How to Specify a PTFE Heater for Use with a Variable Frequency Drive or SCR Power Controller

Process heating is increasingly being accomplished with solid state devices, such as SCR power controllers or variable frequency drives (VFDs) for motor driven pumps. PTFE heaters are really simple resistive loads but how they interact with non-sinusoidal waveforms from these devices is something to consider. If not managed effectively, the properties of these waveforms, such as harmonics and voltage spikes, might damage the heater's performance and lifespan.

Basics of SCR and VFD Power Control
SCR controllers are used extensively to control the power delivered to resistive heating elements. They work by phase angle firing which cuts the A.C. waveform to alter the power going to the heater. This method creates voltage spikes and high frequency harmonics that can stress the heater insulation and produce problems such as increased leakage current and insulation breakage.

The problem with VFDs (widely used to regulate a motor driven pump) is the electrical noise they add to the grounding system. This noise can interfere with temperature sensors and GFCI circuits that are important to the safe operation of the heating system.

SCR VFD Compatible PTFE Heater
In installations with SCR control the non-sinusoidal waveform of the phase-angle firing may induce voltage spikes over the nominal line voltage. The PTFE heater sheath has a high dielectric strength (usually > = 15 kV/mm); nonetheless, voltage spikes must be carefully considered for their effect on the heater's insulation. In case of long cable lengths the effect of the reflected wave is increased and higher stress is applied to the insulation.

The compatibility evaluation with PTFE heater SCR VFDs includes a study of the insulation class to ensure the heater can withstand the peak voltages generated by SCR controllers. The terminal insulation and the internal connections also need to be qualified to tolerate these transient voltage peaks.

The big problem with VFD applications is the electrical noise that the drive can inject into the system which can cause inconsistent behaviour of associated components, notably temperature sensors. The VFD's noise can also interfere with GFCI protection circuits, leading to false trips or inconsistencies in temperature control. To combat these dangers it is important to properly insulate, filter and ground the heater and its accompanying control components.

Important Heater Specification Considerations for SCR Control
When selecting a PTFE heater for SCR service, there are numerous critical considerations that must be handled to guarantee reliable and safe operation:

Insulation Class. Compare the heater insulation rating with the peak voltages produced by phase angle fired SCRs. PTFE insulation is normally quite rugged, however the particular voltage and frequency ranges should be considered throughout the choosing process.

• Dielectric Strength Check that the dielectric strength of the PTFE sheath is sufficient to withstand the voltage spikes, typically 15 kV/mm. This is an important consideration since the peak voltage from SCRs can be higher than the nominal line voltage.

Terminal Insulation: The terminal connections and any internal wiring should be rated for the anticipated peak voltage and frequency. The connections have to survive the fast voltage swings without breaking down.

Cable Length: Long cable lines increase the likelihood of reflected waves and higher insulation stress. When long cables are utilised, it may be essential to include a load reactor or other voltage-smoothing devices to reduce the effects of the voltage spikes.

Grounding and Shielding Proper shielding and grounding are important in VFD applications so that electrical noise does not have a negative effect on the heater and control system. A strong grounding system keeps noise from degrading the performance of temperature sensors and GFCI circuits.

Decision Guidance: SCR Control Heater Specification Checklist
If you are specifying a PTFE heater to be used with an SCR power controller, the following checklist can assist you assure compatibility:

* Check PTFE heater insulation class for SCR output voltage spikes.

Terminal insulation and internal connections have to tolerate maximum voltages and frequency fluctuations.

For lengthy runs of cable, a load reactor or other protective measures should be considered to limit the reflected wave effects.

PTFE sheath dielectric strength should be ≥15 kV/mm to provide effective protection against voltage spikes.

-- Properly grounded and shielded to minimise electrical noise generated by the VFD and prevent interference with sensors and safety circuits.

Choose filtering devices for mitigating noise in the power system and control circuits, particularly in VFD applications.

Conclusion.
The application of PTFE heaters with current power control devices such as SCR controllers and VFDs is highly desirable but specification must be considered carefully in order to avoid operational problems. Attention to insulation class, terminal insulation and voltage handling capability helps reduce the dangers from high frequency harmonics, voltage spikes and electrical noise. System-wide measures such as grounding, shielding and noise filtering are also included to ensure that the heater and accompanying controls operate reliably in difficult settings. At the end of the day , the entire electrical system from the power source to the heater must be designed and specified together for long-term compatibility and performance .