Motor used – A.C. series motor
Voltage – 15 KV to 25 KV
This is the voltage of overhead wire and not of motor.
- Motor operates at 400 V or 300 V. So, the 25 KV voltages from overhead wire is step-down to 300 V-400 V by a step-down transformer. This transformer is present in the locomotive (train) only.Low frequency improves power factor, efficiency and reduces sparking in motor. Hence, 25 HZ frequency is used for this.
- A.C. series motor gives best performance when low frequency supply is given to it.
Advantages of 25 KV AC single phase system
- Light Overhead system: As transmission voltage is high, the current through line will be less. As current is less, the size of conductor is less.
- Due to low current, the size of conductor is less so, weight of conductor is also less.
- As weight of conductor is less, the supporting tower becomes lighter.
- Due to low current, the I2R losses are reduced. So transmission efficiency increase.
- Voltage drop I.R reduces. Hence, distance between two substations can be increased. Hence, less no. of substations are required for 25 KV AC system than 3000 V DC system.
(Note: Distance between two substations depends on voltage drop i.e. ↓ Voltage drop in conductor × 1/ distance between 2 substations ↑).
- We can locate the substations near to high voltage Grid which runs close to main railway routes and so, the initial cost of transmission reduces.
Disadvantages of 25 KV AC 1 phase system
- As system voltage is high (25 KV) so its Insulation cost is more. (Insulation depends on voltage and size of wire depends on current)
- Torque obtained by 1 phase AC series Motor is not uniform like DC series motor because of presence of frequency. So it affects driving quality.
- AC series motor cannot handle much overload. While DC series motor easily handle overload.
- Weight of AC series motor is 1.5 times more than weight of DC series motor both of same HP.
- AC system causes interference (disturbance) in the nearby telephone lines.
- 1 phase traction load produces current unbalance and voltage unbalance or power system.
- Because of current unbalance the devices like Alternators, motors etc. gets overheated.
Three (3) Phase AC System:
Motor used –
- 3 phase slip. Ring Induction motor
- Linear Induction motor (3 phase).
- Voltage – 3.3 KV at 25 HZ frequency.
- Overhead wire – 2 Overhead wire and rail is used as 3rd wire.
- No. of substation required – More substation are required because voltage is less so Current is more so I. R voltage drop is more so no. of Substation required is more.
- Equipments used in substation –
- Step-down transformer (220KV/3.3KV)
- Frequency changer (50HZ to 25HZ)
- Protective equipments and switchgear (CT, PT, relays etc.)
- Control Panel.
Disadvantages of 3 Phase System
- 2 overhead wires are required. So cost increases.
- System voltage 3.3 KV is less as compound to 25KV so because of low voltage, current is high.
So, I2R losses are more and less transmission efficiency.
- More current, more I.R (voltage) drop. So distance between 2 substations is less. So we require more no. of substation.
- Composite System:
1 phase AC, 3 phase AC and DC system have some advantages and some disadvantages. So, we try to develop a system which has combined the advantages of any two systems.