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Direct starting of induction motors

Direct starting is the method of driving electric motors of alternating current, where the motor is connected directly to the electrical network, that is, it happens when we apply the nominal voltage on the windings of the motor stator, in a direct way. For the implementation of this starting system, we use a contactor as a switching device and protection devices such as fuses and overload relays or circuit breakers.

It is necessary to illustrate the main diagram, also called force or power for the direct starting system 23024 skf-bearing, and we need to energize the coil of the contactor C1 so that it can activate the main contacts of the contactor by starting the motor. For this, it is important to observe the value of the supply voltage of the coil. Normally coils are fed which are supplied with voltages with the same value of the supply network (220 Vac). Protection fuses, overload relay terminals (e4), a normally open push-button for motor startup (B1) and another NF pushbutton (normally closed) are used to switch the motor off (B0 ).

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Single-phase motor and starting capacitor

The single-phase induction motor has a main winding connected directly to the mains supply and a lagging auxiliary winding, usually at 90 ° from the main winding and connected in series with a capacitor. Both the auxiliary winding and the capacitor are inserted into the supply circuit only during the starting period of the motor. The capacitor allows a larger angle of lag between the main and auxiliary winding currents, thus providing high starting torque. The main winding alone develops almost the same torque as the combined windings.

22222 e/c3 SKF-Bearing

For higher velocities between 80% and 90% of the synchronous speed, the curve of the conjugate with the combined windings passes the curve of the conjugate of the main winding so that for speeds above this point, the motor has more conjugated with the auxiliary circuit switched off. Because the crossing of the curves does not always occur at the same point, and still, the centrifugal switch does not open at precisely the same speed, it is common practice to cause the aperture to occur on the average just before the curves intersect. After disconnection of the auxiliary circuit, its operation is the same as that of the split-phase electric motor.

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Cogging Torque

Cogging torque is a result of the permanent interaction of the magnet produced with the permeability of the air gap manifested by the tendency of the rotor to look for stable positions, even when the motor is off, which results in a pulsating torque that does not cooperate for the total torque of the system. The cogging torque is quite harmful when it comes to precision applications because it introduces small variations in speed that generate vibrations in the engine that are felt when it operates at low speeds and with light loads.

In order to reduce this effect, some changes are made in the design of the machine, such as determining the width of the openings of the stator slots, the angle of inclination of those grooves and creation of auxiliary teeth. In cases where the cogging torque is not very extraordinary, it is customary to tilt the stator grooves, at most one groove ac motors. Theoretically, it is possible to eliminate the cogging torque by using a stator configuration which is based on the construction of a flat stator where the windings are not housed in grooves. It is a configuration that would be good for running at high speeds, as iron losses would be very low and the engine would see its weight reduced.

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Influence of the environment

The ambient conditions where the engine will be installed influence its operation, since dust that deposits in its housing, when it absorbs moisture or particles of oil, forms a crust that will hinder the release of heat and because of this, the internal temperature of the engine will rise. One of the consequences is to increase the value for the winding resistance and decrease the efficiency of the motor. Cereal and steel processing areas are examples of environments that need to be taken care of with engine cooling, due to the presence of dust, dust and suspended particles.

The increase in temperature due to the presence of dirt on the housing also causes deterioration of the lubricant, oil or grease used in the bearing or bearing because its lubricity reduces with increasing temperature vem3709t. Humidity is one of the main causes of insulation failures. In humid environments, corrosion and deterioration of the insulation occurs, since humidity facilitates the deposit and absorption of dust and chemicals. Motors operating in humid environments have more failures in their insulation system than those working in dry environments subject to the same type of dirt, dust, and chemical agents.