Single Phase AC System vs Three Phase AC System (with Picture)

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

1. 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.
2. Due to low current, the size of conductor is less so, weight of conductor is also less.
3. As weight of conductor is less, the supporting tower becomes lighter.
4. Due to low current, the I2R losses are reduced. So transmission efficiency increase.
5. 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 ↑).
6. 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

1. As system voltage is high (25 KV) so its Insulation cost is more. (Insulation depends on voltage and size of wire depends on current)
2. 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.
3. AC series motor cannot handle much overload. While DC series motor easily handle overload.
4. Weight of AC series motor is 1.5 times more than weight of DC series motor both of same HP.
5. AC system causes interference (disturbance) in the nearby telephone lines.
6. 1 phase traction load produces current unbalance and voltage unbalance or power system.
7. Because of current unbalance the devices like Alternators, motors etc. gets overheated.

Three (3) Phase AC System:

Motor used –

1. 3 phase slip. Ring Induction motor
2. 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 –

1. Step-down transformer (220KV/3.3KV)
2. Frequency changer (50HZ to 25HZ)
3. Protective equipments and switchgear (CT, PT, relays etc.)
4. Control Panel.

Disadvantages of 3 Phase System

1. 2 overhead wires are required. So cost increases.
2. 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.

3. More current, more I.R (voltage) drop. So distance between 2 substations is less. So we require more no. of substation.
4. 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.