Watlow Power Series
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Benefits
Key benefits of the Power Series
Save money with SCR control

More information
Specification sheet (PDF)
User's manual (PDF)
Ordering information
Power controller case histories
SCR Power Controls brochure (PDF)
SCR Power Control education (PDF)
PowerPoint® library
Video clip collection
Watlow web site

Go directly to information about...
Agency approvals
Alarms
Amperage ratings
Communications
Current limiting
Field-selectable input
Hardware overview
Heater bakeout
Heater diagnostics option
Heater tolerance detection
Installation features
Maximum rate of change
Operator interface
Ordering information
Phase angle control
Phase configurations
Retransmit
SCR control
Selection chart, control methods
Soft start
Voltage compensation
Zero cross control

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Watch a video clip of soft start on an oscilloscopeSoft start

Soft start protects the power controller and heater from being damaged by high inrush current during startup. Soft start is required for variable-resistance heater elements such as:

Tungsten
Molybdenum
Silicon carbide (e.g., Globar®)
Carbon graphite

Soft start is available for single-phase and three-phase, three-leg models only, and requires phase angle control.

Why is soft start required?
How does soft start work?

Watch soft start on an oscilloscope


Why is soft start required?

Some heater elements show large resistance-value changes with temperature (i.e., a high hot-to-cold ratio). If full power is applied when the element is cold, there will be an inrush of high current, which will blow semiconductor fuses and degrade the heater.

For example, suppose that we have a 230V tungsten heater. At operating temperature, the resistance of the heater element is 2.3 ohms. Ohm's Law tells us that Current = Voltage/Resistance, therefore the current through the element is 100 Amps (230/2.3).

When the heater is shut off, the resistance of the element drops to 1/15 its hot value, or 0.153 ohms. If full power is applied, the current will be about 1500 Amps (230/0.153).

Soft start prevents this large inrush current, which lengthens heater life and prevents blown semiconductor fuses.

Note: Some heater elements also require current limiting.

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How does soft start work?

Soft start allows the heater element to warm up before full load voltage is applied to the heater.

Soft start allows only a small amount of current through the heater. The current is gradually increased until the requested power level is achieved.

Soft-start waveform

You can field-select a soft-start duration between 0.1 and 120 seconds.

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Watch soft start on an oscilloscope

Watch a video clip of soft start on an oscilloscopeWatch a video clip of soft start on an oscilloscope. The video shows two two-second soft starts.

Note: If you cannot hear the sound, you may skip the first 19 seconds of this video.

Click here if the video will not play or if you cannot hear the sound.

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