Part 2-Shaft Voltage And Bearing Current Introduction Motor bearing life has historically been from six to ten years with sinusoidal 60 Hz power to the motor. However, with modern adjustable speed drives (ASDs), some users are seeing bearing damage in as little as one week, damage caused by electric currents flowing through the bearings from shaft to motor frame, as the result of a voltage potential induced between the rotor and stator. This electric potential is associated with the use of solid-state switches, called transistors, which turn on and off thousands of times per second to control the voltage applied to the motor windings. These new switching devices, turning on and off very fast with extremely short rise and fall times of the applied voltages, have dramatically increased stress on the motor windings and on the bearings. And this damage is not limited only to AC motors, but also can stress DC motors, when driven by ASDs. Bearing damage caused by electric currents flowing through them is called electrical discharge machining or EDM. What causes bearing damage? Causes of bearing damage, for the most part, can be broadly classified within three categories: a. Lubrication b. Mechanical c. Electric Discharge Machining (EDM) or Bearing Currents It is important to seek to identify the specific cause of failure in order to not repeat the failure, often within a short period of time. Bearing current failures, for example can occur in as short a time as one week after installation. Others, such as insufficient grease, can take several years to develop into a problem. Table 15 lists the most common causes of bearing failure and evidence to look for, during operation in some cases, and others may require an “autopsy” to look for evidence. Bearing or shaft current damage is difficult to prove, short of cutting the bearing apart and examining it under a microscope.

Figure 15 Bearing Failure Causes Failure Category

Estimated % of Failures

Inadequate Lubrication

35-40

Wear Improper Mounting

15-20 10-15

Corrosion

5-10

Fatigue

5-10

Other Causes

20

Further breaking down the leading causes of failure helps to identify the root cause and provide a guide for corrective action. Figure 16 lists the primary causes of failure and evidence supporting the assumptions. Shaft currents are the fastest growing cause of bearing failure today, because of the rapid deployment of adjustable speed drives in industry, air conditioning and ventilating systems, plastic extruders, etc. Its root causes will be presented and analyzed to provide guidance for prevention of recurrence. Service centers play an important role in helping their customers identify and fix causes of premature motor failure. It helps to know the history of failures or repairs, but which are sometimes just not available or the piece of equipment transferred from one site to another.

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Table 16 Bearing Failures/Malfunctions Cause

Evidence

Overload

Spalled Races/Balls, Rollers

Foreign Matter

Noise, Embedment

Excessive Preload

Excessive Heating

Excess Grease

Immediate High Temperature

Insufficient Grease

Delayed High Temperature

Cocked Bearing

Noise, Skewed Ball Path

Shaft Currents

Spherical Craters or Fluting

Shaft Voltages and Bearing Currents There are three primary causes of shaft voltages and bearing currents in drives today. • Electrostatic rotor voltage caused by power supply asymmetry resulting from: a. Unbalanced line voltages b. Common mode voltage caused by odd number of semiconductor switches on at one time (AC inverter driven) This is electrostatic asymmetry. • Fast switching (high frequency ) PWM power supplies a. Fast voltage transients (high dv/dt) b. High frequency of PWM Carrier • Axial rotor voltage generation caused by motor magnetic asymmetry or flux imbalance (Figure 1) due to: a. Rotor static or dynamic eccentricity b. Rotor slotting c. Axial cooling holes in rotor d. Stator eccentricity For AC drives the most common cause of shaft voltages and bearing currents is the generation of electrostatic potential induced on the rotor, as a result of common mode voltage. PWM Adjustable Speed Drive and Topology Figure 17 shows a block diagram of a typical PWM adjustable speed drive, and Figure 18 the simplified drive schematic, with the input diode rectifier bridge, a DC bus filter capacitor, and an output or inverter stage. Figure 17 Block Diagram – PWM AC Motor Control

AC MAINS

AC Motor

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Figure 18 Basic Adjustable Speed Drive Schematic

AC MAINS

To Motor

Three-Phase Sine Waves Referring to Figure 19, at any instant in time the sum of the three 60 Hz line voltages V A, VB, and VC add to zero. Hence the common mode voltage for the system VCM = 0. For all balanced three-phase 60 Hz line power, the Common Mode Voltage (CMV) is theoretically zero. Figure 19 Three-Phase Sine-Wave Voltages VA

