Engineering House
Share
Your Engineering Reference - Your Guide to your Career - If you dream of becoming a successful engineer, Engineering House will be your destination
The engineering profession is now exceptionally wide-ranging; it covers every kind of product from dams to chemical plant to silicon chips, all having this in common: they have to be understood, designed, and made. What's more, they have to be made economically, efficiently, and better than the last time (or better than the competition). The qualities of a good engineer include
They are typically two main devices able to interrupt fault currents, circuit breakers and fuses :
The circuit breakers must be associated with a protection relay having three main functions:
Measurement of the currents
Detection of the faults
Emission of a tripping order to the breaker
The fuses blow under certain fault conditions.
☆ Braking of Induction Motor
-------------------------------------------------
★ Dynamic Braking:-
------------------------------------
1] AC dynamic braking
------------------------------------
The first classification of dynamic braking of induction motors is :
AC dynamic braking any one of the supply phase is disconnected from the supply and then it is either kept open or connected with the other phase.
》The first type is known as two lead connection and
》the second one is known as three lead connection.
To understand this braking method clearly we can assume the system to be a single phase system.
Now the motor can be considered to be fed by positive and negative sequence voltages.
That’s why when the rotor resistance is high the net torque is negative and braking can be acquired.
☆ Braking of Induction Motor
-------------------------------------------------
■ Plugging of induction motors
---------------------------------------------------
is done by interchanging any two of the supply terminals.
When the terminals are reversed the operation of the machine changes from motoring to plugging.
● From technical point of view and for better understanding, it can be said that:
the slip changes from ‘s’ to (2-s),
which indicates that due to reversal of the terminals the torque also changes its direction and braking occurs.
☆ Regeneration braking for induction motor
-----------------------------------------------------------------
Here, θs is the phase angle between the stator voltage and stator current,
→ whenever this phase angle exceeds 90° (i.e θs>90°) regenerative braking can take place.
To explain this more clearly and easily we can say that :
▶▶whenever the speed of the rotor exceeds synchronous speed, regeneration braking occurs.
▶▶ That is because whenever the rotor rotates at a speed more than synchronous speed there is a reverse field occurs which opposes the normal rotation of the motor and therefore braking takes place.
★ Main disadvantage of this type of braking is that:
the speed of the motor has to exceed synchronous speed which may not be possible every time.
★To acquire regenerative braking at a lower speed than synchronous speed, variable frequency source can be used.
28/10/2017
☆ Braking of Induction Motors
----------------------------------------------------'
When it comes to controlling an electric machine by electric drivers braking is a very important term
→ because it helps to decrease the speed of the motor according to will and necessity.
Braking of induction motors can be classified mainly in three types
◀Regenerative braking▶
◀plugging or reverse voltage braking▶
◀Dynamic braking▶ which can be further classified as:
● AC dynamic braking
● Self-excited braking using capacitors
● DC dynamic braking
● Zero sequence braking
04/10/2017
Conditions for protecting branch or distributed circuits for power distribution in LV
--------------------------------------------------------------------
5. Exemption from protection against overloads
Exemption from protection against overloads
Figure – Exemption from protection against overloads
The diagram above illustrates three examples of tap-offs (S1, S2, S3) where it is possible not to provide any overload protection or simply not to check whether this condition is met:
Busbar system S2 is effectively protected against overloads by P1 and the busbar system does not have any tap-offs or power sockets upstream of P2
Busbar system S3 is not likely to have overload currents traveling over it and the busbar system does not have any tap-offs or power sockets upstream of P3
Busbar system S4 is intended for communication, control, signaling and similar type functions and the busbar system does not have any tap-offs or power sockets upstream of P4.
03/10/2017
Conditions for protecting branch or distributed circuits for power distribution in LV
--------------------------------------------------------------------
4. 3-metre rule applied to overload protection devices
When protection device P1 placed at the head of line S1 does not have any overload protection function or its characteristics are not compatible with the overload protection of the branch line S2 (very long circuits, significant reduction in cross-section) – it is possible to move device P2 up to 3 m from the origin (O) of the tap-off as long as there is no tap-off or power socket on this portion of busbar system and the risk of short circuit, fire and injury is reduced to the minimum for this portion (use of reinforced insulation conductors, sheathing, separation from hot and damaging parts).
