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PdM-2006 & LubricationWorld online at http://www.maintenanceconference.com

Copyright 2006 Reliabilityweb.comKadon Electro Mechanical Services Ltd. www.kadon.ca Page 2 of 22

Table of ContentsDynamic Electric Motor Testing of DC Motors ................................................................. 1

Table of Contents................................................................................................................ 2

Introduction......................................................................................................................... 3DC Motor Primer ................................................................................................................ 4

DC Motors Circuit Analysis ............................................................................................ 4

Shunt wound DC motor .................................................................................................. 4Performance of DC motors when operated under load................................................... 5Summary of load characteristics - DC Motors ............................................................... 6

Other Field Windings.......................................................................................................... 7

Commutating windings................................................................................................... 7

Compensating windings.................................................................................................. 7Field windings - summarized.......................................................................................... 7

Motor Speed Control........................................................................................................... 7

Adjustment of the flux (field current) via shunt field rheostat ....................................... 7Adjustment of armature circuit resistance ...................................................................... 7

Adjustment of armature terminal voltage ....................................................................... 7

SCR/Diode Bridge circuits supply armature and field ................................................... 8Motor Starting..................................................................................................................... 8

Complex Analysis............................................................................................................... 8

Linear analysis vs. saturation.......................................................................................... 8

Transient analysis............................................................................................................ 8Acceleration linear and angular ................................................................................... 9

Torsional inertia and stiffness - resonance...................................................................... 9

Summary of DC motor complexities .............................................................................. 9

DC Motor Testing............................................................................................................. 10Performance testing: efficiency evaluation................................................................... 10

Troubleshooting DC motor faults ................................................................................. 11Test Data ........................................................................................................................... 13

250 Volt - 25 KW - DC shunt wound no load ........................................................... 13

Zero the Hall Effect current clamps.............................................................................. 13DC In-Rush/Start-Up test data ...................................................................................... 13

DC SCR drive armature circuit.................................................................................. 14

Analysis of phase A current.......................................................................................... 14

Tips when troubleshooting DC motors fed by SCR bridges......................................... 16Power drive input AC side...................................................................................... 18

In Closing.......................................................................................................................... 22References..................................................................................................................... 22
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IntroductionBefore we jump into software data analysis specifics and test result data, we felt it

appropriate to first present a brief primer on DC motors, so that we can better appreciatethe data we are about to look at.

After all, what good is data if it does not make sense? Furthermore, we have maintainedfor many years, that simply going out and collecting data in the field, then coming back

to the office to interpret the data, is a flawed process. This is an important difference

between test instrumentation that efficiently, but blindly collects and instrumentation thatalso efficiently collects, but lets the technician see and evaluate the data, as it is being

collected, and enter notes, conclusions, and flags. This is the argument forinstrumentation that permits in-depth troubleshooting if it is required. Lets finish the job,

not leave it partly done!

We strongly believe that we should be interpreting the data as we collect it, this has somany advantages. If we see some data that is not right, we are in the ideal position to

follow up immediately while we are in the plant, connected to the circuit. Follow upaction might mean taking additional, more specialized measurements. It could meansimply entering the appropriate condition code into the software, so that the motor and

circuit are flagged for extra attention. It might mean that the motor/circuit are removed

from service right now, for detailed dismantling, inspection, and overhaul and repair ifneeded. It might mean the motor/circuit is flagged for detailed inspection/overhaul on the

next shutdown. It might even mean that the data is wrong, and should be checked and

retaken immediately. In our business there are few things worse than bad data. Suspectdata should be discussed within the group involved (customer and vendor), so we can all

learn from it. Certainly, thinking about and analyzing data as it is collected makes the job

so much more interesting and challenging.

What is the alternative? Collect the data blindly and quickly. Store it away in the

computer until you get time to look at it. The worst case is when a machine fails, and then

we look back at the data and realize we could have caught the problem before it turnedinto a blow; if we had looked and if we had understood what we were looking at.


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DC Motor PrimerHow deeply do we have to go into DC motor operation? The intent is that these notesshould provide answers to our questions as we test various kinds of DC motors.

Especially when we do online testing of motors driving variable loads or motors fed by

SCR bridges, we see waveforms and spectra which may not make sense at first, but withsome explanation become easy to understand.