VB

VC

0 Volts

Common-Mode Voltage VCM = 0

VCM 0 Volts

Inverter Technology and Common Mode PWM ASDs generate common mode voltage because of their six-switch three-phase topology, in which three of the six transistors are on at any given time. Referring to Figure 20, closing Switches SW-1, SW-4 and SW-5 as shown in Figure 21, gives the equivalent circuit shown in Figure 22. The resulting voltage from the capacitor neutral to the motor neutral at time T1 then can be calculated as VCM = VB/6 Figure 20 Basic Six-Switch Inverter/Three-Phase Motor Circuit Configuration

+ VB

MOTOR

+

SW-1

Motor Neutral

SW-5

SW-3

VB/2 Effective System Neutral VB/2

+ SW-2

SW-4

CW

SW-6

Ground

Windings 0-Ref

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Motor Frame

Figure 21 Circuit at Time T1 when SW 1, 4, 5 Closed +VB

MOTOR

SW-1

SW-3

SW-5

SW-2

SW-4

SW-6

Motor Neutral

System Neutral

0-Ref

Ground

Frame at Earth Ground = Approx. System Neutral Potential

Figure 22 Equivalent Circuit at Time T1 with Switches 1. 4, 5 closed

+

RW

RW

VB/2

System Neutral

+

Motor Neutral

+ VB/2

Voltmeter VCM

RW

Common Mode Voltage VCM = VB/6 Six-Step Inverter By successively closing different combinations of three switches different values of CMV are generated as shown in Figures 23 through 26. Three of the six switches are always closed (in theory) to generate an output voltage. The control strategy used, “six-step”, sine-triangle, adjacent states, space vector modulation determine the output voltages to the motor. As an example, by closing three switches in sequence of 1-6-32-5-4, etc., and always maintaining three closed switches at any given time generates six-step voltages as shown in Figure 27. The name “six-step” is derived from the six voltage levels displayed in the Phase-toNeutral voltage waveform VAN. This type of inverter is NOT a PWM inverter because its RMS output voltage is fixed by the DC bus and not by switching on and off to create a variable duty cycle. Sine-Triangle PWM Using three-phase sine waves as references (φA, φB, φC) shown in Figure 28 and modulating them with a higher frequency triangle wave, pulse-width modulated “sine-weighted’ digital signals are generated. When the reference signal for phase A, for example, is greater than the triangle signal, the upper switch SW-1 is

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closed. And when the triangle signal is greater, the lower switch SW-2 is closed. The resulting phase-tophase voltage waveform VA – VB = VAB at the output of the inverter or drive is a sinusoidally-weighted PWM voltage of peak amplitude VBUS with an effective switching frequency twice that of the triangle carrier frequency fC and a fundamental frequency fO which is that of the sine reference signal. This PWM algorithm has been one of the more popular PWM strategies, but is rapidly giving way to higher performance algorithms such as Space Vector modulation, with the ready availability of higher performance microprocessors and DSPs.

Figure 23 Circuit at Time T2 when SW 2, 4, 5 Closed

+VB

MOTOR

SW-1

SW-3

SW-5

SW-2

SW-4

SW-6

Motor Neutral

System Neutral

Ground

Frame at Earth Ground = Approx. System Neutral Potential

Figure 24 Equivalent Circuit at Time T2 with Switches 2. 4, 5 closed

+

RW

VB/2

Motor Neutral System Neutral

+

+ Voltmeter VCM

VB/2

RW

Common Mode Voltage VCM = -VB/6

5-26

RW

Figure 25 Circuit at Time T3 when SW 1, 3, 5 Closed

Motor Neutral

+VB MOTOR SW-1

SW-3

SW-5

SW-2

SW-4

SW-6

System Neutral

Groun d

Figure 26 Common Mode Voltage with Six-Step Inverter Operation (Switching sequence 1-6-3-2-5-4) Phase A Switching +VB/2

