Induction motors are as much a part of North America’s production backbone as ever, consuming 40-50% of the electrical power in the USA today. If you rely on synchronous motors in your power generation or manufacturing facility, you know how critical they are to your daily operations, and the importance they have on profitability. Keeping them running well is not only a challenge, it is essential. Some considerable rotor issues affecting production are broken or cracked rotor bars and end-ring faults. Broken or cracked rotor bars increase thermal loading and stator temperature causing motor burn out in a short period. Stop/Starts will burn out the stator even sooner. End-ring faults, though not as severe, can cause rotor imbalance and stator damage.
When do these issues need to be detected? As soon as possible. Even though, there may be no immediate failure if only one or two rotor bars are broken, several damaged bars will prevent startup and cause sparking, overheating, torque and speed variation, deterioration in other motor components and driven equipment. Rotor bars can also lift out and strike the stator causing catastrophic failure, rotor core damage and costly downtime. Therefore, another important tool in your preventative maintenance (PdM) toolbox should be Motor Current Analysis (MCA) to detect faults. This method ingeniously uses the motor itself as a transducer.
The concept dates back to the early 1970s, when the US Nuclear Regulatory Commission searched for non-intrusive ways to evaluate machine condition. Over the past 40 years, this method has proven itself time and again. L&S Electric provides MCA services to help our customers verify and quantify cracked rotor bars or an open in a synchronous motor’s rotor tested on thousands of motors of various sizes, types, configurations, and loads, motor current analysis has become a proven circuit. We do this by attaching a clamp-on current ammeter to a secondary current line. Our analyzer reads the current signature looking for sidebands around 60 HZ of pole-pass frequency. Using simple math and a tachometer it is possible to calculate a motor’s synchronous speed and actual speed.
However, the challenge lies in determining the severity of your motor rotor problem. This is achieved by calculating the motor’s pole-pass frequency relative to its running speed components, based on the amplitude of the sideband (e.g., the equivalent of 10 cracked rotor bars). That’s where we come in.
The MCA measurements we make can be used to identify the following rotor issues:
- Rotor Bar Damage
- Misalignment and imbalance
- Shorts and opens
- Bad or missing bearings
Our team is expertly trained in motor rotor current analysis. Our analysis can be performed at the motor control center and away from moving mechanical equipment, on main phase circuit or secondary control circuits. When we are done, we provide you with a report of our findings, along with the solutions we believe are necessary to fix the problems we encounter.
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