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Deftly Deploy

Drives

MOTORS AND DRIVES eHANDBOOK

TABLE OF CONTENTSMake the Most of Adjustable Speed Drives 4

Understanding the system curve is crucial to avoid problems

Rethink Options for Large Drivers 6

Variable-speed electric motors can offer significant advantages

Drive Retrofit Ups Water Treatment Plant’s Efficiency 12

Compact stand-alone drives replace 18-pulse drive

Additional Resources 17

AD INDEXYaskawa America, Inc. • www.yaskawa.com 3

MOTORS AND DRIVES eHANDBOOK: Deftly Deploy Drives 2

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Adjustable speed drives are

well-known but often misunder-

stood because little attention

is paid to the system as a whole. Note,

I used the terminology “adjustable

speed drives” (ASDs) and not “variable

frequency drives” (VFDs). Most engi-

neers associate speed changes with an

inverter-based VFD. That’s true in most

applications, but in the process indus-

tries we have a significant amount of

steam demand that allows us to deploy

back-pressure, extraction and condens-

ing steam turbines. These turbines are

inherently ASDs and can accommodate

shaft-horsepower and process-flow-rate

demand very efficiently. More can be said

on the topic, but realize that we have

several options to adjust operating speed

of rotating equipment.

So why should we be interested in adjust-

ing the speed of any rotating equipment?

The answer is simple — we design and build

processes and units for a certain specified

condition. At these design operating con-

ditions, when the system is started for the

first time, the system works at its optimized

configuration. However, as time goes on,

processes change, demands alter and our

system needs to adjust to accommodate

the off-design and the “new normal” oper-

ating conditions. As engineers, we also

over-design our systems by using safety

factors to compensate for any shortcom-

ings due to equipment or oversights. Lastly,

we design for reliability of operations and

redundancy and that should never be sacri-

ficed. Hence, a strong need exists to control

our systems at their optimized operation

over the range of operating conditions.

Make the Most of Adjustable Speed DrivesUnderstanding the system curve is crucial to avoid problemsBy Riyaz Papar, Energy Columnist



A very interesting two-part series in Chem-

ical Processing discussed optimizing

electrical systems (see, “Optimize Electrical

Systems, Part I (http://goo.gl/ym7SYl) and

Part II (http://goo.gl/4nlxZd)). The two col-

umns highlighted some options to adjust

electric motor speed. Centrifugal machines,

such as pumps, compressors and fans, are

energy-intensive rotating equipment and

follow the cubic law relationship between

flow and the power needed. Because flow

is proportional to the speed of the rotat-

ing equipment, this cubic law relationship

applies directly to speed and also power.

Hence, if speed is reduced to 80% of operat-

ing conditions, the power demand is reduced

to 51.2% of the operating shaft power.

The most common methodology for con-

trolling flow is to operate the rotating

equipment at a constant speed and regulate

the flow required by a restriction mechanism

(throttle valve, damper) or a recirculation

mechanism (bypass valve). This incurs a sig-

nificant energy loss while overcoming the

restriction or in recirculating the fluid. Adjust-

ing the speed of the rotating equipment to

a level that provides the necessary flow will

eliminate almost all of this energy loss. The

rotating equipment’s actual performance

curves should be obtained from the equip-

ment manufacturer to understand constraints

and minimum and maximum operating speeds.

Now comes the part where we need to

understand the system curve — or more

importantly, the difference between static

head and dynamic (frictional) head. Every

process is unique and will have its own

system curve. The intersection of the system

curve and the equipment’s performance

curve is the operating point. The cubic

law relationship works very well when all

the head developed by the equipment is

dynamic — purely frictional — such as in

closed-loop configurations. But as the static

head increases, a time will come when the

rotating speed can no longer be reduced

even though the flow required warrants a

speed reduction. This is because the system

static head needed is high enough that the

rotating equipment can’t develop it at that

lower speed. Avoid such misunderstandings

and misapplications at all costs.

So in summary, walk through the plant and

look for all those throttled valves, recircu-

lating flows, etc. and ask yourself, Are any

of them a good fit for adjustable speed

drives? Then, complete additional due dil-

igence by taking a systems approach to

implement ASDs using VFDs on electric

motors or steam turbine drives. Remember,

how you drive a car; press the accelerator

for the speed needed. You don’t floor the

pedal and control the car speed with the

brake – isn’t that what is happening with

fixed speed operation and control with a

throttle valve?

RIYAZ PAPAR is a former CP energy columnist. He can be

reached at [email protected].



