What this page solves

This page is a planning space for thermal sensing and control in servo drives and inverters. It clarifies where temperature must be monitored, which sensor type is suitable at each point, how the analog front-end should be built, and which IC roles fit NTC, RTD and on-module sensors.

The content helps structure a complete path from temperature sensors into AFEs and monitors, then on to fan or pump controllers, protection logic and data logging. Instead of treating thermal design as an afterthought, the page turns it into a repeatable checklist that can be reused across different motor and motion platforms.

Along the way the text keeps a clear separation of responsibilities: this page focuses on thermal sensing, thermal AFEs and cooling actuators. Short hooks are provided toward over-current and over-voltage protection, STO, soft-stop sequences and predictive maintenance, but the detailed implementations stay on their own dedicated pages.

Where temperature sensing sits in a motor drive

A motor drive turns AC input into a controlled DC link and then into three-phase power for the motor. Along this chain several blocks concentrate losses and become thermal risk points: the power module and DC-link capacitors, the heatsink or cold plate, the motor itself, and hot spots on the control and power boards. Thermal sensing is the layer that observes these points so that protection, derating and cooling decisions are based on real data instead of guesswork.

In a typical servo or inverter system the most critical measurement is close to the semiconductor junction in the power module. Additional sensors on the heatsink or cold plate show whether the cooling system is keeping up, while sensors in or on the motor track stator and bearing temperature. Board-level sensors around gate drivers, eFuses and front-end power supplies complete the picture by catching local hot spots that could shorten lifetime.

Each of these locations lends itself to a different sensor technology. Module and heatsink points are usually served by NTC sensors or module-integrated temperature pins, motor windings often use RTDs for higher absolute accuracy, and board hot spots can be covered by small NTCs or digital temperature ICs. The rest of this page builds on this map and turns it into concrete AFE choices and IC roles.

• Power module & DC-link: monitors junction and capacitor temperature to prevent over-stress and lifetime loss.
• Heatsink / cold plate: tracks the cooling path and provides a reference for fan or pump speed control.
• Motor stator / bearing / housing: limits continuous torque and protects insulation and mechanical components.
• Control and power board hot spots: guards gate drivers, eFuses and auxiliary supplies against local overheating.

NTC & on-module sensing AFEs

NTC sensors and module-integrated temperature pins are the most common way to observe thermal stress in motor and motion drives. Typical locations include the power module baseplate, bolt-on sensors on the heatsink or cold plate, and NTCs placed next to DC-link capacitors and power board hot spots. This section focuses on how these sensors are wired into analog front-ends and monitors that convert resistance into usable voltages, alarms and data for the control system.

For many cost-sensitive designs a simple resistor divider into an ADC channel already provides adequate protection. Higher power levels and tighter derating requirements often justify dedicated NTC AFEs and temperature monitor ICs with precision references, built-in linearisation and programmable thresholds. The choice between these options depends on the number of channels, allowed error budget, required response time and whether hard wired over-temperature signals are needed.

Typical NTC and temp-pin use cases

• Power module / IPM / SiC temp pin: tracks junction temperature by using the module’s internal NTC or temperature pin as the main proxy for derating and shutdown.
• Bolt-on NTC on heatsink or cold plate: measures cooling performance and provides a stable reference for fan or pump speed control.
• NTC near DC-link capacitors: monitors capacitor can temperature to control ripple current, lifetime and maintenance intervals.
• Board-level NTCs: placed near gate drivers, eFuses and front-end power supplies to catch local hot spots and guide layout or cooling changes.

Simple divider vs dedicated NTC front-end

The simplest NTC interface is a resistor divider from a stable reference or supply into an ADC input. Divider values are chosen so that the voltage swing is most sensitive in the temperature band of interest, while still keeping self-heating and current consumption under control. This approach fits non-critical locations where moderate error is acceptable and where the MCU can handle resistance-to-temperature conversion.

Dedicated NTC AFEs and temperature monitor ICs add value when the number of channels grows or when thresholds must be enforced in hardware. These devices integrate precision references, configurable excitation, linearisation networks and digital interfaces. Many variants include programmable over-temperature and under-temperature comparators that drive open-drain alarm pins wired into gate drivers, eFuses or PMIC fault inputs, reducing software latency for critical events.

Multi-channel architectures and MUX trade-offs

Several NTCs can share one ADC by using an analog multiplexer, or each sensor can be assigned its own front-end channel. A MUX-based architecture reduces cost and pin count but introduces channel settling time, crosstalk risk when resistors are shared, and limited time resolution for fast transients. Independent channels provide better isolation and consistent timing at the expense of silicon area and external components.