Math has been minimized, with the exception of some circuit equations, which may serve

as a useful review for some people.

DC motors Circuit AnalysisWhy have DC motors

been so popular over

the many years?DC motors have a greatvariety of performance

characteristics, offered

by the possibilities ofshunt, series, and

compound excitation,

and in the relativelyhigh degree of

adaptability to control.

[1, p. 31]

Shunt wound DC motor

Counter emf (Cemf) increases with field current and RPM.Summing the voltages around the armature circuit:

Ea Cemf L di/dt - Ia Ra = 0 (Transient conditions) [1, p. 537 (12-4)]

If the speed, load, and supply voltages are constant, then the inductance termL di/dt = 0

and can be removed, thus:Ea Cemf = Ia Ra (Steady State Conditions)

Replacing Cemf with (k x RPM x If), the approximate steady state relationship can berewritten as:Ea (k x RPM x If) ~= Ia Ra

Solving for speed: RPM ~= (Ea Ia R) / (k x If) [also see 1, p. 284]

In a shunt wound DC motor (R) is small and the steady state relationship between speed,

armature volts, and field current can be approximated even further, to give us this usefulsteady state concept: RPM ~= constant x Ea / If (Steady State Conditions)

If

Rf Ef

Lf

A

Ia

Cemf

Ra

L

Ea

Figure 1: DC Motor Circuit


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RPM ~= constant x Ea/If DC Shunt Wound - Steady State

This equation can be stated, DC Shunt wound motor speed is approximately

proportional to armature voltage and inversely proportional to field current.

The following graph of this relationship illustrates the very useful, controllable, widespeed range available in a DC shunt wound motor.

Performance of DC motors when operated under load

Loaded Condi tions Shunt Wound DC Motor

Increased torque is accompanied by a small decrease in speed resulting in decreased

counter emf, which allows increased current through the small armature resistance.

Typically, shunt wound DC motors are constant speed, having only about a 5% drop in

rpm from no load to full load.

Figure 3: DC Motor Shunt Wound RPM vs. Load (% of Rating)

RPM

Decreasing Field Volts

Field weakening

Const. Arm. Volts

Increasing Armature Volts

(Constant Field Volts)

RPM ~= K x Ea / If

% of Rated RPM:

100%

80%

60%40%

20%

0%

0% Load 100%

Figure 2: DC Speed Control


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Loaded Conditions - Series Wound DC Motor

1. At the moment of connecting a series motor to full armature volts, RPM = 0 andthe rate of rise of DC amps is limited by Inductance (L di/dt).

2. Armature will accelerate; Acceleration = (Available Torque)/Inertia; where

(Available Torque) = (Total Torque) (Torque required to drive the load).3. As armature rpm increases, the reducing armature (field) current causes Total

Torque to decrease.

4. The motor stops accelerating when (Total Torque) = (Torque required to drive theload).

5. Therefore series motors must never be run unloaded, as they will keep onaccelerating well past maximum speed. Failure will normally occur.

Series DC Motor

Rated: 1000 RPM - 10 Amps

0.01

0.1

1

10

100

0 1000 2000 3000 4000 5000

RPM

Amps&

PerUnitTorqu

e

Amps

PU Torque

Figure 4: DC Motor Series Wound Amps & Torque vs. RPM

Series motors are thus variable speed motors with a steeply drooping speed-loadcharacteristic. This is excellent for hoist, crane, and traction motor applications.

Loaded Condi tions - Compound Wound DC Motor

A compound wound DC motor has both a shunt field and a series field. The series field

may be connected either cumulatively (commonly used) or differentially (rarely used).

The cumulative connection has a speed-load characteristic intermediate to the shunt and

series connected motors and does not have the disadvantage of very high light-load speedof the series motor.

Summary of load characteristics - DC Motors

As load is added to the motor shaft:

The shunt motor operates at almost constant speed.

The series motor operates at speeds which decrease rapidly.

The compound motor operates with any degree of droop between these extremes,depending on the relative strengths of series and shunt fields.


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Other Field WindingsAdditional field windings are used to compensate for the effects of sparking at the

brushes and to permit good operation under heavy duty conditions.

Commutating windings

Sparking at the brushes causes excessive local heating of the brushes and commutator,

leading to burning away of the copper and carbon, and possibly more severe effects.

