be used to provide some degree of overload protection which standard fuses cannot.

The NEC allows time delay fuses to be sized up to a maximum of 175% of a motor’s FLA for overcurrent protection

Time-delay fuses will hold 500% of their amp rating for 10 seconds which will allow most motors to start without opening the circuit.

Under normal conditions, a 100-amp time-delay fuse will start any motor with a locked-rotor current rating of 500 amps or less.

Standard (Non-Time Delay) Fuses:Standard fuses protect against short circuits and ground faults using thermal features to sense a heat buildup in the circuit. The NEC allows standard fuses as overcurrent protection devices sized up to a maximum of 300% of the motor’s FLA to allow the motor to start.

Standard Fuse Response hold 500% of their current rating for approximately one-fourth of a second.

In order for a standard fuse to used as motor overload protection, the motor would have to start and reach its running speed in one-fourth of a second or less.

Standard fuses will not generally provide any overload protection for hard starting installations because they must be sized well above 125% of a motor’s FLA to allow the motor to start.

CIRCUIT BREAKER RATING based on NEC 430-52

No Type Of Motor Instantaneous Trip Breaker

Inverse Time Circuit Breaker

1 Single Phase 800 % 250 %

2 3 Phase 800 % 250 %

3 Synchronous 800 % 250 %

4 Wound Rotor 800 % 150 %

5 Direct Current 200 % 150 %

Max Size of Instantaneous Trip Circuit Breaker : 800 % x Full Load Current : 8 x 19.7 = 158 A

Max Size of Inverse Trip Circuit Breaker : 250 % x Full Load Current : 2.5 x 19.7 = 49 A

Inverse Time Circuit Breakers:Inverse time circuit breakers have both thermal and instantaneous trip features and are preset to trip at standardized levels. This is the most common type of circuit breaker used.

The thermal action of this circuit breaker responds to heat.

If a short should occur, the magnetic action of the circuit breaker will detect the instantaneous values of current and trip the circuit breaker.

The National Electrical Code requires inverse time circuit breakers to be sized to a maximum of 250% of the motor FLA.

The rating of an inverse time circuit breaker can be multiplied by 3 and this total amperage will start any motor with less locked-rotor amperage.

The time it takes to reach the 300% level varies with the amperage and voltage ratings of the breaker.

Instantaneous Trip Circuit Breakers:Instantaneous trip circuit breakers respond to immediate (almost instantaneous) values of current from a short circuit, ground fault, or locked rotor current.

The National Electrical Code allows instantaneous trip circuit breakers to be sized to a maximum of 800% of a motors FLA value.

They are used where time-delay fuses set at five times their ratings or circuit breakers at three times their rating will not hold the starting current of a motor.

Some instantaneous trip circuit breakers have adjustable trip settings. The instantaneous trip ratings of an instantaneous trip circuit breaker can be adjusted above the locked-rotor current of a motor to allow the motor to start and come up to its running speed.

Make: SIEMENSModel: 3RV10414JA10

CIRCUIT-BREAKER SIZE : S3

FOR MOTOR PROTECTION, CLASS 10

A-REL. 45...63A,

Current range: 45-63 A

Thermal Overload RelayMin Thermal Overload Relay setting: (According to part load conditions)

Full Load Current : 19.7 A

Running Load Ampere : 14-17.1 Amp

Relay Setting : 17.5 A ( 88 % of FLC)

Max Thermal Overload relay setting : 120 % of FLC

Full Load Current : 19.7 A

Relay Setting : 23.6 A ( 120 % of FLC)

Siemens 3RU1136-4BB0 Thermal Overload Relay, for induction motors with a rating of 7.5 kW with a set current value of the inverse-time delayed overload release of 14 .. 20 A..

No APPLICATION CONTACTOR MAKING CAPACITY

1 Non Inductive, Slightly Inductive, Resistive Loads

AC 1 1.5

2 Slip ring Motor AC 2 4

3 Squirrel Cage Motor AC 3 10

4 Rapid Start/Stop AC 4 12

5 Switching of Transformers AC 6a 12

6 Switching of Capacitor Banks AC 6b 12

7 Motor Load in House hold application

AC 7b 8

Type of Contactor: AC 3Size of Contactor : 100 % x Full Load Current = 1 x 19.7 = 19.7 AMaking / Breaking Capacity of contactor : Full Load Current x Valu in Chart

: 19.7 x 10 = 197 A

CONTACTOR SIZE & TYPE

Contactor: 3RT10-26-1 XB40-OLA2

Make: SIEMENS

Rated Operational current at 400 VAC : 25 Amp

Rating of Induction Motor at 50 Hz: 11 kW

Rated Control supply voltage: 24 VDC

THANKS

Problem: Vibrating Screen having two unbalanced motor (each 10 kW), one motor rotate CW and other CCW.

Report: While Overload Relay is tripped, electrician went to the equipment for investigation and found motor is already burnt. He came back to switchboard and checked fuse and power contactor is intact.

Problem: As with same arrangement other equipment is running trouble free, all the running data also normal so why particular one motor is burning frequently ? What could be the preventive measures ?

Issue: Out of two, one particular motor burnt out three times since commissioned (five months before).