Selecting Motor Controlsfor Mining Process Torque Demands Slide 1
Selecting Motor Controlsfor Mining Process Torque Demands
Selecting Motor Controlsfor Mining ProcessTorque Demands
Bill Horvath, PETMEIC CorporationRoanoke, VA 24153
Mining Electrical Maintenance Association [MEMSA]June 2013 Technical Symposium
Clearwater, Florida
Selecting Motor Controlsfor Mining Process Torque Demands Slide 2
Selecting Motor Controlsfor Mining Process Torque Demands
Selecting Motor Controlsfor Mining Process Torque Demands
Selecting Motor Controlsfor Mining Process Torque Demands Slide 3
Selecting Motor Controlsfor Mining Process Torque Demands
Purposes Today
• View typical mine equipment & processeswith torque-speed characteristics
• Review typical motor speed and torquecapabilities
• Review key motor control methods and howthey affect motors and loads
• Provide comparisons and matches betweenmotors, controls and mine loads
Selecting Motor Controlsfor Mining Process Torque Demands Slide 4
Selecting Motor Controlsfor Mining Process Torque Demands
Controlling Speed• Some processes work well at fixed
speed – control for these motors isfocused on reliable, safe starting.
• Some processes can benefit fromvariable speed - control for motors isfocused on optimum speed setting
• Many mine processes have widelychanging loads - control for motors isfocused on adapting speed to load
Selecting Motor Controlsfor Mining Process Torque Demands Slide 5
Selecting Motor Controlsfor Mining Process Torque Demands
Controlling Torque - Basics• Mine process torque profiles are all over
the map – some wildly swinging, somesmoothly predictable.
• Torque and motor current are roughlyequivalent.
• Torque Control for motors focuses on:
Providing adequate torque for load over alloperating points
Protecting equipment and people fromexcess torques and currents.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 6
Selecting Motor Controlsfor Mining Process Torque Demands
Equipment ProtectionA Proper Motor Control Must Work Within:
• Driven equipment mechanical limits[couplings, gearboxes, pressures, speeds …]
• Driven equipment electrical limits [voltages,currents, frequencies …]
• Process limits [temperatures, flows, materialfeed, viscosities …]
Selecting Motor Controlsfor Mining Process Torque Demands Slide 7
Selecting Motor Controlsfor Mining Process Torque Demands
Power System Effects & LimitsA Proper Motor Control Must:
• Recognize and work withinavailable system voltagelimits
• Be able to start and run theirloads with actual voltagesavailable after system voltagedrop
• Live inside limits on utilitysystem voltage and currentharmonic distortion
Selecting Motor Controlsfor Mining Process Torque Demands Slide 8
Selecting Motor Controlsfor Mining Process Torque Demands
Motor Characteristics & LimitsA Proper Motor Control Must:
• Recognize and work withactual motor torque, voltageand current characteristics
• Protect motors fromdamaging currents andvoltages
• Feed voltage, frequency, andcurrent to the connectedmotor to produce the requiredspeeds and torques.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 9
Selecting Motor Controlsfor Mining Process Torque Demands
Drive & Motor Characteristics & Limits
• Drive & Motor Characteristics: Torque control,voltage & speed control, response to load, powersystem volts and amps and power factor,harmonics, motor speed range
• Drive Limits: maximum torque, max & minspeeds, voltage levels in / out, environment
• Drive & Motor Energy Consumption – speed &torque matching to load & resulting electricalenergy consumed by process
Selecting Motor Controlsfor Mining Process Torque Demands Slide 10
Selecting Motor Controlsfor Mining Process Torque Demands
Load, Speed, Torque – General
• The load type,physical load, andequipmentdetermines the curve
• Actual conditions[physical load, uphill,sets the level.
+ Speed or RPM+
To
rqu
e
Selecting Motor Controlsfor Mining Process Torque Demands Slide 11
Selecting Motor Controlsfor Mining Process Torque Demands
Constant Torque vs. Variable Torque LoadsHow Torque Varies with Operating Speed
Selecting Motor Controlsfor Mining Process Torque Demands Slide 12
Selecting Motor Controlsfor Mining Process Torque Demands
Mining Loads & Their Categories
Constant Torque
Grinding Mills
Excavators
Hoists
Conveyors
Crushers
Recip Compressors
Variable Torque
Variable Torque
Pumps
Fans
Rotary compressors
Mixed / In between
Slurry Pumps
Separators
Selecting Motor Controlsfor Mining Process Torque Demands Slide 13
Selecting Motor Controlsfor Mining Process Torque Demands
Load, Speed, Torque – Quadrants
• Q1, Q4 Motoring= Torque and Speedare in same direction
• Q2, Q4 Regeneration= Torque and Speedare opposite direction
Selecting Motor Controlsfor Mining Process Torque Demands Slide 14
Selecting Motor Controlsfor Mining Process Torque Demands
Mining Load “Family Tree”
Selecting Motor Controlsfor Mining Process Torque Demands Slide 15
Selecting Motor Controlsfor Mining Process Torque Demands
Variable Torque Loadsand “In-Between” Loads
Selecting Motor Controlsfor Mining Process Torque Demands Slide 16
Selecting Motor Controlsfor Mining Process Torque Demands
• Flow Rate VariesProportionally withSpeed
• Pressure & LoadTorque Varies as theSquare of the Speed
• Motor shaftHorsepower variesas the Cube of theSpeed
Variable Torque Loads – Pumps & Fans& Centrifugal Compressors
