Greetings, our workshop recently received a motor from our plant with the following specifications: Voltage: 3.3 kV Power: 300 kW No load current: 25A Full load current: 68 A Solo run (no load) current: 33.4A Brand: ABB RPM : 989 RPM The issue at hand is that the current during the solo run exceeds the permissible value by 48%, which should ideally be 20% of the full load current (63A), equating to 33.4A. Despite conducting a successful megger test with a 5kV test voltage (resulting in over 100 MegaOhm), performing maintenance tasks such as cleaning the stator, rotor balancing, bearing replacement, and metal spraying did not resolve the issue. The motor had not been used since 2003 until recently, and we are unsure of what may be causing the problem. Any assistance or insights would be greatly appreciated.
Pisemude: High current under no-load conditions (assuming the motor is not coupled) can be attributed to various factors. While you have addressed many of these factors, there are additional areas that should be examined to assess the situation. Firstly, consider if the motor has undergone rewinding in the past as changes to the windings may lead to high current due to a subpar repair job. Additionally, a damaged core can also result in elevated no-load current as it affects circuit impedance and motor losses. To further troubleshoot the issue, ensure that the motor is operating at the specified nameplate voltage. Any deviation from this voltage can cause an increase in no-load current, with noticeable changes starting at around 3% above or below the nameplate value. Voltage imbalances between phases can also contribute to this issue. Rotor malfunctions can lead to reduced torque and subsequent high current under no-load conditions. Various methods such as growler testing, single phasing, infrared testing, vibration analysis, and dye testing can be employed to evaluate the rotor's condition during repair processes. With advancements in technology, motor circuit analysis and current signature analysis have become more accurate in detecting rotor bar issues. Growler testing, for example, involves inducing power into the rotor while using metal filings or magnetic paper to inspect rotor bars. Single phase testing, infrared testing, and vibration analysis are other effective methods to detect rotor faults. Analog current meters and inductance testing can also be utilized for diagnosis purposes. Current signature analysis remains a reliable technique for identifying severe rotor bar conditions.
Valuable insights from Howard. One point of contention is the idea that low voltage can lead to high current during no-load operation. In reality, reducing voltage will always result in decreased current in such situations. It is crucial to ensure that the voltage does not exceed the nameplate rating by more than 10%. Additionally, it is important to verify balanced currents, as any imbalance could be linked to power system voltages or the motor itself. It is worth considering if a wye motor has been incorrectly connected in delta, as this could significantly increase current. While a no-load current of 33A~ 50% FLA may seem high for a 4-pole motor, the criteria of 20% FLA appears overly strict. The source of the listed "No load current: 25A" is unclear and seems to surpass the 20% threshold you mentioned.
The relationship between voltage and current in an electric motor is complex, with various factors influencing the results. Higher voltages can lead to increased current due to excitation of the stator and rotor cores, as well as circuit impedance. On the other hand, lower voltages may cause the motor to generate a weaker magnetic field, resulting in more slip and increased losses. During motor starting, there is a significant spike in current, sometimes reaching up to 21 times the nameplate current. This is known as the 'half-cycle peak.' The initial high current is a result of the locked rotor condition, with both the stator and rotor experiencing full line frequency. As the motor reaches higher loads, losses play a smaller role, while at lower loads, losses become more significant. This is reflected in a standard NEMA chart showing higher running currents at lower than nameplate voltages. Issues such as broken or damaged rotor bars can also lead to higher currents, as the motor struggles to generate enough torque, causing increased slip frequency and pulsating current. The question of where a specific idle current value of 25 Amps comes from is raised, noting that different motor types may have varying idle currents. Rule of thumb estimations are common, but it is important to consider the specific characteristics of the motor in question. In the USA, similar 6-pole machines often have idle currents around half or just over half of the full load current.
