DC Shunt Motor MCQ Quiz in मराठी - Objective Question with Answer for DC Shunt Motor - मोफत PDF डाउनलोड करा

Concept:

Efficiency is given by, \(\eta = \frac{{{P_{OUT}}}}{{{P_{IN}}}}\)

PIN = V × IL

Back emf is shunt motor given by

Eb = V – Iara

Ia  = IL - If

Where

If = Field current

V = Terminal voltage

Ra = Armature resistance

Calculation:

Given that,

Ra = 0.9 Ω

Rf = 180 Ω

V = 440 volt

\({I_f} = \frac{{440}}{{180}} = 2.44\;amp\)

\({P_{IN}} = \frac{{9}}{{0.9}} = 10\;kW\)

\({I_L} = \frac{{10000}}{{440}} = 22.727\;A\)

Ia = 22.727 – 2.44 = 20.287 A

DC shunt motor:

• In case of DC shunt motor, the flux per pole is considered to be constant, torque increases with the increase of load current.
• If the load current increases then the armature current also be increased and the speed slightly falls due to increase in voltage drop in armature.
• As the torque is proportional to ϕ (flux) and I_a (armature current).
• Flux and armature currents are almost constant, so torque is almost constant from no-load to full-load, can be seen same from the graph also.
• The ratio of starting torque to full load torque is nearly one and least among all motor. 
• It is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps, fans, woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines, etc.

The speed-torque characteristics of different DC motors are as shown in the below figure.

Key Points

Applications of different DC motors are given below.

DC Series motor: 

• It has a very high starting torque.
• Hence it is used for heavy-duty applications such as electric railways, mine hoists, continuous conveyors cranes, rolling mills, metallurgical works, etc.

DC Cumulative compound motor:

• It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing the load.
• Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators, freight elevators, etc.

DC Differentially compound motor: 

It has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.

• In DC series motors, torque increases as the square of armature current. Hence these motors are used in high starting torque and variable speed applications.
• In DC shunt motors, torque is directly proportional to armature current. It is a constant speed motor. It is used for slightly or constant speed applications.
• The compound motors are used where higher starting torque and fairly constant speed is required.

Concept:

In a DC shunt generator, generated emf is given by

Eg = Vt + IaRa

In a DC shunt motor, back emf is given by

Eb = V – IaRa

Where, Ia is armature current

Ra is armature resistance

Calculation:

Armature resistance (Ra) = 0.5 ohm

Voltage (V) = 220 V

Back emf (Eb) = 200 V

Eb = V – IaRa

⇒ 200 = 220 – Ia(0.5)

⇒ Ia = 40 A

DC shunt motor consists of armature and field circuit in parallel as shown in fig.

Vt = terminal voltage DC

IL = line current

IF = field current

For DC shunt motor, speed equation is

Eb ∝ Nϕ

\(N \propto \frac{{{E_b}}}{\varphi }\)

Where Eb is back emf, N is speed and ϕ is flux.

In a DC shunt motor, flux (ϕ) is approximately constant.

∴ Eb ∝ N

So, the back emf is directly proportional to speed.

Hence, when the speed of a DC shunt motor is increased, then the back emf of the motor will increase.

DC shunt motor:

• In case of DC shunt motor, the flux per pole is considered to be constant, torque increases with the increase of load current.
• If the load current increases then the armature current also be increased and the speed slightly falls due to increase in voltage drop in the armature.
• It is used for driving constant speed line shafts, lathes, constant speed head centrifugal pumps, fans, woodworking machines, reciprocating pumps, laundry washing machines, milling machines, grinders, small printing presses, paper making machines, metal cutting machines, etc.

The speed-torque characteristics of different DC motors are as shown in the below figure.

Key Points

Applications of different DC motors are given below.

DC Series motor: 

• It has a very high starting torque.
• Hence it is used for heavy-duty applications such as electric railways, mine hoists, continuous conveyors cranes, rolling mills, metallurgical works, etc.

DC Cumulative compound motor:

• It has high starting torque and has varying speed within limits i.e. it has self-adjustable speed with changing the load.
• Hence, it is used for driving compressors, pressure blowers, door lifts, circular saws, passenger elevators, freight elevators, etc.

DC Differentially compound motor: 

It has low starting torque and has a constant speed, but the dangerously high speed at no load under the circumstances it is seldom used.

• In DC series motors, torque increases as the square of armature current. Hence these motors are used in high starting torque and variable speed applications.
• In DC shunt motors, torque is directly proportional to armature current. It is a constant speed motor. It is used for slightly or constant speed applications.
• The compound motors are used where higher starting torque and fairly constant speed is required.

The correct answer is option 1):(210 V)

Concept:

The EMF generated is 

E_g = V_t - I_aR_a

E_g is the generated EMF 

V_t is the terminal Voltage

I_a is the armature current

R_a is the armature resistance

Calculation:

Given

V_t = 220 V

R_a = 0.2 Ω 

I_a = 50 A

Eg = Vt - IaRa

= 220 - ( 0.2 × 50 )

= 210 V

Concept

In DC shunt motor, the speed (N) is directly proportional to back emf (Eb) and inversely proportional to flux (ϕ).

\(N \propto {E_{b} \over \phi}\)

\(N \propto {V-I{_a}R{_a} \over \phi}\)

By neglecting armature reaction,

\(N \propto {V\over \phi}\)

Flux is directly proportional to the applied voltage. As applied voltage (V) is varying flux is also varied.

⇒ N is constant.

Calculations

Let applied voltage =  V

Reduced applied voltage =  V / 2

No-load speed = 1500 rpm

Field control method:

By adding external resistance in the field circuit, we can control the speed of DC shunt motor above the normal speed.

• In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field.
• An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed.
• This method of speed control is independent of load on the motor. Power wasted in controlling resistance is very less as field current is a small value.

We know that,

\(N \propto \frac{{{E_b}}}{ϕ }\)

\(N \propto \frac{{V - {I_a}\left( {{R_a}} \right)}}{ϕ }\)

ϕ ↓  → N ↑ 

EMF equation of DC motor

The EMF of the DC motor is given by:

\(E={NPϕ Z\over 60 A}\)

where, E = Generated EMF

N = Speed

P = No. of poles

ϕ = Flux per pole

Z = No. of conductors

A = No. of parallel paths

The no. of poles, conductors, and parallel paths are fixed for the DC motor. Hence, these always remain constant irrespective of the magnitude of the generated EMF.

Hence, the generated EMF depends upon the flux and the speed.

Calculation

\({E_1\over E_2}={N_1\over N_2}\times {\phi_1\over \phi_2}\)

For the DC shunt-wound generator, the flux remains constant.

Hence, \({E_1\over E_2}={N_1\over N_2}\)

Given, E₁ = 200 V at N₁ = 800 RPM

N₂ = 1500 RPM

\({200\over E_2}={800\over 1500}\)

E₂ = 375 V

Applications of DC motors

• The speed of the DC shunt motor almost remains constant on various loads, thus it suits perfectly the application where the speed requirement is constant like in machine tools.
• Series DC motors are generally used where high starting torque is required, and speed variations are possible. These are used in traction systems, cranes, air compressors, vacuum cleaners, sewing machines, etc.
• Cumulative compound motors give high starting torque like a series motor and reasonably good speed regulation at high speeds like a shunt DC motor, making compound DC motors suitable for a variety of applications such as business machines, machine tools, agitators, shears, presses, and reciprocating machines.