Protections & Fusing

This guide provides you with best practices and steps to implement electrical protections and fusing when integrating a motor controller into an application. Proper protection ensures safety, reliability, and longevity of the system, especially in battery-operated environments where overcurrent and thermal events can have serious consequences.

Critical Safety Notice: Integrator's Responsibility

Safety is the sole responsibility of the system integrator. The design, implementation, and validation of all safety-related aspects of the final product rest with the integrator.

This document provides general application level guidelines and describes some protective features within the controller. However, it is not a substitute for a comprehensive, system-level safety analysis based on your specific application's requirements, including but not limited to: - Compliance with local and international safety standards (e.g., ISO, IEC). - A formal risk assessment (e.g., FMEA - Failure Mode and Effects Analysis). - Validation and testing of all safety functions.

Failure to implement adequate protections can result in equipment damage, serious injury, or death.

Document Scope and Inquiries

This document provides guidance on common protection strategies but is not an exhaustive list of all possible solutions. Technology and best practices evolve. If you have questions about a specific application, or if you require a feature not described here, please contact your ASI representative for the most current information and support.

Prioritization of Standards and Guidelines

In the event of conflicting guidance, always prioritize the source with the highest level of authority and stringency. We recommend following this hierarchy when determining which source to rely on:

Industry Regulations – e.g., ISO, UL, CE, EN Industry Standards – e.g., IEEE, ANSI Application-Specific Guidelines – tailored to the particular use case or product General Guidelines and Good Engineering Judgment – including this article

Protection Goals

Prevent overcurrent and short circuits
Protect against reverse polarity
Prevent overvoltage and undervoltage
Ensure thermal safety
Isolate faults to avoid cascading failures

Fuse Selection and Placement

Purpose of Fuses

Fuses act as sacrificial protection devices that disconnect the power path during an overcurrent condition to prevent thermal event, equipment damage, or battery hazards.

Recommended Fuse Types

Fuse Type	Use Case
Blade Fuses	Automotive, medium-current loads
Resettable PTCs (Positive Temperature Coefficients)	Reusable protection in low- to mid-power applications
Fast-Blow Fuses	Sensitive electronics needing quick disconnection
Slow-Blow Fuses	Motor startup surges (inrush current)

Fuse Rating Guidelines

Critical Fuse Parameter: Interrupting Rating (AIC)

For high-power systems, the battery can deliver extremely high short-circuit currents. The fuse's Ampere Interrupting Capacity (AIC) – also known as Breaking Capacity – must be higher than the maximum potential fault current from your battery system. An inadequate AIC can lead to fuse explosion, fire, or failure to clear the fault safely. Always consult your battery manufacturer's specifications for maximum discharge/fault current.

Voltage Rating: Must be greater than or equal to the system voltage (for example, a 24 V system requires a 32 V rated fuse).
Current Rating: The fuse must protect the system wiring according to its ampacity under all conditions. It should be rated above the maximum continuous operating current and allow for expected peak operational currents without nuisance tripping. However, it must blow before exceeding the applications absolute maximum current ratings (both peak and continuous, considering duration) or causing wiring damage. A general starting point might be 125-150% of maximum continuous system current, but this must be verified against detailed application specifications and wiring ampacity charts for the specific application.

Fuse Placement

Primary Fuse: Positioned between the battery positive terminal and the motor controller input.
Secondary Fuse (optional): Placed between the controller output and the motor.

Controller Overcurrent Protection

Our controllers already disable the output if overcurrents are detected in software.

Motor-Side Fusing Caution

Fusing individual motor phases on a 3-phase motor (like BLDC or PMAC) is generally not recommended. If one phase fuse blows, the controller might continue to operate on the remaining two phases, potentially causing severe damage to the motor or controller, or creating an unsafe condition. At power levels up to 16kW, such an event can be particularly destructive. The controller's internal overcurrent and fault protections are designed to protect the motor and controller system. If external motor-side protection is deemed essential, a 3-phase linked circuit breaker or fuse system that disconnects all phases simultaneously should be considered.
I/O Protection: Use inline fuses on input/output logic power sources. For example, for low-side switches, key-out, and light-out.

Additional Electrical Protections

Reverse Polarity Protection

Protecting against accidental reverse battery connection is crucial. For high-power systems, solutions must handle significant current with minimal voltage drop and must not interfere with regenerative braking.

Our controllers require current to flow freely bi-directionally between the controller and the DC source. Even during normal operation with regenerative braking disabled, small motor position errors can lead to regen current.

Bidirectional Current Required

ASI motor controllers support regenerative braking, meaning current must be able to flow from the motor back to the DC source (battery). Any external reverse polarity protection circuitry on the main battery input must be designed to allow this bidirectional current. A simple series diode is not suitable as it will block regenerative current and can lead to controller overvoltage faults or damage when the motor is attempting to regenerate.