A decrease in voltage results in a decrease in no-load current. This is due to the fact that, in equivalent circuit terms, as we approach a no-load condition where the synchronous reactance (s) approaches zero, the rotor branch R2/S essentially disappears (as if it were an open circuit). At this point, only the stator leakage reactance and magnetizing reactance remain, creating a constant impedance below saturation levels. Therefore, reducing the voltage across this constant impedance will lead to a decrease in current flow.
When considering motor performance, it is important to note that assuming the voltage is at the nameplate value may not always be accurate. The S~0 method of conducting a no-load test suggests that the motor will operate at synchronous speed even at lower voltages. This method simplifies calculations for comparing with blocked-rotor tests and estimating performance. For smaller motors with light rotors, such as integral or fractional horsepower motors, this assumption holds true until the voltage decreases significantly. However, larger motors may exhibit different dynamics. Therefore, it is crucial to consider the actual idle current to accurately assess motor performance.
The increased no-load current could be due to a misalignment or an issue with the bearings that's leading to higher friction, although it is odd that replacing the bearings didn't resolve it. Another probable cause could be the insulation break down leading to stray losses. Though the Megger test results seem fine, it might not be able to pick up localized weak spots. It's also worth considering the age of the motor, as it has been idle for a long period of time, there could be issues with the windings or rotor bars. A more thorough investigation involving checking alignment, bearing installation, and a detailed inspection of the rotor bars and windings could give better insights.
Hey there, it definitely sounds like you've been thorough with your troubleshooting. Given that the motor hasn't been used since 2003, it's possible that the insulation has degraded causing the magnetic field to leak, which in turn might be increasing the current draw during the solo run beyond the acceptable limit. Some actual motor testing such as efficiency testing, load testing and thermography could help pinpoint if that's the case. It might even be a good idea to have ABB themselves or a certified professional take a look, especially since you've done most of the simpler maintenance tasks already and haven't had any success in resolving the issue.
From your detailed observations, it sounds like you've done a thorough job of trying to troubleshoot this issue. One possible cause you might not have considered is that the quality of the insulation might have deteriorated after many years of disuse. Even though your initial megger test passed, it could still be a culprit. Another standard check would be to look at the current balance-- unbalanced voltages can lead to an increase in current. Also, consider the possibility of issues with the power supply or load conditions which might trigger excessive current. Hopefully, these suggestions can give you a fresh perspective. Good luck, and keep us updated!
It sounds like you've done a thorough job with the maintenance and testing already, which is great! Given that the motor has been out of service since 2003, it could be a problem related to insulation breakdown or issues with the windings that aren't immediately apparent even with a megger test. Have you considered checking the connections and contact points for corrosion or degradation? Also, it might be worthwhile to look into the motor's operating environment; if it’s been exposed to moisture or contaminants, that could also lead to higher current draw. Have you tried running the motor under various loads to see if the issue persists across different conditions?
It sounds like you’ve already done a thorough job troubleshooting the motor, which is great! Given that the solo run current is high despite the megger test showing excellent insulation, it might be worth investigating the rotor-related issues, like winding defects or possibly a problem with the rotor's magnetic circuit. Also, consider checking the power supply quality—voltage sags or unbalanced phases can sometimes cause higher than expected currents. Lastly, since it hasn't run since 2003, some internal components could be experiencing wear or degradation that aren’t visibly apparent. Have you also checked the motor for any signs of excessive heat or physical damage to the windings that could point towards electrical faults?
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Answer: - The current during the solo run is exceeding the permissible value due to it being 48% higher than the expected value of 20% of the full load current.
Answer: - Maintenance tasks such as cleaning the stator, rotor balancing, bearing replacement, and metal spraying have been performed on the motor to address the issue.
Answer: - The megger test conducted on the motor with a 5kV test voltage resulted in over 100 MegaOhm, indicating successful insulation resistance.
Answer: - The motor had not been used since 2003, and its age might be a factor contributing to the high amperage issue during the solo run.
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