Power Systems
Power Systems
Lesson Objectives
When you finish this lesson you will
understand:
• Control of the high electrical power
systems, transformer control
• Timing functions
• Calculation of the heat
• Troubleshooting
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line
workbook
5. Do Homework
Keywords
Power Systems, Single Phase AC, Single Phase Rectified, Three
Phase, Three Phase Rectified, Frequency Changer, Inverter,
Transformer, Tap Switches, Saturation, Hysteresis, Reactance,
Inductance, Resistance, Impedance, Power Factor, Duty Cycle,
Timing Controls, Synchronous, Phase Shift Heat Control, SCR, half
wave rectified, Full Wave Rectified
Machines and Equipment
• Cooling System
• Mechanical System
– Force generation system
• Pneumatic/Hydraulic system
• Motor or foot-operated system
– Electrode application system
• Rocker arm or pinch type
• Press or direct action type
• Portable system
• Electrical System
– Single-phase system
– Single-phase rectified
system
– Three-phase rectified
system
– Frequency changer
system
– Frequency inverter
system
Block Diagram of Single-Phase
Spot Welder
Contactor
Main Power Line
Spot Weld
Configurations for Spot Welding
Transformers
(a) Stacked, Shell Type
(b) Stacked, Core Type
(c) Wound Type
W. Stanley, Resistance Welding
McGraw-Hill, 1950
N=np/ns
Vs= Vp/N
Is = Ip N
Parallel/Series Tap Switches
Es
(a) Parallel
S2
P2
S1
P1
Es
S1
P1
Vp
(b) Series
S2
P2
Es
Es
Vp
Transformer Tap Switch
Es
Vp
Induced Magnetic Filed
Saturation Induction
Remnant Induction
Coersive Force
Structure & Properties of Materials,
Wiley, 1967
Applied Magnetic
Field
Rp
X = reactance in Ohm
L = inductance in Henry
f = frequency in Hertz
Xs
Rs
Xp
X  2fL
Z R X
2
2
Z = impedance in Ohm
R = resistance in Ohm
Power Factor
DC Current
Ohm’s Law
V
I
R
I= Current
V= Voltage
R= Resistance
Power = VI
=KW (kilowatts)
AC Current
V
I
Z
Z  X2  R2
Z= Impedance
X= Reactance
• Frequency of AC
• Geometry of Secondary
• Magnetic Material
Power = KVA
Power Factor
P
P
V
I

V
I
(a) Purely Resistive Load
(b) Resistive and Reactive Load
Pav = VavIav cos()

X=Reactance
Z=Impedance
R=Resistance
Power Factor = Cos 
  45 o
If: R=X
pf = 70.7%
If: I=10,000 amps Kva=100
V=10 volts
Kw=70.7
Variables Affecting Power Factor
• Machine Design
• Single-Phase versus Three-Phase Current
• Frequency
• Reactance/Resistance of the Secondary
Loop
• Magnetic Material in the Throat
Increased Throat Length
R  2K  2L
X  KL
X
R
Increased Angle
Decreased Power Factor
Resistance Welding Manual,
RWMA, 1972
Ferrous Material in Throat
X
R
Reduced Power Factor
Resistance Welding Manual,
RWMA, 1972
Power Factor Correction Capacitors
AC Power
Supply
Contactors
(Switched Simultaneously)
High Impedance DC
Charging Circuit
(a) Shunt Capacitors
Welding
Contactor
AC Power
Supply
Normally-Closed
Contactor
Discharging
Resistor
Tuned Capacitor
(b) Series Capacitors
Duty Cycle
Percent Rated Load
is the power level at which a transformer can operate for
some fraction of time without overheating
kvamax
7.07kvarated

DutyCycle
DutyCycle 
100%
50%
Percent Duty Cycle
TimeOn
TimeOn  TimeOff
Timing Controls
At what point in time do we close the contactor
• Non-synchronous Controls
• Synchronous Controls
• Microprocessor Control = Synchronous
Synchronous Timing Accuracy
Electronic Control Res. Weld,
McGraw-Hill, 1943
Transient Currents with Non-Synchronous Timing
Electronic Control Res. Weld,
McGraw-Hill, 1943
Electronic Control Res. Weld,
McGraw-Hill, 1943
Control of Secondary Current
• Transformer Tap Setting (Turns Ratio)
• Phase Shift Heat Control
60 Cycle
AC
Gate
Cathode
Anode
Rules for SCR Conduction
• can conduct when anode is + with respect to cathode
• once initiated, can only be stopped by removal of anode voltage
• initiated by a pulse on ignitor
Welding Handbook
“Idealized” Phase Shift Heat Control
(a) Current Controlled by Tape Settings
In Actual Case,
Current can not
instantaneously
change
(b) Current Controlled by Phase Shift Setting
Actual
Condition