Advances in high-speed electric

motor technology along with

improvements in the cost and the

performance of variable speed drive (VSD)

systems make direct coupling of a gearless

electrical motor to a turbocompressor or

pump worth considering for many services

requiring large drivers. Brushless synchro-

nous motors with two-pole rotors often suit

high performance duties. Special applica-

tions may benefit from other options such

as induction electric motors.

When using an electric motor driver,

full power is available instantly over the

entire site ambient temperature range

and train speed range (including startup).

The number of successive and cumula-

tive start/stop and load cycles generally

isn’t critical.

Variable-speed electric motors in the

upper-megawatt power ranges (say, over

20 MW) usually have energy efficien-

cies exceeding 97% over the entire useful

speed range (typically 70–105% of the

rated speed). In a combined-cycle power

plant, the electric drive’s efficiency gen-

erally is 15–25% better than that of typical

heavy-frame gas turbine drivers. In addi-

tion, some of today’s electric motors don’t

need scheduled maintenance for periods of

up to 6 years of continuous operation and

even after that don’t require replacement of

costly parts.

Large electric drives always are custom

engineered for an application, allowing,

e.g., a turbocompressor to be optimized in

capacity and speed for the process, rather

than being limited by a given gas turbine

Rethink Options for Large DriversVariable-speed electric motors can offer significant advantagesBy Amin Almasi, mechanical consultant



rating. The rotor design and overall fea-

tures of the motors closely match those of

electrical generators; design and manufac-

turing of large (over 100 MW) generators

is well established, and numerous units are

operating successfully. However, motors

are variable speed while generators usually

are constant speed, and motors suffer from

oscillating shaft torques during operation

(particularly when starting).

MOTOR ISSUESWhen designing large high-speed electric

motors, mechanical and dynamic prob-

lems must be solved carefully. Mechanical

stresses, vibration level, losses and cool-

ing restrictions can limit the capacity and

the maximum speed of a large electri-

cal motor.

In any high-speed electric motor drive

application, mechanical excitations, elec-

trical pulsations, rotor dynamics issues,

balance problems and mechanical-dynamic

considerations in general are of paramount

importance in ensuring a smooth-running

rotating train over the entire speed range

and during all normal and transient opera-

tions. Also, prior to ordering, it’s essential

to know the behavior of the train during

any electrical fault conditions (the most

severe probably being a short circuit at the

electric motor terminals). VSD-fed electric

motors continuously produce some small

torque oscillations over the entire speed

range. So, the design phase should include

careful analysis of the effects of such torque

pulsations, along with other excitations,

particularly torsional ones.

A large electric motor requires a complex

and heavy rotor assembly. For example,

the assembly can weigh 6–35 tons for

20–120-MW units. Balancing such a rotor

assembly is an extremely difficult job.

(Some expensive assemblies actually have

been scraped after many unsuccessful

attempts to balance them.) Coarse balanc-

ing of an electric motor rotor usually gets

to within 0.015–0.03 mm of mass center

offset; final balancing for some high-speed

electric motors, for example, may require

getting to within around 0.002 mm of

mass center offset. Advanced systems

UNDERLYING ADVANCES BOOST MOTORSSome major developments have made large

(>20 MW) electric motor drivers possible:

• better understanding of rotor dynam-

ics, advanced balance technologies and,

more importantly, use of advanced bear-

ing options;

• progress in materials such as high-tensile

steels for motor high-stressed and critical

components; and

• advanced finite-element analysis, methods,

e.g., for advanced electromagnetic calcula-

tions, and improved analytical approaches

to predict electric motor performance

parameters.



such as active magnetic bearings also

could be used to further improve the vari-

able-speed electric motor driver.

LCI TECHNOLOGYMost variable-speed electric-drive sys-

tems rectify alternating current (AC)

to direct current (DC) and invert DC to

variable frequency AC. For a VSD system

with a rated output of over 60 MW, two

popular and field-proven inversion options

are a load commutation inverter (LCI)

and a gate commutated turn-off thyristor

(GCT). Other options, such as a voltage

source inverter (VSI), may not be mature

enough for rating above 60 MW. A grey

area where both VSI and LCI technologies

are feasible exists between 30 MW and

60 MW.

Today, LCI technology is the most pop-

ular VSD converter system. It’s a mature

technology; disadvantages and solutions

to minimize its problems are well known.

It commonly is teamed with dual-star

two-pole synchronous motors with supply

frequencies between 50 and 80 Hz.

If the electric power supply is interrupted

(for example, due to a temporary problem

in a generator or a power transmission mal-

function), the turbocompressor or other

driven equipment will decelerate rapidly

and may trip a protection system (e.g., for

lubrication oil low-pressure or anti-surg-

ing). This may prevent the unit from

re-accelerating when power is restored.

