Medium voltage motor specification - Mike Holt's Forum

My company has a very old medium voltage (V) motor specification that I am reviewing/updating. None of the people who created it are still with the company.

There are two requirements that I have been looking into:

• Windings shall withstand a minimum of full voltage starts.

• Outboard end bearing shall be electrically insulated from the motor frame to eliminate shaft currents. Shaft current potential not to exceed 200mv during full load operation.

I'm having difficulty finding a source/justification for these numbers. Does anyone know of any design standard which would recommend those values? Is there an existing standard or recommendation for how many full voltage starts a medium voltage motor should withstand? I agree that motor ODE bearings should be insulated, but what about the 200mV shaft current potential? I'm thinking about eliminating these requirements but would like to see what others think. I've never seen a minimum number of starts specified before, interesting approach. Most likely this spec is from some specific mfr who put what we call "spec hooks" into the wording in order to drive users into their specific motor. Unfortunately without knowing who's spec that is, it might be something that gets you no bids, or exceptions. If not, and I had to guess, I'd say it might have been an indirect way to make sure they use phase paper in the slots, and that they dip the windings. Most will do that on an MV motor anyway, but maybe they just intended to eliminate the bottom feeders.

As to the 200mV, that seems arbitrary too, in fact that would be considered high if this were an inverter duty motor. Are you going to run this from an MV VFD? If so, consider grounding the shaft.

I've never seen a minimum number of starts specified before, interesting approach. Most likely this spec is from some specific mfr who put what we call "spec hooks" into the wording in order to drive users into their specific motor. Unfortunately without knowing who's spec that is, it might be something that gets you no bids, or exceptions. If not, and I had to guess, I'd say it might have been an indirect way to make sure they use phase paper in the slots, and that they dip the windings. Most will do that on an MV motor anyway, but maybe they just intended to eliminate the bottom feeders.

As to the 200mV, that seems arbitrary too, in fact that would be considered high if this were an inverter duty motor. Are you going to run this from an MV VFD? If so, consider grounding the shaft.

No, this is for across-the-line motors. From the manufacturers and repair shops I have spoke with an insulated ODE bearing is common practice on medium voltage across-the-line motors. Most of ours have it. I guess motors at that voltage level are more susceptible to induced shaft voltages? I'm not sure.

I've continued to dig and actually just found this EASA white paper on bearing fluting:

If the shaft to frame voltage exceeds 100 millivolts AC for a ball or roller bearing, or 200 millivolts AC for a sleeve bearing, the shaft current is probably high enough to degrade the bearings.

Could be where they grabbed the 200 mV from... still, this doesn't seem to be a heavily documented practice for manufacturing specs (that I can find).

My company has a very old medium voltage (V) motor specification that I am reviewing/updating. None of the people who created it are still with the company.

There are two requirements that I have been looking into:


I'm having difficulty finding a source/justification for these numbers. Does anyone know of any design standard which would recommend those values? Is there an existing standard or recommendation for how many full voltage starts a medium voltage motor should withstand? I agree that motor ODE bearings should be insulated, but what about the 200mV shaft current potential? I'm thinking about eliminating these requirements but would like to see what others think.

Do you what the power rating is and what application it's being used on?

No, this is for across-the-line motors. From the manufacturers and repair shops I have spoke with an insulated ODE bearing is common practice on medium voltage across-the-line motors. Most of ours have it. I guess motors at that voltage level are more susceptible to induced shaft voltages? I'm not sure.

I've continued to dig and actually just found this EASA white paper on bearing fluting:


Could be where they grabbed the 200 mV from... still, this doesn't seem to be a heavily documented practice for manufacturing specs (that I can find).

I've read mentions of bearing fluting in relation to PWM drives in a Gambica report. The paper you cited also mentions variable speed drives. I've dealt with drives most of my working life and never come across this phenomenon.
Shaft voltage may damage the bearings and/or the bearing lubricant resulting in failure.

I've read mentions of bearing fluting in relation to PWM drives in a Gambica report. The paper you cited also mentions variable speed drives. I've dealt with drives most of my working life and never come across this phenomenon.
Shaft voltage may damage the bearings and/or the bearing lubricant resulting in failure.

Did a quick Google, there's some good explanations here on why manufacturers might supply an insulated ODE bearing for non-VFD applications.

Most motor OEM's provide insulated bearing for NDE regardless of VFD application of very large motors. As I undsrtand it, the reason is that stator lamination material comes in standard size sheets... if motor stator lamination is too large it cannot be constructed from a single piece and instead several segmented stator laminations are used in a given plane. This introduces magnetic asymmetries due to variations in the gaps where lamination segments butt against each other, which can cause power frequency bearing circulating current... only necessary to insulate one bearing to avoid it since the power frequency is not capacitively coupled like vfd's.

VFD-induced bearing wear is just a fact of life here, as it is in most industrial facilities. Older VFDs don't seem to be as problematic due to the lower switching frequencies. Short cable lengths, appropriately rated cable, and load filters/reactors can all mitigate the effects.

Do you what the power rating is and what application it's being used on?

This is a generic specification for V motors.

Did a quick Google, there's some good explanations here on why manufacturers might supply an insulated ODE bearing for non-VFD applications.

Appreciated, thank you.

VFD-induced bearing wear is just a fact of life here, as it is in most industrial facilities. Older VFDs don't seem to be as problematic due to the lower switching frequencies. Short cable lengths, appropriately rated cable, and load filters/reactors can all mitigate the effects.

