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Current drops when motor starts?


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Out on site a customer has a 900hp motor he want us to monitor the current, voltage and harmonics.


The basic layout of his system is: 72kv/4.16kv xfmr, isolation xfmr, capacitor, soft start, metering and motor.


We connected to the customers metering and monitored before the motor started. Before startup we read 150A (this would include lights and a few small motors) seems high to me. Once the 900hp motor started it immediately dropped the currrent to 34A. As the motor came up to speed the current sat at 90A. We double checked this at the utility's meter and it was te same.


What is causing the current to drop? and why?


I'm new to the power quality field, bare with me.



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Hello lhowes


Sounds to me like you may have a lot of power factor correction capacitance permanently connected.

The only other scenario would be if the motor is being over driven by the load and is acting as an induction generator.


Can you give us more details??


Best regards,

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Thanks for your interest so far.

I got the motor size mixed up with another job I was doing, it is actually a 3000hp not 900hp motor.


The customer was complaining because their harmonic filter bank was tripping the motor starter (soft start) on start up. It did not do this when we were on site.


Their site is split into two voltages. Their 4160 comes off their main 72kV / 4160V transformer, runs through a large isolation transformer the feeds their 3000hp compressor motor. 480V is also used. It is transformed down before the isolation transformer and appears to run small lube pump motors, coolant motors, that sort of thing. In total we added about 15hp of small motors. Their only real load is these small motors and the one large 3000hp compressor motor


We were connected at their main incoming that would monitor both the comp. motor and the 480V motors. The load current was about 150A with the big motor off. This was verified on two of their meters, my fluke and my ACE meter, and later on utilities 72kV metering. When the motor started up the current dropped to ~30A. Again this was verified on all the meters including utility metering.


They use a harmonic filter bank for the 5th, 7th and 11th order which is left on all the time. After taking my readings we had to do another start up once wenoticed the large amount of even harmonics being produced. It appeared the filter bank was doing it’s job with it’s odd harmonics, but we’ve never before seen the even order show up.


The voltage waveform is a notched sine wave and the current waveform is a triple peaked notched wave. (These are the worst waves I saw, these are only present while the motor is starting). Before startup the waveforms are ideal.


The current profile: Sits steady @ 150A before start up, Motor starts current goes up to 200A and immediately drops to ~40A, Slowly comes back up to 90A, a small peak back up to 150A, and levels off @ 90A until we shut the motor down.


Voltage profile: Sits steady @ 4160V before startup, Motor starts voltage drops immediately down to 3950V, the voltage wavers around the 4100V mark and then levels out @ 4140V until the motor is shut down.


Its too bad I couldn't attach the voltage and current profiles.


We couldn’t record the current flowing through the filter bank. Five different meters read the same results. Do you have any ideas to why the current would show a decrease with the motor running, or why it is so high with the compressor motor off?



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a) It sounds to me as though the filter is very capacitive and that there is a high level of capacititive current flowing into the filter. This would give a high leading power factor at the point of supply. When the motor is running, there is some lagging current due to the inductive nature of the motor and this is helping to neutralize the capacitive current. In effect, the motor is power factor correcting the filter.


B) the plant has a poor power factor and the large motor is a synchronous motor and it has been set up to correct the power factor of the load.


Have you looked at the power factor at the point of supply? if you monitor voltage and current at the same time, you can get an indication of the displacemtn power factor by the angles between the current and voltage zero crossings.


I am curious about the even order harmonics. Modern soft starters employ reverse parrallel connected SCRs and should not produce significant even harmonics.


Best regards,

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