Electric Drives at Universität Wien | Flashcards & Summaries

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Lernmaterialien für Electric Drives an der Universität Wien

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TESTE DEIN WISSEN

What determines the speed at which the magnetic field rotates?

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The frequency of the applied voltage and the number of stator

poles per phase

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What are applications of induction motors?

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• grinders

• lathes

• drill presses

• pumps

• compressors

• conveyors

• printing equipment

• farm equipment

• electronic cooling

• other mechanical duty applications

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​Why are induction motors so widely used in the industry?

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• Simple construction

• Reliable

• Rugged

• Robust

• High efficiency

• Require less maintenance

• Rotates itself without external assistant.

• Less expensive than direct current motor of equal power and speed.

• Good power factor

• Easy to start

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TESTE DEIN WISSEN

What are disadvantages of induction motors?

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• Low starting torque if compared to dc shunt motor.

• Speed will be reduced when load increased.

• Speed can’t be changed without frequency converter.

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TESTE DEIN WISSEN

What happens to the motor under load?


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• When we apply mechanical load to the shaft of the motor, it

will begin to slow down and the rotating flux will cut the

rotor conductors at a higher and higher rate. The induced

voltage and resulting current in rotor conductors will

increase progressively, producing greater and greater

torque.

• The motor and mechanical load will soon reach a state of

equilibrium when the motor torque is exactly equal to the

load torque. When this state is reached, the speed will cease

to drop any more and the motor will run at the new speed at

a constant rate.

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TESTE DEIN WISSEN

What is the starting torque?


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TESTE DEIN WISSEN

• The rotor circuit of an induction motor has low resistance

and high inductance.

• At starting, the rotor frequency of the motor current is

equal to the stator frequency (i.e., 50 Hz) so that rotor

reactance is large compared with rotor resistance.

Therefore, rotor current lags the rotor e.m.f. by a large

angle, the power factor is low and consequently the starting

torque is small.

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TESTE DEIN WISSEN

What does motor operation look like?

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• The motor would always operate between the maximum torque or breakdown

torque point and the synchronous speed point which is represented by the

intersection with the horizontal axis.

• The maximum starting torque may actually be much less than full-load torque.

This may provide enough torque for starting some loads such as a centrifugal

load, but other loads such as a constant torque load will require higher

starting torques.

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Switch

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  • Purpose of Use: Component for connecting or disconnecting electrical conductors to enable, interrupt or divert an electric current
  • Operation principle: Movable contact points are either pressed together or separated from each other to enable or disable a certain path for the electric current. Types range from manually to electromechanically operated switches with a variety of configurations of contact points and possible connections between them.
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Fuse


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  • Purpose of Use: Component for opening a circuit in the case, the allowed maximum current is exceeded for a to large timespan. Depending on the needs, the time delay characteristics of a fuse are chosen (fuse class). àProtects other components and conductors from damage
  • Operation principle: Simplest fuses consist of a metal strip that heats up when current passes through it. If to much current flows through it, the strip melts and therefore breaks the circuit.
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Switch-Fuse
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  • Purpose of Use: Used for applications, where changing of a blow fuse in the otherwise live circuit would be too dangerous.
  • Operation principle: A switch that is able to disconnect the fuse from both sides of the circuit.
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TESTE DEIN WISSEN

Automatic fuse/Automatic circuit breaker


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  • Purpose of Use: Same as general fuse, but this type can be resetted after activation
  • Operation principle: Often, there are to protective elements in an automatic fuse: a thermal (for longer times, where the current exceeds the rated current only slightly) and a magnetic element (for cases like short circuits, where there is a sudden rush of current), that trigger the fuse to shut down the circuit.
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Residual current circuit breaker
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TESTE DEIN WISSEN
  • Purpose of Use: Protection of life and components
  • Operation principle: The device compares the current that is “entering” the circuit to the current that is “leaving” it. If it detects a discrepancy between these two, that means, that there is another conductive path to the earth (maybe somebody has touched a bare conductor) and so the device shuts the connected circuit off.
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Q:

What determines the speed at which the magnetic field rotates?

