Synchronous Motors MCQ Quiz - Objective Question with Answer for Synchronous Motors - Download Free PDF

Explanation:

Maximum Torque Angle in Synchronous Motor

Definition: The torque angle (δ) in a synchronous motor is the angle between the rotor's magnetic field and the stator's magnetic field. This angle is crucial in determining the torque produced by the motor. The torque angle is a critical parameter that affects the motor's stability and performance.

Working Principle: In a synchronous motor, the rotor and the stator have magnetic fields that interact to produce torque. The torque angle (δ) is formed due to the difference in the positions of these magnetic fields. The torque produced in the motor is directly proportional to the sine of the torque angle (T ∝ sin δ). For the motor to operate stably, the torque angle must be within a specific range.

Maximum Torque Angle: The maximum value of the torque angle in electrical degrees for a synchronous motor is 90 degrees. This means that the angle δ can vary from 0 to 90 degrees. Beyond 90 degrees, the motor becomes unstable, and the torque starts to decrease, leading to a potential loss of synchronism. Therefore, the maximum torque angle is crucial for the stable and efficient operation of the motor.

Advantages of Operating Within the Maximum Torque Angle:

• Ensures stable operation of the synchronous motor.
• Optimizes the torque production, ensuring efficient energy conversion.
• Prevents the motor from losing synchronism, which could lead to operational failures.

Disadvantages of Exceeding the Maximum Torque Angle:

• Risk of losing synchronism, leading to motor instability.
• Reduction in torque production, affecting the motor's performance.
• Potential damage to the motor due to unstable operation.

Applications: Synchronous motors are widely used in applications where constant speed is required, such as in industrial machinery, compressors, and conveyors. Maintaining the torque angle within the maximum limit is essential for these applications to ensure reliable and efficient operation.

Correct Option Analysis:

The correct option is:

Option 2: 90

This option correctly represents the maximum value of the torque angle in electrical degrees for a synchronous motor. The torque angle should not exceed 90 degrees to maintain stability and optimal performance of the motor.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: 360

This option is incorrect because a torque angle of 360 degrees is not feasible for synchronous motor operation. The torque angle is defined within a range of 0 to 90 degrees for stable operation. A torque angle of 360 degrees would imply a complete cycle, which is not applicable in this context.

Option 3: 45

This option is incorrect as it represents a possible value within the torque angle range but not the maximum value. The maximum torque angle is 90 degrees, and 45 degrees is just a mid-range value.

Option 4: 180

This option is incorrect because a torque angle of 180 degrees exceeds the stable operating range of the synchronous motor. Beyond 90 degrees, the motor becomes unstable, and 180 degrees would lead to a complete loss of synchronism.

Conclusion:

Understanding the torque angle and its maximum value is essential for the stable and efficient operation of synchronous motors. The maximum torque angle is 90 degrees, beyond which the motor cannot maintain synchronism and becomes unstable. This knowledge is crucial for applications requiring precise and reliable motor performance.

Explanation:

Typical Losses in a Synchronous Motor

Definition: Synchronous motors are a type of AC motor where the rotation of the shaft is synchronized with the frequency of the supply current. They operate at a constant speed regardless of the load. Various losses occur in synchronous motors during their operation, which affects their efficiency.

Working Principle: Synchronous motors operate on the principle of magnetic field interaction. The stator produces a rotating magnetic field (RMF) when supplied with AC power. The rotor, which is typically supplied with DC current, generates a magnetic field that locks onto the rotating magnetic field of the stator, causing the rotor to rotate at the same speed as the stator's RMF.

Typical Losses in Synchronous Motors:

• Friction Loss: Friction losses occur due to the friction between the rotating parts of the motor, such as the bearings and the air gap. These losses are inevitable in any rotating machine and result in energy dissipation as heat.
• Copper Loss: Copper losses, also known as I²R losses, occur in the stator and rotor windings due to the resistance of the copper conductors. These losses depend on the current flowing through the windings and the resistance of the conductors.
• Iron Loss: Iron losses, also known as core losses, occur in the magnetic core of the motor due to the alternating magnetic field. These losses consist of hysteresis losses and eddy current losses. Hysteresis losses are due to the lagging of the magnetic field behind the magnetizing force, while eddy current losses are due to circulating currents induced in the core.

