Motor control Centre specifications

A motor control center (MCC) is an assembly of one or more enclosed sections having a common power bus and principally containing motor control units. Motor control centers are in modern practice a factory assembly of several motor starters.

Motor control centre specifications

Following are the general specifications for MCC panel. End user can update the specification as per specific requirement.

 1. Refer scheme drawings for wiring details of each feeder or feeder type. Scheme drawings are guidelines and following details shall be met.

a. Ammeter for motor feeders shall be part of display unit
b. Motor space heater power supply (interlocked type) MCB shall be provided for motor feeders rated 45kW and above.
c. Earth leakage relay & CBCT shall be provided for all motor feeders
d. Thermister relay (may be part of micrprocessor based OLR) shall be provided for motor feeders rated 75kW and above.
e. For high inertia loads such as fans, blowers, compressor etc.relay shall be provided proper trip curve to avoid nuisance trip during starting.
f. Direct on line starters shall be provided for motors rated up to and including 22kW. Star delta starters shall be provided for the motors rated 30kW and above.

2. Stud type terminals along with nut and bolt shall be provided in the feeders cable size of 16 sq.mm or more will be terminated. All terminals blocks shall be fingure touch proof.

3. The sleeves of busbar shall be of heat shrinkable PVC material. The busbar joints shall be shrouded.

4. Control supply required for each motor feeder shall be tapped before switch using SP MCB & neutral link.

5. Termination of the cables on any feeder terminals shall be safely possible when other feeder of the same cable alley are live. For this purpose, the feeder terminals shall be segregated with hylam sheet and staggered

6. Unused vacant modules shall be fully equipped with hinged door.

7. MCC shall be complete with inter-panel wiring including the necessary wiring between shipping sections.

8. All switches shall be interlocked with door and have defeat interlock mechanism.  Switches shall have also padlocking facility in OFF position.

9. Removable undrilled gland plate shall be provided at bottom of each panel. Distance between gland plate and cable terminals shall be 800 mm minimum for incomer feeders and 400 mm minimum for outgoing feeders.

10. Indicating lamps for motor ON, OFF and TRIP, earth leakage indication and an emergency STOP push button shall be provided on door of each motor starter feeder as per scheme diagrams.

11. Bill of material of each MCC type of feeder and each kW shall be submitted separately for approval. 

12. Cable clamping supports to be provided in cable alley.

13. Stop PB for each starter feeder shall be mushroom head stayput type.

14. Vendor shall provide marashalling panel with terminals for each motor starter module and interpanel wiring from individual module to these terminals. Marshalling panel shall be provided for all MCCs. Marshalling box to be provided one per shipping section of the MCC.

15. Vendor shall provide cables from each feeder module upto corresponding terminal in marshalling box. Feeder side of this cable shall be terminated at factory & marshalling box side of this cable shall be tagged & kept loose which shall be terminated as per marshalling box termination schedule submitted by MCC vendor. This site work at site shall be done by MCC vendor person.

16. Vendor shall try to provide PSF/SFU feeder at the bottom.

17. Vendor shall furnish GA drawing showing the following details for each MCC along with offer.
a. Overall dimensions of the panel.
b. Length of each shipping section
c. Height of each module
d. Cross, sectional view of panel showing busbar arrangement.
e. Foundation plan indicating size and gland plate
f. Weight of each shipping section

SWITCH DISCONNECTOR FUSE

1. General

1.1. Switches should be tested for disconnection function and to be called as switch disconnetor fuse.

1.2. The switches will conform to the requirements of IEC 947.1 and IEC 947-3 & IS 13947 part 1 and part 3
− the switches will have an impulse withstand voltage of 8 kV.
− the switches will have a rated operational voltage of 660V AC (50/60 Hz)
− the switches will have a short circuit withstand current (Icw) for 1 second of 2 kA for ratings up to 63A, 5kA up to 200A and 10kA beyond 200A upto 630A.

1.3. The switches will be of the positive contact indication type (according to IEC 947 – 3) to the exclusion of all other mechanisms. This function is to be certified by tests carried out by the constructor.

1.4. The range of switches will be available in 3 and 4 pole versions with full rated fused switched neutral within the same frame size.

2. Construction and operation

2.1. The switch operating mechanism will ensure rapid opening and closing (operator independent) and will conform to §2-12 of IEC 947-3. The closing of all poles, including the neutral, will be simultaneous as required by IEC 947-3.

