Sub-Circuit protection

Sub Circuit Protection

H R C fuses

When a motor is first connected to the line, the power consumed is very much higher then after the motor comes up to speed. The starting current of a motor may be 5 to 8 times the full-load current. For example, an ordinary single phase 1H.P,230V motor consuming only 8A while running at full speed and full load, will require as much as 48 A, when motor is started. When motor is first connected to the power line, it is not turning, so that locked-rotor condition does exists, until the motor starts to turn. The wire to the motor must then be big enough to carry easily its starting current momentarily, its running current continuously and its normal current plus a considerable overload current for short periods. There fore it is advised to use circuit breaker or fuses. The fuses or circuit breakers which are installed in branch circuit of motor, protect the branch circuit against overload condition. The rating of these fuses or circuit breakers always depends on the size of the wire in branch circuit. These fuses or circuit breaker have amperage rating sufficient to carry the starting current of the motor and rarely will protect the motor against damage from continued overloads.
In sub-circuit, if we use fuses for protection the H.R.C fuses are mostly preferred, because it's fusing factor (The ratio between minimum fusing current to rating current is called a fusing factor) is 2. In some circuits, the starting current of the motor is large, in these circuits circuit breakers are used for protection. The small motors are feeding from the multi-way distribution boards, always use H.R.C fuses on these distribution boards.
When we choose H.R.C fuses for a 3-phase motor circuit than be carefully select the rating of fuses. Because, if we choose the low value of the H.R.C fuse, then the fuses protecting the circuit are not large enough to carry the starting current for a sufficient time it is possible that one may operate, thus causing the motor to run on two phases. This could serious damage to the motor. 

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Posted at 1:32 AM |  by Unknown

What is Motor Circuits?

Before learning about the motor starter circuits, first we learn about some important devices, which are used in motor starter circuits. Starter consists of a magnetic contactor, thermal relay, push buttons, light indicators and some time we use fuses or circuit breakers inside the starter, but his condition is rarely used .
A motor is rated in horse power (H.P). This means that is will deliver the horse power stamped on its name plate. Motors deliver far  more power while starting than after they are up-to full speed. The proportion varies with the type of motor, some types have starting torques 4 or 5 times greater than at full speed.
Naturally, the watts consumed during the starting period are much higher than while the motor is running at full speed.. That means the motor will heat up quickly, if it does not reach full speed because of too heavy starting load.
Remember these important points, when you make the connections of a sub-circuit of the motor.

(A) Always use the fuses or circuit breaker in sub-circuit of the motor.

(B) Always use the start and stop buttons in motor circuit, a controller is also used, because it insolates the motor from supply. In motor starter circuits relays must be used because it protects the circuit against overloading. A under voltage protection is also used with motor circuit.

(C) The value of volt-drop is not exceeding to 5% in controlling and protecting devices and cables of the motor.

(D) The rating of the final sub circuit is 15A. We can also derive the more than one motor on the final sub-circuit but the total load of the motor always remains less than 15A.

(E) The Motors, whose current ratings are greater than 20 or equal to 20, are always derive with distribution fuse board, where fuses and circuit breaker are installed.

A motor circuit is shown in below diagram. In this circuit three blocks are shown, in first block sub-circuit protection is marked, in second isolator is marked and in third starteris marked.

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Posted at 1:32 AM |  by Unknown

Motor Starters Information

The poly-phase induction motor is most often used to drive essentially constant loads at constant speed. Direct-on-line (D.O.L) starting is the method most commonly used. Remember that the starting current of a motor may be 5 to 8 times the full-load current at the rating of the windings is proportional  to the square of the current. In many cases, induction motors can be started by simply connecting them to the power line.
However, there are sometimes good reasons for not doing this. For example, the starting current required may cause such a acceptable. The starter is an electrically operated switch. When the operating coil is energized by pushing the start button, it closes the three main contacts and starts the motor. It also closes a small auxiliary contact at the same time. Study the D.O.L later and then you will see that pushing the starting button lets current flow through the operating coil, which energizes it; it then closes all four contacts. That starts the motor and as you remove your finger from the start button, the circuit opens but the coil remains energized because the circuit to the coil, at first energized by current flowing through the start button, now remains energized by current flowing through the auxiliary contacts.
The current ratings  of cables in a circuit feeding a motor must be based upon the full load current of the motor and not upon the starting current.

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Posted at 1:30 AM |  by Unknown