The loss of three-phase AC motor can be divided into copper loss, aluminum loss, iron loss, stray loss, and wind loss. The first four are heating loss, and the sum is called total heating loss. The proportion of copper loss, aluminum loss, iron loss and stray loss to the total heat loss is expounded when the power changes from small to large. Through the example, although the proportion of copper consumption and aluminum consumption in the total heat loss fluctuates, it generally decreases from large to small, showing a downward trend. On the contrary, iron loss and stray loss, although there are fluctuations, generally increase from small to large, showing an upward trend. When the power is large enough, the iron loss stray loss exceeds the copper loss. Sometimes stray loss exceeds copper loss and iron loss and becomes the first factor of heat loss. Re-analyzing the Y2 motor and observing the proportional change of various losses to the total loss reveals similar laws. Recognizing the above rules, it is concluded that different power motors have different emphasis on reducing temperature rise and heat loss. For small motors, copper loss should be reduced first; for medium and high-power motors, iron loss should be focused on reducing stray losses. The view that “stray loss is much smaller than copper loss and iron loss” is one-sided. It is especially emphasized that the greater the motor power, the more attention should be paid to reducing stray losses. Medium and large capacity motors use sinusoidal windings to reduce harmonic magnetic potential and stray losses, and the effect is often very good. Various measures to reduce stray loss generally do not need to increase effective materials.
Introduction
The loss of three-phase AC motor can be divided into copper loss PCu, aluminum loss PAl, iron loss PFe, stray loss Ps, wind wear Pfw, the first four are heating loss, the sum of which is called total heating loss PQ, of which stray loss It is the cause of all losses except copper loss PCu, aluminum loss PAl, iron loss PFe, and wind wear Pfw, including harmonic magnetic potential, leakage magnetic field, and lateral current of the chute.
Due to the difficulty in calculating the stray loss and the complexity of the test, many countries stipulate that the stray loss is calculated as 0.5% of the input power of the motor, which simplifies the contradiction. However, this value is very rough, and different designs and different processes are often very different, which also hides the contradiction and cannot truly reflect the actual working conditions of the motor. Recently, the measured stray dissipation has become more and more popular. In the era of global economic integration, it is the general trend to have a certain forward-looking how to integrate with international standards.
In this paper, the three-phase AC motor is studied. When the power changes from small to large, the proportion of copper loss PCu, aluminum loss PAl, iron loss PFe, and stray loss Ps to the total heat loss PQ changes, and the countermeasures are obtained. Design and manufacture more reasonable and better.
1. Loss analysis of the motor
1.1 First observe an instance. A factory exports E series products of electric motors, and the technical conditions stipulate the measured stray losses. For ease of comparison, let’s first look at 2-pole motors, which range in power from 0.75kW to 315kW. According to the test results, the ratio of copper loss PCu, aluminum loss PAl, iron loss PFe, and stray loss Ps to the total heat loss PQ is calculated, as shown in Figure 1. The ordinate in the figure is the ratio of various heating losses to the total heating loss (%), the abscissa is the motor power (kW), the broken line with diamonds is the proportion of copper consumption, the broken line with squares is the proportion of aluminum consumption, and the The broken line of the triangle is the iron loss ratio, and the broken line with the cross is the ratio of the stray loss.
Figure 1. A broken line chart of the proportion of copper consumption, aluminum consumption, iron consumption, stray dissipation and total heating loss of E series 2-pole motors
(1) When the power of the motor changes from small to large, although the proportion of copper consumption fluctuates, it generally changes from large to small, showing a downward trend. 0.75kW and 1.1kW account for about 50%, while 250kW and 315kW are less than The proportion of 20% aluminum consumption has also changed from large to small in general, showing a downward trend, but the change is not large.
(2) From small to large motor power, the proportion of iron loss changes, although there are fluctuations, it generally increases from small to large, showing an upward trend. 0.75kW~2.2kW is about 15%, and when it is greater than 90kW, it exceeds 30%, which is greater than copper consumption.
(3) The proportional change of stray dissipation, although fluctuating, generally increases from small to large, showing an upward trend. 0.75kW ~ 1.5kW is about 10%, while 110kW is close to copper consumption. For specifications greater than 132kW, most of the stray losses exceed copper consumption. The stray losses of 250kW and 315kW exceed the copper and iron losses, and become the first factor in the heat loss.
4-pole motor (line diagram omitted). The iron loss above 110kW is greater than the copper loss, and the stray loss of 250kW and 315kW exceeds the copper loss and iron loss, becoming the first factor in the heat loss. The sum of copper consumption and aluminum consumption of this series of 2-6 pole motors, the small motor accounts for about 65% to 84% of the total heat loss, while the large motor reduces to 35% to 50%, while the iron consumption is the opposite, the small motor accounts for about 65% to 84% of the total heat loss. The total heat loss is 10% to 25%, while the large motor increases to about 26% to 38%. Stray loss, small motors account for about 6% to 15%, while large motors increase to 21% to 35%. When the power is large enough, the iron loss stray loss exceeds the copper loss. Sometimes the stray loss exceeds the copper loss and iron loss, becoming the first factor in the heat loss.