3-metre rule applied to overload protection devices
Figure – 3-metre rule applied to overload protection devices
02/10/2017
Conditions for protecting branch or distributed circuits for power distribution in LV
--------------------------------------------------------------------
3. Checking the protection conditions using the ‘TRIANGLE’ rule
The short-circuit protection device P1 placed at the origin a of the line can be considered to effectively protect branch S2 as long as the length of the branch busbar system S2 does not exceed a certain length, which can be calculated using the triangle rule.
The maximum length L1 of the conductor with cross-section S1 corresponds to the portion of the circuit AB that is protected against short circuits by protection device P1 placed at point A.
The maximum length L2 of the conductor with cross-section S2 corresponds to the portion of the circuit AM that is protected against short circuits by protection device P1 placed at point A.
Triangle rule
These maximum lengths correspond to the minimum short circuit for which protection device P1 can operate.
S1 corresponds to the cross-section of the main conductor and S2 to the cross-section of the branch conductor.
The maximum length of the branch conductor with cross-section S2 that is protected against short circuits by protection device P1 placed at point a is represented by segment ON. It can be seen using this representation that the protected length of the branch line decreases the further away the tap-off point is from protection P1, up to the prohibition of any S2 smaller cross-section tap-off at the apex of the triangle, B.
This method can be applied to short-circuit protection devices and those providing protection against overloads respectively, as long as device P2 effectively protects line S2 and there is no other tap-off between points A and O.
01/10/2017
Conditions for protecting branch or distributed circuits for power distribution in LV
--------------------------------------------------------------------
2. Checking the protection conditions of the branch line(s) with regard to the thermal stresses
For branch lines with smaller cross-sections (S2
30/09/2017
Conditions for protecting branch or distributed circuits for power distribution in LV
--------------------------------------------------------------------
1. General principle for checking thermal stress
For insulated cables and conductors,
(the breaking time of any current resulting from a short circuit occurring at any point) must not be longer than (the time taken for the temperature of the conductors to reach their permissible limit.)
This condition can be verified by:
checking that (the thermal stress K2S2 that the conductor can withstand ) is greater than (the thermal stress (energy i2t) that the protection device allows to pass).
17/09/2017
Question of the Day
_________________
What about the "Automatic power factor correction"?
Answer:
_______
➡️ Automatic power factor correction
-----------------------------------------------------
💡 In most installations there is not a constant absorption of reactive power for example due to working cycles for which machines with different electrical characteristics are used.
Figure 1 - ABB's power factor monitoring, type 'Dynacomp'
💡 In such installations there are systems for automatic power factor correction which, thanks to a monitoring varmetric device and a power factor regulator,
🔌 allow the automatic switching of different capacitor banks,
🔌 thus following the variations of the absorbed reactive power
🔌 and keeping constant the power factor of the installation constant.
💡 An automatic compensation system is formed by:
🔹 Some sensors detecting current and voltage signals;
🔹 An intelligent unit which compares the measured power factor with the desired one and operates the connection and disconnection of the capacitor banks with the necessary reactive power (power factor regulator);
🔹 An electric power board comprising switching and protection devices;
🔹 Some capacitor banks.
Figure 2 - Automatic power-factor correction panel (photo credit: eamfco.com)
💡 To supply a power as near as possible to the demanded one, the connection of the capacitors is implemented step by step with a control accuracy which will be the greater the more steps are foreseen and the smaller the difference is between them.
16/09/2017
Question of the Day
_________________
What about the "Combined power factor correction"?
Answer:
_______
➡️ Combined power factor correction
-----------------------------------------------------
💡 This solution derives from a compromise between the two solutions of distributed and centralized power factor correction and it exploits the advantages they offer.
💡 In such way,
🔌 the distributed compensation is used for high power electrical equipment
🔌and the centralized modality for the remaining part.
💡 Combined power factor correction is prevailingly used in installations where
🔹 large equipment only are frequently used; in such circumstances their power factor is corrected individually,
🔹 whereas the power factor of small equipment is corrected by the centralized modality.
Click here to claim your Sponsored Listing.
Category
Contact the school
Address
Cairo