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Why Insulated Bearing is used for VFD driven Motor?

Insulated bearing for motor is used to prevent the circulating current through the motor bearing. The circulating bearing  current can damage the bearing. When we run induction motor by variable frequency drive, it is apt to fail caused by bearing failure. In this article we will  discuss how the circulating current flows through the motor bearing when VFD drives the induction motor. We will also discuss how to prevent the circulating current in order to prevent the bearing damage.

How do VFDs Cause Bearing damage?

In large rating motor and alternator induced shaft voltage cause flowing of  circulating current from the motor shaft to outer race, rollers or balls of the bearing. The circulating current can damage the bearing. That is why we ground the motor shaft  through the carbon brush so that the circulating  current finds its path to ground and it does not pass through the bearing. The same phonephone happens with an induction motor driven by VFD. Magnitude of circulating bearing current with VFD is much more and it cause bearing damage.

VFD induced Shaft current in AC Motor

Let us understand how the shaft voltage is build up when PWM current flows through the motor winding.

The motor has insulation and conductive parts, therefore capacitance is formed between the insulating and conductive parts of the motor. The parasitic capacitance formed in between the various parts of the motor become more relevant when the motor is driven by a PWM voltage source inverter. The equivalent circuit of the motor considering all the parasitic capacitance is as given below.

Parasitic capacitances in an induction motor

The various parasitic capacitance formed in the motor are termed as;

CSF – Stator to Frame Capacitance
CSR– Stator to Rotor Capacitance
CRF – Rotor to Frame Capacitance
CB   –  Bearing Capacitance

All the above capacitance is responsible for inducing the shaft voltage in an induction motor. The capacitance value is low when we run induction motor with pure sinusoidal voltage. However, with VFD the value of the parasitic capacitance is  different and it cause voltage across the  motor shaft.

CSF – Stator to Frame Capacitance

This is the primary capacitance formed between the stator winding and the grounded frame of the motor. The value of capacitance is largest among all the parasitic capacitance.The most of the common mode current caused by high dv/dt of common mode voltage of PWM waveform flows through this path.

CSR – Stator to Rotor Capacitance

This capacitance is formed in between the stator winding and rotor frame. The value of the capacitance is rather small, but it has more significance, in view of it charge the shaft which is physically connected to the rotor and this parasitic capacitance is responsible for shaft voltage. Therefore, the value of this capacitance is very paramount for evaluating the magnitude of shaft voltage.

CRF- Rotor to Frame Capacitance

The rotor to frame capacitance completes the charging path started from the stator winding to the rotor.The value of the CRF is about ten times that of the stator winding to rotor surface capacitance( CSR). The most of the applied common mode voltage appears CSR and only small voltage developed across CRF. The voltage developed across the rotor to frame capacitance is called the shaft voltage.Thus, the CRF is vital in establishing the shaft voltage.

CB –Shaft to Frame Capacitance

The motor shaft rests on the bearings. When the motor rotates the balls in the ball bearing or roller bearing float up and occupy the space in between the inner and outer race of the bearing.An insulating film is formed by lubricating medium in which ball or roller is floating. The capacitance is transient and forms only when motor rotates.

The value of the shaft to frame capacitance or bearing capacitance CB depends on the shaft speed, type of lubricant used, the temperature of lubricant, the surface area of the ball or roller in the bearing and mechanical load on the shaft. The value of the capacitance is important because the magnitude of the bearing current depends on the bearing capacitance and life of the bearing depends on the bearing current.

All the above  parasitic capacitance is shown in an electrical model as shown below.

Mechanism behind high-frequency circulating bearing currents

The common mode voltage is responsible for current flowing  in various parts of the motor. The various current caused to flow when motor is driven by PWM inverter are as follows. The bearing current can be categorized into four different current according to their generation mechanism.

Induced AC motor shaft current & bearing failures

There are 4 types of  significant current  flows in VFD driven AC motor.

1. Capacitive bearing Current(i1)

High common mode voltage cause pulse current flow to the rotor through the parasitic capacitance between stator to rotor CSR. The current gets divided into two paths.The first path is the return path formed by the capacitance between the rotor and the frame CRF and the second path is formed by the capacitance between the inner and outer race of the bearing CB.Since CSR is much smaller than the parallel combination of CRF and CB, the amplitude of i1 is smaller than the total common mode current(ISF).

However, the consistent flow of electric current through the bearing current cause heat in the lubricating system and cause failure of bearing.

2. Electric Discharge Machining(EDM) bearing Current(i2)

An electric charge is stored in the capacitance (CRF) due to common mode voltage. The CRF is formed across the rotor body and frame. The voltage across CRF keep  building up and finally  reach such a level that may cause the insulation of lubricating film to breakdown. This dielectric breakdown results in the charge stored across CRF to discharge through the insulating film of the bearing  and creates EDM bearing current. The energy stored in the capacitance CRF may cause bearing damage.

3.  Common mode shaft current (i3)

If the motor frame is poorly grounded the current finds the path through the mechanical load which has much lower grounding impedance.

4. Circulating bearing current (i4)

The circulating current flows along the axis of the rotor, through the bearings and circulates through the stator frame and returns back from the other bearing end. The circulating current is generally insignificant.

Preventing VFD-induced bearing damage in electric motors 

The main cause of bearing damage in VFD driven motor is circulating current. Circulating current passing through the motor bearings needs to prevent in order to prevent the damage of the motor bearings. The followings are the ways and method to hinder the circulating current through the motor  bearings.

1. Shaft Grounding Devices

Shaft grounding devices minimize shaft voltage and prevent bearing damage caused by circulating bearing current. The shaft of the motor is grounded by connecting  a carbon brush between the motor shaft and the motor frame. The current finds lowest impedance path through this circuit and does not flow through the bearing. The regular maintenance/replacement of the carbon brush is required for reliable operation.

2.  Insulated bearings

Insulated bearing at NDE side of the motor can be used to prevent the current flowing through the bearing to ground. The insulated bearing  hinders the path of the circulating current which can flow from the shaft,inner race,bearing balls or rollers, outer race to motor frame.

3. External passive/active common mode filters

Common mode noise filters are used to cancel common mode noise of the system.These filters are bulky and expensive  and cause voltage oscillation if passive components are not properly tuned.

3. Multi level inverter technology

The multi level inverter has low common mode voltage and thus the bearing current gets reduced drastically.