Channel architecture is usually aligned with the criticality of each measurement. Module temp pins, heatsink NTCs and DC-link sensors are strong candidates for dedicated channels in a multi-channel monitor IC, while cabinet air temperature or secondary board hot spots can share a multiplexed path. The ADC architecture itself is covered on current sensing or MCU focused pages; this section concentrates on the resistor network and channel topology.

Common-mode range, noise and isolation

NTC and temp-pin signals live on local references that may be close to the high-voltage domain. Front-end inputs must tolerate the actual common-mode voltage and remain within absolute maximum ratings over surge and fault conditions. Long sensor leads across the inverter or cabinet pick up mains frequency and switching noise, so simple RC filters near the sensor and at the AFE input are standard practice.

Isolation strategy is another key decision. When the power module sits close to the control board and creepage constraints are manageable, NTC leads can often be routed back to a low-voltage AFE. For remote modules or distributed cabinets it is frequently safer to place the AFE on the high-voltage side and send a digitised or isolated signal across the barrier. This avoids long, noise-sensitive analog runs while still giving the controller precise temperature information.

IC roles and mapping checklist

• Multi-channel temperature monitor ICs with several NTC inputs, digital readout and programmable thresholds.
• Module monitor ICs that combine NTC sensing with DC-link and fault handling functions.
• Over-temperature switch devices that expose a dedicated alarm output for direct wiring into gate driver, eFuse or PMIC fault inputs.
• System monitors and PMICs that include one or more NTC channels alongside voltage and watchdog supervision.

RTD & precision thermal AFEs

RTD-based temperature sensing is reserved for locations where absolute accuracy, stability and lifetime tracking are critical. Typical examples include motor stator windings in high-end servo and traction drives, and bearings or gearboxes in heavily loaded mechanical systems. These sensors enable precise torque limits, insulation protection and early fault detection that go beyond what simple NTCs can provide.

A precision RTD chain relies on controlled excitation, low-noise differential measurement, careful wiring and a clear error budget. The analog front-end often uses a constant-current source and a precision reference resistor, followed by a differential amplifier and a high resolution ADC or integrated RTD front-end. Three-wire and four-wire configurations are used to cancel line resistance and maintain accuracy over long cable runs.

Where RTDs make sense in motor drives

• Motor stator windings: protect insulation, enforce continuous torque ratings and feed thermal models that predict allowable overload.
• Bearings and gearboxes: reveal lubrication loss and mechanical wear through slow but consistent temperature rise, often before vibration limits are reached.
• Large frames and critical power modules: provide a stable absolute temperature reference for long-term health monitoring and trending.

Classic RTD front-end architectures

The most common RTD AFE uses a precision constant-current source that flows through a reference resistor and the RTD element. The resulting voltage across the RTD is then measured with a differential input. This topology converts resistance into voltage while keeping the transfer function predictable and directly related to the accuracy and drift of the reference resistor and current source.

Dedicated RTD front-end ICs integrate current sources, programmable gain amplifiers, channel multiplexers and diagnostics. These parts reduce design effort for three-wire and four-wire connections and provide consistent behaviour across multiple axes or drive sizes. In other designs a precision ADC with built-in excitation and sensor-bias features can implement the RTD measurement, sharing resources with voltage and current sensing.

Three-wire and four-wire compensation

Two-wire RTD connections are affected directly by line resistance, making them suitable only for short runs and relaxed accuracy targets. Three-wire schemes use matched leads and a specific bridge or measurement configuration so that most of the line resistance cancels out. This is the standard compromise in industrial drives where cable lengths are moderate and installation cost must be controlled.

Four-wire RTD connections route separate pairs for excitation and sensing. The sense pair carries almost no current, so voltage drop along the leads is negligible and line resistance has minimal impact on accuracy. This configuration is well suited to long cable runs and demanding applications such as traction motors, high duty-cycle servo axes and test benches. Many RTD AFEs provide dedicated three-wire and four-wire modes to support these layouts.

Error sources and budgeting

A realistic RTD error budget covers several contributions: line resistance, reference resistor tolerance and temperature coefficient, amplifier gain and offset error, ADC non-linearity and noise, and long-term drift of the sensing elements. Each term consumes part of the allowed temperature error, for example ±2 °C across the operating range. Selecting a front-end without considering this budget often leads to either unnecessary cost or insufficient accuracy in the field.