Selecting Motor Controlsfor Mining Process Torque Demands Slide 17
Selecting Motor Controlsfor Mining Process Torque Demands
Loads In-BetweenConstant Torque &Variable Torque
• Pump loads with solids – slurry pumps
• Fan loads with heavy concentrationsof dust / solids – cyclones & separators
Slurry Pump Courtesy Weir Pumps
Selecting Motor Controlsfor Mining Process Torque Demands Slide 18
Selecting Motor Controlsfor Mining Process Torque Demands
TypicalConstantTorqueMiningLoads
Selecting Motor Controlsfor Mining Process Torque Demands Slide 19
Selecting Motor Controlsfor Mining Process Torque Demands
Mining Conveyors
Application Issues
• Starting Torque
• Belt Tension overwide weather range
• Protection of beltfrom over torque
• Variation in materialweight
CONSTANTTORQUE
Selecting Motor Controlsfor Mining Process Torque Demands Slide 20
Selecting Motor Controlsfor Mining Process Torque Demands
Conveyor Mechanical ApplicationConsiderations
Stretch, Length,Belt weight,load weight,speed
Friction
T1
T2
T3
T4
D1
D2 D3
Tension Ratios,Dynamic Response,Programmed Torque, LoadSharing
Typical Application Factors, Long Conveyor
Selecting Motor Controlsfor Mining Process Torque Demands Slide 21
Selecting Motor Controlsfor Mining Process Torque Demands
Uphill / Level Conveyor or Mill LoadingT
OR
QU
E
SPEED, RPM
BREAKAWAYTORQUE
Typical InductionMotor Char on60 Hz power
Selecting Motor Controlsfor Mining Process Torque Demands Slide 22
Selecting Motor Controlsfor Mining Process Torque Demands
Downhill Conveyor - Example(+
)T
OR
QU
E
SPEED, RPM
BREAKAWAYTORQUE
Typical InductionMotor Char on60 Hz power
(-)
conveyordemand
Downhill:Net (-) Torque =Regeneration
Selecting Motor Controlsfor Mining Process Torque Demands Slide 23
Selecting Motor Controlsfor Mining Process Torque Demands
Crushers
• Breaks materials intomore uniform size
• High Peak Torques!
• Potential for jamming
• Often used withcontrolled speedfeed conveyor
CONSTANTTORQUE
Selecting Motor Controlsfor Mining Process Torque Demands Slide 24
Selecting Motor Controlsfor Mining Process Torque Demands
Grinding Mill Types & Operation
Autogenous Mill
[Ore is crushed bycollisions with itself]
Semi-Autogenous [SAG]Mill
[Ore is crushed by steelballs & collisions with itself]
Ball Mill
[Ore is crushed bycollisions with steel balls
CONSTANTTORQUE
Selecting Motor Controlsfor Mining Process Torque Demands Slide 25
Selecting Motor Controlsfor Mining Process Torque Demands
SAG Mill Viewed From Top
Motor #1of 2
Gearbox #1of 2
Mill Drum
Motor #2of 2
Selecting Motor Controlsfor Mining Process Torque Demands Slide 26
Selecting Motor Controlsfor Mining Process Torque Demands
Locked
Rotor Tq
Pull Up
Torque
Peak
[Breakdown]
Torque, BDT
Rated Torque
Sync
RPM
RPM
Rated SlipRPM =
Sync - Rated RPM
Rated
RPM
To
rqu
e
Sync Rpm = 120 x Freq.#Poles
Typical Induction Motor Speed-Torque Profile
Mill Starting canbe a challenge!!
MILL TORQUENEEDED!
Selecting Motor Controlsfor Mining Process Torque Demands Slide 27
Selecting Motor Controlsfor Mining Process Torque Demands
High Pressure Grinding Roll(HPGR)
• Material is crushedbetween twoindependently driven rolls
• Hydraulic pressure isapplied to maintain aspecified gap
• An autogenous layerforms on the rolls toprotect the roll surface
CONSTANTTORQUE
Selecting Motor Controlsfor Mining Process Torque Demands Slide 28
Selecting Motor Controlsfor Mining Process Torque Demands
HPGR Machine Design
Maintaining the same roll tangential velocity is important:
•Helps maintain the autogenous layer•Minimizes roll wear•Increases uptime
RR11 RR22R1 R2
Selecting Motor Controlsfor Mining Process Torque Demands Slide 29
Selecting Motor Controlsfor Mining Process Torque Demands
Excavators:Draglines & Shovel Motions
• Loads are “constanttorque”
• BUT loads vary widelyand often wildly
• All are 4 quadrant loads– motoring andregenerating
• Require responsivecontrols
CONSTANTTORQUE
Hoist & DragExample
Selecting Motor Controlsfor Mining Process Torque Demands Slide 30
Selecting Motor Controlsfor Mining Process Torque Demands
Excavators:Electric Blast Hole Drills
• Rotary, Propel and pulldown motions
• High peak torque
• Special modes: reversedirection high torquebreaking [unscrewing] ofdrill stem
CONSTANTTORQUE
Selecting Motor Controlsfor Mining Process Torque Demands Slide 31
Selecting Motor Controlsfor Mining Process Torque Demands
MOTORS CHARACTERISTICS
Selecting Motor Controlsfor Mining Process Torque Demands Slide 32
Selecting Motor Controlsfor Mining Process Torque Demands
Basic Motor Characteristics
• DC Motors still in wide use in Draglinesand shovels - specialty applications
• For today we will consider only AC motors
Induction
Synchronous
Utility Fed – Fixed Speed
VFD fed – VFD Start only & Variable Speed
Selecting Motor Controlsfor Mining Process Torque Demands Slide 33
Selecting Motor Controlsfor Mining Process Torque Demands
INDUCTION MOTORS
Selecting Motor Controlsfor Mining Process Torque Demands Slide 34
Selecting Motor Controlsfor Mining Process Torque Demands
Induction Motor Model
STATOR
RS
LS
LM
LM
MagnetizingCurrent
POWER SOURCE
One Phase Model
•AC Power on stator sets up rotating field magnetic flux
•Rotor acts as shorted transformer secondary, currentproduces induced rotor field flux, torque results
•Rotor voltage dependent on difference between stator wave& rotor rpm = slip NO SLIP= NO POWER!
