PROBLEM STATEMENT
The main problem of this milling machine is when the grains is finished and the operator wants to put another for another product then the motor is still rotating in the same speed as before (no load speed is equal to full load speed).
Due to this problem, designing of a circuit for controlling speed of the motor will be done.
OBJECTIVES
To minimize cost of electricity by designing a circuit for controlling motor speed.
SIGNIFICANT OF THE PROJECT
To minimize cost of electricity.
It will reduce motor overheating.
To improve efficiency of the machine.
LITERATURE REVIEW
Literature review include existing system and proposed system.
EXISTING SYSTEM
DISADVANTAGE OF EXISTING SYSTEM
When there is no load in the machine then the motor is still rotating in the same speed as it was before.
PROPOSED SYSTEM
The following are data collected from the field of study
Thermal overload protection
3phase miniature circuit breaker
Cartridge fuses 100A
Motor ratings
Motor rated voltage –380 — 400V
Rated power – 7.5kW/10HP
Current taken – 17.7A full load
Motor speed – 1450RPM
Motor type – AC Three phase Induction motor
Contactor size – 220V AC coil voltage
Power factor – 0.87
Frequency – 50Hz
Connection – Direct on line (DOL)
Size of cable in control circuit 2.5mm2
Analysis of the motor
Specification of the motor
Rated power = 7,5 kW/10HP
Current taken 16.7A
Power factor (cos ɵ) = 0.87
Motor full load speed = 1450 rpm
Therefore,
Full load line current =HP × 0.746 × [1000 ÷ (1.732 × V)]
= 10 × 0.746 × [1000 ÷ (1.732 × 415)
= 10.4A
Full load phase current = line current ÷ 1.732
= 10.4 ÷ 1.732
= 5.9A
Rated torque
Tfl =5252×HP×rpm
But for motor less than 30kW
Starting torque =3×Ifl
=3×16.7A
=50.1Nm
Torque developed
T= (applied voltage)2
T=V12
Transformer selection
Then the capacity of Autotransformer to be connected in the motor is as follows
Motor rated = 10HP/7.5Kw
Voltage applied = 415V
Current = 16.7A
Speed = 1450rpm
So Current at instant of starting (Iis) takes 5 to 7 full load line current
Iis = 5×16.7A
Iis = 83.5A
But according to the tapping of autotransformer 65% tapping would reduce the Iis to 42% of Iis using full voltage starting
That is
Iis = 83.5A ×42%
= 35.07A
Voltage during starting period Vs = 415V ×65%
= 269.75V
Then kVA rating of autotransformer (S) is given by
S = 1.732 × Iis ×Vs
= 1.732 × 83.5A × 269.75V
= 16384.9 VA or 16.3849 kVA
But the available autotransformer to be used is of 20kVA
Analysis of motor starter
Contactor sizing
Rated voltage=380V then
Coil voltage= 220V ac
Coil resistance= 22ῼ
Then current taken by contactor
I=V/R
=220V/22ῼ
=10A
Overload relay
Thermal overload relay setting = 100% × full load current (line)
= 1 × 10.4AS
= 10.4A
therefore
Minimum setting=70% ×full load current (phase)
=70% × 5.9A
=4.13A
then Maximum setting = 120% × full load current(phase)
= 120% × 5.9A
=7.08A
Start and stop push button
=10A
Fuse selection
From the table above Therefore,
Maximum size of time delay fuse
= 300% × line Ifl
=3 × 10.4
= 31.2A
then
Maximum size of non-time delay fuse
=175% × line Ifl
= 1.75 × 10.4
=18.2A
Size of Circuit breaker
Therefore From NEC 430-52 table
Maximum size of instantaneous trip circuit breaker
=800% × line Ifl
= 8 × 10.4A
=83.2A
Then Maximum size of inverse trip circuit breaker
=250% × line Ifl
=2.5 × 10.4A
=26A
ADVANTAGES OF PROPOSED SYSTEM
The efficiency of the machine will be improved
It will reduce the cost of electricity
CONCLUSION
From the result of the circuit above, so it’s conclusion that the proposed system diagram of the project is working properly and it can be implemented / build into the circuit board ready for use. But the main obstacle is the availability of autotransformer components which is difficult for me to get in the market. Also the designed system meets the objectives of the project in several percentages i.e. to increase efficiency and reduce overheating of the particular machine.
RECOMMENDATION
Most of the milling machine do not use this method of speed control therefore, the company should use this method because it is very economic to the environment so as to improve the efficiency of the machine and avoid some cost which can be occur due to overloading or overheating of the motor.