Although this website is focused on PLCs, I have a question that I believe many knowledgeable individuals can provide advice on. I am currently using a 2HP DC motor with a CMC Pacemaster 1 drive, which is equipped with a 30 amp breaker. The issue I am encountering is that the breaker trips about 50% of the time when the motor is starting up. Once the motor starts running, it continues without tripping the breaker. During idle moments with the power off, I have measured between 1 and 2 ohms from Arm to Arm, and consistently measure 930 ohms from Arm 2 to ground. Occasionally, I notice more than 2 ohms from Arm to Arm, likely due to incomplete cleaning of the commutator with soapstone. The 930 ohms to ground has raised concerns for me, leading me to suspect a faulty motor. However, since it is not a dead short, I am unsure if a high resistance to ground is acceptable. My initial assumption was that the starting current might be too high, but the setup has remained the same for years. Seeking additional opinions to alleviate my concerns. Thank you. - Russ
When testing motors, it is important to take notice of any readings to ground, as this could indicate a potential issue. An armature to ground short may be intermittent, causing the motor to run sporadically. The discrepancy in armature readings could be due to the type of meter used; typically, armatures read 1-5 ohms, but larger HP DC motors may have higher readings. It is essential to consider the voltage and current rating of the motor being tested, as well as whether it has field windings. Any weakening of the field could lead to an overcurrent problem with the armature. Additionally, DC motors retain magnetism, which can affect startup performance.
One important step in troubleshooting startup issues is monitoring the current flow. By doing this, you can gain a clearer understanding of the underlying problem at hand. In some cases, a specialized breaker designed for "difficult start" scenarios may be necessary. These types of breakers can be adjusted to accommodate challenging applications by altering the trip delay time.
An Instantaneous Over-Current (IOC) during startup can be caused by various factors such as armature grounding, weak field, mechanical binding, or dirty commutator. It is important to ensure proper maintenance and troubleshooting to prevent IOC issues. When checking for armature grounding, aim for a resistance reading of at least 1.5M ohms, although some motors can still operate with lower readings. Cleaning the motor regularly with compressed air can help remove carbon dust buildup from brushes. Verify field connections with a voltmeter and ensure they are secure to prevent IOC occurrences. Mechanical binding issues may stem from motor bearings, gearbox, or excessive load on the motor. Watch out for worn key-ways on the motor shaft as this can cause free-wheeling. These are just some key points to consider when troubleshooting an IOC scenario. It is recommended to seek advice from others in the field to pinpoint the exact cause tailored to your specific application. Regular maintenance and attention to detail can help prevent IOC issues in the future.
Thank you for your input, everyone. This motor is a 180VDC permanent magnet motor with no field windings. I have already checked the current when running, and it appears to be fine. However, when the breaker trips, I am unable to determine the exact reading due to my digital meter. The suggestion of checking the keyway is interesting and something I will look into after separating the motor from the gearbox. Currently, there is some load on the motor as it is turning a gearbox and shaft, but there are no other restrictive loads. Due to the lack of a significant load, I suspect the motor itself may be the issue. I have cleaned it out and checked for any brush dust that could be causing a short, but it did not change the reading. Thank you once again for your insights. - Russ
When diagnosing motor problems, monitoring the armature current directly is the most effective method. Most DC drives can display this information by correctly adjusting the drive display settings. By monitoring the armature current, you can determine if it exceeds 150% of the nameplate amps during startup. If it falls within or below this range, adjusting the circuit breaker for the starting inrush may be all that is needed. However, it's important to note that even small changes in starting torque can greatly impact the current level. Additionally, acceleration rates play a crucial role in this process. For testing purposes, consider extending the acceleration time to observe how the current behaves under different conditions. It's possible that simply adding 1 second to the accel time could resolve the issue. While an armature to ground resistance of 930 ohms is considered low, it may not necessarily be the root cause of the problem. If significant current is flowing to ground, the armature and motor would likely overheat when running at full speed. If overheating is not an issue, suspect the motor as the potential culprit without deeming it faulty just yet.
Hey Russ, it sounds like you’ve done some solid troubleshooting already! The tripping breaker during startup could definitely be due to inrush current—DC motors tend to draw significantly more initial current when starting. If your measurements are showing that the resistance isn't a dead short, that's a good sign, but it does raise a flag with that 930 ohms to ground. It might be worth checking the connections and cleaning the commutator more thoroughly, as even small inconsistencies can lead to increased resistance. Also, consider whether the breaker is rated specifically for DC applications, as sometimes AC breakers can trip unexpectedly in DC circuits. If everything checks out, you might want to monitor the current during startup to see if it’s actually hitting that 30-amp limit or something higher. Good luck, and I hope you get it sorted!
Hi Russ! It sounds like you're dealing with a classic case of inrush current causing the breaker to trip during startup, especially since the motor runs fine once it's up and going. A 2HP DC motor can draw up to five times its rated current on startup, which might explain those trips. You might want to look into adding a soft starter or a current-limiting device to help manage that initial surge. Also, double-check that your connections are clean and secure—sometimes, bad contacts can exacerbate the issue. As for the resistance readings, while 930 ohms isn’t a dead short, it’s always good to keep an eye on potential insulation issues, especially if you notice any intermittent behavior. Hope this helps!
Hi Russ, it sounds like you’re experiencing a pretty common issue with DC motors, especially during startup when they draw a higher inrush current. The 30 amp breaker might be tripping because the motor is momentarily drawing more current than it can handle. Since you've measured low resistance between Arm to Arm and see some fluctuations, it's worth checking for any carbon buildup or other wear on the commutator, as that could contribute to inconsistent performance. Also, since the motor has been working for years, the brushes could be worn or not making proper contact. If you haven’t already, try using a different breaker rated for a higher inrush current or look into soft starters that can help mitigate the startup surge. It’s also good practice to consult the drive’s manual to ensure everything is set up correctly. Hope this helps!
Hi Russ, it sounds like you're dealing with a classic case of inrush current causing the breaker to trip. DC motors often draw a significantly higher current during startup, which can exceed the breaker rating, especially if it's an older system. The 930-ohm reading to ground is a bit concerning, as it could indicate some leakage, but it's worth checking if the motor is grounded properly or if there's any wear on the insulation. You might also want to consider soft starters or an inrush current limiter to ease the strain on the breaker during startup, but I'd definitely recommend consulting with a motor specialist if you suspect any major issues. Best of luck!
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Answer: - The breaker trips due to a high starting current drawn by the motor, causing an overload on the circuit.
Answer: - A high resistance to ground, such as 930 ohms, can indicate a fault in the motor and raise concerns about its condition.
Answer: - Incomplete cleaning of the commutator with soapstone can lead to higher resistance between the motor components, affecting its performance.
Answer: - To prevent the breaker from tripping during startup, you can consider reducing the starting current or checking for any faults in the motor components.
Answer: - Variability in resistance measurements, especially between Arm to Arm and Arm to ground, can indicate potential issues in the motor that may require further investigation.
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