Why the sensor choice matters

Servo motor temperature sensing protects windings from overheating, enables derating strategies, and feeds fault logic in the drive. The two dominant sensor types in industrial servos are PT100 RTDs and thermistors (PTC or NTC). The choice affects accuracy, wiring, drive compatibility, and total cost. This article gives you the criteria to specify and source correctly.

PT100: precision and linearity

A PT100 is a platinum resistance temperature detector with 100 Ω at 0 °C. Its resistance rises nearly linearly with temperature (≈0.385 Ω/°C). Key characteristics:

• Accuracy: Tight tolerances available (Class A, Class B). Typical error ±0.15 °C to ±0.3 °C at 0 °C, depending on class.
• Range: Suitable for −50 °C to +200 °C and beyond; well matched to servo winding hotspots.
• Linearity: Near-linear output simplifies drive firmware; no lookup tables needed for basic protection.
• Interchangeability: Standardized curve (IEC 60751) means sensors from different suppliers are drop-in compatible.
• Signal conditioning: Requires excitation current and amplification; 2-, 3-, or 4-wire configurations. 3-wire is common in motors to compensate lead resistance; 4-wire gives the highest accuracy.

PT100s suit applications where you need reliable absolute temperature readings, precise derating, or compliance with customer specifications that mandate RTDs.

Thermistors: fast, simple, low-cost

Thermistors are semiconductor resistors with a large resistance change over temperature. Two families dominate:

• PTC: Resistance rises sharply at a threshold (e.g., 120–155 °C). Used as a binary over-temperature switch. Simple, robust, and cheap; no calibration needed. Drive detects a step change and trips.
• NTC: Resistance falls with temperature. Higher sensitivity than PT100, but highly non-linear. Requires a lookup table or Steinhart-Hart equation in firmware. Tolerance and drift are generally worse than PT100; interchangeability is limited unless you specify a tight curve.

Thermistors are smaller, faster responding (lower thermal mass), and lower cost. They are ideal for compact motors and high-volume applications where a simple trip point is sufficient.

Selection criteria for servo applications

Match the sensor to the drive’s input capability and the protection strategy:

• Drive compatibility: Check the drive’s temperature input. Many servo drives accept PT100/PT1000 directly or via option cards; others expect a PTC switch or NTC. Retrofitting a different sensor type may require signal conditioning or firmware changes.
• Protection vs. measurement: If you only need over-temperature trip, a PTC is sufficient and cheapest. If you need continuous temperature for derating, fan control, or predictive maintenance, choose PT100 (or PT1000 for lower lead-resistance sensitivity).
• Accuracy and tolerance: Specify sensor class and tolerance. For PT100, state Class A or B per IEC 60751. For NTC, specify resistance at 25 °C and beta value with tolerance; for PTC, specify trip temperature and tolerance.
• Response time: Smaller sensor bodies respond faster. If the winding hotspot is critical, specify sensor placement (embedded in winding vs. end-winding) and thermal coupling.
• Wiring and insulation: Motor environments are noisy and hot. Specify lead wire insulation (e.g., PTFE, silicone) rated above maximum winding temperature, and shielding/twisted pair for signal integrity. For PT100, 3-wire is a practical compromise; 4-wire if accuracy is critical.
• Mechanical integration: Sensor must fit the slot or end-winding without damaging insulation. Specify lead length, connector type, and strain relief. For high-volume orders, confirm the sensor is potted or encapsulated for vibration resistance.

What to specify before ordering

Provide the motor manufacturer or sensor supplier with a clear specification:

• Sensor type and standard (PT100 Class A per IEC 60751; PTC trip point; NTC R25 and beta).
• Operating temperature range and maximum exposure temperature.
• Wiring configuration (2/3/4-wire for RTD) and lead length.
• Connector type and pinout; insulation material and voltage rating.
• Mechanical dimensions, mounting method, and encapsulation requirements.
• Test and acceptance criteria: resistance at reference temperature, insulation resistance, and hi-pot test.
• Traceability and batch consistency requirements for volume production.

Cost considerations

Unit cost differences are small at the component level but can be significant in total system cost:

• Sensor cost: PTC is typically the lowest; NTC slightly higher; PT100 highest, especially tight-tolerance or 4-wire versions.
• Signal conditioning: PT100 requires excitation and amplification, adding cost to the drive or an interface board. PTC needs only a comparator; NTC needs an ADC and firmware.
• Assembly and wiring: 3- or 4-wire RTD adds lead cost and assembly time. PTC with two leads is simplest.
• Inventory and interchangeability: Standardized PT100 curves reduce spare-part complexity. Custom NTC curves lock you into one supplier.
• Warranty and failure risk: A robust PTC may be sufficient for basic protection; over-specifying PT100 where not needed adds cost without benefit. Conversely, under-specifying can lead to nuisance trips or missed overheating.

Sourcing tips from Chinese factories

China supplies a large share of motor temperature sensors. To get consistent quality at volume:

• Request the curve: For PT100, ask for calibration data or tolerance class certificate. For NTC, request R-T table and beta tolerance. For PTC, request trip-point distribution.
• Verify insulation and lead materials: Cheap silicone may not meet temperature class. Ask for UL or equivalent recognition if required.
• Sample and test: Order samples and perform your own resistance check at 0 °C and 100 °C (ice bath and boiling water) or with a dry-block calibrator. Check insulation resistance after thermal cycling.
• Audit the process: For volume, confirm the factory has calibration equipment and batch traceability. Ask for PPAP or first-article inspection reports.
• Clarify lead time and MOQ: Custom lead lengths and connectors often carry MOQs. Standard PT100 sensors may be available from stock; custom assemblies take longer.
• Plan for second source: Standardized PT100 and common PTC types are easier to dual-source. Avoid proprietary NTC curves unless necessary.

Bottom line

Choose PT100 when you need accurate, linear, interchangeable temperature measurement for derating or diagnostics, and your drive supports RTD input. Choose a PTC thermistor for simple, low-cost over-temperature protection with minimal signal conditioning. NTC sits in between but demands firmware support and tighter supplier control. Specify the sensor type, tolerance, wiring, and test criteria explicitly, and validate samples from your Chinese supplier before committing to volume. The right choice is the one that matches your drive’s input, your protection strategy, and your total cost target—not simply the lowest sensor price.