Option 3 : the amount of load met by the motor

__Synchronous Motor on Load:__

- In DC motors and induction motors, when the load increased causes the motor speed decreases.
- The decrease in speed reduces the counter EMF which causes additional current is drawn from the source to carry the increased load at a reduced speed.
- This action cannot take place in a synchronous motor because it runs at a constant speed (i.e., synchronous speed) at all loads.
- When we apply mechanical load to a synchronous motor then the rotor poles fall slightly behind the stator poles while continuing to run at synchronous speed.
- The angular displacement between stator and rotor poles (called torque angle α ) causes the phase of back EMF Eb to change with respect to supply voltage V. This increases the resultant EMF Er in the stator winding.
- Consequently, stator current increases to carry the load.

The following points may be noted in synchronous motor operation:

- A Synchronous motor runs at the synchronous speed at all loads. It meets the increased load not by a decrease in speed but by the relative shift between stator and rotor poles i.e., by the adjustment of torque angle α.
- If the load on the motor increases, the torque angle α also increases (i.e., rotor poles lag behind the stator poles by a greater angle) but the motor continues to run at synchronous speed.
- The increase in torque angle α causes a greater phase shift of back EMF Eb with respect to supply voltage V.
- The greater phase shift of back EMF Eb with respect to supply voltage V cause increases the net voltage Er in the stator winding. Consequently, the armature current Ia increases to meet the load demand.
- If the load on the motor decreases, the torque angle also decreases. This causes a smaller phase shift of Eb with respect to V. Consequently, the net voltage Er in the stator winding decreases and so does the armature current Ia.
- Where Armature Current, \({I_a} = \frac{{V - {E_b}}}{{{Z_s}}} = \frac{{{E_r}}}{{{Z_s}}}\)

And Zs is Synchronous Impedance

**Here, α is torque angle or coupling angle**

Option 1 : decrease

**Concept:**

**Effect of changing field excitation for constant load:**

For a synchronous motor, as excitation (E) is increased :

- PF (i.e. cos ϕ) decreases and becomes more and more leading
- Armature current, I
_{a}increases.

As we move from normal excitation ( E = V) to overexcitation (E > V), I_{a} increases from a minimum at normal excitation (Unity PF)

__ Explanation__:

For synchronous motor:

E = V - I_{a} (R + j X)

From phasor approach,

**Under-excitation (lagging pf)**

**Over-excitation (leading pf)**

As we go from under-excitation → normal excitation → over-excitation, E moves anticlockwise and δ increases.

__Additional Information__

Excitation | Power factor |

Under excitation | Lagging power factor |

Normal excitation | Unity power factor |

Over excitation | Leading power factor |

Option 3 : fall back in phase by some angle but it still continues to run synchronously

__Explanation:__

**Synchronous Motor on Load:**

- In DC motors and induction motors, when the load increased causes the motor speed decreases.
- The decrease in speed reduces the counter EMF which causes additional current is drawn from the source to carry the increased load at a reduced speed.
- This action cannot take place in a synchronous motor because it runs at a constant speed (i.e., synchronous speed) at all loads.
**When we apply mechanical load to a synchronous motor then the rotor poles fall slightly behind the stator poles while continuing to run at synchronous speed.**- The angular displacement between stator and rotor poles (called torque angle α ) causes the phase of back EMF E
_{b}to change with respect to supply voltage V. This increases the resultant EMF E_{r}in the stator winding. - Consequently, stator current increases to carry the load.

__Important Points__**The following points may be noted in synchronous motor operation:** - A Synchronous motor runs at the synchronous speed at all loads. It meets the increased load not by a decrease in speed but by the relative shift between stator and rotor poles i.e., by the adjustment of torque angle α.
- If the load on the motor increases, the torque angle α also increases (i.e., rotor poles lag behind the stator poles by a greater angle) but the motor continues to run at synchronous speed.
- The increase in torque angle α causes a greater phase shift of back EMF E
_{b}with respect to supply voltage V. - The greater phase shift of back EMF E
_{b}with respect to supply voltage V cause increases the net voltage E_{r}in the stator winding. Consequently, the armature current I_{a }increases to meet the load demand. - If the load on the motor decreases, the torque angle also decreases. This causes a smaller phase shift of E
_{b}with respect to V. Consequently, the net voltage E_{r}in the stator winding decreases and so does the armature current I_{a}. - Where Armature Current, \({I_a} = \frac{{V - {E_b}}}{{{Z_s}}} = \frac{{{E_r}}}{{{Z_s}}}\)

And Zs is Synchronous Impedance

Option 3 : 750 rpm

**Synchronous Speed:**

- The synchronous speed is the speed of the revolution of the magnetic field in the stator winding of the motor.
- It is the speed at which the electromotive force is produced or at which the magnetic field of the stator is rotating by the alternating machine.

The Synchronous Speed is given by the relation shown below.

It is given by,

\(N_s=\dfrac{120f}{P}\)

According to the question.

P = 8

f = 50 Hz,

Hence, the speed at which the magnetic field of the stator is rotating or synchronous speed will be,

\(N_s=\dfrac{120f}{P}=\dfrac{120\times 50}{8}=750\) rpm

Option 1 : The machine draws a leading power factor current.

