Unit-5
1. List out important consideration regarding motor installations. (Jan-2016)
Solution: - Important consideration regarding motor installation wiring
These are as detailed below.
1. All equipment used in power wiring shall be of iron clad construction and wiring shall be of the armoured cable or conduit type (IE Rule 51).
2. Woodwork shall not be used for mounting of switchgear.
3. Looping of conductors and use of the joints shall not be done,
4. The length of exible conduit used for connections between the terminal boxes of motors and starters, switches and motors shall not exceed 1.25 metres.
5. Every motor, regardless of its size shall be provided with a switch fuse placed near it. [IE Rule 50 clause (d)]
6. In addition to switch fuse all motors shall be provided with suitable means for starting and stopping (starters) placed at convenient places. The starters are used to limit the starting current to a desirable value. Direct-on-line starters, star-delta starters, auto transformer-starters (or rotor resistance starters in case of slip-ring induction motor)are used for ac motors of rating up to 0.75 kW, above 0.75 kW and below 11 kW and above 11 kW respectively.
7. The conduit should preferably be laid in covered trenches to facilitate operator movement (safe).
8. Laying of cables must be in separate conduits for separate motors.
2. The Fig. shows the plan of workshop. One 15 HP, 3 phase, 415 y induction motor is Installed. Show the key diagram and estimate quantity of material required. (Jan-2016)
Solution: - Assumptions made:
1. The motor and starter are to be procured through separate contract.
2. Motor disconnects switches and main switches are to be supplied by wiring contractor.
3. All the conduits are to be run exposed on walls.
4. The main switch, motor switch and starter shall be mounted at a height of 1.5 metres from ground level.
5. Two earth wires will be run side by side for earthing the motor, starter and switches.
6. The motor shall be installed on suitable foundation, 0.2 m above the floor level.
7. Motor efficiency 85% and power factor 0.8 (lagging).
Starting current = 1.5 times full-load current = 1.5 X 15.06 = 22.6A. Hence three-core PVC 1100V grade, 6 mm2 aluminium conductor cable of current carrying capacity 24 A may be used. The main switch and motor switch to be used will be 32 A. 415V TPIC switches. As from meter board to main board and main board to motor control board only one 3-core cable is to be run so a HG conduit of size 25mm will be run from meter board to main board and from main board to motor control board. From, motor starter to motor two 3-core cables carried so HG conduit of size 31 mm will be used. Flexible conduit of size 25 mm will be used for connecting motor switch and motor starter and of size 31 mm will be used for connecting heavy gauge conduit to motor.
Length of 25mm HG Rigid Conduit
From meter board to main board = 0.3 m
from main board to motor switch (mounted on control board) = 22.5 metres
Total = 22.8 metres
Wastage 10% = 2.3 metres
Total = 25.1 metres = 25 metres (say)
Length of 31 mm HG Conduit
From motor starter to ground = 1.5 metres
Below ground level = 0.2 metres
Along ground up to foundation = 1.0 metres
Up to top of motor foundation = 0.2 + 0.2 = 0.4 metre
Total = 1.5 + 0.2 + 1.0 + 0.4 = 3.1 metres
Wastage 10% = 0.31 metre
Total = 3.1 + 0.31 = 3.41 = 3.5 metres (say)
Length of 25 mm exible conduit required for connecting motor switch to motor starter= 0.25m
Length of 31 mm exible conduit required for connecting heavy gauge conduit to motor = 1 metre
Length of 3 core, 1100 V grade, 1/2.80 mm (6 mm2) aluminium conductor PVC cable
(i) 1 Length from meter board to main board = 0.3 m
(ii) 1 Length from main board to motor switch = 22.5 m
(iii) l Length from motor switch to motor starter = 0.25 m
(iv) 2 Lengths from motor starter to motor terminal box = 2 (3.2 + l) = 8.2 m
Total Length= 31.25 m
Wastage and for connections, 10 96 = 3.13 m
Total = 34.38 = 36 metres (say)
Length of Earth Wire According to IE rules, the motor frame, motor switch, motor starter, main switch are to be earthed by means of two separate and distinct connections. Hence two separate earth electrodes will be provided for earthing purpose.
From the table for size for earth wire, for 10 HP motor 8 SWG GI wire will be required as earth wire. Length of earth wire required
3. Explain determination of input power, current to motors & rating of cables.(Jun -2015)
Solution: - Step 1: Data Gathering
The first step is to collate the relevant information that is required to perform the sizing calculation. Typically, you will need to obtain the following data:
Load Details
The characteristics of the load that the cable will supply, which includes:
Load type: motor or feeder
Three phase, single phase or DC
System / source voltage
Full load current (A) - or calculate this if the load is defined in terms of power (kW)
Full load power factor (pu)
Locked rotor or load starting current (A)
Starting power factor (pu)
Distance / length of cable run from source to load - this length should be as close as possible to the actual route of the cable and include enough contingency for vertical drops / rises and termination of the cable tails
Cable Construction
The basic characteristics of the cable's physical construction, which includes:
Conductor material - normally copper or aluminium
Conductor shape - e.g. circular or shaped
Conductor type - e.g. stranded or solid
Conductor surface coating - e.g. plain (no coating), tinned, silver or nickel
Electrical Design Estimation and Costing 10EE82
Department of EEE, SJBIT Page 34
Insulation type - e.g. PVC, XLPE, EPR
Number of cores - single core or multicore (e.g. 2C, 3C or 4C)
Installation Conditions:
The cable will be installed, which includes:
Above ground or underground
Installation / arrangement e.g. for underground cables, is it directly buried or buried in conduit for above ground cables, is it installed on cable tray / ladder, against a wall, in air, etc.
