In Motor Talk blog from the Snell Group, this week’s Tales from the Field, written by ‘Motorguy,’ shares an anecdote regarding maintenance people not making a serious effort to determine the root cause of a failed motor and how a scheduled monitoring of your equipment can reveal potential problems.
I was working out of a motor shop in Charlotte, North Carolina thirteen years ago when I witnessed a 1500 HP synchronous motor arriving at the shop. You could easily distinguish the smell of burnt insulation. I could see a rewind in that motors future.
About two weeks later I was back in the shop and saw the same motor. “You guys haven’t rewound this yet?” I inquired. They had rewound the motor and it failed three days after installation. The motor was used on an overhead crane at a steel mill. “Did anybody check out the rotor circuit when the motor was installed?” I asked. They answered “No.” “It may have been a problem in the resistor circuit in the rotor that caused the motor to fail initially and again after rewind.” I suggested. My observation was ignored and unanswered. The guys in the shop were of “the old school,” and they didn’t have much understanding or appreciation of Motor Circuit Analysis (MCA). The motor was rewound and you guessed it, it failed again.
I wanted to shout, “The problem is in the rotor!” Well, they wouldn’t get another opportunity to work on that motor. The customer sent the motor to a rival motor shop in Rock Hill, SC. They basically sent me there as well, because I was laid off that week. I left there on a Friday and started to work for their competitor on Monday. Guess what I saw when I was touring the shop with my new boss? It was the 1500 HP wound rotor motor. I informed them about the history of the motor and my repeated appeals to check out the rotor circuit. I had changed jobs to another motor shop with people of the same mentality as the last. I was again ignored.
Let’s take a minute and discuss how a wound rotor motor works. A wound rotor motor is an induction motor, but unlike a standard motor with a squirrel cage rotor, it has a wound three phase rotor that is connected via slip rings and brushes. During startup, resistance is placed in series with the rotor which reduces the high starting current. With the resistance in series with the rotor, high pull up torque is available even at 100% slip. During normal operation, various resistance taps can be inserted via control circuitry to reduce rotor speed and current. This gives, in this circumstance, the crane operator several RPM selections.
Now, the rest of the story: Two weeks later, the motor was again in the shop. You guessed it, another rewind. The plant manager ordered a new motor and this time I was sent in to test it after installation. The rotor circuit had a resistance unbalance of over 40%. The technicians at the steel mill had tapped the rotor resistance on differing taps because of burnt connections from previous hookups. Whenever they found a burnt tap, they simply moved the rotor connection to the adjacent tap. I had the steel mill order a new resistor bank. I installed it and made equal tapped connections for the rotor. The motor ran without problems for at least the next five years. It may still be running for all I know!
There are two valuable lessons here. Number one; motors fail for a reason. You should make every effort to find the root cause of failure and when found, make the necessary repair or circuit modifications to prevent recurrence. Number two; periodic monitoring of the motors operation can reveal developing problems that may allow repairs prior to failure and prolong motor longevity.
Always explore all reasons for failure and don’t give up!