So, all protection system issues deserve

detailed study.

The main issues for the VSD converters are:

• size (very important);

• redundancy of the equipment;

• control system details (alarms, diagnos-

tics, reliability, etc.);

• guarantees for disturbance “ride

through” capability;

• harmonic mitigation, harmonic filter and

torque oscillation; and

• converter cooling-system requirements.

To provide “ride through” capabilities —

a standard feature for an LCI converter

with synchronous motor drive — a secure

uninterruptible power supply should back

up the power to the control system. The

arrangement and layout of the converter

system should prevent a domino effect (i.e.,

the loss of one part shouldn’t disturb other

parts as far as practical).

Like any nonlinear system, a frequency

converter produces harmonic currents.

Therefore, conducting a harmonic study

(and usually providing a harmonic filter

package) makes sense. The analysis

should look at the complete electrical net-

work (including VSD converter) over the

entire frequency spectrum, calculating the

voltage total harmonic distortion (THD)

under all system operating and upset con-

ditions. Usually, a network short circuit



when the system is under no-load (or the

minimum load) conditions constitutes the

worst case.

A harmonic filter is connected to the net-

work or to a third secondary winding of

the system transformer. The choice mainly

hinges on cost and usually depends on

the network voltage level. For 33 kV and

below, the connection most often is on

the network. For 110 kV and above, a third

secondary winding generally is selected. In

between, the decision must be made on a

case-by-case basis. (The design and manu-

facturing of large power transformers with

three secondary windings is a difficult tech-

nical challenge; only a limited number of

manufacturers are capable of implementing

such designs.)

To minimize the harmonics effect (par-

ticularly on the electrical network), large

LCI systems usually have 12-pulse topol-

ogy. Even if an LCI system has multiple

pulse rectifier configurations to reduce

the harmonic current level emission, the

reactive power consumption of the LCI

rectifier may require use of a power-factor

compensation system (usually a capacitor

harmonic filter). In LCI-type convert-

ers, the harmonic excitation generates

a constant nominal flux in the motor air

gap, which could result in train mechanical

excitations.

The main issues related to the harmonic fil-

ters are:

• sizing (which requires extensive data

about the entire electrical network);

• possible running without one harmonic

filter rank; and

• switching a filter during normal operation

and over-compensation at special operat-

ing points.

Harmonic studies should provide drive

output current spectra, harmonic details

(order, amplitude and phase), and how

these vary with the compressor train

speed. In multi-drive installations, the

superimposition of individual harmonics

and the sizing of harmonic filters for an

entire installation require special calcula-

tions and simulations. Calculated harmonic

levels must be compared against standard

limits (for example, those in IEEE 519).

The harmonic study contains two parts:

one dedicated to calculating the electrical

natural frequencies, and the other aimed

at minimizing the harmonic distortion to

optimize the design of the harmonic fil-

ters. The study should determine potential

Transformers play an important role in any VSD system.



resonances in the entire system. Power

generators usually give rise to some

harmonics that could interact with VSD

systems. Restrictions should be imposed

on train torque ripple (usually under 1–2%

peak-to-peak) to preserve the torsional

stability. The THD of the line-side voltage

should be within certain limits (most often

2–3%) to minimize disturbances to the

other electrical loads connected to the

same plant electrical network.

VSI TECHNOLOGYLCI technology suffers from some well-

known drawbacks — e.g., high torque ripple,

poor power factor, relatively high losses

and harmonic pollution. These disadvan-

tages can make LCI-based variable-speed

drives inadequate to reach the increasingly

demanding performance required in some

applications. In such cases, a VSI may pro-

vide the solution for turbocompressors and

pump drivers.

Indeed, quadruple-star four-pole syn-

chronous motor technology fed by four

pulse-width modulation (PWM) multilevel

VSIs is getting considerable attention.

Based on today’s targets for low torque

ripple and low harmonic distortion (par-

ticularly low grid-side harmonic pollution),

the PWM-VSI-based variable-speed drive

design has been selected for several large

turbocompressor projects. A cascaded mul-

tilevel converter topology usually is chosen.

Each converter phase is obtained by series

connecting several transistor cells. The

choice of this topology makes it possible to

attain some important goals like:

• voltage output (converter output to an

electric motor) that approaches the sinu-

soidal waveform as the number of cells

is increased — providing the possibility

of operating the electric motor at a near-

unity power factor;

• tolerance to single cell faults by imple-

menting a faulty-cell bypass function; and

• low harmonic injection.

In fact, with LCI-based drives, having more

than two supplying converters may be the-

oretically feasible, although this may pose

commutation overlapping issues.