Ambient or soil temperature of the installation site
Cable bunching, i.e. the number of cables that are bunched together
Cable spacing, i.e. whether cables are installed touching or spaced
Soil thermal resistivity (for underground cables)
Depth of laying (for underground cables)
For single core three-phase cables, are the cables installed in trefoil or laid flat?
Step 2: Cable Selection Based on Current Rating
Current flowing through a cable generates heat through the resistive losses in the conductors, dielectric losses through the insulation and resistive losses from current flowing through any cable screens / shields and armouring. The component parts that make up the cable (e.g. conductors, insulation, bedding, sheath, armour, etc) must be capable of withstanding the temperature rise and heat emanating from the cable.
The current carrying capacity of a cable is the maximum current that can flow continuously through a cable without damaging the cable's insulation and other components (e.g. bedding, sheath, etc). It is sometimes also referred to as the continuous current rating or ampacity of a cable. Cables with larger conductor cross-sectional areas (i.e. more copper or aluminium) have lower resistive losses and are able to dissipate the heat better than smaller cables. Therefore a 16 mm2 cable will have a higher current carrying capacity than a 4mm2 cable.
Base Current Ratings
International standards and manufacturers of cables will quote base current ratings of different types of cables in tables such as the one shown on the right. Each of these tables pertain to a specific type of cable construction (e.g. copper conductor, PVC insulated, 0.6/1kV voltage grade, etc) and a base set of installation conditions (e.g. ambient temperature, installation method, etc). It is important to note that the current ratings are only valid for the quoted types of cables and base installation conditions. In the absence of any guidance, the following reference based current ratings may be used
4. A 10 HP (metric), 415 v, 3, 50Hz squirrel cage IM is to be installed in a flour mill, the plan of which is shown in fi Shows the wiring dia of the layout and estimate the quantity of materials required and its cost. (Jun -2015)
Solution: - We depend on electricity to light our homes, turn on our television sets, and even cook our meals. When the power goes out because of a storm, a short circuit, or another problem in the electrical circuit, understanding what the basic components of an electrical system is a must. Your homes’ electricity starts with the electrical service connection. This is where the electric company connects their wires to your homes’ feeder wires that attach to the meter on your home or power pole.
This is the device that measures the amount of electricity your home uses and determines the amount of money the electric company charges you on a monthly basis. From here your meter either feeds a disconnect switch or a main breaker or fuse panel. A typical home has a single phase service consisting of an “A” phase and a “B” phase, a neutral and a ground wire.
Disconnect Switch
A disconnect switch is mounted on the outside of your home close in proximity to the meter on the outside of your home or power pole. The advantage of having a disconnect switch is for safety. In the event of a fire or flash flood, you can shut the power off from the outside of your home verses having to enter a burning home or a flooded basement. The other instance is having a transfer switch in which you can switch between live power and a generator for backup power.
Main Breaker
A breaker panel consists of a main breaker that is sized according to your homes’ load needs. It is used to turn the power on and off to the entire electrical panel. Typically, homes have a 100amp or a 200 amp service. A main breaker of 100 amps will only allow 100 amps to flow through it without tripping. In a tripped state, no current will flow throughout the panel. It is interrupt between the service and the branch circuits of the panel. This main breaker protects the main service wires from damages that would occur given an overload. In that case, the wires would heat up and eventually could cause a fire.
Branch Circuit Breakers
Breakers that feed lighting, outlets, central air conditioning and sub-panels are considered branch circuits. These circuits can either be 120 volts or 240 volts. The 120 volt circuits require a single pole breaker, using only one phase of the electrical service. These circuits provide power to lighting, outlets and furnaces.
The breakers are usually sized at 15 or 20 amps. In a 240 volt circuit, a two-pole breaker uses both phases of a circuit. Examples of 240 voltappliances would be an electric range, an electric stove or central air conditioning.
These appliances don’t work unless both “A” and “B” phases are working. Most of these examples would require a two-pole 30 amp breaker. Remember to size your breaker by the name plate rating on the device you are connecting to.
Switches
Switches are the devices that turn on and off lights and fans in your home. These switches come in many different styles and colours to suit your design needs. There are single-pole, three-way, four-way and dimmer switches. Their purpose is to alter the flow of current to your lights and fans in a home.
Outlets
Electrical outlets are used to plug portable devices into. Televisions, lights, computers, freezers, vacuums and toasters are all good examples of devices that can be plugged into an outlet. Outlets consist of a hot feed, a neutral and a ground. Some outlets are used especially for wet areas.
Wiring
Your home’s wiring consists of romex, BX cable or wiring concealed in conduit. Romex is a brand name for a type of plastic insulated wire. The formal name is NM that means non-metallic sheath. This is suitable for use in dry, protected areas (inside stud walls, on the sides of joists, etc.), that are not subject to mechanical damage or excessive heat.
Bx cable is known as armored cable.
Wires are covered by aluminum or steel flexible sheath that is somewhat resistant to damage. Single strands of conductor wire are pulled through conduit that is the safest method for wiring for durability purposes. These different types of wiring carry electrical current from the panel to the device being fed. Wiring is sized according to the load demand required. Check the rated required load requirements marked on each device to determine the needed size wire to carry the needed load.
5. List any eight important considerations regarding motor installation wiring (Jan-2015)
Same Ans as 1que of unit -5
6. A 10 HP (metric), 415V, 3 Phase, 50 Hz induction motor is to be installed in a workshop, the plan of which is shown in below fig. show the layout of the wiring (key dia) & estimate the quantity of material required. The wiring is to be surface conduit. Assume efficiency of motor = 85% & powerfactor 0.8 lagging. (June-2014)