SCR Control for Molybdenum Disilicide (MoSi2) Heating Elements

Molybdenum disilicide (MoSi2) heating elements, such as Kanthal Super, are widely used in high-temperature industrial furnaces operating up to 1800°C. However, controlling the power delivered to these elements requires specific types of Silicon Controlled Rectifier (SCR) or thyristor power controllers due to the unique electrical characteristics of MoSi2.

 The Challenge: MoSi2 Resistance Characteristics

The primary challenge in controlling MoSi2 heating elements is their extreme Positive Temperature Coefficient (PTC) of resistance.

 Unlike standard resistance wires (like Nichrome) which maintain a relatively constant resistance, MoSi2 has an extremely low resistance when cold—acting almost like a short circuit. As the element heats up, its resistance increases dramatically, often by a factor of 10 to 16 times its cold resistance

 If full voltage is applied to a cold MoSi2 element, it will draw a massive inrush current (up to 15 or 16 times the nominal operating current). This massive current surge can easily blow fuses, trip circuit breakers, saturate coupling transformers, or destroy the SCR controller itself 

 Required SCR Controller Specifications

To safely and effectively control MoSi2 heating elements, the SCR power controller must be equipped with specific firing modes and protective features. Simple on/off or zero-cross firing controllers are completely unsuitable and will fail or cause system damage.

The SCR controller must possess the following capabilities:

 1. Phase Angle Firing

The controller must support Phase Angle Firing (also known as phase-angle control). In this mode, the SCR turns on for only a portion of each AC half-cycle. By varying the firing angle, the controller smoothly regulates the RMS voltage and current applied to the load. This is essential for gradually applying power to the low-resistance cold element . Zero-cross firing cannot be used during cold start because it applies full line voltage for complete AC cycles, causing immediate overcurrent .

 2. Soft Start

A Soft Start feature is mandatory. Soft start automatically restricts the firing angle when power is first applied, then gradually advances it over a set duration (typically a minimum of 2 to 3 seconds). This gradual ramping of voltage prevents massive inrush currents and protects both the heating element and the power transformer from saturation

 3. Current Limiting

Because the cold resistance is so low, soft start alone is not always sufficient. The SCR must have an active Current Limit function. The controller continuously monitors the RMS current and automatically throttles back the phase angle to keep the current below a safe, pre-set maximum limit, regardless of the temperature controller's power demand. The controller stays in current limit mode until the element heats up and its natural resistance rises enough to limit the current on its own.

 4. Transfer to Delayed Triggering / Burst Firing (Optional but Recommended)

Phase angle firing produces a poor power factor and generates significant electrical harmonics. Advanced modern SCR controllers (such as the Eurotherm EPower or Watlow Power Series) offer a hybrid or "Transfer Mode" .

• During cold start, the SCR uses Phase Angle Firing with Current Limit.

• Once the element reaches operating temperature and its resistance stabilizes, the controller automatically switches to Zero-Cross / Burst Firing (or delayed triggering if using a transformer). This drastically improves the power factor and reduces harmonic distortion during the long steady-state soaking periods.

 Transformer Considerations

MoSi2 elements typically operate at very low voltages and high currents, so they are almost always driven via a step-down isolation transformer.

 When an SCR is driving the primary side of a transformer:

•         The SCR must be designed to handle inductive loads.

•         The soft start and delayed triggering functions are critical to prevent the transformer core from saturating, which would otherwise cause massive current spikes and blown fuses [1] [2].

 Summary of Recommendations

When selecting an SCR power controller for MoSi2 heating elements, you must specify a unit that features:

•Phase Angle Firing capability

•Soft Start (adjustable, minimum 2-3 seconds)

•Current Limiting (adjustable)

•Support for Inductive/Transformer Loads

• Ideally, an Automatic Transfer Mode (from phase angle to zero-cross/burst firing) for better energy efficiency and power factor.

Wiring Diagram