1.2 R series 2-pole motor, measured stray loss
According to the test results, the ratio of copper loss, iron loss, stray loss, etc. to the total heat loss PQ is obtained. Figure 2 shows the proportional change in motor power to stray copper loss. The ordinate in the figure is the ratio of stray copper loss to the total heat loss (%), the abscissa is the motor power (kW), the broken line with diamonds is the ratio of copper loss, and the broken line with squares is the ratio of stray losses . Figure 2 clearly shows that in general, the greater the motor power, the greater the proportion of stray losses to the total heat loss, which is on the rise. Figure 2 also shows that for sizes greater than 150kW, stray losses exceed copper losses. There are several sizes of motors, and the stray loss is even 1.5 to 1.7 times the copper loss.
The power of this series of 2-pole motors ranges from 22kW to 450kW. The ratio of the measured stray loss to PQ has increased from less than 20% to nearly 40%, and the change range is very large. If expressed by the ratio of the measured stray loss to the rated output power, it is about (1.1~1.3)%; if expressed by the ratio of the measured stray loss to the input power, it is about (1.0~1.2)%, the latter two The ratio of the expression does not change much, and it is difficult to see the proportional change of the stray loss to PQ. Therefore, observing the heating loss, especially the ratio of stray loss to PQ, can better understand the changing law of heating loss.
The measured stray loss in the above two cases adopts the IEEE 112B method in the United States
Figure 2. Line chart of the ratio of copper stray loss to total heating loss of R series 2-pole motor
1.3 Y2 series motors
The technical conditions stipulate that the stray loss is 0.5% of the input power, while GB/T1032-2005 stipulates the recommended value of the stray loss. Now take method 1, and the formula is Ps=(0.025-0.005×lg(PN))×P1 formula PN- is rated power; P1- is input power.
We assume that the measured value of the stray loss is equal to the recommended value, and re-calculate the electromagnetic calculation, and thus obtain the ratio of the four heating losses of copper consumption, aluminum consumption and iron consumption to the total heating loss PQ. The change of its proportion is also in line with the above rules.
That is: when the power changes from small to large, the proportion of copper consumption and aluminum consumption generally decreases from large to small, showing a downward trend. On the other hand, the proportion of iron loss and stray loss generally increases from small to large, showing an upward trend. Regardless of 2-pole, 4-pole, or 6-pole, if the power is greater than a certain power, the iron loss will exceed the copper loss; the proportion of stray loss will also increase from small to large, gradually approaching the copper loss, or even exceeding the copper loss. The stray dissipation of more than 110kW in 2 poles becomes the first factor in the heat loss.
Figure 3 is a broken line graph of the ratio of four heat losses to PQ for Y2 series 4-pole motors (assuming that the measured value of stray loss is equal to the above recommended value, and other losses are calculated according to the value). The ordinate is the ratio of various heating losses to PQ (%), and the abscissa is the motor power (kW). Obviously, iron stray losses above 90kW are greater than copper losses.
Figure 3. The broken line chart of the ratio of copper consumption, aluminum consumption, iron consumption and stray dissipation to total heating loss of Y2 series 4-pole motors
1.4 The literature studies the ratio of various losses to total losses (including wind friction)
It is found that copper consumption and aluminum consumption account for 60% to 70% of the total loss in small motors, and when the capacity increases, it drops to 30% to 40%, while iron consumption is the opposite. %above. For stray losses, small motors account for about 5% to 10% of the total losses, while large motors account for more than 15%. The laws revealed are similar: that is, when the power changes from small to large, the proportion of copper loss and aluminum loss generally decreases from large to small, showing a downward trend, while the proportion of iron loss and stray loss generally increases from small to large, showing an upward trend. .
1.5 Calculation formula of recommended value of stray loss according to GB/T1032-2005 Method 1
The numerator is the measured stray loss value. From small to large motor power, the proportion of stray loss to input power changes, and decreases gradually, and the change range is not small, about 2.5% to 1.1%. If the denominator is changed to the total loss ∑P, that is, Ps/∑P=Ps/P1/(1-η), if the motor efficiency is 0.667~0.967, the reciprocal of (1-η) is 3~30, that is, the measured impurity Compared with the ratio of input power, the ratio of dissipation loss to total loss is amplified by 3 to 30 times. The higher the power, the faster the broken line rises. Obviously, if the ratio of the stray loss to the total heat loss is taken, the “magnification factor” is larger. For the R series 2-pole 450kW motor in the above example, the ratio of stray loss to input power Ps/P1 is slightly smaller than the calculated value recommended above, and the ratio of stray loss to total loss ∑P and total heat loss PQ is 32.8%, respectively. 39.5%, compared to the ratio of the input power P1, “amplified” about 28 times and 34 times respectively.
