供电电压不平衡
根据三个线路读数求出电压或电流不平衡度。
尝试一个工程 4
每项工作都会预填示例数值。请替换为您自己的数值。
- Check the voltage unbalance at a motorThe unbalance of 415, 402 and 409 V at a motor starter.
- Voltage unbalance of a rural supplyHow far out a long rural line is at 400, 384 and 396 V.
- Check the current unbalance of a motorThe current unbalance of a motor drawing 31, 27 and 29 A.
- Find the motor heating from unbalanceHow much a 3 % unbalance adds to the winding temperature rise, by the rule of thumb.
输入
使用默认输入值的示例计算。
- 读数
- 线电压
- A 至 B 电压
- 405 V
- B 至 C 电压
- 398 V
- 电压 C 至 A
- 411 V
偏差不平衡
1.65%
- 序分量不平衡度
- 1.86%
- 三者的平均值
- 404.67V
- 与平均值的最大偏差
- 6.67V
- 绕组温升增加，经验法则
- 5.4%
此结果仅供设计参考。请对照现行标准进行核验。
计算步骤
- 三者的平均值404.67V
- 与平均值的最大偏差6.67V
- 偏差定义：100 × 6.67 ÷ 404.67结果1.65%
- 序列定义：100 × 负序 ÷ 正序，由三个量值计算1.86%
- 电机发热：2 × 1.65²5.4%
关于此计算器
How this is calculated
A three‑phase supply is balanced when its three line voltages are equal and 120° apart. Long rural lines, single‑phase loads spread unevenly and a failing connection all pull the three apart. The tool puts a number on how far apart they are, in the two ways the trade uses.
The deviation definition is the simple one. The tool takes the mean of the three readings and finds the reading furthest from it. The unbalance is that deviation over the mean, as a percent. This is the definition NEMA uses for motors.
The sequence definition comes from symmetrical components. Any three phasors split into a positive sequence, which turns a motor forward, and a negative sequence, which turns against it. The unbalance is the negative sequence over the positive sequence, as a percent. This is the IEC definition. The tool works it from the three magnitudes alone, with a closed formula that holds for line-to-line voltages. For a small unbalance the two definitions are close.
For voltages the tool also gives a motor heating figure. A widely published rule of thumb puts the percent increase in the winding temperature rise at twice the square of the percent unbalance. It is an estimate, not a standard's curve. AmpSize holds no standard for unbalance, so the tool cites none.
The same sums work on three line currents. Current unbalance is often several times the voltage unbalance behind it.
| Step | Table or clause | What it decides |
|---|---|---|
| Mean | The sum of the three readings over three | The reference the deviation is taken from |
| Deviation definition | Largest deviation over the mean | The NEMA style unbalance |
| Sequence definition | Negative over positive sequence, from the magnitudes | The IEC style unbalance |
| Motor heating | Twice the square of the unbalance | A rule of thumb for the increase in the winding temperature rise |
| A reading that cannot close | One reading larger than the other two together | No sequence figure, check the readings |
Worked examples
- A motor starter reading 415, 402 and 409 V is 1.63 % out by the deviation definition and 1.84 % by the sequence definition.
- A rural supply at 400, 384 and 396 V is 2.37 % out by the deviation definition and 2.43 % by the sequence definition.
- A supply at 400, 388 and 412 V is 3 % out by the deviation definition and 3.47 % by the sequence definition. The rule of thumb puts the increase in the winding temperature rise at 18 %.
- A motor drawing 31, 27 and 29 A has a current unbalance of 6.9 % by the deviation definition and 7.99 % by the sequence definition.
Limits
The tool takes three readings at one moment. A supply that swings through the day needs readings at its worst hour, or a logger.
It does not give a motor's rating at an unbalance. That curve is in NEMA MG 1 and in the maker's data, which AmpSize does not hold. Use the maker's figure for a motor that runs near its full load.
Phase-to-neutral readings can carry a zero sequence part that the sequence formula leaves out. Enter line-to-line voltages.
See also: Neutral current, Three phase power and Motor.
问题
结果仅供设计参考。请依据现行标准核对。
如何计算电压不平衡？
取三个线电压的平均值。找出与平均值偏差最大的读数。不平衡度为该偏差除以平均值，以百分比表示。405、398 和 411 V 的平均值为 404.67 V，偏差为 1.65 %。
偏差定义与序分量定义有什么区别？
偏差定义采用与平均值的最大偏差。序分量定义将负序电压除以正序电压。不平衡度较小时，两者接近。工具同时给出两者的结果。
电压不平衡为何会使电动机发热？
负序电压会驱动一个与转子方向相反的电流。它产生损耗和热量，却不做功。一条公开的经验规则认为，绕组温升的百分比增幅约为 2 × 不平衡度百分比的平方。
应测量线电压还是相电压？
请使用线电压。序分量公式需要三个首尾相接成三角形的相量，线电压即满足此条件。相对中性线的读数可能含有该公式未计入的零序分量。
它能计算电流不平衡吗？
是的。选择线电流并输入三个读数。电流不平衡度通常为引起它的电压不平衡度的数倍，因此电动机上较小的电压不平衡可能表现为较大的电流不平衡。
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