Correct Option Analysis:

The correct option is:

Option 1: Capacitive Loss

This option correctly identifies a type of loss that is not typical in synchronous motors. Synchronous motors do not experience capacitive losses in their operation. Capacitive losses are more relevant to capacitors and other components in AC circuits but are not a typical consideration for the losses in synchronous motors.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Friction Loss

Friction losses are indeed a typical loss in synchronous motors. These losses occur due to the friction between the moving parts of the motor, such as the bearings and the air gap, resulting in energy dissipation as heat. Friction losses are unavoidable in any rotating machinery and affect the overall efficiency of the motor.

Option 3: Copper Loss

Copper losses, also known as I²R losses, are typical in synchronous motors. These losses occur due to the resistance of the copper windings in both the stator and the rotor. The amount of copper loss depends on the current flowing through the windings and the resistance of the conductors. Copper losses result in the dissipation of electrical energy as heat, affecting the motor's efficiency.

Option 4: Iron Loss

Iron losses, also known as core losses, are a typical loss in synchronous motors. These losses occur in the magnetic core of the motor due to the alternating magnetic field. Iron losses consist of hysteresis losses and eddy current losses. Hysteresis losses occur due to the lagging of the magnetic field behind the magnetizing force, while eddy current losses are caused by circulating currents induced in the core. These losses contribute to the overall energy dissipation and affect the motor's efficiency.

Conclusion:

Understanding the various losses in synchronous motors is essential for evaluating their efficiency and performance. While friction losses, copper losses, and iron losses are typical losses in synchronous motors, capacitive losses are not. Capacitive losses are more relevant to capacitors and other AC circuit components but do not apply to the typical losses experienced by synchronous motors. By recognizing these distinctions, one can better understand the factors that affect the efficiency and operation of synchronous motors.

The net emf per phase of an alternator does not depend on Rotor resistance.

Concept:

EMF Equation of Alternator:

The emf induced by the alternator or synchronous generator is three-phase alternating in nature. Let us derive the mathematical equation of emf induced in the alternator.

Let,

Z = number of conductors in series per phase.

Z = 2T, where T is the number of coils or turns per phase. One turn has two coil sides or a conductor as shown in the below diagram.

P = Number of poles.

f = frequency of induced emf in Hertz

Φ = flux per pole in webers

K_p= pitch factor, K_d = distribution factor

N = Speed of the rotor in rpm (revolutions per minute)

N/60 = Speed of the rotor in revolutions per second.

Time is taken by the rotor to complete one revolution,

dt = 1/(N/60)= 60/N second

In one revolution of the rotor, the total flux ϕ cut by each conductor in the stator poles,

\({\bf{d}}ϕ = ϕ {\bf{P}}\;{\bf{weber}}\)

By faraday’s law of electromagnetic induction, the emf induced is proportional to rate of change of flux.

Average emf induced per conductor = \(\frac{{{\bf{d}}ϕ }}{{{\bf{dt}}}} = \frac{{ϕ {\bf{P}}}}{{\frac{{60}}{{\bf{N}}}}} = \frac{{ϕ {\bf{NP}}}}{{60}}\)

We know, the frequency of induced emf

\({\bf{f}} = \frac{{{\bf{PN}}}}{{120}}\;,\;{\bf{N}} = \frac{{120{\bf{f}}}}{{\bf{P}}}\)

Submitting the value of N in the induced emf equation, We get

Average emf induced per conductor = \(\frac{{ϕ {\bf{P}}}}{{60}} \times \frac{{120{\bf{f}}}}{{\bf{P}}} = 2ϕ {\bf{f}}\;{\bf{volts}}\)

If there are Z conductors in series per phase,

Average emf induced per conductor = \(2ϕ {\bf{fZ}} = 4ϕ {\bf{fT}}\;{\bf{volts}}\)

RMS value of emf per phase

= Form factor x Average value of induced emf

= 1.11 x 4 Φ f T

RMS value of emf per phase = 4.44 Φ f T volts

The obtained above equation is the actual value of the induced emf for full pitched coil or concentrated coil. However, the voltage equation gets modified because of the winding factors.

Actual induced emf per phase (E) = 4.44 K_p K_d Φ f T volts.