Commutating or interpole windings are used, with brushes in the neutral position, toobtain good commutation and minimize sparking at the brushes.

Compensating windings

Short term heavy overloads, rapidly changing loads or operation with a weak main field

can cause excessive cross-magnetizing armature reaction, and the coil voltage may be

high enough to break down the air between the adjacent segments to which the coil isconnected resulting in flashover or arcing between segments. [1, pp. 244-246]

Compensating or pole-face windings are used to mitigate these effects on machinessubject to severe duty cycles (e.g. steel mill motors).

Field windings - summarized

Shunt and series field windings act along the axis of the main poles.

Commutating and compensating field windings act along the armature axis.

Thus, control of air gap flux around the armature periphery is achieved. [1, p. 247]

Motor Speed ControlMethods of DC motor speed control are summarized as follows:

Adjustment of the flux (field current) via shunt f ield rheostat

Common with shunt and compound wound motors

Constant power

Adjustment of armature circuit resis tance

Common with series motors

Constant torque

Adjustment of armature terminal voltageWard Leonard System

MG set: AC motor drives a DC generator;

Output of the generator feeds the DC motor armature;

Control is achieved by varying the generator field voltage

Common in passenger elevator control


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SCR/Diode Bridge circuits supply armature and field

Very common today, SCRs are very powerful, efficient, cost effective,controllable, and lend themselves to feedback control schemes

Feedback signals are processed causing SCR firing to be phased forward or backthus varying armature voltage/field current

Resulting current and voltage waveforms can have very high ripple

Motor StartingHistorically, the series resistance in the armature circuit limited initial inrush current, as

without rpm, there is no counter emf to limit the current. Some older starters were quite

complex with timers and DC contactors acting to switch resistance out of the armaturecircuit as the motor accelerated up to speed.

Modern starters utilize SCR control, permitting gradual application of voltage to the

armature, permitting it to come up to speed without experiencing high starting current.

Complex Analysis

Linear analysis vs.saturation

Motor magnetic circuits operate at the knee of the curve (in saturation). [1, pp. 232-234].

The hysteresis loop illustrates the nonlinear relationship between volts (rate of change offlux) and amps (mmf). Mathematical analysis of nonlinear circuits is quite complex. To

simplify analysis linearity is generally assumed, even though this does not give perfect

answers. Thus, we depend on real life measurements such as can be made with the PdMAtest equipment for collection of operating data. Results can then be examined and

adjustments made for the effects of nonlinearity and saturation.

Transient analysis

When conditions are changing, whether they are changing speed, changing load,

changing armature voltage, changing armature current, changing field voltage, orchanging field current, we have to include a rate of change term in the circuit equations,

so as to take into account such changes.

For example, when the field is fed by a rectifier with significant ripple in the voltage and

current, instead of writing: ef = if x Rf, we include an L di/dt term: ef = if x Rf + L di/dt

This assumes linearity thus, is an approximate relationship.

Non-steady field current has an effect on armature current. Recall: Cemf increases with

field currentthus, field current ripple is reflected in Cemf, thus affecting armature

current and therefore, instantaneous torque (torque ripple).


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Acceleration l inear and angular

When we apply a force to a mass, the mass accelerates:F = M (Mass) x A (Acceleration)Similarly, when motor torque exceeds that required by the load, the load accelerates

according to:Available Torque = I (angular inertia) x (Angular Acceleration).

When the armature and field voltages call for the load to go faster (or slower), the changein speed does not happen instantaneously; it takes time for the masses and angular

inertias to accelerate or decelerate.

Torsional inertia and sti ffness - resonance

It is possible for systems to resonatetorsionally. This can happen if the motor and loadtorsional stiffness and angular inertia result in torsional natural frequencies close to motor

torque ripple frequency or harmonics thereof.

Resonances are a back and forth exchange of energy between energy storage mediums;

such resonances can involve not only mechanical components (springs, mass-velocity),

they may also involve the electrical system (inductance Li^2, capacitance Cv^2).When we suspect the electrical power system is involved in such a resonance, we need

instrumentation, which will let us look for modulation of power flow.

