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Dutch Fluid Power Conference 2016

Electrical, mechanical and integration aspectsof Hydraulic System Design

Marten Fluks

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Topics

Background of speaker

System Architecture

building systems – hydraulic systems

speaker’s perspective

Electrical aspects

hydraulic and electric motors characteristics

efficiency aspects

hydraulic system choice and electric motor for HPUoperating cycle

hydraulic system alternatives

E-motor size

overload prevention

overall efficiency of variable speed drives

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Topics

Mechanical aspects

mechanical energy transformation

mechanical-hydraulic drive instead of full-hydraulic

optimizing hydraulic system by changing cylinder position

enhancing vehicle hydraulic drive performance by addition of mechanical direct-drive

hydraulic system with mechanical energy storage in fly-wheels

Integration aspects

overall system and next higher system level

snow-ball effect of drive system choice

clear communication functional system description

modes of operation and transitions

document change management2

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Speaker’s background

Education: Mechanical Engineering – Hydraulic Drive System Technology; TU Eindhoven; 1968

Royal Netherlands Navy (mil. service): technical staff at MoD

Engineering, Technical Development and Management functions at:

IHC

Stork Group

Hydraudyne

Bosch Rexroth

Design of simple or complex equipment and systems, including hydraulic, electric or hybrid drive technology

Engineering and Consulting activities since 2008: Conseon

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Portfolio, small selection

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System Architecture

5 [1] Refers to external form, not to basic choices of a system

Aspects Building

System

Architecture

Hydraulic

System

Architecture

Art and Aesthetics Very high content Very low content

Appreciation Like/dislike

Highly disputable Few discussions

Visibility for many people

Very high Very low

Integration with environment

Visually very Important

Functionally very important

Skill & knowledge Moderate - high High - very high

Fit for purpose Minimum

requirements must be met

Very important

Form follows function

“No”; highly

depending on personal style

Yes, low personal style influence [1]

Creator’s satisfaction

May be high May be high

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Architecture examples

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Dutch Embassy Residence in Ljubljana (Bevk, Peroviand Kanduk Architects)

Lou Ruvo Center for Brain Health in Las Vegas (Architect Frank

Gehry)

Hydraulic Power Unit (Direct Industry)

Hydraulic Power Unit (Rosso Hydraulic)

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System Architecture

Speaker’s perspective:

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DriveSystem

Architecture

HydraulicSystem Design

HydraulicDetailDesign

Pro

gres

s o

f D

esig

n P

roce

ss

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Focus of presentation

Aspects of hydraulic system design 18 items identified in Vademecum Hydrauliek 2014

now 3 more issues are treated:

electrical

mechanical

integration

Awareness of these 3 aspects may contribute to better and cheaper overall system solutions

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Electrical aspects

Increasing tendency to use variable speed motors instead of constant speed squirrel cage motors

Many hydraulic applications: preference still exists for constant speed E-motors

Smaller size motors and electrical installations are often possible, compared to “conventional” selection

This presentation is restricted to squirrel cage motors

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Motor characteristics

Full and partial loads

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HMEM

Load

(%)

Hydromotor

∆p (%)

Electric motor

I (%)

100 100 100

50 50 60

36 36 50

0 “0” 36 !

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E-Motor current

Influenced by power factor cos φ

Siemens, 55 kW motor, 1500 rpm

Main reason for heat generation

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I/In

0,5

0,36 T/Tn P/Pn

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Motor Efficiency

Hydromotor Electric motor

Calzoni MRD 300; 54 kW Report IEA 2011; 55 kW

E-motors exist in different efficiency classes

IE1; IE2, IE3; IE4

E-motor efficiency decreases at lower nominal power

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E-Motor Efficiency classes

IE1 standard

IE2 high

IE3 premium

IE4 super premium

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Energy cost reduction

Replacement of E-motor may be cost-effective

Calculation example:

motor power 22 kW

2-shift operation => approx. 4000 hrs per year

running at nominal load

efficiency IE1: 90,0 %; IE4: 94,5 %

energy cost: € 0,12/kWh

energy cost savings over 5 years:5 x 4000 x 22 x (0,945 – 0,900) x 0,12 = € 2.376,--

this equals twice the purchase price of a new motor !

