Shaft Currents in VFD driven motors

[mshields]

I have a couple of Chilled water pumps whose bearings after only 6 months of operation are shot.

These motors are VFD driven and it has been suggested that Shaft Current induced by the VFD are the culprit. I've never heard of this. The suggested solution by the HVAC contractor is to install an AEGIS SGR ring on the motors. This provides a path from the shaft to the frame of the motor hence creating a lower resistance path than the path via the bearings.

Makes sense! But I'm wondering why the motor isn't designed to do this without this extra part. I'm also dubious because the only empirical data I have on this SGR device is a white paper by the manufacturer.

I could say, sure, go ahead and install these things on the two motors whose bearings failed but then the questions becomes what about all of the other VFD driven motors on the job; should we install these devices on all of them?

Your input would be greatly appreciated.

Thanks,

Mike

 

[iwire]

it has been suggested that Shaft Current induced by the VFD are the culprit. I've never heard of this.

There is a ton of info out there about this.

Try 'Googling' Fluted Motor Bearings

This provides a path from the shaft to the frame of the motor hence creating a lower resistance path than the path via the bearings.

This solution alone seems at best to be able to only reduce the current not eliminate it.

I had to help with four 500 HP motors that where eating bearings, at full speed they found 18 amps flowing on the motor bearings.

In this case they installed ceramic coated bearing races that isolated the shaft from the frame so no current could flow through the bearings.

They also had us install grounding brushes on the rotating shaft to ground the now electrically isolated rotating parts of the motor.

The GE motor guys who rebuilt the motors on site said they spend a lot of their time fixing these problems from VFDs.

 

[mshields]

thanks Bob

thanks Bob

Raises the questions. What should one be specifying to avoid the problem in the first place.

I'll try that google search you suggested

Mike

 

[Jraef]

Aegis are not the only solution by the way. Inpro/Seal, who has been making the best bearing isolators for years, has released a new shaft grounding solution as well, called a "Current Diverter Ring".

As far as specifications, MOST good quality "Inverter Duty" motors now come with one of these technologies or can be ordered with them. A simple statement in a specification such as "all motors for use with VFDs shall include provisions to mitigate shaft current buildup and subsequent bearing damage, vendor shall show proof that a system is included" should suffice I would think.

 

[Besoeker]

From another thread on this topic:

The first few mitigation measures from the GAMBICA guide:

1. Ground potential equalisation (i.e. high frequency bonding) between VSD, motor and load. The principle is to ensure the lowest possible impedance path on the shield connection to avoid currents travelling through the bearings and back to ground. Rigorously follow the manufacturers installation procedures.
2. Reduce the PWM switching frequency if practical.
3. Use insulated bearings.
4. Use an inverter output choke or filter.

 

[mickeyrench]

Is there anything that can be done when a vfd is powering a single motor / pump unit? A sump pump 50-75-100hp units besides using ceramic bearings,since the pump motor are one piece.

 

[Besoeker]

Is there anything that can be done when a vfd is powering a single motor / pump unit? A sump pump 50-75-100hp units besides using ceramic bearings,since the pump motor are one piece.

See measures 1,2, & 4 in the above post.
Item 1 is good installation practice anyway.
Number 4 is easy to implement and somethine we routinely do on variable frequency inverters.

 

[cadpoint]

Three questions:
What equipment is required to fine voltage on the Shaft?

Beside the use of a VFD is there other motors or even future critical failures where this might
be present and measureable?

Are certain type of motors AC/ DC (realizing that DC voltage is put on the stater which is the shaft)
more susceptible? Or is this only a VFD influenced problem?

How would or should the above average electrician or even a maintaince electricain be aware and
properLY test for this? Granted it should be part of a maintain/history route, just wondering what it's all about!

 

[TxEngr]

Circulating shaft currents have been around a lot longer than the introduction of PWM drives - my introduction was on a 30MW generator where we ate up the bearings due to a failed grounding assembly.

To measure shaft currents, you need a 1 ohm non-inductive resistor, probably in the range of 1/2 to 1 watt. These are available through specialty suppliers. You measure the voltage drop across the resistor and get the current through calculation. The resistor is connected to the shaft with some sort of brush assembly - an easy one is some braided copper resting on the shaft. The other end is connected to ground. The key thing here is you don't want your hand anywhere near the rotating shaft when taking measurements - it's just not safe. There are a number of other assemblies sold by motor manufacturers to ground shafts - Reliance has one that is a brush that mounts to the inboard housing and rides the shaft. As mentioned, InPro insulated seals are good as well.