Summary of DC motor complexities

Real world complexities make theoretical analysis of DC machines difficult. To review

some of these:

1. Commutator ripple2. Sparking at the brushes3. Demagnetizing effect of brush shifting off neutral4. Varying length air paths in inter-polar space5. Cross-magnetizing armature reaction6. Magnetic saturation effects cause non-linear relationships between volts and

amps; mathematical analysis methods generally make the assumption of linearity.

7. Diode and SCR bridges power modern DC machines with harmonically richvoltage applied to the field and armature certainly not smooth DC voltage!

(Transient behavior)8. When a DC armature is supplied by a captive transformer, transformer impedance

can cause large voltage drop when each SCR is switched on, six times per cycle

(360 Hz). (Transient behavior)9. If multiple motors are fed by one transformer, SCR switching can affect the

supply voltage of other motors fed by the transformer.

10.Such chopped up and variable voltage may also be applied to the field circuits ofeach motor, resulting in even more transient effects on armature current.

11.Modern DC drives generally have feedback control, with feed back signalscoming from RPM, armature current, current limit, field current, process

variables, etc. If control instability occurs, amplitude modulation of the appliedvoltage can result, even when driving very stable loads.

12.Torsional resonance may be excited by the above noted instabilities, resulting ineven greater instability.


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For these reasons, in the practical world, mathematical analysis of DC machines becomesless important than practical measurements and practical understanding. Therefore we

need to take reliable measurements on DC machines to determine if they are working

correctly and if they are not, to point us in the direction that will get them working

correctly.

DC Motor Testing

Performance testing: efficiency evaluation

Direct measurement of individual losses

On site conditions lend themselves to the loss segregation approach in evaluating

efficiency, as it can be more practical than taking the difference between output power

and input power. Accuracy of this method is good, despite having to assume a value forstray load loss.

Shaft power to load = input power minus:1. Shunt field I2R loss2. Series field I2R loss3. Brush contact loss

Assume 2.0 volts for brush/brush contact drop4. Armature I2R loss5. No-load rotational loss6. Stray load loss

Assume 1% to 2% of output power

The PdMA MCEMAXwith offline and online DC machine measurement capabilitiescan readily make the following measurements:

Copper loss: I2R loss of individual windingso Measure resistance of individual windingso Subtract 2.0 volts for brush/brush contact dropo Correct winding resistances to operating temperature

No load rotational losses = Core loss - friction and windageo No load rotational losses: Input power with machine operating at normal

speed and excited to produce the calculated internal voltage under full

load condition (subtract voltage drops across brushes and series fields).o Friction and windage: Power input at normal speed with machine

unexcited


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Troubleshooting DC motor faults

DC online shows promise in identifying the following types of DC motor system faults,

online:

Poor commutation, sparking at the brushes

Improperly set kick neutral Incorrectly connected fields

Problems with driven equipment

Grounded armature circuit

Grounded field circuit

Open circuited or cracked commutator risers

Open circuited armature coil

Open circuit drive SCRs, Diodes, fuses

Improper SCR firing circuit operation

Improperly functioning control circuit

Control instability Flashovers that have not progressed into complete armature failure

Data from both the AC side and DC side of the drive can be collected, observed, notes

made, and data saved for trending and comparison with future or previous data.

The PdMA test equipment has proven to be very handy as a powerful and easy to use

troubleshooting tool. On the AC side we liken it to a six-channel digital storage scope,

with analytical software and signal conditioning optimized for power systemmeasurements of three phase voltages and currents including spectrum analyzer

capabilities with automatic anti-alias filters on each channel.

DC online capability increases the utility of this instrumentation for engineering andmaintenance people responsible for maintaining and troubleshooting DC motors, in manyindustries cement, chemicals, mining, pulp and paper, steel, transportation, and

elevators to name but a few. The instrument is practical for electrically trained

professionals to connect to DC motors, capture, analyze, and store online voltage and

current data, given knowledge of how DC machines work, and what their characteristicsare.


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Test Selection

We can do offline and online tests of field andarmature.

