Omega Products SCR19 Installation Manual

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SCR POWER CONTROLLERS
TABLE OF CONTENTS

General Description

and Specifications .................1

Firing Modes .......................2

Installation and Wiring ...............4

Operation .........................9

Troubleshooting....................15

Parts Lists and

Ordering Codes ..................18

1

OPERATING INSTRUCTIONS

Series 19 and 39 SCR Power Controllers
Series 91 and 93 SCR Power Controllers

Section 1. General Description

Introduction
SCR Power Controllers are designed to regulate ac power to electrical

heating processes, such as ovens, furnaces, heat sealers, etc. (Note:
They are not designed to drive transformers, coils or other inductive­type loads.)

The controller accepts an input signal, such as 4-20 mAdc from some
signal conditioning device, e.g., a temperature controller. For most
processes, the combination of a temperature controller and SCR
power controller will provide very accurate, automatic temperature
control. For manual operation, a manual control option with a remote
potentiometer is available.

General Specifications

Inputs: 4-20 mAdc standard, or as ordered (see

serial number) minimum voltage
requirements 10 Vdc; all inputs
electrically isolated via optical coupling

Supply Voltage: 110/120; 208/240; 440/480, 575/600

Vac, or as ordered (Phase connection
not critical on 3-phase units)

Frequency: 50/60 Hz
Ambient Temperature: 30

o

to 122oF for listed power ratings
Cooling: Convection
Protection: Sub-cycle, current-limiting fuse; transient

voltage suppresion

Load Resistive, 1- or 3-phase - 3-wire Wye or Delta

All specifications subject to change.

2

0.2 Second
Time Base

On

Off

Figure 1. Time-proportioned, zero-cross burst.

Section 1. Firing Modes

Zero-Crossing Control

A zero-crossing switched (zer-switched or burst fired) SCR
power controller works by triggering at the moment when the
value of the ac sine wave is at the baseline or “zero” voltage
point (Figure 1.) This results in a “burst” of full line voltage with
no RFI. SCR power controllers utilize a patented trigger
circuit that turns on the SCRs as close as possible to the ac
zero voltage point. Proportioning action is obtained by varying
the number of cycles on to the number of cycles off. The
output will vary from a few cycles on and a large number of
cycles off at low input, through halh the cycles on and half off
at half input, to all cycles on at maximum input. This output
is integrated by the heaters which produce a smoothly
proportioning heat output that varies directly with the input
signal.

Phase-Angle Control

A phase-angle type SCR power controller works by delaying
the trigger pulse to some point in the half cycle of the ac
wave. This trigger point, from 0 to 180 degrees, is referred to
as the phase angle (Figure 2). The SCR will turn on when
triggerred, and remain on for the rest of the half cycle.
Increasing the control signal will cause the trigger pulse to
occur earlier in the half cycle, thus delivering a greater portion
of the wave to the load.

Because it provides an extremely fast response, phase-angle
control should be used in low-mass element applications that
require high switching speeds, such as tungsten elements,
quartz lamps, hot wires and other loads subject to high inrush
currents. (Note: Some RFI can be generated from the phase­angle controller) SCR phase-angle power controllers are avail­able with a soft-start timing option that provides a ramp to
peak voltage, and are available with a voltage limit option that
“clamps” output voltage to a level lower than the supply
voltage.

3

Figure 2. Phase-angle changes and their effect on output power.

Phase angle = 113°, 293°

Power = 25%

0° 360°

180 °

11 3 °

293°

4

Section 3. Installation

3.1 Mounting

Mount the controller, with the heat sinks in a vertical position,
in a reasonably cool location -- 50

o

C (122oF) maximum.
Some space should be left above and below the unit to allow
for air circulation. If the controller must be placed in an

environment where the ambient temperature exceeds 50oC
(122

o

F), it will be necessary to derate the unit. If derating is
not possible, venting or an exhaust fan must be used to keep
ambient temperatures at an acceptable level. (See Figure 3
for cooling calculations).

Formula for minimum metal enclosure size

for convection cooling

.72 x AMPS x # of Controlled Legs Min. Exposed

=

122

o

F - Ambient oF Sq. Ft.

.40 x AMPS x # of Controlled Legs Min. Exposed

=

50oC - Ambient oC Sq. Ft.

Formula for forced air cooling

2.2 x AMPS x # of Controlled Legs

= Min. CFM

50

o

C - Ambient oC

3.8 x AMPS x # of Controlled Legs

= Min. CFM

122oF - Ambient oF

Figure 3. Calculations for determining cooling requirements

3.2 Wiring

The wiring components of SCR power controllers consists of
line voltage, heater, load, and signal input. Follow the wiring
diagrams on the following pages (Figures 4 and 5) and the
terminal labels on the unit. On three-phase controllers (Series

39), it is not necessary to connect the phases to any
particular terminal. Because the controllers are
phase-to-phase controllers, either Wye or Delta connected
loads may be used.

Wire gauge for power and load connections will vary depend­ing on the size of the load. Standard electrical code
procedures should be followed. Do not exceed the voltage
and ampere ratings indicated on the controller’s label. Before
connecting the controller to a heater, we recommend that the
heater be connected directly to the power line to ensure that
the current rating is correct and that no shorts exist.

5

CAUTION

On Wye connected loads, do not connect the

center terminal to the line or to the ground.

6

CAUTION:

Possible Shock Hazard -- Exposed high voltage exists on heat

sinks and other parts of these units.
To prevent possible electrocution, the
controller must be locked in a secure
enclosure during operation. Solid
state devices do not completely
remove power from the load, even in
the OFF state. This leakage current
presents a potential shock hazard at
all unit and load terminals. All power
must be completely off before servicing

.
Only qualified personnel should be
allowed access.

Possible Fire Hazard -- Because SCR power controls and

associated equipment are not fail-safe
devices, an approved temperature
and/or pressure safety control should
be used to ensure safe operation.