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Load conditions and E-motor size

Motors seldom run at nominal load all the time

Some applications comprise extreme low (average load)/(maximum load) ratio:escalators must be capable of transporting more than 1 person at every step over the full length; most of the time only a couple of people are simultaneously transported

Hydraulic power units normally have varying load conditions and thus varying (average load)/(maximum load) ratios

From energy cost and installation price point of view: choose smallest E-motor and let it regularly run in overload conditions

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Example E-motor selection for HPU

Hydraulic system, 3 consumers (A,B and C), HPU with 1 E-motor driving the pump(s)

Operating conditions : working cycle of 145 seconds, and load profile per consumer according table in next slide

Compare 4 different hydraulic system lay-outs:

a. open circuit; prop. valve control (VC); constant pressure pump (CP) and accumulator (AC)

b. open circuit; prop. valve control (VC) ; load sensing pump (LS)

c. consumer A and B: according b); consumer C: closed circuit, pump control (PC); motor with fixed displacement

d. open circuit; motor control (MC) (secondary control);constant pressure pump (CP) and accumulator (AC)

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Operating conditions of consumers

In table:

load profile: transferred to pressure (difference) at the consumer

speed profile: transferred to oil flow at the consumer (for fixed displacement motor)

theoretical nett peak power 30 + 25 + 90 = 145 kW

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Consumer A Consumer B Consumer C

pressure flow time power pressure flow time power pressure flow time power

(bar) (lpm) (s) (kW) (bar) (lpm) (s) (kW) (bar) (lpm) (s) (kW)

250 50 25 20,8 10 5 70 0,1 270 200 40 90,0

180 100 40 30,0 300 40 20 20,0 250 -200 40 -83,3

50 10 80 0,8 100 150 55 25,0 40 400 30 26,7

35 -400 35 -23,3

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Operating conditions of consumers

In diagram:

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Power profile

Nett total power versus time (at consumers)

Theoretical peak power: 145 kW

Real peak power 120 kW

Average power: only 26 kW

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Power profile

Gross total power versus time (at pump)

Case b; LS pump

Maximum pump power: 233 kW => 250 kW standard E-motor ??

Average power: 117 kW => 132 kW standard E-motor ??

Req’d cooling power: approx. 100 kW

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Output power of pump(s)

For 4 hydraulic system lay-outs

Large differences in maximum power and average power

Ratio between average and maximum power:94 %; 50 %; 24 % and 93 % respectively

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pm

ax

Q

max

Pm

ax

Pav

bar

lpm

kW

kW

a) CP (+accu)

340

396

225

211

b) LS 310

560

233

117

c) PC + LS

310

400160

123

30

d) MC (+accu)

340

53 30 28

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Size of electric motor

Two important issues:

Maximum torque at nominal speed, not larger than 1,6 – 1,8 x nominal torque

Effective current (or RMS current), not larger than 1,0 x nominal current at rated speed

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T max

T nom

I nom

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E-motor choice

Case b); standard type (S1; isolation class B)

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Motor size and energy cost

Comparison table for different hydraulic systems

Same functionality => different system solutions => quite different motor sizes and energy cost

Maximum required hydraulic pump power is often not determining for electric motor size

Average required hydraulic pump power is not always determiningfor electric motor size !

So: careful analysis recommended

Savings in energy cost and lower electric system cost may overcompensate for more expensive hydraulic system !24

Pmax Pav E price PEM

kW kW € / a kW

a) CP (+accu) 225 211 57.101 250

b) LS 233 117 31.753 132

c) PC + LS 123 30 8.167 75

d) MC (+accu) 30 28 7.468 30

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Overload prevention

Considerations:

Customer already has some spare capacity in specification?

Standard E-motor mostly larger than necessary (case b: Ieff/Inom = 0,94; so 6 % spare)

Sensitivity analysis recommended

Use isolation class F motors instead of class B

Integrate power limiter with hydraulic pump (hydraulic/mechanical or electronic)

Power limiter to be adjusted at nominal motor power ?!

More ideal: use temperature sensor in motor windings- to gradually limit the power, also based on learning curve

- temporary maximum power can be much higher than withconventional power limiter

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Variable speed drive; overall efficiency

Comparison of hydraulic and electric drives: claim is always that electric drive efficiency is superior

Is this really true ??

Efficiency of E-motor (class IE2) 55 kW, 1500 rpm: 0,93 at 100% load; 0,92 at 50% load

Combined with frequency convertors: motor efficiency gets lower; based on recent literature: estimated reduction factor 0,96 – 0,95

Efficiency of frequency convertors: claimed to be 0,98 –0,96; however this applies to max. load and simultaneous low speeds; estimate at “nominal” speeds: 0,94 – 0,93 (consultation forum LOT 3)

Cabling losses are not considered

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Electric VSD; overall efficiency

Result:

Claim for superior efficiency of electric drive over hydraulic drive needs reconsideration !!

Proposal: execute test program to verify this; important for position of hydraulics in comparison with electrics. Role for FEDA?