1 - ON

1 - ON 2 -ON

-VB/2

Phase B Switching +VB/2 -VB/2

3 - ON

3 - ON 4 - ON

4 - ON Phase C Switching

+VB/2

5 - ON

5 - ON

6 - ON

6 - ON -VB/2 +VB

VAB Phase-to-Phase Voltage

-VB

+2VB/3

VAN Phase-to-Neutral Voltage

-2VB/3 Common Mode Voltage +VB/2 -VB/2

T1

T2

T3

For Fundamental Motor Frequency of f0, Common mode Voltage Frequency is 3 f0

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Figure 27 Sinusoidal PWM Waveform Generation and Common Mode Voltage φA

φB

φC Reference Sine Waves

Phase A

Carrier Signal @ PWM Freq. fC

Phase B

Phase C

+VB/2

Common Mode Voltage +VB/6 -VB/6

0 -VB/2

Common Mode Frequency = Carrier Frequency fC No. of Transitions or Steps = 6 x fC Common Mode Voltage and Current Motor controls have abrupt voltage transitions on their outputs that are inherent sources of radiated and conducted electrical and magnetic noise. Most control manufacturers use IGBTs today because of their efficient fast switching properties, with rise- and fall-times on the order of 20 to 50 nanoseconds. The majority of control related problems are caused by conducted noise currents, the magnitude of which, are a function of the amount of stray capacitive coupling output phases to ground, and the rate-of-change of voltages on the control’s output. These noise currents flow through the parasitic capacitance of the motor cables to the ground lead or conduit, and from motor windings through the parasitic capacitance of the motor to the frame (and rotor) of the motor. These currents flowing through the system grounds and returning ultimately to the source or cause are called common mode currents. It is these currents that cause a voltage drop in the ground circuits, and this voltage drop is referred to as “common mode voltage.” Because of these voltage drops, one piece of equipment’s “reference ground” may be at a substantially different voltage level with respect to another’s “reference ground.”

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By summing the three voltages at the ASD’s output terminals with respect to a reference or ground, a voltage waveform is generated that is a function of the PWM frequency, with rapid transitions corresponding to the switching speed of the IGBT switches. It is this common-mode voltage that is induced in the motor circuit, creating problems as described earlier. For the sine-triangle PWM strategy of Figure 28, a common-mode voltage is generated as repeated in Figure 29. It very much resembles the six-step waveform of Figure 27. Note the mostly uniform transitions or steps with amplitude VB/3, except for an occasional seemingly double transition of two times VB/3 or 2VB/3. It is these larger transitions that create the most trouble for the drive system; high dv/dt, bearing currents, EMI, etc. Referring to the waveforms of Figure 28, it is seen that these “double” transitions correspond to “almost simultaneous” switching of transistors in different phase “legs” of the inverter. Avoiding these “almost simultaneous” transitions by delaying the turn-on or turn-off of the second or following transistor by no more than 5-10 microseconds, a clear step or plateau results and has the effect of minimizing common-mode dv/dt. Figure 28 Sine-Triangle Common Mode Voltage (from Figure 28) +VB/2 +VB/6 0

-VB/6

-VB/2

Almost simultaneous switching of two IGBTs in output phases.

Bearing Currents Inverter-induced bearing currents appear to have two characteristic modes: a. Displacement current through the bearing This is proportional to the derivative of common mode voltage applied to the stator winding. This current is capacitive and occurs to some degree at every switching transition of the inverter. Its magnitude can be affected by anything that affects the rise time (dv/dt) of the applied voltage (current loading of the switching device, dc bus voltage, reflected wave reinforcement/cancellation, etc.) or the instantaneous capacitance of stator to rotor (rotor tooth to stator tooth alignment, air gap eccentricity, etc.). These variations make the capacitive current a somewhat random phenomenon. b. Discharge of electrostatic energy that is stored in the capacitance between rotor and stator. This event also appears to be highly stochastic (non-repetitive or random) and causes peak currents that are one to two orders of magnitude greater than displacement currents. Factors that may play a role in the occurrence of discharge currents are rise time of the applied voltage, asperity of the rolling elements or journals, grease thickness, grease contaminants and bearing loading (mechanical, thermal). This is the current that causes pitting and fluting of the bearing races, and will be the main focus of this discussion.