•Power Factor is always lagging
Selecting Motor Controlsfor Mining Process Torque Demands Slide 35
Selecting Motor Controlsfor Mining Process Torque Demands
Induction Motor Speed-Torque Profile
Locked
Rotor Tq
Pull Up
Torque
Peak
[Breakdown]
Torque, BDT
Rated Torque
Sync
RPM
RPM
Rated SlipRPM =
Sync - Rated RPM
Rated
RPM
To
rqu
e
Sync Rpm = 120 x Freq.#Poles
A
B
C
D
FE
Selecting Motor Controlsfor Mining Process Torque Demands Slide 36
Selecting Motor Controlsfor Mining Process Torque Demands
Starting Induction Motors• Motors connected
across the line at startdraw large currents ~600% rated
• Current remains highthrough most of start
• Torques aredetermined by motordesign.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 37
Selecting Motor Controlsfor Mining Process Torque Demands
Accelerating a Load
Motor AvailableTorque
Full LoadTorque Level
Moto
rT
orq
ue,
Load
Torq
ue
Torque vs SpeedInduction Motor Accelerating a Load
Torque needed by load
= NetinstantaneousTorque availto accel load
1. Top Curve is defined by motor and voltage available2. Lower curve is defined by load [above is typical of a fan, centrifugal
compressor, or pump].3. Inertia of load is accelerated by difference torque.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 38
Selecting Motor Controlsfor Mining Process Torque Demands
Motor Starting Characteristics And Starting Time[Exaggerated to show the process]
Rated torque
No load current
Synchronousspeed
Rated speed
Rated current
Speed
Locked rotorcurrent
Motor current
Load torque
Motor torqueBreak down torque
Cu
rren
t,To
rqu
e
●●
Locked rotortorque
Starting time(s) = Wk2 x Delta RPM / (308 x Torque)
LoadTorque
TOO High!
Selecting Motor Controlsfor Mining Process Torque Demands Slide 39
Selecting Motor Controlsfor Mining Process Torque Demands
Induction Motor Starting and Thermal Curves (2C/1H)Type ICFT Form CHCN Ins.Class F
Pole 4 Voltage(V) 4160 Rating 連続
Output(kW) 2300 Frequency(Hz) 50 Frame No. 500-1800
Rated Current : 370 (A)
1
10
100
1000
10000
100 200 300 400 500 600 700 800
ALLOWABLETIME(sec)
CURRENT(%)
C
HOTCOLD
Type ICFT Form CHCN Ins.Class F
Pole 4 Voltage(V) 4160 Rating 連続
Output(kW) 2300 Frequency(Hz) 50 Frame No. 500-1800
Rated Speed
(min-1
)1480
JM (=GD2/4)
(kgm2)
136.JL (=GD
2/4)
(kgm2)
578.
Starting at 100%V at 80%V Rated Current : 370 (A)
Time(sec.) 13 32
0
250
500
750
0
100
200
300
0 50 100
CURRENT(%)TORQUE(%)
SPEED(%)
LOAD TORQUE
CURRENT ( at 100%V )
CURRENT ( at 80%V )
TORQUE ( at 100%V )
TORQUE ( at 80%V )
Starting Current
Minimum Accelerating Torque
Minimum Thermal Margin
Selecting Motor Controlsfor Mining Process Torque Demands Slide 40
Selecting Motor Controlsfor Mining Process Torque Demands
Wound Rotor Induction Motor [WRIM] BasicsComparing Wound Rotor Machine to Standard Induction Machine
Squirrel Cage rotor,current & Torquefixed by Motordesign
Wound rotor, currentand torque set byexternal resistor orcontrol
Selecting Motor Controlsfor Mining Process Torque Demands Slide 41
Selecting Motor Controlsfor Mining Process Torque Demands
WRIM Popularity & Tradeoffs• Historical Popularity of WRIM vs Standard
Induction Motors WRIM easier to start, & with simple equipment
Higher starting & running controlled torque levels
Lower WRIM inrush amps allows large motors to starton weak power systems
Fixed [full] speed application most common
Wound rotor machines were the earliest Electrical ACAdjustable Variable Speed Drives.
Operation below top speed is possible if rotor powercan be taken off with resistors or sent back to powerline
• Down-side of WRIM Brushes and slip rings – wear and maintenance
Variable speed operation energy wastes energy ifresistors are used.