2.2. To ensure positive contact indication as described in IEC 947-3 § 7-2-7.
− The operating handle will only indicate the O (OFF) position if the main contacts are actually separated. They will be achieved by design of the operating mechanism.
− The switches will be designed to be locked on the OFF position by padlock (with locking in
the ON position possible)
− The distance between open contacts will be greater than 8 mm.

2.3. Fuses should be isolated from both sides. Construction should prevent the possibility of live outgoing when incoming is live but there is no fuse mounted on the switch.
− These auxiliary contacts will be common with all of the range.
− The auxiliary contacts can easily be mounted on site without taking out any part like side
plate etc. and disturbing the mechanism.

2.4. The electrical endurance will be that of category A. It will correspond to an AC23 operational category without a current derating at 415V ac for ratings up to 630A.

2.5. Fuse should be stationery during the switch operation.

3. Contact system

3.1. Contact system should have self wiping feature.

3.2. Separate arcing and current carrying zone to be provided for better thermal performance throughout the life of the switch.

3.3. Contact system should be designed in such a way that during high short circuit fault current the contact pressure increases and switch can withstand the fault. In no circumstances there shall be any repulsion between the contacts during short circuit

4. Installation and auxiliaries

4.1. The switches can be panel mounted.

4.2. The switches should come along with an operating handle with door interlock with defeat and padlock as standard.

4.3. Phase barriers for all switches should be available as standard. Terminal shield upto 63A should be available as standard. For higher ratings provision for fitment should be available.

4.4. Length of operating shaft should be adjustable continuously. This feature should come as a standard.

4.5. The switches should be able to take Aluminum termination.

5. Maintenance and site convertibility

5.1. The switches should be modular type. In case a single pole is damaged it should be possible to replace the entire pole instead of replacing the contacts inside.

5.2. It should be possible to convert 3 Pole to 4 Pole switch and vice versa.

5.3. In case it is required it should be possible to convert a switch from BS type fuse holder to DIN type fuse holder and vice versa at site

Contactor Selection for Motors with long starting time

This note explains contactor selection for motors with long starting time. The note has been divided into three parts for easy understanding of the concepts involved. They are as follows,

1. Understanding Motor Inrush Current

2. Long Starting Time Applications

3. Contactor selection for motors with long starting time

Understanding Motor Inrush Current (Stator current)

A motor generally drives a load through some transmission system. During start, the motor draws a high starting current or inrush current. This current is about 6-8 times the motor rated current and can cause a significant voltage drop. This voltage fluctuation affects other devices connected to the same supply. Hence several other strategies are employed for starting motors to reduce its starting current; the most commonly employed being the Star–Delta starting. The starting value of the current is independent of the load attached; however it must be sufficient to overcome the inertia of the motor load system. However, inertia of the load impacts the starting time of the motor as explained in the next part. As the motor accelerates and nears its rated speed, the current gradually reduces and

settles down to a value equal to motor rated current or less depending on the actual load connected. The typical torque-speed characteristics of an induction motor are as given below,

Long Starting Time Applications

The total time from rest till the motor draws its rated current is called the starting time. The starting time of the motor is a function of the load inertia, load speed and the starting torque developed by the motor. A high inertia load requires an extended time to reach full speed and hence the motor also draws high starting current for a long time. The motor starting time is specified by the manufacturer in the motor data sheet. Since motor starting time is also a function of applied voltage it differs for different starting methods. For example starting time of the motor with Direct-Online starting would be different than with Star-Delta starting. The starting line current in Star Delta configuration is one third of the starting current of the same motor in DOL configuration. However applied voltage and therefore starting torque also reduces, leading to higher starting time

Long starting time applications are generally those applications in which the motor starting time is around 40 to120 secs.

Typical applications involving motors with a high starting time are,

• Induced Draft Fans (ID Fans)

• Forced Draft Fans (FD Fans)

ID and FD fans have a high inertia and hence motors required to drive them will have a long starting time. As a result the motor will draw high inrush current for an extended period of time.

The high inrush current drawn by the motor at start is carried by the contactors that are used for switching. Since, this current flows for an extended period of time, the contactor needs to be selected judiciously. Guidelines for selection of contactor rating is as follows:

Contactor Selection for motors with long starting time

Contactors are selected based on their overload current withstand capability. Overload withstand capability is defined in IEC 60947-4-1.

It means that a contactor with rated operational current equal to or less than 630A can withstand 8 times its rated Ac3 operational current for a period of 10 seconds. This rating is also called as the 10 sec rating of the contactors.

For Example:

Let Rated operational current (AC3 Utilization category) of contactor = 400A.