A:

The frequency of the applied voltage and the number of stator

poles per phase

Q:

What are applications of induction motors?

A:

• grinders

• lathes

• drill presses

• pumps

• compressors

• conveyors

• printing equipment

• farm equipment

• electronic cooling

• other mechanical duty applications

Q:

​Why are induction motors so widely used in the industry?

A:

• Simple construction

• Reliable

• Rugged

• Robust

• High efficiency

• Require less maintenance

• Rotates itself without external assistant.

• Less expensive than direct current motor of equal power and speed.

• Good power factor

• Easy to start

Q:

What are disadvantages of induction motors?

A:

• Low starting torque if compared to dc shunt motor.

• Speed will be reduced when load increased.

• Speed can’t be changed without frequency converter.

Q:

What happens to the motor under load?


A:

• When we apply mechanical load to the shaft of the motor, it

will begin to slow down and the rotating flux will cut the

rotor conductors at a higher and higher rate. The induced

voltage and resulting current in rotor conductors will

increase progressively, producing greater and greater

torque.

• The motor and mechanical load will soon reach a state of

equilibrium when the motor torque is exactly equal to the

load torque. When this state is reached, the speed will cease

to drop any more and the motor will run at the new speed at

a constant rate.

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Q:

What is the starting torque?


A:

• The rotor circuit of an induction motor has low resistance

and high inductance.

• At starting, the rotor frequency of the motor current is

equal to the stator frequency (i.e., 50 Hz) so that rotor

reactance is large compared with rotor resistance.

Therefore, rotor current lags the rotor e.m.f. by a large

angle, the power factor is low and consequently the starting

torque is small.

Q:

What does motor operation look like?

A:

• The motor would always operate between the maximum torque or breakdown

torque point and the synchronous speed point which is represented by the

intersection with the horizontal axis.

• The maximum starting torque may actually be much less than full-load torque.

This may provide enough torque for starting some loads such as a centrifugal

load, but other loads such as a constant torque load will require higher

starting torques.

Q:

Switch

A:
  • Purpose of Use: Component for connecting or disconnecting electrical conductors to enable, interrupt or divert an electric current
  • Operation principle: Movable contact points are either pressed together or separated from each other to enable or disable a certain path for the electric current. Types range from manually to electromechanically operated switches with a variety of configurations of contact points and possible connections between them.
Q:

Fuse


A:
  • Purpose of Use: Component for opening a circuit in the case, the allowed maximum current is exceeded for a to large timespan. Depending on the needs, the time delay characteristics of a fuse are chosen (fuse class). àProtects other components and conductors from damage
  • Operation principle: Simplest fuses consist of a metal strip that heats up when current passes through it. If to much current flows through it, the strip melts and therefore breaks the circuit.
Q:


Switch-Fuse
A:
  • Purpose of Use: Used for applications, where changing of a blow fuse in the otherwise live circuit would be too dangerous.
  • Operation principle: A switch that is able to disconnect the fuse from both sides of the circuit.
Q:

Automatic fuse/Automatic circuit breaker


A:
  • Purpose of Use: Same as general fuse, but this type can be resetted after activation
  • Operation principle: Often, there are to protective elements in an automatic fuse: a thermal (for longer times, where the current exceeds the rated current only slightly) and a magnetic element (for cases like short circuits, where there is a sudden rush of current), that trigger the fuse to shut down the circuit.
Q:


Residual current circuit breaker
A:
  • Purpose of Use: Protection of life and components
  • Operation principle: The device compares the current that is “entering” the circuit to the current that is “leaving” it. If it detects a discrepancy between these two, that means, that there is another conductive path to the earth (maybe somebody has touched a bare conductor) and so the device shuts the connected circuit off.
Electric Drives

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