Induction Motor Speed Control MCQ Quiz in मल्याळम - Objective Question with Answer for Induction Motor Speed Control - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Concept:

Rotor resistance method of speed control:

Under load condition torque approximately

T ∝ (sV₁²) / (R₂ + R_e)

• In this method, some external resistance is inserted under the load conditions.
• Then the slip of the induction motor increases to maintain the load torque constant.
• As slip is increased, the speed of the motor will be reduced to below the rated speed.
• In this method, the motor acts as a constant torque variable power drive.

For torque constant(T = k)

\(\frac{{{{\bf{s}}_1}}}{{{{\bf{R}}_2}}} = \frac{{{{\bf{s}}_2}}}{{{{\bf{R}}_2} + {{\bf{R}}_{\bf{e}}}}}\)

Calculation:

Given that

Rotor resistance R₂ = 0.1 Ω / ph

N₁ = 1440 rpm and N₂ = 1200 rpm

N_s = (120 × 50) / 4 = 1500 rpm

s₁ = (1500 - 1440) / 1500 = 0.04

s₂ = (1500 - 1200) / 1500 = 0.2

\(\frac{{{{\bf{s}}_1}}}{{{{\bf{R}}_2}}} = \frac{{{{\bf{s}}_2}}}{{{{\bf{R}}_2} + {{\bf{R}}_{\bf{e}}}}}\)

⇒ \(\frac{{{{\bf{0.04}}}}}{{{{\bf{0.1}}}}} = \frac{{{{\bf{0.2}}}}}{{{{\bf{0.1}}} + {{\bf{R}}_{\bf{e}}}}}\)

⇒ R_e = 0.4 Ω / ph

(V / f) control or frequency control method: 

It is basically frequency control but to maintain B_max constant

The frequency variations should be done by keeping the (V / f) ratio constant.

Formula:

The synchronous speed of the induction motor in rpm is

\({N_S} = \frac{{120f}}{P}\) rpm 

NS ∝ f

NS = Synchronous speed

f = Frequency

P = No. of poles

Torque produced in the motor is

\(T = \frac{{3 \times 60}}{{2\pi {N_S}}} \times \frac{{sE_2^2{R_2}}}{{R_2^2 + {{\left( {s{X_2}} \right)}^2}}}\)

E2 = Rotor induced emf

R2 = Rotor resistance

X2 = Rotor reactance

s = slip      

\({E_{ph}} = 4.44\phi nf\;\;\)

\(\begin{array}{l} E ∝ V\\ \phi ∝ {B_{max}} ∝ \frac{V}{f} \end{array}\)

The Torque - Speed characteristics:

From the above characteristics, Let motor runs at speed of N_s0 at a frequency of f_o with a slip of S_m0.

Let at frequency f₁ < f₀, the speed of the motor reduced with N_s1 and slip increased from s_m0 to s_m1.

Now at frequency f₂ > f0, the speed of the motor increases with the synchronous speed of Ns2, and slip decreases from s_m0 to sm2.

∴ the slip is inversely proportional to the frequency applied to the motor.

If frequency reduced then slip will increase. 

Given P1 = 4, P2 = 6

f2 = 1 Hz; f = 50 Hz

f2 = s2f1­, f1 = s1f

⇒ f2 = s1s2f ⇒ s1s2 = 1/50 = 0.02

For cumulative cascade

\(\begin{array}{l} \frac{{120f}}{{{P_1}}}\left( {1 - {s_1}} \right) = \frac{{120{s_1}f}}{{{P_2}}}\left( {1 - {s_2}} \right)\\ \frac{1}{{{P_1}}} - \frac{{{s_1}}}{{{P_1}}} = \frac{{{s_1}}}{{{P_2}}} - \frac{{{s_1}{s_2}}}{{{P_2}}}\\ {s_1}\left( {\frac{1}{{{P_1}}} + \frac{1}{{{P_2}}}} \right) = \frac{1}{{{P_1}}} + \frac{{{s_1}{s_2}}}{{{P_2}}}\\ {s_1}\left( {\frac{{{P_1} + {P_2}}}{{{P_1}{P_2}}}} \right) = \frac{1}{{{P_1}}} + \frac{{{s_1}{s_2}}}{{{P_2}}}\\ {s_1}\left( {\frac{{10}}{{24}}} \right) = \left( {\frac{1}{4} + \frac{{0.02}}{6}} \right)\\ {s_1} = 0.608 \end{array}\)