Also, older PWM inverters had somewhat slower IGBTs so the switching dv/dt was lower.

Our UK cable installation practices sometimes differ from that of USA. Power cabling is almost always steel wire armoured - SWAPVC or XLPE with the armour earthed (grounded). EMT (conduit) is rarely used for power. PVC conduit is sometimes used for lighting circuits. Insulated bearings for higher power ratings is quite normal.

I fell foul of this when the bearings on two HP mill motors fail in a month. The motors had pedestal bearings mounted on mica insulators at both ends. The NDE bearing would have a deliberate earth/ground strap, the DE bearing remaining insulated.
After the first failure we tested the bearing insulation weekly @500V DC everything seemed fine. With the 2nd bearing failure we were under the spotlight.

The cause was laughably simple. A new plant operator was given the motor platforms as his cleaning area. A handy place to leave his pry-bar was leaning against the motor DE bearing. The platform vibration wore the paint away and there was our short circuit. I agree with Tony S. regarding the use of insulated bearings. My company is currently looking to install one on a hp synchronous motor due to some modifications to the exciter and the discovery of bearing wear due to circulating current in the shaft.

Regarding the number of starts for the winding to withstand -- IEEE is a standard for re-winding synchronous generators rated 1MVA and above. Appendix G lists a typical value for baseload units as 3,000 lifetime total starts and 10,000 for frequently cycled units. Note these numbers are not explicitly directed at the stator winding. NEMA MG1 may have some language on number of lifetime starts, but my recollection is that the standard deals with number of consecutive starts.

Low-voltage motors are often a preferred choice due to familiarity with products and available services, as well as the typically lower cost of individual components. However, as horsepower (hp) increases, there can be advantages to moving to a medium-voltage motor. Low-voltage motors typically go up to 1,000 hp while medium-voltage motors can cover 250 hp and higher.

If you want to learn more, please visit our website.

Furthermore, in special variable frequency drive (VFD) applications, low-voltage motors can go up to or even over 5,000 hp. This high rating is preferably above the National Electrical Manufacturers Association (NEMA) low-voltage limit of 600 volts but still under International Electrotechnical Commission (IEC) low-voltage limit of 1,000 volts.

Knowing when to select the right motor for an application can save users time, space and money. Here are some areas to consider when choosing between low- and medium-voltage electric motors.

Cabling

In low-voltage motors, as the hp range increases, the size of cabling increases to handle the increase in amps. With conductors being a copper component, this increase in wire gauge can add cost, especially on longer cabling runs across a large facility or over a long distance to a remote pumping station. This increase in diameter also makes turn radii larger, which increases the difficulty in making connections within the terminal boxes. This can be time-consuming and introduce additional risk to the maintenance crew during initial setup of the motor.

A lower current in medium voltage motors allows for smaller cables (leads) even at higher hp. The use of smaller gauge leads reduces the cost per foot for those long-distance connections to remote pumping stations. Also, during the motor connection procedures, the small gauge wires are easier to work with and connect within the motor terminal box. This can reduce the maintenance crew’s time in making the connections and reduce the risk of damage to the cables.

The cost of copper as a commodity and the difference in thickness of leads sized for low-voltage machines versus medium-voltage machines can be so large that this can be the primary determining factor in what voltage service is specified. The higher cost of medium-voltage equipment can easily be offset in applications with long cable runs from distribution.

CHANGLI ELECTRIC MOTOR Product Page

Size

When space is a consideration, more than motor size should be reviewed as the choice between a low- or medium-voltage motor that has an impact on the components in the entire system.

Low-voltage drives are smaller than medium-voltage drives when variable speed applications play a role in the motor selection. However, above 1,000 hp this ratio starts to flip, and drive size may be comparable or even smaller. Due to lower amps, medium-voltage motors also enable the use of smaller supply side switch gear, supply transformer and controls.

Windings

To prevent short circuits and preserve the longevity of medium-voltage windings, they are commonly produced using a form wound insulation system. The insulation system is sealed using a vacuum pressure impregnated (VPI) system, which fills the voids in the coils to protect from contamination. The coils are organized outside of the stator core to ensure the ideal spacing of turns, which allows for air flow around the coils to improve heat transfer. It is a more labor-intensive process but is well suited to the rigors of the voltage impulses of a medium-voltage system. Additionally, due to the smaller conductors used in the windings, there is the possibility of having more turns, so there is greater flexibility in the electrical design, making it possible to achieve specific performance characteristics.

In low-voltage motor windings with larger diameter conductors, there are more limitations to the electrical design but less need for the precisely ordered coils required to withstand medium voltage. Because of this, low-voltage machines can use a more cost-effective random or mush wound design with a thorough dip-and-bake in varnish that is often coupled with a vacuum impregnation of the winding to ensure that the insulating material fills all voids. The result is a low-voltage insulation system that is capable of exceeding industry standards for longevity while achieving the performance characteristics necessary for a broad range of applications.

Like all good questions, whether to pick a low- or medium-voltage motor for a pump system does not have an easy answer. There are several factors to weigh, including site and installation specifics that will impact what voltage service is best for a given project. When selecting a motor for an application, evaluating these three factors should provide the best all-around motor for the facility.  

Wayne Paschall is a product market specialist with ABB Inc., in the large machine and generator division. For more information, visit abb.com.

The company is the world’s best medium voltage electric motor supplier supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.