WRIM MiningApplications
• Pumps
• Ball and Sag Mills
• Cranes
• Fans and Blowers
• Conveyors
Selecting Motor Controlsfor Mining Process Torque Demands Slide 42
Selecting Motor Controlsfor Mining Process Torque Demands
Induction Motor Behavior with VFD• Voltage and frequency are both
adjusted very accurately – to matchprocess needs
• High starting currents andstresses are eliminated, no startsper hour limit
• Utility amp starting surgeeliminated and power factor isgood over whole speed range
• Motor can be controlled to makerated torque (or more) over thewhole speed range
Selecting Motor Controlsfor Mining Process Torque Demands Slide 43
Selecting Motor Controlsfor Mining Process Torque Demands
Induction Motor Torque vs. Speed with VFD
Selecting Motor Controlsfor Mining Process Torque Demands Slide 44
Selecting Motor Controlsfor Mining Process Torque Demands
SYNCHRONOUS MOTORS
Selecting Motor Controlsfor Mining Process Torque Demands Slide 45
Selecting Motor Controlsfor Mining Process Torque Demands
Slide45
Synchronous Motor Model - Starting
POWER SOURCE
• s
One Phase Model
• AC Power on stator sets up rotating field magnetic flux
• For starting, rotor Amortisseur acts as shorted transformersecondary, current produces rotor flux like induction motor
• Torque produced accelerates load to near sync speed
• DC field poles shorted by “discharge” resistor during start
• Near sync speed, DC field is applied, rotor syncs to line
Selecting Motor Controlsfor Mining Process Torque Demands Slide 46
Selecting Motor Controlsfor Mining Process Torque Demands
Slide46
Typical Sync Motor Starting Curves
PulsatingTorque
Average
Torque
Power
Factor
Field
Current
LineCurrent
0.20
0.15
0.10
0.05
PowerFactor
6.00
5.00
4.00
3.00
2.00
1.0050%
100%
150%
100%
140%
PERCENT SYNCHRONOUS SPEED20% 40% 60% 80% 100%0%
Selecting Motor Controlsfor Mining Process Torque Demands Slide 47
Selecting Motor Controlsfor Mining Process Torque Demands
SYNC MOTOR DEFINITIONS - 1Pull-In Torque [Not applicable to VFD Oper]
The maximum connected load torque under which themotor will pull its connected inertia load intosynchronism
at rated voltage, frequency, and
with rated field applied
Pull-In speed [Not applicable to VFD Oper]
The speed at which a motor will bring its load intosynchronism – dependent on
the inertia of the revolving parts
the load torque
Selecting Motor Controlsfor Mining Process Torque Demands Slide 48
Selecting Motor Controlsfor Mining Process Torque Demands
SYNC MOTOR DEFINITIONS - 2Pull-Out Torque
The maximum sustained torque which the motor willdevelop at synchronous speed without stepping out
with rated voltage applied
at rated frequency and excitation.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 49
Selecting Motor Controlsfor Mining Process Torque Demands
Typical Torque/Speed - 4 Pole
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40 50 60 70 80 90 100
% Speed
PU
Torq
ue
Mean
Pulsating
Load
With Resistor
Selecting Motor Controlsfor Mining Process Torque Demands Slide 50
Selecting Motor Controlsfor Mining Process Torque Demands
Typical Torque/Speed - 4 Pole Salient RotorNOTE – Pulsating Torque NOT produced for VFD Start
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 10 20 30 40 50 60 70 80 90 100
% Speed
PU
Torq
ue
Pulsating Tq100-120 Hz
Pulsating Tq0 Hz
Average of InductionAnd pulsating torque
Best to Apply Field@ (+) Torque
Peaks
Selecting Motor Controlsfor Mining Process Torque Demands Slide 51
Selecting Motor Controlsfor Mining Process Torque Demands
Notes on Sync Motor Starting - 1
• Speed of 95-97% is typical field application point
• “Best Angle” field application may not be needed –timed application is often effective & simpler
• Turning on the fields too soon can create excessivetorques at “lock in” to synchronous speed
• Open circuit fields during start creates high voltages[10,000 volts or more] – damage to fields, slip rings!
Either a short circuit, diode rectifier, or a resistor should beused across fields during start.
Using an optimal resistor can give 30-50% more start torque
• Voltage surge protectors across slip-ring type fields
Selecting Motor Controlsfor Mining Process Torque Demands Slide 52
Selecting Motor Controlsfor Mining Process Torque Demands
Notes on Sync Motor Starting - 2• Field application Contactors
connect DC before discharge pathbreaks
• “Reluctance torque” is producedby attraction of rotor iron torotating stator field average pluspulsating peaks
• Sync Motors are stressed bystarting – design limit is 2 coldstarts per hour
• Full voltage 600% inrush amps,@15-20% pf is typical
DCSOURCE
Thyrite[Typical]
MAIN FC TIPSCLOSE BeforeR Ckt Opens
FC
FC
FC
Example DC Slip RingSync Excitation Circuit
Selecting Motor Controlsfor Mining Process Torque Demands Slide 53
Selecting Motor Controlsfor Mining Process Torque Demands
SPECIAL STARTING CASES• When High starting torque is required
Double squirrel-cage design gives higher torquesynchronous motor starting torque [like NEMA C Ind.motor]
Applied on loads requiring high starting and pull-intorques
Can result in oversize and much more costly
• Very large sync motors can be started unloaded with a VFDto synchronous speed and synchronized to utility
Selecting Motor Controlsfor Mining Process Torque Demands Slide 54
Selecting Motor Controlsfor Mining Process Torque Demands
Synchronous Motor Starting and Thermal Curves
Minimum Thermal Margin
Starting Current
Minimum AcceleratingTorque
Selecting Motor Controlsfor Mining Process Torque Demands Slide 55
Selecting Motor Controlsfor Mining Process Torque Demands
Slide55
Sync Motor ModelFully Running
Effect of DC Field
•Sync Motor KVAR
Exported with strong DCfield [leading pf]
Imported with weakerfield [lagging pf]
• Increases torquecapability [power output]
Selecting Motor Controlsfor Mining Process Torque Demands Slide 56
Selecting Motor Controlsfor Mining Process Torque Demands
TorqueAngle d
Rotor follows statormagnetic wave at syncRPM
Like an elastic band
Torque “stretches”band and rotor trailsstator by an anglecalled the torqueangle d
Salient Pole Sync MotorAfter Synchronizing,
With DC Field Applied
Sync Rpm = 120 x Freq. / #Poles
Selecting Motor Controlsfor Mining Process Torque Demands Slide 57
Selecting Motor Controlsfor Mining Process Torque Demands
Sync Motors on VFDsStarting
1. The field is energized FIRST -> AC exciter or slipring DC power
2. The motor is synchronous as it starts to turn.3. This allows for very high starting torque.4. Must know rotor position with absolute encoder / or
by detecting rotor position by calculation5. Amortisseur bars (dampening windings) are
important for impact or erratic loads6. Motor starting characteristics not utilized7. Motor may be transferred to utility by switchgear
when volts, frequency & phase match
Selecting Motor Controlsfor Mining Process Torque Demands Slide 58
Selecting Motor Controlsfor Mining Process Torque Demands
Sync Motors on VFDsRunning
Variable Speed Control by Stator Frequency
Constant volts per hertz applied to maintain constantflux
Can run motor at high frequencies for high RPM
Control changes field current based on load to maintainunity PF (minimum current)
Zero speed [near DC] is difficult for drive power devices.
