Motor Starting
Dynamic Acceleration
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration
Why to Do MS Studies?
• Ensure that motor will start with voltage drop
• If Tst<Tload at s=1, then motor will not start
• If Tm=Tload at s<sr, motor can not reach rated speed
• Torque varies as (voltage)^2
• Ensure that voltage drop will not disrupt other loads
• Utility bus voltage >95%
• 3% Sag represents a point when light flicker becomes visible
• 5% Sag represents a point when light flicker becomes irritating
• MCC bus voltage >80%
• Generation bus voltage > 93%
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 2
Why to Do MS Studies?
• Ensure motor feeders sized adequately
(Assuming 100% voltage at Switchboard or MCC)
• LV cable voltage drop at starting < 20%
• LV cable voltage drop when running at full-load < 5%
• HV cable voltage drop at starting < 15%
• HV cable voltage drop when running at full-load < 3%
• Maximum motor size that can be started across the line
• Motor kW < 1/6 kW rating of generator (islanded)
• For 6 MW of islanded generation, largest motor size < 1 MW
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 3
Motor Sizing
• Positive Displacement Pumps / Rotary Pumps
• p = Pressure in psi
• Q = fluid flow in gpm
• n = efficiency
• Centrifugal Pumps
• H = fluid head in feet
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 4
Motor Types
• Synchronous
• Salient Pole
• Round Rotor
• Induction
• Wound Rotor (slip-ring)
• Single Cage CKT Model
• Squirrel Cage (brushless)
• Double Cage CKT Model
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 5
Induction Motor Advantages
• Squirrel Cage
• Slightly higher efficiency and power factor
• Explosive proof
• Wound Rotor
• Higher starting torque
• Lower starting current
• Speed varied by using external resistances
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 6
Typical Rotor Construction
• Rotor slots are not parallel to the shaft but
skewed
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 7
Wound Rotor
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 8
Operation of Induction
Motor
• AC applied to stator winding
• Creates a rotating stator magnetic field in air gap
• Field induces currents (voltages) in rotor
• Rotor currents create rotor magnetic field in air gap
• Torque is produced by interaction of air gap fields
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 9
Slip Frequency
• Slip represents the inability of the rotor to
keep up with the stator magnetic field
• Slip frequency
S = (ωs-ωn)/ωs where ωs = 120f/P
ωn = mech speed
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 10
Static Start - Example
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 11
Static Start - Example
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 12
Service Factor
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 13
Inrush Current
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 14
Motor Torque – Speed Curve
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 15
Resistance / Reactance
• Torque Slip Curve is changed by altering
resistance / reactance of rotor bars.
• Resistance ↑ by ↓cross sectional area or
using higher resistivity material like brass.
• Reactance ↑ by placing conductor deeper in
the rotor cylinder or by closing the slot at the
air gap.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 16
Rotor Bar Resistance ↑
• Increase Starting Torque
• Lower Starting Current
• Lower Full Load Speed
• Lower Efficiency
• No Effect on Breakdown Torque
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 17
Rotor Bar Reactance ↑
• Lower Starting Torque
• Lower Starting Current
• Lower Breakdown Torque
• No effect on Full Load Conditions
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 18
Motor Torque Curves
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 19
Rotor Bar Design
• Cross section Large (low
resistance) and positioned deep in
the rotor (high reactance).
(Starting Torque is normal and
starting current is low).
• Double Deck with small conductor
of high resistance. During starting,
most current flows through the
upper deck due to high reactance
of lower deck. (Starting Torque is
high and starting current is low).
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 20
Rotor Bar Design
• Bars are made of Brass or
similar high resistance
material. Bars are close to
surface to reduce leakage
reactance. (Starting torque is
high and starting current is
low).
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 21
Load Torque – ID Fan
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 22
Load Torque – FD Fan
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 23
Load Torque – C. Pump
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 24
Double Cage Motor
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 25
Motor Full Load Torque
• For example, 30 HP 1765 RPM Motor
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 26
Motor Efficiency
• kW Saved = HP * 0.746 (1/Old – 1/New)
• $ Savings = kW Saved * Hrs /Year * $/kWh
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 27
Acceleration Torque
• Greater
Acceleration
Torque means
higher inertia
that can be
handled by the
motor without
approaching
thermal limits
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 28
Acceleration Torque
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 29
Operating Range
• Motor, Generator, or Brake
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 30
Rated Conditions
• Constant Power
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 31
Starting Conditions
• Constant Impedance
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 32
Voltage Variation
• Torque is proportional to V^2
• Current is proportional to V
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 33
Frequency Variation
• As frequency decreases, peak torque shifts toward lower
speed as synchronous speed decreases.