The method of observation and analysis in this paper is to take the ratio of 4 kinds of heat loss to the total heat loss PQ. The ratio value is large, and the proportion and change law of various losses can be clearly seen, that is, the power from small to large, copper consumption and aluminum consumption In general, the proportion has changed from large to small, showing a downward trend, while the proportion of iron loss and stray loss has generally changed from small to large, showing an upward trend. In particular, it was observed that the greater the motor power, the higher the proportion of stray losses in PQ, which gradually approached the copper loss, exceeded the copper loss, and even became the first factor in the heat loss. stray losses. Compared with the ratio of stray loss to input power, the ratio of the measured stray loss to the total heat loss is only expressed in another way, and does not change its physical nature.
2. Measures
Knowing the above rule is helpful for the rational design and manufacture of the motor. The power of the motor is different, and the measures to reduce the temperature rise and heat loss are different, and the focus is different.
2.1 For low-power motors, copper consumption accounts for a high proportion of total heat loss
Therefore, reducing the temperature rise should first reduce the copper consumption, such as increasing the cross section of the wire, reducing the number of conductors per slot, increasing the stator slot shape, and lengthening the iron core. In the factory, the temperature rise is often controlled by controlling the heat load AJ, which is completely correct for small motors. Controlling AJ is essentially controlling the copper loss. It is not difficult to find the stator copper loss of the entire motor according to AJ, the inner diameter of the stator, the half-turn length of the coil, and the resistivity of the copper wire.
2.2 When the power changes from small to large, the iron loss gradually approaches the copper loss
Iron consumption generally exceeds copper consumption when it is greater than 100kW. Therefore, large motors should pay attention to reducing iron consumption. For specific measures, low-loss silicon steel sheets can be used, the magnetic density of the stator should not be too high, and attention should be paid to the reasonable distribution of the magnetic density of each part.
Some factories redesign some high-power motors and appropriately reduce the stator slot shape. The magnetic density distribution is reasonable, and the ratio of copper loss and iron loss is properly adjusted. Although the stator current density increases, the thermal load increases, and the copper loss increases, the stator magnetic density decreases, and the iron loss decreases more than the copper loss increases. The performance is equivalent to the original design, not only the temperature rise is reduced, but also the amount of copper used in the stator is saved.
2.3 To reduce stray losses
This article emphasizes that the greater the motor power, the more attention should be paid to reducing stray losses. The opinion that “stray losses are much smaller than copper losses” applies only to small motors. Obviously, according to the above observation and analysis, the higher the power, the less suitable it is. The view that “stray losses are much smaller than iron losses” is also inappropriate.
The ratio of the measured value of stray loss to the input power is higher for small motors, and the ratio is lower when the power is greater, but it cannot be concluded that small motors should pay attention to reducing stray losses, while large motors do not need to reduce stray losses. loss. On the contrary, according to the above example and analysis, the greater the motor power, the higher the proportion of stray loss in the total heat loss, the stray loss and iron loss are close to or even exceed the copper loss, so the greater the motor power, the more attention should be paid to it. Reduce stray losses.
2.4 Measures to reduce stray losses
Ways to reduce stray losses, such as increasing the air gap, because the stray loss is approximately inversely proportional to the square of the air gap; reducing the harmonic magnetic potential, such as using sinusoidal (low harmonic) windings; proper slot fit; reducing cogging , The rotor adopts closed slot, and the open slot of high-voltage motor adopts magnetic slot wedge; cast aluminum rotor shelling treatment reduces lateral current, and so on. It is worth noting that the above measures generally do not require the addition of effective materials. Miscellaneous consumption is also related to the heating state of the motor, such as good heat dissipation of the winding, low internal temperature of the motor, and low miscellaneous consumption.
Example: A factory repairs a motor with 6 poles and 250kW. After the repair test, the temperature rise has reached 125K under 75% of the rated load. The air gap is then machined to 1.3 times the original size. In the test under rated load, the temperature rise actually dropped to 81K, which fully shows that the air gap has increased and the stray dissipation has been greatly reduced. Harmonic magnetic potential is an important factor for stray loss. Medium and large capacity motors use sinusoidal windings to reduce harmonic magnetic potential, and the effect is often very good. Well-designed sinusoidal windings are used for medium and high-power motors. When the harmonic amplitude and amplitude are reduced by 45% to 55% compared with the original design, the stray loss can be reduced by 32% to 55%, otherwise the temperature rise will be reduced, and the efficiency will increase. , the noise is reduced, and it can save copper and iron.
3. Conclusion
3.1 Three-phase AC motor
When the power changes from small to large, the proportion of copper consumption and aluminum consumption to the total heat loss generally increases from large to small, while the proportion of iron consumption stray loss generally increases from small to large. For small motors, copper loss accounts for the highest proportion of total heat loss. As the motor capacity increases, stray loss and iron loss approach and exceed copper loss.
3.2 To reduce heat loss
The power of the motor is different, and the focus of the measures taken is also different. For small motors, copper consumption should be reduced first. For medium and high-power motors, more attention should be paid to reducing iron loss and stray loss. The view that “stray losses are much smaller than copper losses and iron losses” is one-sided.
3.3 The proportion of stray losses in the total heat loss of large motors is higher
This paper emphasizes that the greater the motor power, the more attention should be paid to reducing stray losses.
Post time: Jul-01-2022