As in fixed speed application any load change createsload angle change and instantaneous speed change.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 59
Selecting Motor Controlsfor Mining Process Torque Demands
Induction Motors Compared With Sync Motors
Induction Motors
• AC Rotor field, induced bytransformer action
• Rotor field depends on ACline voltage
• Always turns slower thansync speed by slip %
• Always runs lagging p.f.
• Torque falls ~ Volts2
Synchronous Motors
• DC Rotor Field, ext. fed orgenerated by DC exciter
• Rotor field set independently
• Always turns exactly at syncspeed – NO SLIP
• Can run leading or lagging pf
• Torque falls ~ Volts
Similarities: Follow rotating 3-phase magnetic flux wave, RPM is dependenton frequency of source
Differences:
Selecting Motor Controlsfor Mining Process Torque Demands Slide 60
Selecting Motor Controlsfor Mining Process Torque Demands
SM & IM Speed vs KW Capacity Application
Induction Motor
Synchronous Motor
0 3000 6000 9000 12000 15000 18000 210000
10
20
30
40
50
60
70
80
Cap
acit
y(M
W)
Rotating Speed (min-1)
• Sync motors best suitedfor very high outputs, lowRPM
• Induction motors bestsuited for high speed
• Considerable overlapexists to allow bestchoice to be made foreach application
Selecting Motor Controlsfor Mining Process Torque Demands Slide 61
Selecting Motor Controlsfor Mining Process Torque Demands
Power dips / transfersSYNC MOTORS
• For power loss DC field must be removed and allowedto decay to zero [may be several seconds]
• Can restart as induction machine as if from standstillwith protectives over-riding if needed
• VFDs can restart spinning sync motor as a syncmachine
INDUCTION MOTORS
• Induction machines can begin restart more quickly[after current and volts decayed]
• VFDs can restart spinning induction motors “on the fly”
Selecting Motor Controlsfor Mining Process Torque Demands Slide 62
Selecting Motor Controlsfor Mining Process Torque Demands
Why Pick Sync or Induction MotorTechnology?
Why Induction Motor?
• Lower first cost
• Very High RPM
• Lower complexity [noDC field supply]
Why Sync Motor?
• Higher efficiency
• Low RPM - <200 RPM
• Very high Power - > 20MW to >100 MW
• Correct system powerfactor to reduce voltagedrops
Selecting Motor Controlsfor Mining Process Torque Demands Slide 63
Selecting Motor Controlsfor Mining Process Torque Demands
MOTOR & LOAD CONTROL
Selecting Motor Controlsfor Mining Process Torque Demands Slide 64
Selecting Motor Controlsfor Mining Process Torque Demands
Starting & Controlling Connected LoadsGround Rules and Assumptions
1. Motor available torque must ALWAYS exceed theconnected load Torque requirement During starting
During acceleration
Over peaks during the duty cycle
Considering available utility voltage including the effect ofvoltage drop
2. Motor must remain within its design thermal limits
3. Acceleration time of the load must be acceptable
4. Regenerated load power from the motor and itsload must be controlled if required.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 65
Selecting Motor Controlsfor Mining Process Torque Demands
Will It Start?
100% V
90% V
100% V Rated torque
80% V
To
rqu
e
Cu
rre
nt
90% V 81% Torque
80% V 64% Torque
Speed
ConveyorLoad
PumpLoad
NO!
YES!
Selecting Motor Controlsfor Mining Process Torque Demands Slide 66
Selecting Motor Controlsfor Mining Process Torque Demands
Starting Large Motors
• A “large motor” is usually considered to be1,000 HP or larger
• Why is starting a large motor stressful?Motor must be magnetized, which draws high
current at low pf
The motor and load must be accelerated, whichrequires high current
The motor counter-EMF has to build up, so themotor initially looks like a short circuit
High currents cause high mechanical stresses in themotor
Selecting Motor Controlsfor Mining Process Torque Demands Slide 67
Selecting Motor Controlsfor Mining Process Torque Demands
• Inrush Amps and Duration
• Motor Limit on Number of Starts Per Hour
• Motor Connected Inertia Limits
• Load Mechanical Issues
Pumps, Piping & Hydraulic issues
Coupling Stress
• Starting Torque vs. Load Requirements
Factors To Consider - Motor Starting
Selecting Motor Controlsfor Mining Process Torque Demands Slide 68
Selecting Motor Controlsfor Mining Process Torque Demands
Effects of Motor Starting
• Starting such a large motor Directly Onthe Line (DOL) is stressfulOn the motor due to high current (4 to 6
times rated current) and mechanical stress
On the load (high torques)
On the power system (voltage drop)
On other loads (power interruptions)
• What are strategies for avoiding DOLstarts with large motors?