Bearing Currents Often the only path available for rotor current is through the bearing races and balls to the grounded motor end bell/frame. As static voltage builds up and discharges through the bearings, pitting and scoring of the balls and raceways occur over time and can lead to premature failure of the bearing. If the capacitively stored energy is great enough, this energy concentrated in a very small area raises the temperature of the point of contact, melting the metal and creating a pit. Various solutions have been proposed and some are in production; such as slip ring/brushes (on the shaft to ground it), conductive lubricants, ceramic ball bearings, etc.

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Bearing Current Circuit Model Much has been written regarding the nature of bearing currents and the vast complexity of the large number of variables, including analyses based on quantum mechanics and electron tunneling. Nothing so complex will be attempted here; our goal is to develop a simple circuit model to explain discharge currents in bearings. Consider as our model a steel ball suspended by a thin insulating oil film between two electrically conducting raceways. The oil film acts as a dielectric between two conducting surfaces, forming small capacitors between the inner and outer races, as shown in Figure 30 (b). These two capacitors COR and CIR, effectively connected in series with the steel ball, account for the small displacement current through the bearing. Because the capacitance is very small compared to the other motor parasitic capacitors, they are neglected in the model. The inductance and resistance of the ball and races must be taken into account, and are shown in Figure 30 (c).

Figure 29 Bearing Equivalent Circuit Outer Race SB COR

SOR LB

CIR

RB

SIR

Inner Race

(a)

(b)

(c)

Figure 30 Simplified Bearing Current Model VCM

Motor Coils

VCM

VCM LWR

CWR

VB

VB CWS

Rotor

CWR CWR

LB

CB

SB

SB

LWS

CB

LB

CB CW

RB

RB

Stator

(a) (b) 9-26

(c)

Stator-Rotor Circuit Model Consider the lumped-capacitor stator-rotor model as shown in Figure 31. The rotor is charged to shaft voltage VB through the winding-to-rotor capacitor CWR by the common-mode voltage, effectively on the bearing current model. The critical path for bearing discharge current develops when the switch SB closes as the ball makes contact with both inner and outer races. The capacitive energy ½ CB x VB2 is now dissipated stator windings. Charge is also transferred to the stator by the winding-stator capacitor CWS by the CMV but if the stator is well grounded (as shown) it remains essentially uncharged, with little influence on the through LB and RB, passing a relatively large current through a very small contact surface, and in some instances raising the temperature of the surface above melting, creating small pits in the softer raceways. This displaced metal is trapped in the grease as sediment and further contaminating the grease. Additionally, currents flowing through the grease causing decomposition through chemical activity. Eventually the bearing fails.

Shaft Voltage Shaft voltage relates to bearing current but the relationship is not direct. One cannot assume that twice the shaft voltage will result in half the bearing life for example. However, if the shaft voltage is sufficiently low, little or no bearing current will flow. NEMA MG 1-1993, Section IV, Part 31 states that bearing failure due to electrical arcing on motors with frame sizes less than 500 frame series, can occur if shaft voltages higher than 300 millivolts (peak) are present. Other sources (EPRI, etc.) suggest that voltages greater than 5 volts will result in EDM bearing damage. However, it is assumed that the ohmic discharge of electrostatic energy as described above, is the primary cause of bearing damage. Shaft voltages up to 30volts and bearing currents exceeding 700 milliamp have been measured in the laboratory.

Effect of dv/dt The higher frequency and dv/dt cause voltages to build up on the rotor through charging of parasitic capacitance from the motor windings to the rotor. Since the stator is grounded via the motor frame, voltages appear on the rotor and shaft with respect to the frame. The higher the frequency and dv/dt, and the smaller the air gap, the higher the voltage difference, and it can easily approach 10-15 volts or more. This can cause problems for sensitive loads or measuring equipment connected to the motor shaft, as in high speed cutting tools.

Shaft Voltage Measurement By monitoring shaft voltage and its waveshape with a fast oscilloscope, we may be able to accurately predict the presence of damaging bearing currents. The waveform of Figure 30 shows the relationship between shaft voltage and bearing current observed in the laboratory. An oscilloscope set to trigger on the rising or falling edge of the voltage waveform of shaft voltage will allow observation of the ohmic discharge region. Observations confirm that the higher the voltage at the point of discharge, the greater the bearing current pulse.