Reference resistor specifications are especially important because they scale the entire transfer function. Amplifier and ADC parameters define how much additional error is added on top of the ideal RTD curve. Cable design, shielding and filtering determine how much real-world noise couples into the measurement. When all contributions are listed and summed, it becomes clear whether a simple ADC channel is adequate or whether a specialised RTD front-end is justified.

IC roles: RTD front-ends and mixed-signal monitors

• RTD front-end ICs with integrated current sources, three-wire and four-wire support, diagnostics and digital interfaces.
• High resolution ADCs with built-in sensor excitation and programmable gain for direct RTD measurement.
• Mixed-signal monitor ICs that combine RTD, voltage and current channels for tightly correlated thermal and electrical data.

Fan & pump controllers, thermal control loops

Fan and pump controllers translate measured temperature into cooling action. In motor and motion drives this usually means modulating several cabinet and module fans plus one or more liquid cooling pumps. The focus of this section is the IC roles and signal paths: how temperature channels feed into fan controller and pump driver devices, how tachometer and fault feedback are handled, and how thermal control loops link back to the main controller.

Architectures range from single-channel fan drivers with simple on/off thresholds to multi-channel controllers with PWM outputs, tach inputs and I²C or PMBus interfaces. Liquid cooling adds pump power stages, speed control and flow or pressure diagnostics. Control strategies are usually organised into discrete levels such as on/off, multi-step curves or host-defined profiles written over a digital bus.

Single and multi-channel fan controllers

Modern drives frequently use several cooling zones. One or more fans concentrate on the power module and cold plate, while additional fans move air through the cabinet or dedicated compartments. Fan controllers with multiple PWM outputs and tach inputs allow each zone to be mapped to its dominant temperature sensor while still sharing configuration and monitoring logic across channels.

PWM fans with four-wire interfaces are preferred where low noise and efficient low-speed operation matter. The controller drives a logic-level PWM input while the fan contains its own commutation and speed control circuitry. Two- or three-wire DC fans are still common in cost-sensitive systems, where speed is controlled by varying supply voltage or duty cycle at the power stage. In both cases tachometer inputs enable closed-loop speed control, detection of stalled or disconnected fans and reporting of cooling capability back to the system.

Pump control for liquid cooling

Liquid-cooled cold plates often rely on centrifugal or gear pumps powered from the same DC buses as the drive. At the simplest level a pump runs whenever the drive is enabled, with only undervoltage or overcurrent protection. More advanced designs drive pumps with PWM or controlled DC voltage so that flow can track cold plate temperature, inverter load or ambient conditions. Pump current and, where available, flow or pressure sensors provide additional diagnostics.

Fault outputs from pump controllers or monitoring circuits are wired into the same protection tree as temperature alarms. Typical conditions include stalled pump, blocked flow path, running dry or unexpected power loss. These conditions may not require immediate STO but usually trigger derating and controlled stop sequences, followed by a latched fault until a service action is taken.

Control strategies and host interfaces

Thermal control starts with simple on/off thresholds and evolves into multi-step or host-defined curves as power density increases. A basic scheme uses one or two thresholds with hysteresis to avoid rapid cycling; more advanced schemes implement piecewise linear curves such as low-speed operation from 40 °C, medium speed around 50 °C and full speed from 60 °C upwards. These curves can be implemented inside fan controller ICs or by the main MCU writing target speeds and limits over I²C or PMBus.

Host-controlled profiles keep the thermal loop flexible. The system controller can adapt fan and pump behaviour based on firmware configuration, learned thermal models or remote updates. Actual storage of curves, version control and configuration download mechanisms belong to the system and memory pages; the role of this section is to show how fan and pump controllers expose the hooks needed for such strategies.

IC mapping for thermal actuators

• Multi-channel fan controller ICs with several PWM outputs, tach inputs, internal monitoring and an I²C, SMBus or PMBus interface.
• Local thermal management ICs that integrate a temperature sensor, simple fan PWM output and comparator thresholds for nearby hotspots.
• Pump driver and monitor ICs or power stages with current measurement, speed control and fault outputs.
• PMICs and system monitors that combine temperature channels, power-good monitoring and reset or fault signalling into a single device.

From temperature to protection & derating

Temperature measurements only become useful when they influence torque limits, shutdown decisions and maintenance planning. This section links thermal sensing to three major destinations in a motor drive: derating and soft-stop logic in the controller, hard-wired fault paths in the protection tree, and data logging for predictive maintenance. The focus is on signal routing and threshold concepts rather than the internal details of eFuses, STO modules or safety controllers.

