Copper loss

The copper losses or winding losses are referred to the current heat losses occurring in all the coils in transformers, electric motors, generators and electromagnets. The losses arise mainly through the ohmic resistance of the copper winding.

Basics

Copper losses occur both at direct current and in alternating current. The losses increase with the square of the load.

The current heat losses in accordance with the following formula:

Are calculated, where stands for the coil current and the winding resistance.

The copper losses are determined by the used wire, the winding technique employed, the temperature, and by the current and the voltage. The copper losses are determined by the short circuit test. Caused by alternating fields in the iron core losses are attributed to the iron losses.

Transformers

For transformers are among the copper losses any losses caused by the load current in the respective coils. Although in modern transformers, the windings may also be made ​​of aluminum, while the term copper losses has established itself and is also predominantly used. As seen from the formula of the current heat loss (copper loss) seen, the copper losses are strongly dependent on load. For transformers with multiple windings correspond to the total copper losses of the sum of the respective individual winding losses. By the copper loss, the transformer is heated, this means that the resistivity of the windings increases. Thus, the copper losses are higher due and decreases the voltage on the secondary side when loaded more than in the cold condition of the transformer.

The copper losses or load losses amounted to at Power Transformers:

• Idle approximately 0 percent
• At half load about 0.1 to 0.5 percent
• At full load, about 0.5 to 2.0 percent

In the design of modern power transformers for the power operation, a loss ratio of iron Dissipation: fixed copper power loss at the rated operating point at 0.17 to 0.25. The maximum efficiency of the transformer is located in the operating point in which the copper loss is as large as the iron loss. So about half of the rated power. With transformers for switch mode power supplies, the skin effect also affects the copper losses.

Electric motors

Electric motors are among the copper losses all losses caused by the load current in the respective carrying windings. Permanent magnet motors have only one winding; in the direct-current machine this is in the rotor, in the electrically commutated machine, it is in the stator. When fully executed electrically excited DC machine are the armature windings, the commutating windings, the field winding and the compensation winding. For synchronous machines, the stator winding and the field winding for induction motors, the stator winding and the rotor winding. In phase induction motors the winding losses in the rotor are directly dependent on the slip. There, the slip is equal to one when the motor is at the moment where the rotor does not rotate, therefore the total induced in the rotor is converted into heat. Since the starting current is a multiple of the rated current at phase asynchronous motors, the current heat losses are a multiple of the rated motor power. At too low a power supply voltage falls with a constant load, the engine speed, thus, the slip increases. This leads to the increase the power consumption and thus increases the copper loss.

At high frequencies occurs in the motor windings in addition to the current crowding. In the stator, this effect due to the low field strength in the grooves and forced through the series circuit of the windings equal distribution of the total current of the coils is generally small and can be neglected. Different story in the rotor bars: Here are the conductor connected in parallel across the groove. At higher frequencies, as they occur in the start-up of the engine, the upper layers of the rotor winding or rotor bars can almost fully compensate for the stator and the lower layers do not carry current. By this current crowding leads to a higher AC resistance. Although this higher resistance leads to higher losses, but also to a higher torque at startup and is therefore desirable in large induction motors, because in the nominal operating point, the frequency in the rotor is so low that the current displacement effect does not occur.

Loss reduction

The copper losses can be reduced in various ways. The ohmic resistance of the windings of transformers can be reduced by the number of windings is reduced ( and additionally increased the wire cross-section for a given winding space ). However, this can not vary arbitrarily, since the magnetizing inductance is proportional to the square of the number of turns and therefore the copper losses increase accordingly idle. For coils and transformers, which are used at higher frequencies, but this method is common practice. At a certain frequency Hochfrequenzlitzen be used instead of solid wires for the coils. Thus, the skin effect is reduced. At a certain frequency limit, however, the use of HF stranded wire does not make sense, this frequency limit is dependent on the tube radius. Above this frequency caused by the external proximity effect losses are proportional to the number of cores. Here, either a solid wire or a smaller core radius must be used. In the design of such transformers or coils for the high-frequency range is a compromise between loss of copper and proximity losses is sought.

For motors, the copper losses can not be influenced by a variation of wire cross-section and number of turns for a given load, since the entire flux, the torque determined, regardless of how many conductors it is distributed. The copper losses in the stator can be optimized by minimum conductor length and optimal fill factor. Are these done to yourself understandable steps, they can be reduced only by increasing the stator slots. Copper losses in the rotor of a Asynchonmaschine be reduced by larger rotor bars, copper instead of aluminum and better sized short-circuit rings .. The increase in area for the windings limits for a given motor volume set however, since the copper surface to their room with the river leading iron shares, which by saturation can conduct only limited magnetic flux. Regarding therefore copper losses optimized machines have a low overload. In iron-free air coil machines do not have this problem; the winding height directly reduced the usually excited by permanent magnets air gap induction, which ultimately results in a loss regarding Copper optimum winding thickness. In contrast to the copper losses with respect to optimized grooved motor brings the full optimization in air coil machines, however, no saturation effects and therefore no reduced overload with it.