Speed of cascade set \( = \frac{{120f}}{{{P_1}}}\left( {1 - {s_1}} \right) = \frac{{120 \times 50}}{4}\left( {1 - 0.608} \right) = 588\;rpm\)

Stator voltage control method:

Consider the approximate torque equation

T ∝ \(\frac{sV_1^2}{R_2}\)

Where,

s = slip ,V1 = supply voltage or stator voltage, R2 = rotor resistance

⇒ sV12 = k

⇒ s = \(\frac{k}{V_1^2}\)

And, Nr = Ns(1 - s)

• In this method, by keeping the frequency constant the applied voltage to an induction motor should be reduced below its rated value under load conditions to control the speed.
• If stator voltage is reduced by keeping the frequency constant, the slip of the induction motor increases to maintain the load torque constant.
• As slip increases the speed of the motor falls below its rated speed.
• By using this method speeds below the rated value can be achieved.
• During the stator voltage control method of speed control of the induction motor, the motor acts as a constant torque variable power drive.

Disadvantages:

• As we are reducing the voltage in this method, the induction motor draws high current at low voltages, which may overheat the stator winding.
• That's why this method is not suitable for long-duration speed control.

​Rotor resistance method of speed control:

T ∝ \(\frac{sV_1^2}{R_2+R_{ext}}\)

Where

Rext  = extenal resistance added

• In this method, some external resistance is added in series with the rotor winding under load conditions.
• Here voltage kept constant. To maintain load torque is constant so that slip increases.
• By inserting resistance, the slip of the induction machine increases for constant torque hence the speed reduces.
• By using this method also speeds below the rated value can be achieved.
• During the rotor resistance method of speed control of the induction motor, the motor acts as a constant torque variable power drive.

Disadvantages:

• Due to the presence of additional resistance, copper loss increases leads to a reduction in efficiency of the machines.
• As copper losses increasing heat rise will happen so this method is not suitable for long-duration speed control.

Torque-slip characteristics of ​a three-phase induction motor

Motoring Mode:

• From the curve, the torque and slip is that torque is directly proportional to slip in the low slip region, which is typical in motoring mode.
• This linear relationship holds until the motor reaches its rated speed, after which the torque starts decreasing at higher slips.
   

Generating Mode:

• In generating mode, a three-phase induction motor operates as an induction generator.
• This happens when the rotor of the induction motor is driven faster than the synchronous speed of the rotating magnetic field generated by the stator.
• When in generating mode, the motor converts mechanical energy (from an external prime mover, such as a turbine) into electrical energy, supplying power back to the grid or load.

Braking Mode:

• In braking mode, an induction motor decelerates and absorbs energy from the system, acting as a brake.
• This mode is used to quickly reduce the speed of the motor and is particularly useful in applications where controlled stopping is required.
• There are three common methods of braking in induction motors: regenerative braking, dynamic braking, and plugging.​

Variable Frequency Drives:

• Induction motors are designed for a specific voltage per frequency ratio (V/f).
• Voltage is the supply voltage to the motor, and frequency is the supply frequency.
• The V/f ratio is directly proportional to the amount of magnetic flux in the motor magnetic material (stator and rotor core laminations).
• The torque developed on the motor shaft is proportional to the strength of the rotating flux.
• The type and the amount of magnetic material used in motor construction are factors to define motor power rating.

Torque speed characteristics:

From the characteristics,

• The pullout torque is constant at all points below the rated speed, except at low frequencies.
• At low frequencies, the pullout torque is reduced because of the effect of stator resistance.
• As the frequency approaches zero, the voltage drop due to stator resistance becomes important.
• Flux reduction causes the torque reduction to become prominent.
• This effect is known and easily mitigated by low-speed voltage boosting: increasing the V/f ratio at low frequencies to restore the flux.

The typical set of torque-speed curves for a drive with low-speed voltage boosting:

Therefore, In V/f control of induction motors, the ratio of V/f is boosted during below 5 Hertz

The correct answer is option no "1".