Then the maximum current it can carry for a period of 10 sec = 8 x I = 3200A e

Now let us look at an example, how to arrive at minimum AC3 Ratings of the Star, Main and Delta contactors

Motor specifications

Motor kW Rating: 160 kW

Motor Full Load Line Current: 304A

Motor Starting time in Star-Delta: 85 sec

Solution:

Delta contactor can be directly selected as per type 2 chart specified by the contactor manufacturer. This is because delta contactor is connected only when the motor has reached near its rated speed and motor current has reduced to its full load value

Utilization categories

Contactors are most commonly used in applications concerning control of electric motors. Contactors are used to start, stop, reverse, jog and plug the motors depending upon the application requirement. Contactors along with thermal overload relays also provide protection to the motor against overloads.

The most basic data required for contactor selection is the motor HP rating and it’s rated current. However this data is alone not sufficient. The type of load, duty cycle of the load, switching frequency are some of the factors that influence contactor selection. The switching capability of contactors is majorly dependent on the type of application, and hence international standards (IEC 60947-4-1) specify utilization categories which cover a broad range of applications. These utilization categories and the data associated with them are used by manufacturers to establish contactor ratings.

The utilization categories as per IEC 60947-4-1 are as follow:

AC-1 : Non-inductive or slightly inductive loads, resistance furnaces

AC-2 : Slip-ring motors : starting, switching off

AC-3 : Squirrel-cage motors : starting, switching off motors during running 1)

AC-4 : Squirrel-cage motors : starting, plugging, inching

AC-5a : Switching of electric discharge lamp controls

AC-5b : Switching of incandescent lamps

AC-6a : Switching of transformers

AC-6b : Switching of capacitor banks

AC-7a : Slightly inductive loads in household appliances and similar applications

AC-7b : Motor loads for household applications

AC-8a : Hermetic refrigerant compressor motor to control with manual resetting of overload release

AC-8b : Hermetic refrigerant compressor motor to control with automatic resetting of overload release

AC-15 : Control of a.c electromagnetic lods

DC-1 : Non-inductive or slightly inductive loads, resistance furnace

DC-3 : Shunt-motors : Starting, Plugging, Inching

Dynamic braking of dc motors

DC-5 : Series-motors : Starting, Plugging, Inching

Dynamic braking of dc motors

DC-6 : Switching of incandescent lamps

1) AC-3 category may be used for occasional inching (jogging) or plugging for limited time periods such as machine set-up: during limited time periods, the number of such operations should not exceed five per minute or more than 10 in a ten minute period.

2) A hermetic refrigerant compressor motor is a combination consisting of a compressor and a motor, both of which are enclosed in the same housing, with no external shaft or shaft seals, the motor operating in the refrigerant. 

The utilization categories most commonly encountered in contactor applications are AC-3 & AC-4

Applications under utilization category AC-3 (Normal Switching) are: Compressors, Pumps, Fans, Conveyors, Mixers, Agitators, Air conditioners, Elevators etc

Applications under utilization category AC-4 (Plugging, inching) are: Printing presses, Wire drawing machines, Centrifuges etc

The making and breaking capacities of contactors are dependent on the utilization categories and the standard specifies that the contactors or starters shall be capable of making and breaking currents without failure under the conditions stated.

Over voltage causes and effects

Over voltages or surges in the power system are the abrupt rise in the voltage level in the system. There could be several reasons for over voltage. The normal operating voltage of the system do not stress the insulation severely. But the voltage stresses due to over voltages can be so high that they may become dangerous to both the cables and the connected equipment and may cause damage, unless some protective measure against over voltages are taken. Over voltages occurrence in the system can be categorized by reasons:

1. External over voltages:

These over voltage originate from the atmospheric disturbances, mainly due to lightning. These over voltages take the form of a unidirectional impulse whose maximum possible amplitude has no direct relation with the operating voltage of the system. They may be due to any one of the following causes.

a. Direct lightning stroke

b. Electromagnetically  induced voltages due to lightning discharge near the line

c. Voltage induced due to the changing atmospheric condition along the transmission line

d. Electrostatic ally induced over voltages due to presence of the charge clouds 

e. Due to friction of the charged particles like dust, snow in the atmosphere or due to change in the altitude of the line. 

2. Internal over voltages:

Caused due to changes in operating conditions of the network, further classifieds into two groups.

a. Switching or transient over voltages

    The over voltages are generally of oscillatory nature caused by transient phenomena which appears when the state of network is changed by switching operations or fault condition.

The frequency of oscillation is governed by the inherent inductance and capacitance of the system and may very from few hundres Hz to few kHz

b. Steady state or temporary over voltages

    These are over voltages developed due to the disconnection of loads at power frequency.