Dynamic Balancing In Electrical Motor
Why Balancing in Motor?
The rotating parts need to be balanced because when the parts rotate at high speed the centrifugal forces are developed in the rotating parts and the uneven forces leads to vibrations of the total body which in turn causes wear at the shaft contact points and to the bearings. That causes total damage to the machine, so balancing is must for any type of machine.
However, there are mechanical and electrical faults factors which may generate more unbalance in rotor and Increase the vibrations and sounds and temperature as well as.
☛ Fundamental Train Frequency (FTF).
☛ Ball rotation frequency (BSF).
☛ Ball passage frequency, inner race (BPFI).
☛ Ball passage frequency, outer race (BPFO).
Shaft ovality, shaft wear or damage
Bearing housing looseness:
Eccentric Rotor: An eccentric rotor, which means the rotor core OD is not concentric with the bearing journals. Unsymmetrical Air Gap Around Rotor reference with stator produce uneven force at rotational speed (1x Speed).
Electrical motors have the same mechanical faults that like other rotating machines, but there are also some specific faults for electrical motors.
Sometimes the rotor skid on the axle, depending on the temperature. This generates a vibration 1xRPM and harmonics. Factors provoking this phenomenon are sudden changes in load or line frequency.
If the stator windings are slightly loose the vibration level at 120 Hz will increase. This causes scraped thread isolation which leads to short circuit between the windings. It can also be short circuit between the windings and earth, which leads to stator breakdown.
Thermal bending of the rotor:
Uneven electrical circuits in the rotor bars generates an uneven heat distribution in the rotor. This causes a deformation, bending, of the rotor which results in unbalance.
Eccentricity in the air gap:
If the air gap is not uniform it generates unbalance forces on the rotor which in turn induce a vibration at 120 Hz (2xLF).
If the rotor is not entirely round this will cause vibrations at 1xRPM. It will also generate unbalanced magnetic forces resulting in vibrations at a frequency corresponding to the slip frequency multiplied with the number of poles.
Problem with rotor bars:
An important cause of problem in electrical motors is that the rotor cracks. This mainly happens to motors often exposed to starts with load. During start the current in the rotor bars become very high as the speed of the rotor is much lower than the synchronous speed. The electric current causes heated rotor bars, causing them to expand in relation to the rotor itself. The difference i resistance between the rotor bars result in turn in uneven heating and uneven expansion. This leads to cracks in the joint between the bars and the shorting rings. When a crack appears the resistance in the bar increases, which in turn increases the heating and thereby enlarge the crack. The current also becomes larger in the other rotor bars as the current has been reduced in the broken bar.
Benefits and Advantages If your Motor is in well balance condition:
☛ Your Motor will run in normal current sometimes if rotor is unbalance than motor’s Amperes increase in running condition.
☛ Motor temperature will remain in normal and not increase which that there are high chances of rise in temperature if rotor is unbalance. If rotor unbalance than generates high vibration and stress at motor housing cover and on bearing and thus temperature rise in this all area.
☛ Motor overall efficiency will increase.
☛ Motor and its parts like housing cover, bearing etc life increase.
☛ Reliability increase. All of sudden breakdown chances are very low.
☛ Motors condition will normal like vibration, temperature and noise level will be in permissible limit.