Rated Secondary Voltage

The nominal secondary voltage is defined according to ABNT [9] as “the voltage appearing at the terminals of a nominal load imposed on the CT at 20 times the rated secondary current, without the ratio error exceeding the specified value.” This means that the CT must withstand a maximum current in its secondary circuit proportional to 20 times its rated current without conducting errors higher than the one specified by its accuracy class.

The rated secondary voltage represents the maximum voltage that the CT must withstand under overcurrent conditions. The value 20 is set by the ABNT as the standardized value for the overcurrent factor. Only the protective duty CTs reach rated secondary voltage. In measuring CTs, the core saturates well before the secondary current reaches this value.

The ratio error depends on the impedance connected to the secondary circuit. The calculation of the total impedance of the circuit for practical purposes of specifying the

TC uses the nominal nominal loads as reference.

However, the CTs for protection service also have a consideration in calculating the impedance of the secondary circuit. ABNT subdivides these CTs into two classes for their impedance.


Synchronous Motors – Operation

Synchronous electric motors have two plausible excitation factors:

– direct current in the field winding;

– from the three-phase source of alternating current in the armature.    Gates Rubber 16/A42

The synchronous electric motor when operated in the overexcited and empty condition is called the synchronous capacitor and performs the same function as a capacitor bank, being more efficient than the static capacitors. If its field current is sufficient to compose the essential magnetomotive force, not requiring magnetization current or reactive power, the motor acts with unit power factor.

If your field current is less than is said necessary, saying that the motor is underexposed, the magnetomotive force deficiency needs to be completed by the armature, so the motor works with a delayed power factor. If your field current is greater than is declared necessary, we say that the electric motor is overexcited, the surplus of magnetomotive force must be carefully balanced in the armature and an advanced current device is present, the motor works with power factor in advance.



Bimetallic Relays

The bimetallic relays used to deal with overload are generally used with the intention of indirectly protecting from possible overheating different equipment that operate from electricity such as motors and transformers.

The possible overheating of an engine can be caused by factors such as mechanical overload at the shaft end, too high starting time, blocked rotor, lack of a phase, excessive voltage deviations and grid frequency. In case of any of these factors, the current increment, the overcurrent, in the motor will be controlled in all phases by the overload relay.          32664

It is extremely important to remember that all the terminals in the auxiliary circuits of the relay are properly indicated in the same way as the terminals of contactors, which present specific functions. Consequently, the sequence must have nine numbers and if there is a second sequence, it will be identified as number zero. Relays are devices based on the principle of dilatation of thermoelectric parts, also known as bimetallic. The operation of a relay is based on the various expansions that the metals present, if subjected to a temperature variation.


DC Generator or Dynamo

In 1866, Werner Siemens, using an electromagnet, created a machine that transformed mechanical energy into electric called a DC generator or dynamo. It had the ability to self-induce because it removed from the rotor’s own winding the voltage needed to generate the magnetism.

When a continuous electrical voltage is applied at its terminals, the rotor is in motion, similar to that of the electric motor.

The dynamo was developed by the Italian Antonio Pacinotti, however, according to some historians, he presented the invention to a workshop to obtain the patent and the owner, the Belgian Zenobe-Theophile Gramme, patented it with his name in 1871, after making some improvements.

The large ring at the top of the Gramme machine is a permanent magnet and at the bottom there is a magnet and, between the magnet poles, a disc attached to a handle. Next to this crank are coils consisting of copper wires with coils around a circular shaped iron core.

By turning the crank, the copper coils cross the uniform magnetic field, thus generating electric current. It is known that, by the principle of electromagnetic induction, this current will be alternated, but, with the use of the commutator, the current obtained was continuous.


The Nominal Loads

Current transformers must be specified according to the load to be connected to their secondary. In this way, NBR 6586/92 standardizes secondary loads.

For a current transformer, the secondary load represents the ohmic value of the impedances constituted by the different apparatus connected to its secondary, therein including the interconnecting conductors.