Figure 31 Shaft Voltage and Bearing Current Relationship

Shaft Voltage

≤ 30 Volts

Ohmic Discharge

Time t in μsec.

Bearing Current

10-26

Figure 32 Illustration of Relationship of CMV, Shaft Voltage and Bearing Current 480 VAC Drive with 660 VDC Bus +330-Volts

Common-Mode Voltage Transitions +110-Volts

0 -110-Volts

High CMV dv/dt Induces Shaft Voltage

-330-Volts

Shaft Voltage Time t Bearing Current

5 μs

Measurement of Bearing Currents In order to obtain a measurement of the current through the bearing under actual operating conditions, the bearing must be electrically isolated from the frame and an alternate conducting path must be provided from bearing to the frame. The alternate conducting path, usually a lead wire jumper from stationary bearing outer race to motor end bracket, serves as an access point for a shunt or high bandwidth current probe. Not only does this modification require extensive rework of the motor under test; it also alters the high frequency impedance of the bearing current circuit. Additionally, because of the stochastic nature of bearing currents, it is not possible to accurately define such current with one number, such as peak current. Instead, statistical measures such as average, median, standard deviation (assuming a normal distribution) and frequency of occurrence may be more meaningful. Such measures require the phenomenon to be observed over relatively long periods of time with high sample rates in order to capture the true waveshape of each individual current pulse. Statistical post-processing is required, above and beyond what would normally be found on a digital storage oscilloscope.

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Having said all of the above, it is possible to estimate relative peak magnitudes of bearing current based on knowledge of the shaft voltage and the stator-to-rotor capacitance. A discussion of shaft voltage and techniques for observing and/or measuring it and stator-to-rotor capacitance follows. Bearing Facts An Oil film between bearing and raceway is typically 0.1 to 1 μm thick, as depicted in Figure 32. Assume average is 0.5 μm. A typical sheet of paper is around 0.1 mm thick. It would take 200 oil films stacked on top of each other to equal the thickness of one sheet of paper.

Figure 33 Cross Section of Bearing Ball Bearing 0.1 to 1 μm

Raceway

Types of damage observed (Figure 33) • • •

Frosting Pitting Fluting

Low Speed Motors operating at low speeds, where the balls tend to maintain direct contact with the races, usually sustain little severe damage from bearing currents. Because of this almost constant contact, the shaft voltage cannot rise high enough to cause melting and pitting of the races. Frosting (burnishing) of the raceways may be the best indicator of sustained low speed, low voltage operation. Consequently the only warning may be a slight increase in bearing noise over a period of time. There is an exception to low bearing damage at low speed, and that is if the drive is operating at high torque, requiring high voltage to the motor. In this case severe damage can occur in the form of a random pitting pattern, shown in Figure 33 c

Varying Speed By varying the motor speed over even a small range, the oil film tends to be less uniform and the discharges more random. The peak bearing currents have been observed to initially drop by more than 50% when motor speed is stepped from a fixed speed to a new RPM. If speed is again held constant the bearing current spikes begin to rise as the oil film once again becomes uniform. After several minutes at that new speed the current spikes are back to their initial amplitudes. This would imply that introducing a small dither in the speed of the motor would reduce bearing damage, and reports from industrial installations support this hypothesis.

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Higher Speed It has been observed in the laboratory that speed does influence bearing currents and resulting failure. At motor frequencies below 25 Hz, shaft voltages are low and Bearing currents are small. As motor frequencies are increased above 25 Hz, discharge amplitudes grow rapidly up to 40-45 Hz, rising slightly until around 50-55 Hz, but the incidence of discharges decreases as the bearings float on the more uniform oil film. Above base speed, the discharge amplitudes again decrease as the PWM waveform of applied voltage is overmodulated, and the incidence of discharges continue to decrease. Sustained operation near base speed can result in severe pitting because of the relatively uniform oil films created. This uniform film allows shaft voltage to build up to fairly high levels before arcing, yielding fewer but more severe discharges. Techniques that Protect the Bearings Here are some of the ways one can protect the motor bearings with a relatively high degree of success, listed in order of preference (for a variety of reasons). Some, such as the shaft grounding brush, require periodic inspection or maintenance. • Use Shaft Grounding Brush • Insulate Both Bearings • Isolated Bearings/Both (Ceramic) • Use Bearing with Conductive Greases • Dv/dt Filter (also called Sine Wave Filter) • Use a Motor with Faraday Shielded Stator Winding • Common Mode Transformer (Passive) • Common Mode Filter (Active) • Dual PWM Inverter (12 Switch) • Motor with Specially Wound Stator for CMV Cancellation Techniques that Reduce Bearing Currents, but not eliminate the potential for them. • Reduce PWM Frequency to lowest acceptable value • Install 3-5% Inductor between Drive and Motor • Securely ground motor frame with low inductance/impedance cable plus conduit back to drive • Reduce Drive Input Voltage to Lowest Acceptable Value • Always use motor under load (minimize no-load operating time)