Conclusion: Hence, EMF induced in the alternator is independent of Rotor resistance.

• Mechanical Support: It helps keep the coils in place within the armature slots, preventing movement or damage caused by vibration or mechanical stress.
• Insulation: It acts as an additional layer of insulation between the winding and the core, improving the dielectric properties of the system.
• Extension Beyond the Core: The packing strip typically extends beyond the core to provide added protection at the ends of the winding.
   

Other options:

• Coil separator: This refers to a device or material used to separate coils in some configurations but is not the correct term for the insulation paper between the slot liner and wedge.
• Dielectric separator: This term is more generic and refers to any insulating material between conductive parts but is not specific to the situation described.
• Coil insulation: This refers to the overall insulation of the winding coils, but the specific function of the paper between the slot liner and wedge is better described as a packing strip.

A synchronous motor running with normal excitation adjusts to change in load by change in armature current.

Explanation: If we consider a case when a synchronous motor is operating with normal excitation, with the increase in load:

• The motor continues to run at a synchronous speed as it is a constant speed motor
• The torque angle δ increases
• The excitation voltage E_f remains constant
• The armature current I_a drawn from the supply increases due to increase in resultant voltage across the armature
• The phase angle ϕ increases in the lagging direction
   

Alternate MethodEffect of Increased Load with Constant Excitation (Normal Excitation)

Fig (a) shows the condition when the motor is running with a light load so that

(i) torque angle α₁ is small
(ii) so E_R1 is small
(iii) hence I_a1 is small and
(iv) φ₁ is small so that cos φ₁ is large

Now, suppose that load on the motor is increased as shown in Fig. (b).

For meeting this extra load, the motor must develop more torque by drawing more armature current.

Unlike a DC. motor, a synchronous motor cannot increase its Ia by decreasing its speed and hence E_b because both are constant in its case.

Observation:

1. Rotor falls back in phase i.e., load angle increases to α₂ as shown in
2. Fig. 38.15 (b),
3. The resultant voltage in armature is increased considerably toa new value E_R2, which is increased.
4. As a result, I_a1 increases to I_a2, thereby increasing the torque developed by the motor,
5. φ1 increases to φ₂, so the power factor decreases from cos φ1 to the new value cos φ₂.

Comparison between Induction Motor and Synchronous Motor for power range 35 kW to 2500 kW:

• The power factor of a motor closely depends upon the loading of the motor.

Power factor = cosϕ = (P)/(S)

• When the load on a motor increases, the active power increases while the reactive power remains constant. Thus, the angle of ϕ decreases and the power factor (cos ϕ) improves slightly.
• The PF of a motor is lower when the motor is under-loaded and is significantly reduced when the motor load is less than 70%.
• Closely matching the motor to the load is the best way to keep the power factor close to the motor design rating, which is typically 80% to 85% PF.

The correct answer is option 2):(Leading current)

Concept:

Synchronous Motor The inverted V curve for synchronous motor is given below:

From the inverted V curve:

• The synchronous motor works at a lagging power factor when it is under excited.
• The synchronous motor works at a leading power factor when it is over-excited.
• The synchronous motor works at a unity power factor when it is normally excited.

• A synchronous motor is capable of operating at all types of power factor i.e. either upf, leading or lagging power factor.
• Lagging power factor: If field excitation is such that Eb < V the motor is said to be under excited and it has lagging power factor.
• Leading power factor: If field excitation is such that Eb > V the motor is said to be over excited and it draw leading current. So that power factor improves.
• Unity power factor: If field excitation is such that Eb = V the motor is said to be normally excited.
• 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 synchronous condenser.
• The synchronous condenser is used in power lines to improve power factor, power factor correction by connecting it along with transmission lines.
• V -curve for synchronous motor is shown below

Important Points:

• Under excited alternator works at the leading power factor
• The normal excited alternator works at the unity power factor
• The over excited alternator works at lagging power factor

The correct answer is option 2
The synchronous motor is not a self-starting machine we need damper winding or an auxiliary motor 
Hence statement 1 is true 

But the Auxilary motor method is expensive and adds additional loss so damper winding starting is preferred over auxiliary winding therefore statement 2 is false.

