The motor cannot start due to high voltage drop
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This post was last edited by myhx1668 on 2010-9-20 at 17:09. Our company currently has a 37KW motor; due to the long distance of the power lines (380V, over 1000 meters), an autotransformer is used to reduce the voltage for starting the motor. It used to start normally in the past, but suddenly stopped working half a month ago. When the start button is pressed, the contactor keeps opening and closing repeatedly, like a machine gun. I hope experts can help analyze why this is happening How to solve this problem? {It should be noted that the voltage is 400V when there is no load; at the moment of reduced-voltage startup, the voltage drops to around 300V.} Here’s an update: I tried it again recently, and it started up once. During the next attempt, both 16A fuses in the control circuit blew, so I didn’t dare to use the UPS to try it. What’s going on? I’m becoming more and more confused now. (In my circuit, half of it uses VV22 3×70+1×35 cables, while the other half consists of 70-mm diameter aluminum-clad steel conductors installed overhead; at the connection point between these two types of conductors, there is a 200A horn-shaped fuse.) ）
This post was last edited by zhaohh3211 on 2010-11-15 at 16:47. When the starting contactor does not engage, the voltage is high; once the contactor engages, the voltage drops. When the contactor releases, the voltage rises again, causing the contactor to engage once more, and this cycle repeats. I. If possible, reduce the voltage level of the autotransformer by one more step. II. Replace it with a thicker cable. III. Connect another identical cable as well.
This post was last edited by zyz8438 on 2010-9-17 00:47. All three methods mentioned above are feasible.
First of all, I thank those on floors 2 and 3 for their help, but I still want to know: 1. Why could it be started before, but not now?； 2. The autotransformer has already been set to 60%; it cannot be adjusted any lower. Replacing or connecting cables is not either a simple task. Are there any other methods?
Well, this is also a question worth considering! It worked fine at first but then stopped working. I suspect that the control equipment (especially the coil of the contactor) is gradually aging, which causes the release voltage of the contactor to increase over time. As a result, at around 300V (78.9% Ue), the contactor can no longer remain in its closed state and thus releases. Once the load is removed, the voltage rises, causing the contactor to close again, resulting in it closing repeatedly like a machine gun.
Of course, it’s also possible that there’s a problem with the control circuit! First, disconnect the cable that is located below the contactor (in other words, remove the load), and then try to open and close the contactor to see if the problem of frequent opening and closing of the contactor still persists. If the problem persists, it’s not a voltage drop issue but a problem with the control circuit itself!
This post was last edited by liuei on 2010-9-17 10:10. Check the wiring of the control circuit again to ensure it is correct! If it is confirmed that the issue is caused by voltage drop, you can try this solution: use a small UPS to supply power to the control circuit; if the control circuit operates at 380V, then a 220/380 V control transformer is necessary!
Could the time at which the control voltage is switched also have an impact??
This post was last edited by zhaohh3211 on 2010-11-15 at 16:52. I have a question: what is the capacity of the transformer that feeds the original poster’s equipment? If the capacity is sufficient, it shouldn’t be possible for the voltage to drop by so much, right?
I think the key is still to solve the problems on site. Option 1: The release voltage of an AC contactor is generally within the range of 70-80% of Un. Although it was possible to start operation in the past, it was actually likely already approaching the critical value for release voltage. Consult the relevant manufacturers to see if there are AC contactors with a lower release voltage, and replace the contactor accordingly. Option 2: Replace the coil of the AC contactor with a DC coil, and change from AC control to DC control. Because the decrease in AC voltage caused by motor startup does not affect DC. Technically, this solution is very good, but it is estimated that a DC power supply cannot be obtained, so its feasibility is low.
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