Recommended methods for high-power applications:

Ideal Diode Bridge using N-Channel MOSFETs: Specialized ICs can drive N-Channel MOSFETs to emulate an ideal diode with very low voltage drop, often supporting bidirectional current. Ensure the chosen solution is explicitly rated for bidirectional flow or has appropriate control logic.
Contactor with Polarity Detection: A system can use a contactor that only engages if the polarity is correct, often controlled by a simple detection circuit.

Simple series P-Channel MOSFETs are generally not suitable for the main power path at high power levels due to current limitations and thermal management challenges.

Overvoltage/Undervoltage Protection

For battery protection, utilize Battery Management System (BMS) features at the source, the battery. Controller features are secondary to the battery BMS.
Configure the motor controller's Undervoltage Lockout (UVLO) and Overvoltage Protection (OVP) settings. See Faults and Warnings. E.g.: faults bit 0 controller over voltage, faults bit 6 controller under voltage, faults bit 12 instantaneous controller over voltage, faults bit 15 instantaneous under voltage, warnings bit 7 low battery voltage foldback, and warnings bit 8 high battery voltage foldback.
Use Transient Voltage Suppressors (TVS) on input power or signal lines.

Overcurrent/Over-temperature Protection

We configure motor controllers in software from the factory to protect against output instantaneous overcurrents and limit output based on temperature. See Faults and Warnings. E.g.: faults bit 1 phase over current, faults bit 9 instantaneous phase over current, faults bit 4 controller over temperature, and warnings bit 10 controller over temperature foldback.
You should configure the controller for:
Motor power and current limits. E.g.: All 4 Rated motor power... & Rated motor current
Battery current and temperature limits. E.g.: Battery current limit, Regeneration battery current limit, and warnings bit 14 Low temperature battery and warnings2 bit 8 Hot battery foldback.
Motor temperature limits or overload accumulation based (I2T) protections. See Motor foldbacks.

Capacitor Inrush Current Protection

Understanding Capacitor Inrush Current

Capacitor inrush current occurs when the input capacitors in the motor controller charge from 0 V to the supply voltage during initial startup. These capacitors can draw a large current in a short amount of time, typically several times higher than the steady-state current. If left unprotected, this inrush can damage components like fuses, the motor controller's power transistors, capacitors, and even the battery pack.

Importance of Capacitor Inrush Current Protection

This protection is important because it: - Protects sensitive components like power transistors and electrolytic capacitors from high surge currents. - Prevents fuse blowouts during the startup phase. - Improves system stability by preventing excessive voltage drops and current spikes at power-up.

Methods for Capacitor Inrush Current Protection

Pre-Charge Circuit (Recommended for High Power): For high power systems, a pre-charge circuit using a power resistor and a contactor is the most robust method. The resistor limits current during initial capacitor charging. Once the capacitor voltage is near the supply voltage, the main contactor closes, bypassing the resistor for normal operation. This method is essential to prevent damage to main contactors, fuses, and the controller itself.
- How it works: A resistor is placed in series with the motor controller during the initial power-up. Once the controller capacitors reach near the supply voltage, the pre-charge circuit closes, bypassing the resistor, and full voltage is applied to the motor controller.
NTC (Negative Temperature Coefficient) Thermistor: While NTCs can limit inrush, for higher power systems, they may need to be very large, used in arrays, or require a bypass contactor/MOSFET to prevent overheating and power loss during continuous operation. Their suitability should be carefully evaluated.
- How it works: An NTC thermistor has high resistance when cold and low resistance when hot. Placed in series with the power input, it limits inrush current initially. As it self-heats due to current flow, its resistance drops, allowing normal operation. For higher power systems, an NTC is often used in conjunction with a bypass relay or MOSFET that shorts out the NTC after a brief delay to improve efficiency and prevent the NTC from overheating during continuous operation.
Soft-Start Circuit: Some systems incorporate a soft-start circuit, which uses a combination of resistors, capacitors, and MOSFETs to limit the inrush current. The circuit starts by slowly ramping up the current to the motor controller, thus avoiding high inrush currents. Dedicated soft-start circuits can also be used but must be rated for the system's peak power and voltage.
- Placement: Install in series with the motor controller’s power input.

Battery Disconnection Protection

Battery disconnections during operation can cause overvoltage conditions, power loss, or erratic behavior of the motor controller and connected components which can damage sensitive components or cause a safety issue.
In systems where hot swapping of batteries might occur (e.g., in outdoor power equipment, or robots), it is important to prevent power drops or spikes due to swapping batteries.
Adequately size your system to prevent unintended battery disconnections during operation. Disconnection, during regeneration, even below motor no-load speed, can result in significant and damaging voltage spikes.