Online tests that can be done on the armature are:

DC Power DC Current Analysis

DC In-Rush/Start-Up

Drive Input (Connect to the AC Side ofthe bridge)

For field tests we have:

DC Power

DC In-Rush/Start-Up

Test Points include:

04: DC Armature circuit

812: DC Field Circuit57: AC Side of the drive

Figure 5: DC Motor Test Selections

Figure 6: DC Motor Test Locations


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Test Data:

250 Volt - 25 KW - DC Shunt Wound no load

Zero the Hall Effectcurrent clamps

This is a requirement of

Hall Effect currentmeasuring devices; they

must be set to cancel the

effects of surrounding

magnetic fields.

DC In-Rush/Start-Up test data

Armature

This plot lets us evaluate the initial

inrush transient current, after

applying the initial voltage to themotor.

RMS current is calculated and

plotted 60 times per second. We cansee the current rising until the

armature starts to rotate, at which

time Cemf acts to limit the current.

The drive continues to automatically

increase the armature voltage and the

motor accelerates to full speed; and

we see the current drop as the motorapproaches full speed.

Figure 7: Pre-test Zeroing of the Hall Effect Probe

Figure 8: DC Motor In-Rush/Start-Up


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Shunt field

This is an online view of energizing current of the shunt field circuit.

The right hand plot (Figure 9) is an expanded view of field amps vs. seconds, duringenergization of the field.

DC SCR drive armature circuit:

Analysis of phase-A current

Refer to Figures 10, 11, 12 & 13.

Assuming ABC phase rotation, the following conduction sequence will be observed:

AB AC BC BA CA CB

Current flow in each phase can be seen as a sequence of six steps:

AB

C

Figure 9: Amps vs. Seconds - Energizing the Field Circuit

Figure 10: Current Flow in Phase A of an SCR Supply to a DC Armature.

This shows current entering via phase A and returning via phase B.


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1. From phase A, through the top left SCR to the (+) bus; through the armature, backto phase B via the bottom center SCR

2. From A returning this time via phase C (bottom right SCR)3. The next 1/6thof a cycle, A carries no current4. B is now (+), current returning to the supply transformer via A, which is now

negative in direction5. C is now (+), current returning to the supply transformer via A6. The next 1/6thof a cycle, A carries no current

Note the motor is unloaded

and the SCR bridge is

phased back, i.e.,

conduction is notcontinuous.

1: Current from AB

4: Current from BA

There is of course, equal

and opposite current on the

positive and negative sides.

Figure 11: Current in Phase A

1 2

3 6

4 5

1 2 3 4

4 5 6

Figure 12: Current in Phase A and B


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1: A returning via B2: A returning via C

3: B returning via C

4: B returning via A5: C returning via A

6: C returning via B

At all instants in time:

I1 + I2 + I3 = 0

This simply says that

current flowing into themotor flows out of the

motor of course!

Tips when troubleshooting DC motors fed by SCR bridges

1. The (+) side of the AC current wave is current going into the load; the (-) side iscurrent returning from the load.

2. It is tempting to make the same assumption regarding voltage; however wecannot, not even when the diodes are conducting. The diode bridge effectively

isolates the line from the armature. Ref: Fig 15 for an example, which illustrates

the power of making practical measurements, as opposed to theorizing.

1 2 3 4 5 6

Figure 13: Current in Phase A, B and C


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DC Power Test: I1 and I2: Armature current and voltage

The DC Power Test utilizes one currentclamp on each of A1 and A2; plus voltage

leads on A1 and A2. Per channel sampling

rate is 12,288 samples / second; fastenough to capture detailed current and

voltage waveforms. Each cycle takes 1/60

sec = 0.017 seconds; screen width of 0.02

seconds is slightly more than one cycle; ina three phase full wave bridge we see six

SCR conductions per cycle. The current is

discontinuous. SCR triggering is phasedback, to be expected since the motor is

unloaded (uncoupled).

Figure 15 Explained

SCR Fires (1); current increases (24). When volts drop to zero (3) the current reachesmaximum (4). The motor becomes a generator and terminal voltage goes negative (36)

1 47

82 3 5

6

Figure 14: Full Wave Rectified DC Current

Figure 135: Detailed DC Current and Voltage Waveforms


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to maintain current flow through the SCR. When the current drops to zero (5) the SCR

switches off. The resulting current discontinuity acts on circuit inductance to causevoltage spike, L di/dt (67). This voltage decays (78) until the SCR fires again (8),

and the cycle repeats.