- A synchronous motor can operate at all the power factors.
- At normal excitation, synchronous motor works at the unity power factor.
- If the field of a synchronous motor is under excited, then the power factor will be lagging and it acts as an inductor.
- If the field of a synchronous motor is overexcited, then it acts as a synchronous capacitor and the corresponding power factor will be leading.
- A synchronous motor running with no load will lead the current i.e. leading power factor like a capacitor. This synchronous motor running without load i.e. over-excited is a synchronous condenser.
- The synchronous condenser is used in power lines to improve power factor, power factor correction by connecting it along with transmission lines.

Option 3 : the amount of load met by the motor

__Synchronous Motor on Load:__

- In DC motors and induction motors, when the load increased causes the motor speed decreases.
- The decrease in speed reduces the counter EMF which causes additional current is drawn from the source to carry the increased load at a reduced speed.
- This action cannot take place in a synchronous motor because it runs at a constant speed (i.e., synchronous speed) at all loads.
- When we apply mechanical load to a synchronous motor then the rotor poles fall slightly behind the stator poles while continuing to run at synchronous speed.
- The angular displacement between stator and rotor poles (called torque angle α ) causes the phase of back EMF Eb to change with respect to supply voltage V. This increases the resultant EMF Er in the stator winding.
- Consequently, stator current increases to carry the load.

The following points may be noted in synchronous motor operation:

- A Synchronous motor runs at the synchronous speed at all loads. It meets the increased load not by a decrease in speed but by the relative shift between stator and rotor poles i.e., by the adjustment of torque angle α.
- If the load on the motor increases, the torque angle α also increases (i.e., rotor poles lag behind the stator poles by a greater angle) but the motor continues to run at synchronous speed.
- The increase in torque angle α causes a greater phase shift of back EMF Eb with respect to supply voltage V.
- The greater phase shift of back EMF Eb with respect to supply voltage V cause increases the net voltage Er in the stator winding. Consequently, the armature current Ia increases to meet the load demand.
- If the load on the motor decreases, the torque angle also decreases. This causes a smaller phase shift of Eb with respect to V. Consequently, the net voltage Er in the stator winding decreases and so does the armature current Ia.
- Where Armature Current, \({I_a} = \frac{{V - {E_b}}}{{{Z_s}}} = \frac{{{E_r}}}{{{Z_s}}}\)

And Zs is Synchronous Impedance

**Here, α is torque angle or coupling angle**

Option 1 : decrease

**Concept:**

**Effect of changing field excitation for constant load:**

For a synchronous motor, as excitation (E) is increased :

- PF (i.e. cos ϕ) decreases and becomes more and more leading
- Armature current, I
_{a}increases.

As we move from normal excitation ( E = V) to overexcitation (E > V), I_{a} increases from a minimum at normal excitation (Unity PF)

__ Explanation__:

For synchronous motor:

E = V - I_{a} (R + j X)

From phasor approach,

**Under-excitation (lagging pf)**

**Over-excitation (leading pf)**

As we go from under-excitation → normal excitation → over-excitation, E moves anticlockwise and δ increases.

__Additional Information__

Excitation | Power factor |

Under excitation | Lagging power factor |

Normal excitation | Unity power factor |

Over excitation | Leading power factor |

Option 3 : fall back in phase by some angle but it still continues to run synchronously

__Explanation:__

**Synchronous Motor on Load:**

- In DC motors and induction motors, when the load increased causes the motor speed decreases.
- The angular displacement between stator and rotor poles (called torque angle α ) causes the phase of back EMF E
_{b}to change with respect to supply voltage V. This increases the resultant EMF E_{r}in the stator winding. - Consequently, stator current increases to carry the load.

__Important Points__**The following points may be noted in synchronous motor operation:** - The increase in torque angle α causes a greater phase shift of back EMF E
_{b}with respect to supply voltage V. - The greater phase shift of back EMF E
_{b}with respect to supply voltage V cause increases the net voltage E_{r}in the stator winding. Consequently, the armature current I_{a }increases to meet the load demand. - If the load on the motor decreases, the torque angle also decreases. This causes a smaller phase shift of E
_{b}with respect to V. Consequently, the net voltage E_{r}in the stator winding decreases and so does the armature current I_{a}. - Where Armature Current, \({I_a} = \frac{{V - {E_b}}}{{{Z_s}}} = \frac{{{E_r}}}{{{Z_s}}}\)

And Zs is Synchronous Impedance

Option 3 : 750 rpm

**Synchronous Speed:**

- The synchronous speed is the speed of the revolution of the magnetic field in the stator winding of the motor.
- It is the speed at which the electromotive force is produced or at which the magnetic field of the stator is rotating by the alternating machine.

The Synchronous Speed is given by the relation shown below.

It is given by,

\(N_s=\dfrac{120f}{P}\)

According to the question.

P = 8

f = 50 Hz,

Hence, the speed at which the magnetic field of the stator is rotating or synchronous speed will be,

\(N_s=\dfrac{120f}{P}=\dfrac{120\times 50}{8}=750\) rpm

Option 3 : 250 rpm

__Concept__:

The synchronous speed of a three-phase synchronous motor is given by:

\({N_s} = \frac{{120f}}{P}\)

Where Ns = Synchronous speed in rpm

f = Supply frequency

P = Number of poles

__Calculation__:

Given P = 12 and f = 25 Hz

\({N_s} = \frac{{120\;\times\;25}}{12}~rpm\)

Ns = 250 rpm