Can a motor keep running with a missing phase?

This post was last edited by zhaohh3211 on 2010-6-22 14:53. If overload protection is installed, it won’t work! If it’s not installed, it will have an impact; I’m not sure about the specifics and need someone more knowledgeable to help.

In the event of a phase loss, the motor will continue to rotate (provided there is no phase loss protection; otherwise, the protection will activate and stop the motor), but its speed will **decrease**, its output will **drop**, and there will be a humming sound due to the current flow.

Without overload protection, it will continue to operate, but the load caused by under-voltage can eventually damage the motor.

What was said upstairs is correct; I’ve encountered this in my work – if there is no phase loss protection, the motor will continue to rotate, but it will soon burn out, and it won’t be possible to restart it after it stops working

Suggested author: KSNW120867; it would be better to consult Wei Changjiang’s work on \"Simulation Study of Operation of Three-Phase Induction Motors with a Missing Phase\", published in Motor Technology, issue 03, 2002. Additionally, in systems where the transformer’s neutral point is not grounded, operation can continue for at most 2 hours after one phase becomes grounded – this time depends on the voltage tolerance and insulation properties of the PT and other equipment, but it certainly will not exceed 2 hours. Because when one phase is grounded, the voltages of the other two phases rise to 1.732 times the rated voltage. Isn’t the high-voltage motor also continuing to operate at this time? However, in the case of low-voltage motors, the secondary side of the transformer is connected in star configuration, which is different from high-voltage motors. When one phase is grounded, the vectors of the other two phases do not change, resulting in an effective phase loss. This leads to uneven rotation of the magnetic field, causing stalling, vibration, and other issues; as a result, the current rises and the motor can be damaged. Of course, some people may have encountered situations where a motor continued to operate without burning out despite having a missing phase; this is related to light load conditions, as the operating current did not exceed the rated value. In some cases, the motor managed to start, albeit slowly at first before gaining speed; in other cases, it failed to start, but would start once manually triggered. Moreover, the motor makes a buzzing sound due to a missing phase.

Excuse me, isn’t that high-voltage motor upstairs also continuing to run at this time? However, for low-voltage motors, the secondary side of the transformer is connected in star configuration, which is different from high-voltage motors; after a single-phase ground fault occurs, the vectors of the other two phases do not change. ” How should this sentence be understood? The secondary side is connected in star configuration; why then, when there is a single-wire short circuit, the two-phase vectors do not change?

This post was last edited by jjli618 on 2010-6-23 at 17:50. Reply to 7# sofine: In a transformer system with an ungrounded neutral point, the neutral point is floating; when one of the vectors changes, the other two vectors change as well. In a transformer system with a directly grounded neutral point, a single-phase ground fault results in a large short-circuit current, which causes the circuit breaker or fuse to trip and thus cuts off the fault source promptly, while the voltage vectors of the other two phases remain unchanged. (The neutral point is firmly clamped at the potential of ground.) ）

There are two cases: 1. If a motor operates at full load with a missing phase, it may stall. The motor will burn out soon. 2. If a phase is missing during light-load operation of the motor, it will continue to run. The current in two phases of the motor rises sharply, and the motor burns out after operating for a while.

Thank you for the answer from floor 8. There is another question here: in the sentence \"...but for low-voltage motors, the secondary side of the transformer is connected in star configuration, which is different from high-voltage motors; after a single-phase ground fault occurs, the vectors of the other two phases do not change...\", it is not mentioned that the star connection refers to a star-grounded configuration. Does this mean that when we see a star-grounded connection in low-voltage systems, we should assume that it is a neutral-point grounded connection? And for high-voltage systems, the opposite is true – there is no grounding? Can it be understood in this way?

The last edit to this post was made by jjli618 on 2010-6-25 at 00:08. Reply 10# sofine: I’m more than happy to answer your questions: 1. In most factories in my country, a power supply system with a voltage of less than 1 KV is used, where the neutral point on the secondary side of the transformer is directly grounded in order to supply power to low-voltage electrical equipment. The TT system is rarely used. The transformer connections are usually of the star/star or delta/star type; this allows for the provision of both 380V and 220V power supplies. Moreover, the delta/star connection method helps to reduce the level of 3N harmonics. 2. For transformers with voltages such as 35/6 KV, the delta/star connection is also used, and the neutral point on the secondary side of the transformer is not grounded. This enables the circuit to continue operating for a certain period of time in the event of a single-phase ground fault. The 220KV/35KV transformers use a Y/Y connection. Furthermore, in the star connection of the primary side of the transformer, the neutral point is directly grounded during operations to prevent overvoltage from occurring and causing insulation breakdown; after the operations are completed, the grounding switch for the neutral point is disconnected, while the secondary side neutral point is grounded through an arc-suppression coil.