1. GP Technologies Ltd.
© 2016
Daniel Lang, P.Eng. dlang@gptechnologies.com
GP Technologies Ltd. 200 9016 51 Ave. Edmonton, AB
GP TECH
Power System Experts
Acronyms:
1. SCR – Silicon Controlled Rectifier
2. GCT – Gate Commutated Thyristor
3. SGCT – Symmetrical Gate Commutated
Thyristor
4. GTO – Gate Turn-Off Thyristor
5. IGBT – Insulated Gate Bipolar Transistor
6. IGCT – Insulated Gate Commutated Thyristor
7. IEGT – Injection Enhanced Gate Transistor
Current Source Inverter (CSI)
A current source inverter uses SCRs, GCTs, or
SGCTs to rectify the input voltage (known as an
active front end). A large inductor forms the DC link,
which stores energy and maintains constant current
for the inverter section. The inverter section uses
GTOs or SGCTs to create the pulse width modulated
output signal.
Voltage Source Inverter (VSI)
A voltage source inverter uses diodes to rectify the
input voltage. A large capacitor forms the DC link,
which stores energy and maintains constant voltage
for the inverter section. The inverter section uses
IGBTs, IGCTs, or IEGTs to create the pulse width
modulated output signal.
Dynamic Response
Def.: system response to change in operating point
from one steady state condition to another, following
a change in system variables (load torque, voltage,
current, etc.)
 Dynamic response of a drive system is limited by
mechanical inertia and electrical inductance.
 Time rate of change of current proportional to
size of inductor
 CSI design includes large inductors for energy
storage and signal conditioning, resulting in poor
dynamic response.
 VSI utilize a DC bus capacitor and are able to
provide instantaneous current, vastly improving
drive dynamic response.
Power Quality
 VSI utilizes a 24 or 36 pulse phase shifting
transformer, which produces very little
harmonics and operates at high power factor.
 The active front end of a CSI produces
significant source harmonics and results in a
large displacement power factor when operating
at low torque or speed.
Common Mode Voltage
Def.: voltage signal offset that is “common” to
multiple output nodes.
 Common mode voltage can occur from
ungrounded delta or wye sources, resulting in the
neutral voltage drifting away from zero. This
results in higher line-neutral voltages and thus
stress on insulation.
 CSI transfers common mode voltage to the
motor due to lack of ground reference in inverter
stack.
 VSI design utilizes isolation transformers and a
clamped neutral point, resulting in no common
mode voltages.
CSI VSI
Inverter is fed from a
constant DC current
source
Inverter is fed from a
constant DC voltage
source
Output voltage is
dependent on load
impedance
Output current is
dependent on load
impedance
Output current is
independent of load
impedance
Output voltage is
independent of load
impedance
Produces large input and
output harmonics
Produces low input and
output harmonics
Variable Frequency Drive Topologies:
Current Source vs. Voltage Source