Damage from the electric current of the bearings in to servomotors
Bearing current damage has become a very common issue in electric motors and servomotors. As a matter of fact this is not a new problem.
Once electric arc bearing damage has begun, excessive vibrations, increased heat, increased noise levels, and the reduced effectiveness of the lubricant, and as a result will combine to shorten a bearing’s service life, but it can also fail within a month of being new.
As a matter of fact everyone knows that servomotor bearings are small or medium sized and people may not pay special attention to this type of motor.
Instead these servos have encoders, but the question is which part will fail first: the encoder or the bearings for the electrical current?
Hence when a servo with an encoder can’t find its position, that’s when problems start.
For example, in the paper industry it is common to find this problem of fluting. And surely many companies around the world have found electric current in the bearings of their large AC motors and put solutions such as shaft grounding ring, etc.
-What are bearing currents and what causes them?
Pulse-width modulation is a fundamental operating principle of variable frequency drives (VFDs), but the high-frequency switching that delivers pulses of voltage from the drive to the motor can cause bearing currents — high-frequency currents that flow through the motor bearings, often leading to damage and premature failure.
To achieve pulse-width modulation, insulated gate bipolar transistors (IGBTs) in the variable frequency drive switch on and off rapidly to create a simulated sinusoidal AC waveform. Because the power is delivered in pulses, rather than as a true sine wave, the power is never balanced — that is, the sum of the three phases of power supplied to the motor is never equal to zero.
– Imbalanced voltage
This imbalanced voltage is referred to as common mode voltage (CMV), and it is one of the primary causes of bearing currents in motors driven by VFDs.
The fast-switching time of the IGBTs creates parasitic capacitance, which causes capacitive coupling between the stator and rotor. This capacitive coupling induces a voltage (due to the common mode voltage caused by the drive) across the air gap between the stator and rotor. As the voltage searches for a path to ground, it travels through the motor shaft (which is connected to the rotor) and across the bearings.
Lubrication in the bearing normally acts as an insulator, but because the common mode voltage has a very high rate of change, known as dV/dt (due to the switching of the IGBTs), it can easily exceed the breakdown voltage of the lubrication — that is, the voltage at which the lubrication no longer acts as an insulator.
When this happens, a burst of current is released, known as capacitive discharge current.
But when it comes to servomotors, what do companies and repair shops do?
How can IoT help?
In an increasingly digitalised, competitive and demanding world, delivery times of new products such as drivers, servomotors or encoders is extremely long. Undoubtedly this is one of the reasons why it is very important to know why our servomotors fail or may fail, and to take the correct measures so we don’t have unexpected stops in our production.
Since January 2017, our company is digitized with IoT.
This allowed us to detect that an Elau servomotor from one of our customers that was at our repair shop had been repaired by us just three months before. That set an alarm.
What did we do? Before disassembling the engine, we detected in the control harmonic vibrations in the bearings and a lot of noise when the engine was turned on. Once the engine was disassembled, our technicians analyzed the bearings and observed the characteristic marks of electric current.
We immediately contacted our customer and sent him the information about the issue. As a preventive measure, we decided to install hybrid bearings in the servo motor.
Hybrid ball bearings (which substitute ceramic balls for steel rolling elements) offer an advanced and practical solution. Since silicon nitride has high resistivity, hybrid bearings provide ideal insulation from electric currents