By definition, nominal secondary load is the impedance connected to the secondary terminals of the TC, whose value corresponds to the power for the guaranteed accuracy, under rated current.

It should be noted that when the nominal secondary current is different from 5A, the values ​​of the loads shall be multiplied by the square of the relation between 5A and the corresponding nominal secondary current in order to obtain the desired values ​​of said parameters.

The load of the devices to be connected to the current transformers must be carefully dimensioned to choose the compatible standardized load current transformer. However, since the devices are connected to the CTs by means of wires, usually of great length, it is necessary to calculate the power dissipated in these conductors and to add it to the power of the corresponding apparatuses.



Operating Basics

Some actions that directly operate in the world of electric motors that everyone should know what they are and what they serve, and this should be considered in order to ensure the improvement of performance and extend its useful life. Among these procedures is balancing, which basically consists of a procedure whereby the mass distribution of a body is accounted for and, if necessary, is also adequate to warrant that residual imbalance or vibrations and forces in the bearings in the frequency of mechanical rotation, and within the limits specified in the international standards.

The balancing intensity has the important task of recommending the peak amplitude of the vibration velocity, which will be reported in mm / s, of a rotor that rotates freely in space, being a specific unbalance item and the rotor angular velocity at maximum speed of operation.          3B58SD Browning

The grounded part is metal parts electrically connected to the grounding system and the live part is incident on a conductor or a conductive part, which is to be energized in normal use and may include the neutral conductor.


Motor Storage

In the case of electric motors that will not be used soon, they must be stored in appropriate environments, such as dry places with an average air humidity of 60%, with a temperature ranging between 5 ° C and 40 ° C, preserved from dust, vibrations, gases and corrosive agents.

Engines that have heating resistance must be energized whenever the engine is not operating. This is also useful for cases where the engine is already installed but not used for a long time. In these situations, depending on the ambient conditions, water condensation may occur inside the engine, which may result in the insulation resistance being reduced.    reelcraft D8800 OLP

Engines must be carefully stored in such a way that drainage is facilitated. The heating elements must never be energized when the engine is running.

Electric motors need to be kept at constant temperature and in the original position, preventing any object from being supported on them. Remove the pulleys from the shaft end and keep them free and with protective grease to prevent corrosion.


Starting Methods OK

The electric motors are commanded by means of starters and the most applied are the direct, reversing starter, which works in the drive of small motors; Star triangle starter, which acts on the drive of large motors that have no load; compensating start-up that drives the big engines with load; and starting with frequency inverter, which acts in the drive of FD84000 OLP small and large motors.

Starters include protection devices such as fuse, thermal relay, circuit breaker, control devices such as buttons, contactors and timers. And there are also signaling devices, such as signal, voltmeter and ammeter.

All of these starting switches have a main circuit and a control circuit, where the main or power circuit, as it is also called, acts on the motor supply, in this way it ends up having the function of connecting the terminals and wires of the motor the power grid. In turn, the control circuit, as its name says, has the function of controlling the power circuit, thus determining when the motor can be turned on or off.

What affects engine performance?

The magnetic cores of electric motors, which are stators and rotors, with rare exceptions, are constructed from thin metal sheets (low carbon steel plates) with a thickness of less than 1 mm, grouped in sheet metal packages. Magnetic nuclei surrounded by coils (windings), where alternating currents circulate, cause a magnetic flux also alternating and for that reason, these nuclei are subject to the action of parasitic currents. These currents are known as Foucault currents, which are responsible for appreciable loss of power in these nuclei.

reelcraft 5635ELP

Some higher performance motors are designed with silicon steel plates, with a percentage of approximately 3% silicon. The total process for making these cores consists essentially of lamination, stamping, a treatment for electrical insulation, packaging and fixing. With regard to low carbon steel plates, the process for insulation is based on a heat treatment, where the packages of plates are placed in kilns for a certain time, there being oxidation of the surface of the plates, and as a consequence, the formation of an insulating layer of iron oxide between the adjacent sheets. Some types of silicon steel sheets are provided by manufacturers with a paint based oxide on one of the surfaces.