In the PWM-VSI technology four convert-

ers commonly are used. The decision to

supply the electric motor with several

(four or more) three-phase converter

units naturally leads to the splitting of

the stator winding into independent

three-phase sets, each to be fed by a

converter. The stator design needed for

this purpose often is referred to as “split-

phase” because it results from splitting

the winding into multiple star-connected

three-phase sets. The most common

arrangement uses four converters; the

associated electric motor design is known

as quadruple-star winding. The phase cur-

rents contain harmonics of orders 5, 7, 11,

13, 17 and 19; all the resulting space har-

monic fields in the electric motor air gap



are very low because of the mutual can-

cellation effects.

Today, turbocompressors and pumps may

benefit from a new electric drive option

based on a VSI-fed quadruple-star 100-Hz

four-pole synchronous electric motor. Com-

pared to traditional LCI-based options, it

provides particular advantages:

• torque ripple typically lower than 1–2%

peak to peak;

• very low vibrations owing to the four-

pole design;

• high fault tolerance due to the four-star

four-converter topology; and

• high electric motor efficiency, usually

above 98%.

Because of the large number of phases

(12) and the four-pole design, even for

high power levels the stator winding can

be done with coil technology (instead of

complex/expensive “Roebel” bar con-

struction) with noticeable manufacturing

and cost benefits. The 100-Hz supply fre-

quency doesn’t give excessive core losses.

Stator phase currents may show fifth and

seventh current harmonic distortions as a

consequence of the electric motor internal

electromotive force. However, these har-

monic distortions don’t negatively impact

torque performance. The design also

could be scalable to relatively high power

levels (above 50 MW) by increasing the

number of supplying converter units and

possibly expanding the electric motor

driver size.

OTHER CONSIDERATIONSTransformers play an important role in

any VSD system. Inrush current limitation

requirements and protection philosophies

of transformers are important.

A VSD electric motor system employs

various cooling water pumps. A cooling

pump’s normal operating point should

be as close as practical to the pump’s

best efficiency point (BEP). Rated cool-

ing flows preferably should be within

20% of the BEP flow. The cooling-pump

characteristic curve is very important for

a trouble-free, smooth and proper opera-

tion. A cooling pump curve should exhibit

the characteristic of stable continuously

rising head from the rated capacity to the

shutoff (preferably 10% head rise from the

rated to the shutoff).

Typically, a VSI system’s footprint is less

than 75% of that of a comparable LCI

system. In addition, it usually weighs less

than 70% of a comparable LCI system.

AMIN ALMASI is a mechanical consultant based in

Sydney, Australia. Email him at [email protected].

iServices Pty. Ltd., Brisbane, Australia. E-mail him at

[email protected].



Retrofits to aging pump drives in water treatment facilities can produce significant effi-

ciency improvements (Figure 1). When a Massachusetts-based municipal water treatment

facility needed to upgrade its aging 350-hp 18-pulse drives, it turned to a local Yaskawa

drives distributor, who recommended a compact and cost-effective retrofit. The water

treatment facility has four 350-hp pumps. The pumps run 25 mgd on average but can reach

75 mgd in less than an hour with heavy rain fall. Three of these pumps can satisfy this peak

demand. The last pump is used as backup in case one of the three were to fail.

Drive Retrofit Ups Water Treatment Plant’s EfficiencyCompact stand-alone drives replace 18-pulse driveBy Chris Jaszczolt, Yaskawa America, Inc. Drives & Motion Division

PUMP DRIVE RETROFITS IN WATER TREATMENT FACILITIESFigure 1. With aging equipment, many water treatment facilities often benefit from retrofit proj-ects that improve efficiency.



Failure of one pump during peak flow with-

out a backup could lead to a precarious

situation. If the backup pump and the desig-

nated lag pump were to fail, the backup of

the influent main could cause overflowing

manholes and flooded streets.

Based on the facility manager’s recommen-

dations, the city decided to move forward

with updating all of the influent drives.

RETROFIT DRIVESThe retrofit project began by upgrading

the facility’s aging 350-hp Robicon 18-pulse

pump drives, which had become time-con-

suming and costly to keep in working order

(Figure 2).

The Yaskawa distributor realized that a

complete replacement likely would require a

different size cabinet. The staff at the facil-

ity also needed time to become familiar with

it, so a retrofit option using the Yaskawa

U1000 Industrial Matrix drive was recom-

mended (Figure 3).

After the facility contacted a sister site that

already was using Yaskawa drives, it agreed

with the retrofit option.