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Electric motor plus frequency convertor, nominal

55 kW @ 1500 rpm

Efficiency

at 100% load

Efficiency

at 50% load

Motor (IE2 class) 0,93 0,92

Reduction factor for motor 0,96 0,95

Frequency convertor 0,94 0,93

Overall efficiency 0,84 0,81

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Mechanical aspects

Mechanical energy transformers are very often used in combination with hydraulics systems:- gear transmissions- lever mechanisms

Gear transmissions at primary (pump)-or at secondary (actuator/motor) side of the hydraulic system or at both sides

Lever mechanisms at actuator side

“Clever” mechanisms may simplify hydraulic system: e.g.only 1 cylinder for 180 deg. arm rotation instead of 2 cylinders and intermediate arm

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Mechanical-hydraulic system

Ore separation equipment (jig with multiple cells) originally comprised eccentric drive with sine curve

For optimal performance: use saw-tooth curve

In “pre-electronic” era: full-hydraulic solution realized, with complicated and vulnerable system; difficult to adjust

Too much down-time, so new approach required

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Mechanical-hydraulic system

Solution: Mechanical-hydraulic system replacesfull-hydraulic system

Cam disc with ideal curve drives a number of single piston pumps around perimeter of cam housing

Hydro-pneumatic accumulator stores energy for power peak during fast upward stroke

Combination of mechanics and hydraulics:extremely simple system, and very reliable

Also understood and maintained by low skilled personnel

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Mechanical-hydraulic system

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Change of cylinder arrangement

Passive part of Drill String Heave Compensator (1250 kN)

Almost flat force-stroke characteristic necessary (3 % dev.)

So: hydro-pneumatic accumulator with large gas volume

Alternative geometry of cylinder arrangement: inclined position

Kinematic force characteristic compensates gas curve

Result: gas volume reduction 5700 dm3 => 2800 dm3

Combination of mechanical and hydraulic design can save money32

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Adding direct mechanical drive

Wheel Drive transmission systems of e.g. agricultural tractors, shovels etc: often use combination of hydraulic and mechanical subsystems.

Examples:

serial power flow parallel power flow (power splitting)

Even more sophisticated and complicated systems are nowadays “common practice”

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Adding direct mechanical drive

Alternative solution: Simpler system reduces cost and additionally enhances the driving performance

full hydraulic drive mode full mechanical drive mode

Note: this applies for road driving conditions; for “working conditions” (e.g. ploughing) some gear stages remain necessary

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Road drive characteristic

Low speeds: full hydraulic drive mode

Then fast and smooth transition to next mode

“High” speeds: full mechanical drive mode

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Energy storage in fly-wheels

Hydro-pneumatic accumulators most widely used in hydraulic systems

Mechanical accumulators (e.g. weight and spring types) rarely are attractive

Fly-wheels may be considered, when low weight and high power density are no main criteria

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Energy storage cost trends For large scale applications:

May not be the same for hydraulic systems37

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Hydraulic system with fly-wheel

Test installation

Simple, reliable and proven long life-time

So: for stationary systems: conventional fly-wheel is interesting alternative as energy storage device

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Integration aspects

Good system solution is based on proper design process

It involves comparison of alternative solutions

Simplified block diagram (and without drawing feed-back loops):

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problem definition

different concept solutions

contact with

customer

requirements

selection criteria choice of concept

further design process

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Integration aspects

Chance to obtain “best” system solution requires alsolooking at next higher or overall system level:

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problem definition

different concept solutions

contact with

customer

requirements

selection criteria choice of concept

further design process

relation withoverall system

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Highest system level

Example: Adjustment and fixation drive

platform structure supported by 12 columns

infinite height adjustment required, and sufficient vertical stiffness

Chosen solution:

electric (winch) drive for height adjustment

hydraulic clamping device at each column

Functionally OK, but …… total price too high

Final solution:

Ten columns in irregular pattern resulted in same dynamicstructure behaviour. Cost within budget.

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Snow ball effect

Example: Truck mounted concrete pump with different hydraulic drives “on board”

Mass of hydraulic cylinders in top boom sections may have substantial influence on:

- mass of boom section(s)

- luffing cylinder dimension (at lower boom)

- slewing bearing size and mass

- overall weight of slewing and luffing parts

- slewing drive size; slewing and luffing power

- outrigger arm size and mass

- reaction forces on ground and jacking cylinder dimensions

- overall vehicle weight, and possibly one (extra or less) axle and set of wheels

- power for hydraulic system and vehicle propulsion

Solution: use aluminium or carbon fibre cylinders42

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Clear communication

Operational description of the intended system

in early stage of engineering phase important for:- review by the customer to check mutual understanding- lead system engineer to verify all functionality is included- explanation of the operation to other engineers

should include different modes of operation and transitionsbetween them

“State diagram” with conditions and restrictions for transition between modes of operation is very useful tool for everyone

Change management of documents

if this is not done properly it can lead to dramatic consequences

advice: maintain document register system43

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What change in what specification??

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Summarizing my “message”

Before starting hydraulic system design: what can mechanicscontribute?

Compare different hydraulic system options, including

“smallest” electric motor

cost of associated electric system

energy cost prediction

Reconsider overall efficiency of electric VSD versus hydraulic drive

Stationary applications: use fly-wheel for energy storage?!

For best end-result: always look atnext higher or overall system level aspects

Be aware of snow ball effects

Make operational system description in early engineering stage

Always take care of clear communication

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Thank you for yourattention !