Selecting Motor Controlsfor Mining Process Torque Demands Slide 69
Selecting Motor Controlsfor Mining Process Torque Demands
Motor Starting Power System Impact
• Processes & equipment in rest of facilitymay suffer from voltage drop
• Utility company restrictions“Being a good neighbor” – to nearby users & utility
power quality guarantees to them
Limitations of utility power delivery & transmissionequipment
• Recent trendsRemote locations on long power lines
Many new motors applications are very high power
Selecting Motor Controlsfor Mining Process Torque Demands Slide 70
Selecting Motor Controlsfor Mining Process Torque Demands
Starting Large Motors Intelligently
• A reduced voltage starting system maybe applicableReactor or autotransformer
Reactor - Capacitor starter
Solid state starter
• A VFD may be requiredWeak power system (relative to load)
High inertia load
Selecting Motor Controlsfor Mining Process Torque Demands Slide 71
Selecting Motor Controlsfor Mining Process Torque Demands
Reducing Starting Current• Various Methods – all
work by reducingmotor terminal volts.
• All apply full utilityfrequency to themotor.
• As volts are reduced:Current falls directlyTorque falls with
1/volts2 [e.g. 65%V =42% torque]
Percent voltage drop tomotor falls directly
Selecting Motor Controlsfor Mining Process Torque Demands Slide 72
Selecting Motor Controlsfor Mining Process Torque Demands
Effects of Reduced Voltage Start (1)
100% V
90% V
100% V Rated torque
80% V
Torque
Current
Load
90% V 81% Torque
80% V 64% Torque
Selecting Motor Controlsfor Mining Process Torque Demands Slide 73
Selecting Motor Controlsfor Mining Process Torque Demands
Effects of Reduced Voltage Start (2)
• Motor torque is proportional to voltage2
Current is proportional to voltage, so voltagemust be reduced to reduce current
Torque drops off quickly as voltage is reduced
• Selecting starting components is a tradeoff betweenallowable current (and bus voltage drop) and torque
• Amps jump to what they would have been at that stage iffull voltage start applied at beginning
• Remaining torque at any time MUST be above loadtorque!
Selecting Motor Controlsfor Mining Process Torque Demands Slide 74
Selecting Motor Controlsfor Mining Process Torque Demands
Reduced Voltage Starters – Reactor Example
• Reactor starter as shownsimply places impedancebetween motor and powersystem
• After a time delay foracceleration, the reactor isshorted out, applying fullvolts
MainContactor
BypassContactor
Selecting Motor Controlsfor Mining Process Torque Demands Slide 75
Selecting Motor Controlsfor Mining Process Torque Demands
Reactor Start
0.00
0.50
1.00
1.50
2.00
2.50
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
PU
Torq
ue
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
PU
Curr
ent
Bypass Rx
Selecting Motor Controlsfor Mining Process Torque Demands Slide 76
Selecting Motor Controlsfor Mining Process Torque Demands
Autotransformer Start
• More effective than reactor as ittransforms & decreases linecurrent while setting motorcurrent
• Select autotransformer forsufficient voltage & torque atstandstill
• Works better to limit line amps,but costs more than reactor
M
Selecting Motor Controlsfor Mining Process Torque Demands Slide 77
Selecting Motor Controlsfor Mining Process Torque Demands
Reactor-Capacitor Start
• Cost effective for hard startingapplications
• Capacitor equals starting motorkVAR, or more, cancels voltagedrop from bad [~0.15 – 0.20]motor starting pf
• Components must be selectedcarefully – see performance plot
M
Selecting Motor Controlsfor Mining Process Torque Demands Slide 78
Selecting Motor Controlsfor Mining Process Torque Demands
Reactor-Capacitor Starter Performance Example
0.00
0.50
1.00
1.50
2.00
2.50
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
P.U. Speed (1 p.u. = 1800 rpm)
PU
To
rqu
e
PU
Vo
ltag
e
0.0
1.0
2.0
3.0
4.0
5.0
6.0
PU
Cu
rren
t
Motor3000 HP, 4000 Volt377 FLA, 650% inrush75% starting torque
Utility 200 MVA, 114kV5 MVA, 11% xfmr
10 MVAR Cap0.6 mH/phase rx
Needs careful study !Check resonance andvoltage surges. Watchthe switching current
Selecting Motor Controlsfor Mining Process Torque Demands Slide 79
Selecting Motor Controlsfor Mining Process Torque Demands
Solid State Starter• Most costly of reduced voltage
starters
• Same as reactor at breakaway,usually better at 70 – 80%speed
• Allows controlled accelerationCan limit current
Accel for preset time
Reduces torque shock tomechanical loads
Controls
MTR
Selecting Motor Controlsfor Mining Process Torque Demands Slide 80
Selecting Motor Controlsfor Mining Process Torque Demands
Using a VFD as a Starter (1)
• A VFD controls frequency and voltageapplied to motor
• VFDs can produce high (>100%) torqueat low speeds, with better control ofmotor current
• A VFD may be more costly, but it canstart the largest inertia loads on weakpower systems
Selecting Motor Controlsfor Mining Process Torque Demands Slide 81
Selecting Motor Controlsfor Mining Process Torque Demands
AC Drives Accelerate Loadby Increasing Volts and Frequency
Operational torquemust be regulatedto remain in theshaded near- linearzones.