Another solution not mentioned is insulated couplings to the driven device. This will prevent damage to the load and can be easier to install than the rest of the tools mentioned. It also remains if you change motors in the middle of the night.

Good Luck.

TxEngr

 

[Jraef]

More reading material that is not selling something...
http://www.usmotors.com/Service/Bulletins/Issue3-Dec02.pdf

 

[elec_eng]

From another thread on this topic:

The first few mitigation measures from the GAMBICA guide:

1. Ground potential equalisation (i.e. high frequency bonding) between VSD, motor and load. The principle is to ensure the lowest possible impedance path on the shield connection to avoid currents travelling through the bearings and back to ground. Rigorously follow the manufacturers installation procedures.
2. Reduce the PWM switching frequency if practical.
3. Use insulated bearings.
4. Use an inverter output choke or filter.

Besoeker,

If I understand this correctly, you are providing a low imepedance path by creating a high frequency bonding to divert shaft current to ground (earth)..Is that correct? I am still having a hard time understand this..

1. Why does this high frequency current go back to ground (earth)? Most current will go back to the source, not to the ground.

2. How do you provide a low impedance path to ground (earth)? Driving a ground rod to earth from the motor frame?

 

[elec_eng]

Aegis are not the only solution by the way. Inpro/Seal, who has been making the best bearing isolators for years, has released a new shaft grounding solution as well, called a "Current Diverter Ring".

As far as specifications, MOST good quality "Inverter Duty" motors now come with one of these technologies or can be ordered with them. A simple statement in a specification such as "all motors for use with VFDs shall include provisions to mitigate shaft current buildup and subsequent bearing damage, vendor shall show proof that a system is included" should suffice I would think.

Jraef,

Are you saying that you just need to isolate (electrically) the bearing from the shaft to prevent the bearing damage?

 

[weressl]

I have a couple of Chilled water pumps whose bearings after only 6 months of operation are shot.

These motors are VFD driven and it has been suggested that Shaft Current induced by the VFD are the culprit. I've never heard of this. The suggested solution by the HVAC contractor is to install an AEGIS SGR ring on the motors. This provides a path from the shaft to the frame of the motor hence creating a lower resistance path than the path via the bearings.

Makes sense! But I'm wondering why the motor isn't designed to do this without this extra part. I'm also dubious because the only empirical data I have on this SGR device is a white paper by the manufacturer.

I could say, sure, go ahead and install these things on the two motors whose bearings failed but then the questions becomes what about all of the other VFD driven motors on the job; should we install these devices on all of them?

Your input would be greatly appreciated.

Thanks,

Mike

NEMA MG-1 Standard

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31.4.4.3 Shaft Voltages and Bearing Insulation​

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Shaft voltages can result in the flow of destructive currents through motor bearings, manifesting themselves
through pitting of the bearings, scoring of the shaft, and eventual bearing failure. In larger frame size
motors, usually 500 frame and larger, these voltages may be present under sinusoidal operation and are
caused by magnetic dissymmetries in the construction of these motors. This results in the generation of a
shaft end-to-end voltage. The current path in this case is from the motor frame through a bearing to the
motor shaft, down the shaft, and through the other bearing back to the motor frame. This type of current can
be interrupted by insulating one of the bearings. If the shaft voltage is larger than 300 millivolts peak when
tested per IEEE 112, bearing insulation should be utilized.

More recently, for some inverter types and application methods, potentially destructive bearing currents
have occasionally occurred in much smaller motors. However, the root cause of the current is different.
These drives can be generators of a common mode voltage which shifts the three phase winding neutral
potentials significantly from ground. This common mode voltage oscillates at high frequency and is
capacitively coupled to the rotor. This results in peak pulses as high as 10-40 volts from shaft to ground.
The current path could be through either or both bearings to ground. Interruption of this current therefore
requires insulating both bearings. Alternately, shaft grounding brushes may be used to divert the current
around the bearing. It should be noted that insulating the motor bearings will not prevent the damage of
other shaft connected equipment.

At this time, there has been no conclusive study that has served to quantify the relationship of peak voltage
from inverter operation to bearing life or failure. There is also no standard method for measuring this
voltage. Because of this, the potential for problems cannot consistently be determined in advance of motor
installation.​

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