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Figure 34 Typical Bearing Current Damage Patterns

14-26

Figure 35 Examples of Damage Due to Bearing Currents (Courtesy SKF)

15-26

Figure 36 Typical ASD Connection Diagram

Adjustable Speed Drive Transformer Secondary

Cable in Conduit Motor Frame AC Motor

Probe to measure CM Current ICM

Common Mode Current Path

Figure 37 Basic AC Motor Cross-Section

Stator

ODE

Bearing

Rotor

Shaft

16-26

DE Bearing

Figure 38 Shaft Voltage and Bearing Current Caused by Axial Asymmetry

Stator ODE Bearing

Rotor

DE Bearing

Shaft

Voltage induced in shaft by magnetic asymmetry causes bearing currents as shown by arrows

17-26

Figure39 Shaft Voltage and Bearing Current Caused by Capacitive Coupling between Stator and Rotor

Stator ODE

Bearing

Rotor

DE Bearing

Shaft

Electrostatic Coupling of Stator Windings to Rotor

Voltage induced in shaft by capacitive coupling to stator windings causes bearing currents as shown by arrows

18-26

Figure 40 AC Motor with Isolated Shaft at One End

Stator

Rotor Insulation

Shaft DE Bearing

ODE Bearing

Insulating Material for Isolating Shaft from Bearing Inner Race Voltage induced in shaft by magnetic asymmetry but no current can flow through bearings Figure 41 AC Motor with Shaft Isolated at Both Ends

Stator

Rotor Insulation

Shaft DE Bearing

ODE Bearing

Insulating Material for Isolating Shaft from Bearing Inner Race

19-26

Figure 42 Outer Race Isolated Bearings From SKF

Figure 43 AC Motor with Hybrid (Ceramic) Ball Bearings

M o

Stator ODE Bearing

Rotor Shaft

Hybrid (Ceramic) Ball Bearings

20-26

DE Bearing

Figure 44 AC Motor with Radial Shaft-Grounding Brush Assembly on ODE

Stator Shaft Grounding Brush Assy

Rotor Shaft

ODE Bearing

DE Bearing

Figure 45 AC Motor with Radial Shaft-Grounding Brush Assembly on Both Ends

Stator Shaft Grounding Brush Assy

Rotor Shaft

ODE Bearing

DE Bearing

21-26

Figure 46 Low Cost Shaft Grounding Brush Components and Installation

22-26

Figure 47 Shaft Grounding Currents Caused by Poor Motor Ground

Figure 48 Common Mode Currents and CM Filter

MOTOR CABLE

Inverter Isolation Transformer Secondary Common Mode Filter

AC Motor

Optimum Path for Common-Mode Current (Green = Good)

Common-Mode Current Flowing through Earth-Ground/Building Conduit and Loads (Red = Not Good)

23-26

Figure 49 Common Mode Mitigation Components

24-26

Figure 50 Sketch of "Triangular-Wire" Cable Heavy Stranded Copper

Insulation

Sheath

Phase T1

Phase T2

Ground

Phase T3

Insulated Twisted Stranded Conductors

Figure 51 Asymmetrical Cabling

25-26

Figure 52 Sketch of "Symmetrical Ground” Three-Phase Cable Heavy Stranded Copper Sheath Earth Grounded

Rubber or Neoprene Insulation Phase T1

Earth Ground Conductors (3)

Phase T3

26-26

Phase T2