A robust design defines clear warning levels, over-temperature shutdown thresholds and sensor-fault behaviours. Warning levels trigger cooling adjustments, HMI alerts and moderate derating. Shutdown levels initiate controlled stop sequences and hardware cut-off. Sensor faults such as NTC or RTD open circuits are treated as risk conditions that drive the system into a safe but conservative state until diagnostics are complete.

MCU and FOC controller: derating and soft stop

Temperature channels delivered to the MCU or FOC controller become inputs to torque and current limiting. When junction or cold plate temperature moves into a warning band, the controller gradually reduces permissible torque or current to keep losses in check. This approach allows continued operation while slowing temperature rise, reducing the likelihood of abrupt hard shutdowns in the middle of a critical move or process.

If temperature continues to climb towards the defined limit, the control firmware transitions into a controlled stop mode. Speed and torque commands are shaped to bring the system to a safe state under supervision of motion and safety requirements. The detailed stop profiles and coordination with Soft-Start / Controlled Stop logic are covered on dedicated pages; this section focuses on the fact that over-temperature events provide one of the triggers for such sequences.

Comparators, latched faults and hardware protection

Many temperature monitor ICs and AFEs expose digital alarm outputs in addition to analog or digital readback. These alarm pins assert when a programmable threshold is exceeded or when a sensor fault is detected. Wiring these alarms into eFuse gates, gate driver enable pins or protection controllers creates a hardware path that does not rely on firmware response times.

Thresholds can be configured for auto-retry or latched behaviour. Auto-retry suits conditions where transient overloads are acceptable and rapid recovery is valuable. Latched faults are reserved for sustained or severe over-temperature conditions that demand inspection before restart. In some architectures over-temperature alarms participate in a wider safety chain, including STO and safety relays, while the implementation details of those functions remain within safety-focused topics.

Data logging and predictive maintenance

Temperature signals also feed data logging and predictive maintenance functions. Capturing thermal histories for modules, motors, bearings and cabinet air builds a record of actual operating stress. Statistical views such as time spent within each temperature band or peak temperature occurrence enable realistic lifetime estimates for power modules, capacitors and mechanical components.

Deviations from expected thermal patterns often reveal emerging faults before hard limits are reached. Examples include increasing cold plate temperature at constant load due to pump or fan degradation, growing winding temperatures that indicate ventilation issues, and bearing temperatures that drift upwards as lubrication deteriorates. Data storage formats, sampling rates and analytics reside in the Data Logging & Predictive Maintenance area; the present page defines which temperature points are worth recording and how they connect to the logging infrastructure.

Threshold levels and actions

• Warning level: triggers increased cooling effort, moderate derating, HMI indication and event logging while the drive continues operating.
• Over-temperature shutdown: initiates controlled stop where possible and then engages hard protection paths such as eFuse shutdown or gate driver disable, often with a latched fault.
• Sensor fault: detects open or shorted sensors and treats readings as unreliable, typically falling back to conservative limits and requesting maintenance rather than allowing full-power operation.

Layout, isolation & safety notes

Thermal sensing only works reliably when sensor placement, isolation strategy and EMC behaviour are considered together. This section highlights how NTCs, RTDs and on-module temperature pins should be positioned on the mechanical stack, how high-voltage sensors hand their information down to the low-voltage domain, and how layout and routing keep these low-level signals clean while meeting creepage, clearance and safety goals.

Sensor placement and thermal response

Power modules with built-in NTCs or temperature pins offer short thermal paths to silicon junctions and usually serve as the primary reference for derating and shutdown. Bolt-on NTCs on heatsinks or cold plates are easier to retrofit and correlate closely with cooling system performance, but see additional thermal lag through the package and interface materials.

Embedded stator RTDs and bearing or gearbox sensors prioritise long-term health and lifetime estimation over fast response. These devices often sit deep inside the mechanical stack with long cabling back to the control cabinet, which makes them excellent indicators for trends and predictive maintenance while being less suited to fast protection triggers. Board-level sensors near MCUs, gate drivers, eFuses and front-end PSUs capture local hotspots and help validate layout decisions as power density increases.

Isolation and high-voltage temperature paths

Temperature sensors attached to high-voltage nodes or cold plates on the primary side must be treated as part of the insulation system. Mounting hardware, connectors and PCB landing areas for these sensors need creepage and clearance compatible with the drive’s insulation requirements. Failure to treat temperature wiring as a high-voltage structure can compromise safety even when the measured signals appear to be low-level analog voltages.