• As frequency decrease, current increases due reduced
impedance.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 34
Number of Poles Variation
• As Pole number increases, peak torque shifts toward lower
speed as synchronous speed decreases.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 35
Rotor Z Variation
• Increasing rotor Z will shift peak torque towards lower
speed.
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 36
Modeling of Elements
• Switching motors – Zlr, circuit model, or
characteristic model
• Synch generator - constant voltage behind
X’d
• Utility - constant voltage behind X”d
• Branches – Same as in Load Flow
• Non-switching Load – Same as Load flow
• All elements must be initially energized,
including motors to start
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 37
Motor Modeling
1. Operating Motor
– Constant KVA Load
2. Starting Motor
– During Acceleration – Constant Impedance
– Locked-Rotor Impedance
– Circuit Models
Characteristic Curves
After Acceleration – Constant KVA Load
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 38
Locked-Rotor Impedance
• ZLR = RLR +j XLR (10 – 25 %)
• PFLR is much lower than operating PD.
Approximate starting PF of typical squirrel
cage induction motor:
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 39
Circuit Model I
• Single Cage Rotor
– “Single1” – constant rotor resistance and
reactance
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 40
Circuit Model II
• Single Cage Rotor
– “Single2” - deep bar effect, rotor resistance and
reactance vary with speed [Xm is removed]
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 41
Circuit Model III
• Double Cage Rotor
– “DB1” – integrated rotor cages
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 42
Circuit Model IV
• Double Cage Rotor
– “DB2” – independent rotor cages
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 43
Characteristic Model
• Motor Torque, I, and PF as function of Slip
– Static Model
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 44
Calculation Methods I
• Static Motor Starting
– Time domain using static model
– Switching motors modeled as Zlr during starting and
constant kVA load after starting
– Run load flow when any change in system
• Dynamic Motor Starting
– Time domain using dynamic model and inertia model
– Dynamic model used for the entire simulation
– Requires motor and load dynamic (characteristic) model
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 45
Calculation Methods II
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 46
Static versus Dynamic
• Use Static Model When
– Concerned with effect of motor starting on other
loads
– Missing dynamic motor information
• Use Dynamic Model When
– Concerned with actual acceleration time
– Concerned if motor will actually start
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 47
MS Simulation Features
• Start/Stop induction/synchronous motors
• Switching on/off static load at specified loading
category
• Simulate MOV opening/closing operations
• Change grid or generator operating category
• Simulate transformer LTC operation
• Simulate global load transition
• Simulate various types of starting devices
• Simulate load ramping after motor acceleration
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 48
Automatic Alert
• Starting motor terminal V
• Motor acceleration failure
• Motor thermal damage
• Generator rating
• Generator engine continuous
& peak rating
• Generator exciter peak rating
• Bus voltage
• Starting motor bus
• Grid/generator bus
• HV, MV, and LV bus
• User definable minimum time
span
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 49
Starting Devices Types
• Auto-Transformer • Y/D Winding
• Stator Resistor • Partial Wing
• Stator Reactor • Soft Starter
• Capacitor at Bus • Stator Current Limit
• Capacitor at Motor – Stator Current Control
Terminal – Voltage Control
• Rotor External Resistor – Torque Control
• Rotor External Reactor
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 50
Starting Device
• Comparison of starting conditions
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 51
Starting Device – AutoXFMR
• C4 and C3 closed initially
• C4 opened, C2 is closed with C3 still closed. Finally C3 is open
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 52
Starting Device – AutoXFMR
• Autotransformer
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 53
Starting Device – YD Start
• During Y connection Vs = VL / √3
• Phase current Iy = Id / √3 and 3 to 1 reduction in torque
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 54
Starting Device – Rotor R
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 55
Starting Device – Stator R
• Resistor
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 56
Starting Device Stator X
• Reactor
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 57
Transformer LTC Modeling
• LTC operations can be simulated in motor
starting studies
• Use global or individual Tit and Tot
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 58
MOV Modeling I
• Represented as an impedance load during
operation
– Each stage has own impedance based on I, pf, Vr
– User specifies duration and load current for each stage
• Operation type depends on MOV status
– Open statusclosing operation
– Close statusopening operation
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 59
MOV Modeling II
• Five stages of operation
Opening Closing
Acceleration Acceleration
No load No load
Unseating Travel
Travel Seating
Stall Stall
• Without hammer blow Skip “No Load” period
• With a micro switch Skip “Stall” period
• Operating stage time extended if Vmtr < Vlimit
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 60
MOV Closing
• With Hammer Blow- MOV Closing
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 61
MOV Opening
• With Hammer Blow- MOV Opening
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 62
MOV Voltage Limit
• Effect of Voltage Limit Violation
© 1996-2009 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 63