Explanation:-

(V / f) control or frequency control method: 

It is frequency control but to maintain Bmax constant

The frequency variations should be done by keeping the (V / f) ratio constant.

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The Torque - Speed characteristics:

• From the above characteristics, Let motor runs at speed of Ns0 at a frequency of fo with a slip of Sm0.

Let at frequency f1 < f0, the speed of the motor reduced with Ns1 and slip increased from sm0 to sm1.

• Now at frequency f2 > f0, the speed of the motor increases with the synchronous speed of Ns2, and slip decreases from sm0 to sm2.

∴ the slip is inversely proportional to the frequency applied to the motor.

If frequency reduced then slip will increase., But the T_max (Maximum Torque) will remain constant.

The speed of the induction motor can be controlled by the following methods:

1) V/f control (or) frequency control

2) Changing the number of stator poles

3) Controlling supply voltage

4) Changing winding resistance by adding rheostat in the stator circuit

In AC locomotives, pole changing method of speed control is used for squirrel cage induction motors.

Important Points:

Rheostatic control is the simplest but the least efficient method of speed control of 3-phase induction motors. This method of speed control is employed in light locomotives and motor coaches where a single economical speed is sufficient and energy consump­tion is of no importance.

Open Loop V/F Control:

• The open-loop V/F control of an induction motor is the most common method of speed control
• Because of its simplicity and these types of motors are widely used in the industry.
• Traditionally, induction motors have been used with open-loop 50Hz power supplies for constant speed applications.
• For adjustable speed drive applications, frequency control is natural.
• However, voltage is required to be proportional to frequency so that the stator flux

Circuit diagram of inverter-fed induction motor in the open-loop:

• The power circuit consists of a diode rectifier with a single or three-phase ac supply, filter, and PWM voltage-fed inverter.
• Ideally, no feedback signals are required for this control scheme.
• The PWM converter is merged with the inverter block.

Some problems encountered in the operation of this open-loop drive are the following:

• The speed of the motor cannot be controlled precisely, because the rotor speed will be slightly less than the synchronous speed and that in this scheme the stator frequency and hence the synchronous speed is the only control variable.
• The slip speed, being the difference between the synchronous speed and the electrical rotor speed, cannot be maintained, as the rotor speed is not measured in this scheme. This can lead to operation in the unstable region of the torque-speed characteristics.

The effect of the above can make the stator currents exceed the rated current by a large amount thus endangering the inverter- converter combination.

• These problems are to be suppressed by having an outer loop in the induction motor drive,
• In which the actual rotor speed is compared with its commanded value and the error is processed through a controller usually a PI controller
• Also, a limiter is used to obtain the slip-speed command

Therefore, Implementation of Volts/Hertz strategy for inverter-fed induction motor in an open loop is used in Low performance applications

(V / f) control or frequency control method: 

It is basically frequency control but to maintain B_max constant

The frequency variations should be done by keeping the (V / f) ratio constant.

Formula:

The synchronous speed of the induction motor in rpm is

\({N_S} = \frac{{120f}}{P}\) rpm 

NS ∝ f

NS = Synchronous speed

f = Frequency

P = No. of poles

Torque produced in the motor is

\(T = \frac{{3 \times 60}}{{2\pi {N_S}}} \times \frac{{sE_2^2{R_2}}}{{R_2^2 + {{\left( {s{X_2}} \right)}^2}}}\)

E2 = Rotor induced emf

R2 = Rotor resistance

X2 = Rotor reactance

s = slip      

\({E_{ph}} = 4.44\phi nf\;\;\)

\(\begin{array}{l} E ∝ V\\ \phi ∝ {B_{max}} ∝ \frac{V}{f} \end{array}\)

The Torque - Speed characteristics:

From the above characteristics, Let motor runs at speed of N_s0 at a frequency of f_o with a slip of S_m0.

Let at frequency f₁ < f₀, the speed of the motor reduced with N_s1 and slip increased from s_m0 to s_m1.

Now at frequency f₂ > f0, the speed of the motor increases with the synchronous speed of Ns2, and slip decreases from s_m0 to sm2.

∴ the slip is inversely proportional to the frequency applied to the motor.

If frequency reduced then slip will increase.