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HUBBELL
Major Components 23
Silicon Controlled Rectifiers – SCRs
Thyristor
Power
Module
Protection
Thyristor power modules
(TPM’s) are SCR/SCR,
full wave assemblies
available in several
horsepower ratings from
10–500 hp in ambients
up to 140°F (60°C).
Each assembly is config-
ured as an inverse paral-
lel bridge with snubbing
circuit and voltage tran-
sient protection as shown
in the illustration. The
TPM’s are the units that
control the applied motor
voltage on primary thyris-
tor controls.
The snubbing circuit,
comprised of a capacitor
and a resistor, is con-
nected in parallel with
the SCR’s across the
bridge. The function of
the snubbing circuit is to
limit the rate of rise of ap-
plied voltage. Transient
voltage protection is pro-
vided by a metal oxide
varistor, MOV, which is
also connected in paral-
lel across the bridge. The
MOV is a nonlinear resis-
tive device that will limit
or clamp the transient
voltage to a safe level.
The snubbing circuit in
conjunction with the
MOV provides complete
transient voltage protec-
tion for the TPM’s.
Major Components
Compact SCR
Bridge Assembly
for 40 hp (460V)
or Less
Thyristor Power Module Protection
SCR 2
MOV
R
Snubber Circuit
C
G2
K2
SCR 1
G1
K1
Line Load
External SCR
Bridge Assemblies
for 50 hp (460V)
or Greater