AQ: Motor Rotor Bar issue in Current Signature Analysis
The condition of the rotor bars will determine how much torque your motors will deliver. As a person who has been in the electric motor repair business all my life it is something I constantly check. Normally when you talk about rotor bar health it refers to open rotor bars however I have found that in aluminium die-cast rotors there can be voids in the end-rings. Todays vibration equipment and your CSA equipment is so sensitive that it will pick up these voids. In a repair shop environment and with a motor with a good stator winding it is relatively simple to check for open rotor bars. if at all possible we will check for open rotor bars before we take a motor apart by performing a single phase rotor test. You apply approximately 20% of line voltage to two phases of the motor. Rotate the rotor through 360 degrees and monitor the current. If the current is steady the rotor is in good health. If you have one or more open rotor bars the current will drop as the open bars pass the energized part of the stator. A 10% swing would indicate open rotor bars.
Just in case there is a second cage in the rotor you can also put a voltmeter across one of the energized phases and the open phase. Just like the current, the voltage should stay steady.
When a motor is developing open rotor bars it will become noisy on start up. Noisier with each bar that becomes open. It can sound like a cement mixer or as if there is no lubrication in the bearings.
I have no idea what a rotor bar health index is. I would assume that it is a severity level that has been developed by the people who manufacture your test equipment.
Neither am i familiar with the Motor Current Signature Analysis. We use a surge tester which has an attachment for checking rotors but I don’t put much faith in it.
Open rotors can be a nightmare for electric motor repair facilities. Open rotor bars are not always visible and can be very difficult to detect. Our core tester has clamps that allow us to induce a low voltage and high current into the rotor cage but it is not conclusive. We could use a growler to energize the rotor and throw iron filings over the core. On a big rotor it takes a bit of time and customers don’t like paying for it, especially when you don’t find any problems.
If your motors are die-cast aluminium and they are starting up every day without struggling to get up to speed and they are not noisy during start up, your equipment might be picking up voids in the aluminium.
If you have copper or copper aloy rotors with brazed end-rings and I might suggest that you be concerned. Once you get one open rotor bar it only gets worse as time goes by.