Moto
rV
olts
->
Motor RPM ->Controlled, Increasing Volts and Frequency
Selecting Motor Controlsfor Mining Process Torque Demands Slide 82
Selecting Motor Controlsfor Mining Process Torque Demands
More Notes on Using a VFD as a Starter
• The VFD can be rated at a fraction ofthe motor rating if the motor can beunloaded at synchronizing speed
• Either synchronous or induction motorscan be started with a VFD
• Synchronous motors are started withfield current applied, so an exciter mustsupply current at standstill
Selecting Motor Controlsfor Mining Process Torque Demands Slide 83
Selecting Motor Controlsfor Mining Process Torque Demands
Typical VFDWith
SynchronizedBypass
M Induction motorConverter-Inverter Rectifier & inverterM1 & M1A Input & pre-chargePT Voltage transformerDrive control With VFD & Sync LogicL-1 Output isolation inductorM2 Drive isolation contactorM3 Bypass contactorCTO Current sensing transf.Relay 25 Synch check relay
Selecting Motor Controlsfor Mining Process Torque Demands Slide 84
Selecting Motor Controlsfor Mining Process Torque Demands
Starter Duty Rating• Point C is about 3900 HP• Point B is achieved by throttling /
valving / baffling flow, and is about1872 hp
• For start duty, VFD can be selectedfor B [2000 HP]
• Since PF for this motor is nearlyconstant from 50% to 100% load,total amps at 50% load is about 50%of full load.
• Load is synchronized to the line withthe VFD, and the flow released torun at C.
A
B
C
Selecting Motor Controlsfor Mining Process Torque Demands Slide 85
Selecting Motor Controlsfor Mining Process Torque Demands
Synchronized Start Sequence Utility Volts
Drive Volts
Motor Volts
UtilityAmps
Drive Amps
Motor Amps
ADrive is matched infrequency, phase andvolts to line andbypass co ntactor isclosed
DRIVE ACCELERATES ->
UTILITY PICKS UP AMPS -> B
DRIVE AMPS END ->
MOTOR AMPS AND TORQUE ISSMOOTH THRU WHOLE PROCESS!
Drive is made ready for operationDrive accelerates load to running speed
required by the processUser process requests load to be transferred
to the utilityDrive ramp accelerates load to speedvoltage and frequency & phase exactly
match utilityPT feedback monitored by control.Relay 25 verifies V & F & phase matchM3 contactor is closed [point A]Current from CTO verifies utility amps to
motor and M2 opensInverter output switches are turned off.Control sequence repeats for each motor to
be synchronized.
MOTOR AMPS AND TORQUE ARE SMOOTHTHROUGH THE WHOLE PROCESS
Selecting Motor Controlsfor Mining Process Torque Demands Slide 86
Selecting Motor Controlsfor Mining Process Torque Demands
VFD Controlled, Synchronized & Utility Fed MotorsSome Notes
• Production Related:No Limit on number or frequency of startsProduction optimization
• Energy RelatedFor < 100% production, slowing process can save
energyAt 100% speed, drive’s ~3.5% energy can be saved.
• Maintenance IssuesSynchronized start reduces motor stress, possibly
extending life of motor and drive train.At lower speeds provided by a VFD, mechanical wear
can drop dramatically – if production allows.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 87
Selecting Motor Controlsfor Mining Process Torque Demands
Synchronous Motors vs Induction on VFDs
• Synchronous Motors always operate at a speedmatching applied frequency.
• Sync motors require rotor field at standstill,
• Sync Motors can be 0.5 -2% more efficient thaninduction motors.
• Sync motors can improve system power factor,but only when fed directly by utility.
While on VFD, sync motor PF is seen only by drive,and utility sees drive PF only
Start mode VFDs quickly move motor to utility.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 88
Selecting Motor Controlsfor Mining Process Torque Demands
Typical VFD Synchronized Bypass System forSynchronous Motors
Inverter-Bypass System
• VFD synchronizing systemincludes contactors andlogic same as inductionmotor system
• Synch motor field iscontrolled by VFD to holdunity pf.
• After connection to utility,control sync motor field isswitched from the drive toan outside control
• After connection to line,sync motor leading powerfactor can benefit the utility.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 89
Selecting Motor Controlsfor Mining Process Torque Demands
Tradeoffs For Multiple Motors per VFD• Complexity grows as number of motors
grows –Switchgear and control, feedbacks and PLC
Complexity costs grow as the system control modes
• FlexibilityMore modes of operation
Potential higher availability from 100% backup ofcritical VFD equipment
• Costs –VFD system costs are affected by performance
VFD system costs sensitive to system configuration.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 90
Selecting Motor Controlsfor Mining Process Torque Demands
Applying ASD Synchronized Starting toMultiple Motors
Two-MotorOne ASD System
• Motor 1 or Motor 2can be operatedeither across the lineor on ASD.
• Connection will“bumplessly” connectto and from utility
• Allows smoothincrease in processoutput
Two-MotorOne ASD System
Selecting Motor Controlsfor Mining Process Torque Demands Slide 91
Selecting Motor Controlsfor Mining Process Torque Demands
• Table does not include the costs of installation nor give credit forprocess improvements, energy savings, or reduced maintenance.
• The 100% cost = FULLY RATED 3000 HP 4160 VOLT VFD, nosynchronized start.