Because the drive is a stand-alone, low-har-

monic drive with regenerative capability,

many unnecessary components inside the

cabinet were eliminated. The circuit breaker

from the previous system was reused. AFTER THE RETROFITFigure 3. After the retrofit, the system was simpler, more compact and more efficient.

BEFORE THE RETROFITFigure 2. Before the retrofit, the drive setup was complex with significant cable and wiring.



EFFICIENCY IMPROVEMENTSThe drive solution meets the input

harmonic current capabilities of the

previous 18-pulse package at rated

power and also provides excellent

harmonic current levels throughout

the load range.

The efficiency improvement enabled

the water treatment facility to

shut off the additional cabinet fans

required to cool the previous system.

It realized an immediate increase in

efficiency reduction because of the

building’s cooling requirements.

Reusing the existing cabinet saved

space but also meant that almost

nothing had changed for the staff. All the

old controls on the door were integrated

easily into the solution, which provided all

of these benefits at a cost of approximately

half of a new 18-pulse configuration. As a

result of this project, the water treatment

facility has moved forward with updating

several more of its outdated 18-pulse drives

with the U1000.

HOW DO MATRIX DRIVES WORK?A Matrix drive design removes the DC

bus by using nine bidirectional insulated

gate bipolar transistors (IGBTs) in a matrix

arrangement to generate the variable fre-

quency AC output directly from the AC

input. This reduces the effects of harmonic

distortion on the output signal dramatically,

producing a smoother, cleaner output

waveform. With no DC bus to charge,

the associated nonlinear input current

draw can be eliminated, providing cleaner

power to the load. Figure 4 shows the

Matrix topology.

Note that in the Matrix design, any input

phase can be connected directly to any

output phase at any time. The key to the

Matrix operation is its ability to turn the

bidirectional switches on and off the at the

correct times to generate the proper output

voltage and frequency required to operate

the motor.

To better understand, look at the three

input phase voltages shown in Figure 5. In

MATRIX DRIVE TOPOLOGYFigure 4. Nine bidirectional IGBTs generate the variable frequency AC output directly from the AC input.



operation, the control built into the Matrix

variable frequency drive (VFD) monitors

the voltage difference continually between

each of the phases.

All drives use a pulse width modulation

(PWM) waveform to generate motor volt-

age, including the Matrix drive. A standard

drive pulses the DC bus to the motor to

create the output voltage waveform. The

time each pulse is ON (the pulse’s width)

helps to determine the output voltage and

final RMS voltage of each cycle of voltage.

Instead of using a DC bus, the Matrix

drive pulses the motor using its input

voltage. This is done in two steps (Figure

6). The low input voltage and mid input

voltage are used for the first step and the

mid voltage and high voltage are used

for the second step. The reverse order is

used to turn the pulse off. This process

is repeated over and over to generate

the PWM output waveform constantly

adjusted along with the ever changing

three-phase input voltage.

The constant use of the input voltage

means current constantly is being drawn.

Therefore, the Matrix drive does not

mitigate harmonics. Instead, it draws

input current naturally with less than

5% iTHD automatically as it provides

motor control.

MATRIX DRIVE REGENERATIONIn addition to the low harmonics and

near-unity power factor, the Matrix

design also provides energy savings

through regeneration.

INPUT PHASE VOLTAGESFigure 5. The Matrix drive pulses the motor using its input voltage.

MATRIX STEPPED PULSEFigure 6. A two-step process is repeated to generate the PWM output waveform.



When a motor is being driven by a load,

as opposed to driving the load, it acts as a

generator, sending the extra power back

to the VFD. The extra regenerative energy

then is put back onto the grid to be dis-

persed to other loads on the grid, which

reduces the utility power demand. Exam-

ples include applications such as pump

jacks, in which the load oscillates between

motoring and regeneration, and downhill

conveyors that are in a continuous regen-

erative state.

In conventional drives, dynamic braking

resistors can be used to divert the regen-

erative energy away from the VFD and

prevent a DC bus overvoltage condition.

The Matrix drive’s nine bidirectional IGBTs

enable the regenerative energy to be

directed back to the supply to be credited

against the user’s power bill (Figure 7).

MONITORINGSome Matrix VFDs can monitor power

in several ways to give instant feedback

on energy saved. When provided with

the $/kwh billing rate, the display on the

drive can show the following information

on request:

• Power output

• Power consumption

• Regenerative power

• Power saved

• Power bill

CHRIS JASZCZOLT, is product manager, Matrix drives,

Yaskawa America, Inc. Drives & Motion Division. He can

be reached at [email protected].

REGENERATIVE ENERGYFigure 7. The the Matrix drive’s regenerative energy translates to power bill savings.



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