1
%Heat 

Firing
Angle
Hangover Angle
Related to R/L ratio

Androvich “Resistance Welding Constant Current Heat Control”,
AWS Sheet Metal ConfV, 1992
IRN = Normalized Available
RMS Current
L
  arctan
R


IRN
1
{
2
  


[sin( t  )  sin(   ) (R / L)(  /  t ) ]2  (t )} 1 / 2

Androvich “Resistance Welding Constant Current Heat Control”,
AWS Sheet Metal ConfV, 1992
Tsai, Experimental Study of Weld Nugget Expan,
Paper B1, Sheet Metal Welding Conf V, AWS, 1992
Link to:
Phase Shift Heat Control Demo
Portable Trans Guns
Transformer Integral with Gun Thus smaller in Size
Nedorezov, J “Using Portable Transguns for Resistance Spot
Welding”, Practical Welding Today, Nov-Dec 1997
Nedorezov, J “Using Portable Transguns for Resistance Spot
Welding”, Practical Welding Today, Nov-Dec 1997
Machines and Equipment
• Cooling System
• Mechanical System
– Force generation system
• Pneumatic/Hydraulic system
• Motor or foot-operated system
– Electrode application system
• Rocker arm or pinch type
• Press or direct action type
• Portable system
• Electrical System
– Single-phase system
– Single-phase rectified
system
– Three-phase rectified
system
– Frequency changer
system
– Frequency inverter
system
Single-Phase Rectified System
Center-Tapped Transformer
Bridge-Type Rectifier
Primary Current
Secondary Current
Single-Phase Rectified System
Advantages
• Higher Power Factor
Disadvantages
• Expensive
• More Uniform Heat Flow Pattern
• Less pf Loss by Material in Throat
• Need Heavy Duty Diodes
•One Electrode Wears Rapidly
• Load on one side of Three Phase Line
• Balanced 3 phase hook-up needed
A
B
C
a
b
b
c
a
Machines and Equipment
• Cooling System
• Mechanical System
– Force generation system
• Pneumatic/Hydraulic system
• Motor or foot-operated system
– Electrode application system
• Rocker arm or pinch type
• Press or direct action type
• Portable system
• Electrical System
– Single-phase system
– Single-phase rectified
system
– Three-phase rectified
system
– Frequency changer
system
– Frequency inverter
system
Three-Phase Rectified System
Half Wave
Full Wave
Primary Current
Primary Current
100% Heat
Secondary Current
10% Heat
Secondary Current
120°
*
*
*
*
Half Wave
120°
Full Wave
120°
Spinella, D., “Al RSW: Capital and Operating Costs”,
AWS Xsheet Metal Conf, 1996
Comparison of Two PF on Three Machines
Roth, “Alternating Current Vs DC in Resistance
Welding” AWS Sheet Metal Conf IV, 1990
High Ripple
Medium Ripple
Low Ripple
Comparison of Two PF on Three Machines
• Both Single Phase and 3 Phase DC is best in low
power factor (high inductive) circuits providing as
much smoothing as possible
• Phase shifting will increase ripple (more in single
phase). Operate on the highest phase shift and lowest
tap setting possible
Advantages
• High Power Factor
• Uniform Heat Flow
• No need to balance hook-up
• No pf loss by material in throat
Disadvantages
• Expensive
• Need Heavy Duty Diodes
• One Electrode Wears
Machines and Equipment
• Cooling System
• Mechanical System
– Force generation system
• Pneumatic/Hydraulic system
• Motor or foot-operated system
– Electrode application system
• Rocker arm or pinch type
• Press or direct action type
• Portable system
• Electrical System
– Single-phase system
– Single-phase rectified
system
– Three-phase rectified
system
– Frequency changer
system
– Frequency inverter
system
Frequency Converter
1
Controller A
2
Controller B
3
Controller C
Frequency Changer System
(a) Frequency-Changer
(b) Rectified
Advantages
• Power Factor > 98%
Disadvantages
• Cost
• Balanced Loading
• Balanced Electrode Wear
• Control Problems
Machines and Equipment
• Cooling System
• Mechanical System
– Force generation system
• Pneumatic/Hydraulic system
• Motor or foot-operated system
– Electrode application system
• Rocker arm or pinch type
• Press or direct action type
• Portable system
• Electrical System
– Single-phase system
– Single-phase rectified
system
– Three-phase rectified
system
– Frequency changer
system
– Frequency inverter
system
Frequency Inverter System
Oscillator
Rectifier
AC
60 Hz
DC
AC
HF
400, 600, or
1200 Hz
Advantages
• High Power Factor > 98%
• Less Core Material
• Lighter & Smaller –
suitable for robots
Disadvantages
• Cost
• Newer Process – Still
Under Development
Duty Cycle