• COLUMN A: 32% cost adds one-motor synchronized starting
Two, 3 or 4 motor synchronized cost adders are 54%, 71%, and 89%respectively
• COLUMN B like COLUMN A except with a 50% rated drive for startingduty only, & max HP and torque limited to 50% of 3000 HP.
Number of
Motors
Continuous ASD
No Sync Start
COLUMN ASynchronized Start
ASD Rated for One motor
at a time at Full 3000 HP
COLUMN BSynchronized Start
Torque Mechanically Limited
to 50% of 3000 HP
1 100% 132% 86%
2 154% 102%
3 171% 113%
4 189% 125%
Single ASD Synchronized Start System
Shared Between Multiple Motors
EquipmentConfigurationComparisons
VFDSynchronizedStarting
Selecting Motor Controlsfor Mining Process Torque Demands Slide 92
Selecting Motor Controlsfor Mining Process Torque Demands
VFD Synchronized StartingOverall Economic Comparisons
• Actual economics are sensitive to site,process, equipment ratings, manufacturer.
• Economics need to consider all aspects:
Equipment costs
Utility supply impact and billing cost savings
Process improvements from variable speed
Increased equipment life and reducedmaintenance.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 93
Selecting Motor Controlsfor Mining Process Torque Demands
Wound Rotor InductionMotor Review
Wound RotorSpeed-Torque
Perc
ent
Full
Lo
ad
Cu
rre
nt
Pe
rce
nt
Full
Lo
ad
To
rque
Selecting Motor Controlsfor Mining Process Torque Demands Slide 94
Selecting Motor Controlsfor Mining Process Torque Demands
WRIM Control Summary
• To Control inrush amps and set torque: CONTROL
ROTOR AMPS.
• Rotor volts start high
• Rotor volts x rotor amps = rotor output power
• As speed increases, rotor volts decrease
• Therefore:High Torque at start = high rotor amps
High volts and high amps = high power out from rotor
As speed increases, rotor power at a particulartorque decreases.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 95
Selecting Motor Controlsfor Mining Process Torque Demands
Applying & Controlling New WoundRotor Motors
• For full, fixed-speed
Resistor / liquid rheostat start
Starting equipment matches load inertia and torque
• For Variable Speed ~60 or 70% to Top Speed
Start with Resistor / liquid rheostat start
Control rotor power with resistors – waste energy
Control & recover rotor power with thyristor drive –poor PF, harmonics – OLD TECHNOLOGY
Control & recover rotor power with LV PWM drive
Selecting Motor Controlsfor Mining Process Torque Demands Slide 96
Selecting Motor Controlsfor Mining Process Torque Demands
Starting & Control Methods for WRIM
WRIM Full Speed Options
Stator DIRECT ON LINE, with rings shorted – actslike standard induction motor, torque determined bymotor design [VERY HIGH INRUSH!].
Stator DIRECT ON LINE with stepped resistor rotorcurrent control to top speed, then short rings.
Stator DIRECT ON LINE with liquid rheostat forrotor current control to top speed, then short rings.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 97
Selecting Motor Controlsfor Mining Process Torque Demands
Example Liquid Resistor Rotor Control
FinalFixedStep
Option
LiquidRheostat
RotorShorted
LiquidRheostat &Contactors
SMOOTH TRANSITION
Liquid RheostatCourtesy Eletele
Provides smooth control of torque as resistance isgradually removed from rotor circuit.
Not very fast response – tank has to fill and emptyto follow a changing load
Selecting Motor Controlsfor Mining Process Torque Demands Slide 98
Selecting Motor Controlsfor Mining Process Torque Demands
WRIM PWM Slip Power RecoverySimplified Diagram
Starting duty rated
Utility InterfaceTransformer
Wound RotorInduction Motor
PWM RotorConverter
PWM SourceConverter
TMdrive-10SPR
Power Flow atnormal speeds
Rheostat
Brushes andSlip Rings
Recovered power
Three-phaseMotor Stator
UtilitySupply
Selecting Motor Controlsfor Mining Process Torque Demands Slide 99
Selecting Motor Controlsfor Mining Process Torque Demands
Example: Coordinating Grinder Loadwith 2 VFDs
• Grinder with independent rollers
• Initially no connection between rollers
• After material is entered, material itself“connects” the rolls.
• Goal: Share load, and minimize rollerwear.
Selecting Motor Controlsfor Mining Process Torque Demands Slide 100
Selecting Motor Controlsfor Mining Process Torque Demands
2 Drives on Same Load
• Give same speed reference toboth drives.
• Drive 2 set to match torque ofDrive 1
• When unloaded, drive 2 willspeed up to max of 5% overspeed ref
• Process control, minmaintenance, smallest motorsresult from system
Roller1
Roller2
Motor1
Drive1
Drive2Speed Ref
TorqueRef
Motor2
Selecting Motor Controlsfor Mining Process Torque Demands Slide 101
Selecting Motor Controlsfor Mining Process Torque Demands
Review of Areas to Consider• Loads – Select Constant Torque, Variable
Torque, peak torques
• Consider System voltages
• Motor selection – OEM or site, with specificssuch as speed-torque curves, inrush
• Controls – Operating mode Starting, Running,fixed speed, variable speed
A Chart might be helpful…
Selecting Motor Controlsfor Mining Process Torque Demands Slide 102
Selecting Motor Controlsfor Mining Process Torque Demands
Chart Relating Controls to Load Type
Selecting Motor Controlsfor Mining Process Torque Demands Slide 103
Selecting Motor Controlsfor Mining Process Torque Demands
Thanks!