When temperature is measured on the high-voltage side and used on the low-voltage controller, the signal can cross the barrier through isolated amplifiers, sigma-delta modulators or digital isolators carrying SPI or alarm lines. For safety-related thresholds it is often preferable to perform the over-temperature comparison on the high-voltage side and then pass only a status or fault signal across isolation. This approach reduces dependency on a single communication link and keeps critical reactions local to the power stage.

EMC-aware routing and coupling

Thermal front-ends operate at low bandwidth and are easily disturbed by switching transients from power stages and gate drives. Sensor wiring and PCB traces should be routed away from half-bridge switching nodes, gate drive loops and current shunt connections. Where long cables are unavoidable, twisted pairs, shielding and differential measurement improve immunity to common-mode noise.

Local RC filtering, common-mode chokes and careful reference routing further stabilise readings, especially when several sensors share a common AFE or ADC. Detailed filter design, grounding and shielding strategies belong in the EMC Subsystem topic, but the thermal sensing chain should always be treated as a weak-signal path rather than just another low-frequency auxiliary connection.

Design checklist & IC mapping

Before finalising a motor drive design, the thermal sensing and control chain benefits from a structured review. The checklist below walks through sensing coverage, front-end topology, actuator control, protection links and layout considerations. The IC mapping list then ties each part of the chain to device categories that can be sourced and compared across vendors.

Design checklist for thermal sensing and control

• Are all critical temperature points covered, including power modules, cold plates, DC-link capacitors, inductors, motor stator and bearings, plus control board hotspots?
• Is it clear which sensor or location defines primary shutdown limits, which controls fans and pumps, and which feeds long-term lifetime and predictive maintenance analysis?
• For each point, is the chosen sensor type appropriate: NTC, RTD, embedded device or local temperature IC, with suitable mechanical mounting and wiring?
• Does the front-end topology provide adequate resolution and accuracy, whether a simple divider into an ADC, a dedicated NTC or RTD AFE, or a multi-channel monitoring IC?
• Has a basic error budget been estimated that includes sensor tolerance, reference accuracy, gain and ADC errors plus cable and connector effects?
• Is fan and pump control implemented with discrete stages or with dedicated controllers, and are tachometer or flow feedback paths in place for fault detection?
• Are temperature-based warning and shutdown thresholds mapped into both firmware logic for derating and soft stop, and hardware paths for fast fault handling where required?
• Is the interface to overcurrent, overvoltage, eFuse and STO protection topics clearly defined so that over-temperature alarms participate in the protection tree without duplicating logic?
• Which temperature channels are logged for predictive maintenance, and do sampling rate and data retention policies match storage and bandwidth constraints?
• Are high-voltage sensors, their connectors and PCB footprints designed with creepage, clearance and insulation levels consistent with system safety requirements?
• Do temperature signal routes avoid gate drive loops, shunt current paths and high dv/dt nodes, with appropriate filtering and shielding for long cables where needed?

IC mapping for thermal sensing and control

Once the architecture is defined, the next step is to associate each function with suitable IC categories. Grouping device roles in this way simplifies supplier discussions and comparison across vendors, and helps avoid gaps or overlaps in monitoring and protection.

• NTC / RTD AFEs and module monitors: devices that provide precision bias, linearisation, gain and diagnostics for NTCs and RTDs, including dedicated front-ends for power modules and DC-link monitoring.
• Multi-channel temperature monitors: ICs with several temperature inputs, on-chip references, programmable thresholds and digital readout for consolidating board-level and remote sensor information.
• Fan and pump controller ICs: multi-channel PWM generators with tach inputs, integrated monitoring and I²C or PMBus interfaces for cabinet and module fans as well as liquid cooling pumps.
• Local thermal controllers: compact devices that combine an on-chip temperature sensor with simple PWM or on/off outputs for nearby heatsinks or local enclosures.
• PMICs and system monitors: power management and supervision ICs that integrate voltage rails, power-good, reset, watchdog and one or more temperature channels into a coordinated system view.
• Safety and insulation monitors: devices that collect key temperature and status signals from high-voltage domains, supervise isolation integrity and provide safety-rated outputs into STO and safety PLC paths.

FAQs: thermal sensing & control

This FAQ collects practical questions that arise when planning thermal sensing and cooling for motor drives. The answers stay concise and implementation oriented so they can be reused in design reviews, internal guidelines, search snippets and future checklists without depending on a specific project or vendor.