DC Field voltage and current

These are voltage and current plots of the DC side of the field supply. The Field SCRfiring can be identified; there are two conductions per 0.017 sec. (field supply is a single

phase SCR bridge).

Power drive input AC side

These tests use three current probes and three voltage probes.

In Figure 17 below, note the unbalanced AC current. This is because the AC input feeds

multiple loads armature, field, and control power. This is normal operation. The table

of volts and amps shows the magnitudes and phase angles of the phase currents, Line-to-Line voltages and Line-to-Neutral voltages. Voltages are balanced.

Figure 16: DC Field Voltage and Current


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Figure 17: Phasor Diagram - Drive Input AC Side - OVERALL

Figure 16: Time Domain - Drive Input - AC Side


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We can see from Figure 18 that the instantaneous sum of all three current waveforms

always equals zero; also that the AC side hooks into two bridges one for fields (runs allthe time) and one for armature (SCR supply switches on and off). Figure 18 illustrates the

effects of one supply feeding two or more loads. When the larger supply turns on, the

voltage drops, and BOTH loads experience a transient voltage. On large drives with

captive transformers, transient voltage drop can cause troublesome control problems.

Figure 19 is a plot of the field

current (AC side). The field isfed by I1 and I2 (I3 = 0).

Figure 20 is a plot of instantaneous power

into the field (AC Side).

I1 is zero (1-phase load).

I3 is delivering almost zero power in total;some power is (+), some power is (-).

I2 is delivering most of the power, being

predominantly above zero.

Figure 19: Field Current AC Side

Figure 20: Instantaneous Power AC Side


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This AC side phasor diagram shows the three voltages at 120 degrees to each other

(normal). I1 is zero, as previously noted. I2 and I3 are of course equal in magnitude withopposite phase angles (1- phase field supply).

Caution: Even though Power = the sum of the power delivered by individual phases =(V1 x I1 x Cos Angle 1) + (V2 x I2 x Cos Angle 2) + (V3 x I3 x Cos Angle 3), be

cautious with this calculation as the voltages and currents have harmonics, which

contribute to Power. Figure 21 is the more accurate as it is the instantaneous product ofvolts x amps, regardless of their frequency.

Figure 21: Phasor Diagram - Field Circuit - AC Side


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Copyright 2006 Reliabilityweb com

In ClosingWe have presented a brief description of the various types of DC motors, how they work,

and how they are controlled, along with a description of their electrical characteristics.

We have identified reasons why DC motors are difficult to analyze from a theoreticalperspective.

DC Motors are not linear machines

The iron operates in magnetic saturation

Power supplies are generally SCR or diode bridges with non-sinusoidalwaveforms

DC Motors are often used in variable speed applications, requiring transientanalysis

Drive systems have feedback circuits that can give problems

Multiple motors are often fed from one transformer, and if one motor or drive isgiving problems, it causes voltage drops which affect other drives fed by the same

transformer.

Furthermore, the waveforms can be intimidating to the troubleshooter or maintenance

person.

We have presented a number of example waveforms from the AC and DC sides of drives,

with comments. We have discussed how to make these measurements.

We have only scratched the surface of the subject of testing and evaluating DC machines

with the PdMA online and offline testing capability.

We are confident that, as technicians gain experience working on DC machines with thisinstrumentation, significant progress will be made towards better predictive, proactive,

and corrective maintenance of DC machines and drives.

References1. Fitzgerald and Kingsley, Electric Machinery, 1stedition, McGraw-Hill, New York Toronto London,

1952

2. The Staff of Research and Education Association, Dr. M. Fogiel, Electrical Machines Problem Solver,Research and Education Association, New York, 1983

3. Electrical Maintenance Hints, Westinghouse Electric Corporation, Trafford, Pa, 19744. Maintenance Hints, Westinghouse Electric Corporation, Apparatus Service Divisions, Pittsburgh, Pa,

(undated)

5. Factory Testing of Electrical Apparatus, Westinghouse Electric Corporation, 2ndEd, East Pittsburgh,Pa, 1942

6. Robert Rosenberg, Electric Motor Repair, 2ndEdition, Rinehart Press, New York 1946 / 19707. Frank F. Fowle, Standard Handbook for Electrical Engineers, 6thEdition, McGraw-Hill Book

Company, Inc., New York and London, 1933

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