High Voltage Spikes on Battery Disconnect During Regen

Disconnecting the battery while the motor is actively regenerating (e.g., during braking) can cause extremely high voltage spikes on the controller's DC bus. This is because the regenerative energy has nowhere to go. These spikes can far exceed the battery voltage and may damage the controller or other connected components, even if the controller has OVP. The energy involved can be substantial, and resulting voltage spikes can be extremely destructive. Ensure robust battery connections and consider system-level interlocks if hot-swapping is a requirement.

Methods for battery disconnection protection

Capacitor Buffering (Energy Buffer):
Add a bulk capacitor (often an electrolytic or supercapacitor) across the battery input to the motor controller. This capacitor provides a temporary power supply if the battery is disconnected, preventing the motor controller from losing power immediately.
How it works: When the battery is disconnected, the capacitor temporarily supplies power to the controller, maintaining steady voltage until the battery reconnects or the controller shuts down safely.
Battery Disconnect Detection:
Implement a controller undervoltage faults and foldbacks that detects when the battery voltage drops below a safe threshold, signaling a disconnection, undervoltage or power loss.
These features can trigger a graceful shutdown of the motor controller, or at least prevent sudden power loss that could cause system instability.
These features triggers faults and warnings that can be logged for diagnostics.
Inrush Current Limiting During Battery Reconnection:
If a battery reconnection occurs after disconnection, inrush current limiting must be implemented to prevent damage. This can be done by incorporating pre-charge circuits or soft-start as previously mentioned.
These protections will ensure that when the battery is reconnected, there are no excessive current spikes that could damage components.

Thermal and Mechanical Protection

Ensure the motor controller heatspreader is mounted to a proper heatsink via a thermal pad.
Avoid enclosing the controller in sealed enclosures without ventilation.

Wiring and Connectors

Wire Gauge Selection

Match wire gauge to peak current; consider voltage drop. Selecting the correct wire gauge (e.g., AWG) is critical to handle peak currents and minimize voltage drop. Always consult ampacity tables based on conductor temperature rating, ambient temperature, and bundling. Undersized wires are a fire hazard.
Wire insulation temperature rating is also critical and must be appropriate for the application and expected temperatures.
Use American Wire Gauge (AWG) size calculators or follow Underwriters Laboratories (UL) standards for automotive/industrial wiring, or equivalent.

Connector Protection

Use locking connectors with polarization features. Use high-quality connectors specifically rated for the peak currents and system voltage. Poor connections at high power levels lead to significant resistive losses, excessive heat generation, and potential failure points. Ensure connectors are properly torqued and secured.
Add in-line connectors for easy maintenance or fuse replacement.

Integration Checklist

Step	Checkpoint
1	Battery voltage and current specifications defined.
2	Fuses selected (including AIC) and placed appropriately.
3	Overvoltage/undervoltage thresholds configured.
4	Reverse polarity protection method verified to be compatible with bidirectional current flow (regeneration).
5	Capacitor inrush current protection (pre-charge resistor, NTC thermistor, or soft-start circuit) implemented.
6	Battery disconnection protection (capacitor buffer, detection circuit) implemented.
7	Proper heatsinking and airflow established.
8	Wiring (gauge and insulation) rated for full load current and temperature.
9	Connectors rated for current/voltage and properly installed.
10	Emergency stop or kill switch available.
11	Controller’s firmware safety parameters configured.

Testing and Validation

Pre-Power Checks

Verify wiring polarity and continuity.
Check fuse installation and orientation.
Ensure inrush protection is functional (for example, verify pre-charge resistor or NTC thermistor limits the current at startup).

Power-Up Testing

Use a current-limited bench power supply for initial testing to observe the capacitor inrush behavior.
Monitor current draw, voltage rails, and temperature during startup, especially focusing on the first few milliseconds of power-up.
Monitor temperature, current draw, and voltage rails under load.

Fault Simulation

Simulate short circuits, overcurrent, or thermal events to verify protection circuits respond correctly.
Simulate capacitor inrush conditions to verify that protections (for example, pre-charge resistor, NTC thermistor, soft-start circuit) correctly limit the current without tripping prematurely.
Ensure that overcurrent protection is still capable of activating under sustained overcurrent conditions.
Simulate battery disconnection and reconnection to verify that protections correctly prevent damage.
Ensure that the motor controller shuts down gracefully if a disconnection is detected, and that the system recovers correctly upon reconnection.
Verify Overvoltage Protection (OVP) and Undervoltage Lockout (UVLO) trigger points and system response.
If feasible and safe, test system behavior during regenerative braking into a nearly full or temporarily high-impedance battery to observe OVP engagement.

Documentation and Labeling

Label all fuses and protection devices clearly.
Provide a schematic with fuse ratings and protection locations.
Include user-accessible instructions for fuse replacement and maintenance.