AC Drive Repair with Full Load Dynamometer Test

Check out how our technician walks through an Allen-Bradley AC drive repair from evaluation to a full load test on our 3 tiered dynamometer. We fully load test all AC and DC drives after repair on one of our dynos and we back all of our repairs with an 18 month in-service warranty. Find out more information on our website, or give us a call!

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At Global Electronic Services, we can repair AC and DC drives down to the component level. We consider the job is only half complete without a true load test. Done to ensure a drive is fully tested. This is the key to a successful repair.

We have a two-step process for true load testing drives. Let's watch as this drive gets repaired. The technician will first ohm the drive to check for shorts. When oming the output transistors, we are checking the diode reading between the bus and the output. We check the positive and negative sides of the bus between the input and output on all three phases.

This reading is a zero between the bus and an output connection. This means there is a short so there is at least one bad transistor in this drive. As a standard, we change out all three transistors when one is found bad to ensure that all three will function properly, prolonging the life of the
drive. As the technician breaks down the drive to get to the bad transistor he will visually inspect each board as they are removed from the drive to make sure there isn't further damage. Now he is at transistor level. He will scope the transistors to verify the short this reading is at zero confirming the short . He is removing the three transistors from the drive and will replace them with new ones. Again it is important to change out all three transistors when one is shorted or weak, because once one has gone bad the other two have been compromised, and will usually short out or fail when placed back under full load again.

We stock thousands of transistors. Now he is ready to install the new transistors. He will first apply thermal grease or heat sink compound, which is a thermal insulator. The reason for this is to eliminate air gaps or spaces from the interface area to maximize the heat transfer. Now the technician is reassembling the drive and reconnecting all the circuit boards. Now he is ready to sculpt the output again. As you can see, the readings are good across all three phases.

Prior to running this drive under a full load, he will perform step one in our drives test, the initial run test . For this initial test he will run a motor without a load. This allows us to verify there aren't any weak components before placing the drive under a 100 percent full load. First he is connecting 460 volt power to the input of the drive. Now he is connecting the motors power leads to the output of the drive. The technician is now ready to turn on the 460 volt power. Here he is using the keypad to give the drive of frequency speed reference. Now he is scoping the output and checking the PWM or pulse width modulation. This ensures the gates are firing sequentially. If they were not firing correctly, one line would be missing from the scope. He is also checking to verify that there is not ripple on the bus. If there was ripple, we could see a squiggly type of line. Next he will power down, discharge the bus, disconnect the motor, and head over to the dyno for the final step in our drives test, a 100% full load test.

On our three-tiered dyno, he will select the appropriate tier based on what the drive is rated for. A dynamometer or dyno is two motors coupled together running in opposite directions to create resistance. This resistance is the load we can control, the amount of the load, and bring it over and under its rated amount. The ability to run this drive at its full load gives us an advantage to truly test the drives performance. Without this test the failures would occur at the time of install for the customer, leaving them a useless drive that only runs without a load on it. The technician is hooking up the 460 volt power to the input of the drive, and the dyno leads into the drives output. Now he is powering on the dyno. First he is using an amp meter to make sure that the current levels on each phase are balanced before engaging the load. The technician will now turn on the load and will adjust the load torque according to the drives output amp rating. Then he checks each phase again while under load to ensure that each are balanced. This drive is successfully running and it's fully rated load. After it runs for a one-hour period at 30 and 60 Hertz, it will be ready to be cleaned and shipped back to the customer you.

[Pynenberg]

Its that last testing step that is so vital. I could just replace all output transistors on a dead drive and just call it a day but without a true load test for a duration of time I'd be just guessing.