Hello everyone, I am seeking assistance in troubleshooting an issue with an older reliance DC drive that is powering a 50 HP DC motor for a main machine drive in my facility. Initially, the machine would only turn on after jogging and then pressing the start button. However, it has now completely stopped turning on. After testing the motor leads and replacing the motor with no success, we suspected a fault in the main drive. A similar drive from another plant was tested and yielded the same results. The drive in question is a 500VDC armature drive with a field voltage of 300VDC. While the field voltage is correct, the voltage readings across the drive's wires are inconsistent. Despite correct 3 phase 480v input, we are unable to obtain the proper voltage output from the drive. Fuses have been checked and are intact. We have consulted electricians without success and are in need of expertise with older DC systems. Any assistance provided will be highly appreciated. Thank you in advance for your help. Please refer to the image labeled "Wiring diagram" for clarification - note that the drive is actually 50 HP, not 40 HP as indicated.
Have you checked the disconnected motor leads for 'megging' at the drive?
Based on your description, it seems like the power section in your drive may be limited by a 3 SCR - 3 Diode configuration, restricting control. Ideally, you would use a scope to analyze the output waveform, but if that's not possible, we will need to explore alternative methods. It appears that your motor is overloaded, causing issues when starting up. The drive ramps up voltage gradually, but when the current reaches its limit, it stops. However, when you jog the drive, the ramp is bypassed and the motor receives full speed/voltage directly, providing the necessary torque to start moving. Over time, the motor may struggle to provide enough breakaway torque. To troubleshoot, try running the motor without the load to check for any mechanical issues like a faulty bearing or gearbox. Before investing in costly repairs, consider upgrading to a new VFD and motor for improved performance and lower maintenance costs.
After troubleshooting, we were able to successfully start the motor using the drive. It's uncertain whether the issue stemmed from a faulty motor or drive. However, on Friday evening, we discovered that the main contactor was malfunctioning. Although the coil side was receiving voltage, it failed to engage the contactor. This was a new problem, as the contactor had been working properly until then. Moreover, there were also faulty contacts within the contactor. Presently, the motor is running at full speed despite having a potentiometer connected to the drive for speed control. Testing the speed reference terminals showed that they are functioning correctly, with a range of 0-10v when adjusting the knob. Therefore, the problem might be due to a missing jumper, causing the motor to run at full speed without the ability to adjust its speed.
I quickly responded to your comment as well as the other guys' comments.
When dealing with a DC motor, it is crucial to check both the voltage and current levels on the armature and the field. A common issue is a broken wire, which can cause the motor to go into runaway mode, with the only limitation being the torque. I have witnessed motors with an open field experience a sudden increase in RPMs, leading to motor failure. Therefore, always ensure proper connections and thorough inspections to prevent potential motor damage and accidents.
It sounds like you're facing a frustrating situation! Since you've already checked the basics, I would suggest looking closely at the control circuitry within the drive. Sometimes older drives have issues with aged components like capacitors or relays that can lead to inconsistent voltage readings. Additionally, make sure all connections are tight and free from oxidation, as that can often cause intermittent issues. If you have access to a multimeter or oscilloscope, it might also be worth monitoring the control signals to see if the start command is actually reaching the drive. Good luck, and I hope you get it up and running soon!
✅ Work Order Management
✅ Asset Tracking
✅ Preventive Maintenance
✅ Inspection Report
We have received your information. We will share Schedule Demo details on your Mail Id.
Answer: - Possible reasons could include faults in the main drive, inconsistent voltage readings across drive wires, issues with the input/output voltages, or problems with fuses.
Answer: - You can start by checking the motor leads, replacing the motor, testing with a similar drive, verifying correct input voltages, inspecting fuses, and ensuring the field voltage is correct.
Answer: - Older DC systems may have unique characteristics and troubleshooting methods that require specialized knowledge and experience to identify and resolve issues effectively.
Answer: - Yes, the provided image labeled "Wiring diagram" can be referenced for clarification and to ensure correct connections and configurations are being used for troubleshooting.
Join hundreds of satisfied customers who have transformed their maintenance processes.
Sign up today and start optimizing your workflow.