Concept:
A synchronous motor is not a self-starting machine. Therefore the various methods to start the synchronous motor are:

• Using a small D.C machine coupled to it
• Using pony motors
• Using damper winding
• As a slip-ring induction motor

Damper winding:

The damper winding in a synchronous motor performs two functions:

• Prevents hunting (damped out oscillations)
• Provides starting torque (made self-starting)

Under normal running conditions, damper winding does not carry any current

The starter is used for any high-rated machines to limit the starting current. So, here to limit starting current start-delta starter is preferred.

Therefore starting the synchronous motor is not so easy. 

Hence, we use these motors in specific applications where the load is to be driven at a constant speed and in applications of infrequent starting.

Important Points

Applications of synchronous motor:

• Power factor correction device
• Conveyor belts, paper-making machines, ball mills
• Phase modifier, to bring down the terminal voltage

• An electric motor is an electrical machine that converts electrical energy into mechanical energy.
• Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of rotation of a shaft.
• An electric generator (Dynamo) is mechanically identical to an electric motor, but operates in the reverse direction, converting mechanical energy into electrical energy.
• An uninterruptible power supply (UPS), also known as a battery backup, provides backup power when your regular power source fails, or voltage drops to an unacceptable level.
• A UPS allows for the safe, orderly shutdown of a computer and connected equipment.
• SMPS stands for switch-mode power supply. Its job is to convert wall-voltage AC power to lower voltage DC power.
•  Most computer chips in modern computers require power generally of 1.2 - 3.3V, with some older devices requiring between 5 - 12V DC.

Concept:

Input power of 3 phase motor = 3 V_P I_P cos ϕ

Where,

V_P = phase voltage

I_P = phase current

cos ϕ = power factor

For different phase rating,

Input power of 3 phase motor = V_p1 I_p1 cos ϕ + V_p2 I_p2 cos ϕ + V_p3 I_p3 cos ϕ

Calculation:

Input power of 3 phase motor = (230 X 5 X 1) + (230 X 4.5 X 1) + (230 X 5.5 X 1)    

Synchronous torque: It is the torque that acts on the shaft of a synchronous machine when the rotational speed of the rotor deviates from the synchronous speed and that keeps the machine in synchronism.

Pull out torque: It is the maximum value of torque which allows a synchronous motor to remain in synchronism without pulling out of step or synchronism.

• A static frequency converter or changer is a device that alters the frequency of the input signal according to the input set point. It consists of solid-state switching devices which are either on or off according to the input control signal.
• A static frequency changer is a type of frequency changer used for speed control of AC motors such as used for pumps and fans.
• The speed of an AC motor is dependent on the frequency of the AC power supply, so changing frequency allows the motor speed to be changed.
• This allows fan or pumps output to be varied to match process conditions, which can provide energy savings.

Concept:

In a salient pole synchronous motor, the power flow is given by,

\(P = \frac{{EV}}{{{X_d}}}\sin \delta + \frac{{{V^2}}}{2}\left[ {\frac{1}{{{X_q}}} - \frac{1}{{{X_d}}}} \right]\sin 2\delta \)

The torque equation is given by

\(T = \frac{{60}}{{2\pi {N_S}}}\left[ {\frac{{EV}}{{{X_d}}}\sin \delta + \frac{{{V^2}}}{2}\left[ {\frac{1}{{{X_q}}} - \frac{1}{{{X_d}}}} \right]\sin 2\delta } \right]\)

Here,

\(\frac{{EV}}{{{X_d}}}\sin \delta\) is electromagnetic torque

\({\rm{and}}\frac{{{V^2}}}{2}\left[ {\frac{1}{{{X_q}}} - \frac{1}{{{X_d}}}} \right]\sin 2\delta = Reluctance\;torque.\)

Explanation:

As we know that Vt = terminal voltage depends on the load.

Xd and Xq depend on the power factor angle which also depends on the load.

So, we cannot change the value of the three-parameter.

The value of sin 2δ always varies in the range +1 to -1.

For the maximum value of TR sin 2δ should be maximum

⇒ sin 2δ = + 1

⇒ 2δ = sin-1 (1) = 90°

⇒ δ = 45°

Torque angle δ is 45° for maximum reluctance torque to be produced in a synchronous motor.