Startup protection for a coreless brushless motor is not just about setting a current ceiling; it's a controlled energy-delivery problem.
A coreless brushless motor's startup current must be high enough to accelerate the load but low enough to avoid damaging the winding, driver, or power supply.
Coreless brushless motors can respond very quickly due to their low rotor inertia and, in many compact designs, low-inductance windings. This is their key advantage in dynamic applications. However, the same low-inertia construction that enables fast acceleration also makes driver behavior during the first milliseconds of startup especially important. At zero speed, with no back EMF to oppose the applied voltage, current can rise rapidly.1 Too much current can cause excessive winding heat, stress the driver, and trip the power supply. Yet, setting the current limit too low is also a problem—the motor may never generate enough torque to start the load. The engineering goal is to find the right balance.
Why Startup Is Electrically Different From Normal Running
Let's start from the zero-speed condition. The electrical behavior of a motor at startup is fundamentally different from when it's running at a steady speed.
Startup is not simply "normal operation at zero RPM." It is a transient condition where the absence of back EMF allows current to rise much more rapidly than during normal operation.
During normal rotation, current is influenced by a balance of factors, with back EMF playing a key role in opposing the supply voltage. At the moment of startup, however, the speed is zero, which means the back EMF is also zero.2 The initial current rise is therefore limited primarily by the winding's own electrical properties—resistance and inductance—and the applied voltage. This brings us to the core OEM questions: How much current is actually required to start the real load, and how much transient current can the complete system safely tolerate?
Why Startup Torque Depends on Available Current
In a BLDC motor's normal operating region, torque is approximately proportional to current (T ≈ Kt × I). This direct relationship is central to startup.
More available current translates directly into more electromagnetic torque. The startup current limit must be high enough to generate torque that exceeds all resistive forces.
Before the motor can accelerate, its generated torque must overcome several forces:
- Static friction (stiction) in bearings and gears
- Seal friction or mechanical preloads
- Any external load already applied to the system
- Torque required to accelerate the load's inertia
The basic relationship is:
Available Motor Torque - Resisting Torque = Torque Available for Acceleration
If the driver's current limit is set too low, the motor cannot generate enough torque to break away from the static load, leading to a stall.
Finding the Safe Startup Current Window
There is no universal "safe startup current" for all coreless BLDC motors. The correct value exists within a window defined by the application's specific needs and the system's limitations.
Startup current is a current-time operating window, not a single number. It must be above the minimum required for acceleration but below the maximum the system can safely handle.
The Minimum Required Startup Current is the current needed to reliably overcome all static loads and friction, with enough margin to achieve the target acceleration.
The Maximum Allowed Startup Current is constrained by the transient current-time capability of the weakest element in the complete system. This could be the motor winding's thermal limit, the driver power stage, the power supply's transient capability, the battery/BMS, wiring, or connector ratings.
At stall or near-zero speed, little or no useful mechanical output power is produced while current may remain high.3 Winding copper losses and driver losses can therefore rise rapidly. This is why the allowable startup current cannot be separated from the duration of the event.
BODENMOTION's currently documented coreless brushless motor platform spans approximately Ø10–50 mm, 5–48 V nominal voltage, 0.10–32.65 A nominal current, and 0.32–668.79 mNm nominal torque. These values describe the range of currently documented product configurations, not startup-current limits.
Even within one mechanical frame, winding choice can substantially change the electrical profile4. The BDCL1525 coreless brushless DC motor, for instance, uses the same Ø15 × 25 mm frame but has multiple winding versions:
| Version | Nominal Current | Stall Current |
|---|---|---|
| 9V | 2.71 A | 24.0 A |
| 12V | 1.86 A | 22.6 A |
| 18V | 1.68 A | 15.0 A |
| 24V | 1.15 A | 10.0 A |
Real Integration Challenge: These stall-current figures are specified zero-speed current values under defined datasheet conditions. They should not automatically be used as driver startup-current limits. This data illustrates why a "one-size-fits-all" limit is meaningless and why the final setting must be evaluated for the specific motor, driver, power source, and load.
Current Limit, Ramp Time, and Acceleration Must Be Designed Together
Setting a current ceiling is just one part of the solution. A soft start, which controls how torque is applied over time, is equally important. However, these ramp methods are not electrically equivalent.
The objective is not the lowest possible startup current; it is a controlled current-time profile that accelerates the load reliably within an acceptable time while minimizing unnecessary electrical and thermal stress.
- PWM-Duty Ramp: Progressively changes the average voltage applied to the motor.
- Current-Reference Ramp: Directly controls torque-producing current and requires a driver with current regulation.
- Speed-Reference Ramp: Changes the speed command, while a closed-loop controller determines the required torque/current. A speed-reference ramp does not automatically guarantee low startup current; if the speed loop demands high torque to follow the commanded acceleration, the current controller may still reach its configured limit.
The choice of ramp method depends on the controller architecture and can result in very different peak-current and acceleration behavior.5 Speed-loop tuning, current limits, mechanical load, and the driver's control strategy must therefore be considered together when defining the startup profile.
Why Startup Duration and Repetition Frequency Matter as Much as Peak Current
A single, short current peak is very different from a long, sustained current plateau. This is where many simple protection schemes fall short.
A startup profile that is safe for one isolated event may not be safe when repeated dozens or hundreds of times within the motor's thermal time constant.
A complete startup protection strategy must consider three dimensions:
- Current Amplitude: How high does the current go?
- Current Duration: For how long does the motor stay at this high current?
- Repetition Frequency: How often does the startup event repeat?
An actuator in a pick-and-place robot might start and stop hundreds of times per minute. The heat can accumulate much faster than it can dissipate, so the analysis must shift from peak current alone to overall RMS current, cooling intervals, and the system's thermal duty cycle6.
What OEM Engineers Should Provide Before Setting Startup Current Limits
To move from guesswork to a robust startup strategy, the conversation needs to be much more detailed than "What current limit should I set?"
The ideal startup profile depends on the motor, the driver, the power supply, and the load. Providing a complete system profile is the key to a successful integration.
To evaluate the requirements properly, your motor supplier needs to know:
- Motor Characteristics: Resistance, inductance, torque constant.
- Power Supply Details: Voltage range, power-source impedance, battery/BMS limits.
- Driver Capabilities: Peak current, allowable peak duration, continuous current rating, and current-control method.
- Mechanical Load: Static friction, preload, and inertia.
- Application Requirements: Target acceleration time and startup frequency.
- Thermal Environment: Ambient temperature and cooling conditions.
Power-source capability should not be evaluated from nominal current rating alone7. The engineering question is also: What happens to the DC bus voltage when the required transient current is demanded?
BMS over-current protection, source impedance, wiring and connector resistance, and battery state of charge can all influence the voltage available to the driver during startup. Excessive bus-voltage sag may trigger driver under-voltage lockout (UVLO) or reset the system controller8.
For an OEM startup evaluation, providing these inputs allows the motor and startup strategy to be evaluated as one complete system.
A Practical Startup Protection Sequence for Coreless Brushless Motors
A robust startup protection scheme follows a logical sequence: Define → Accelerate → Verify → Protect → Recover.
Successful startup protection delivers enough energy to accelerate the load while preventing excessive current, excessive time at low speed, and excessive repetition.
- Define Max Allowed Current: Set a peak-current boundary based on the transient capability of the weakest element in the system.9
- Confirm Min Required Current: Verify that the selected current limit provides enough torque to overcome the static load and achieve the required acceleration.
- Set a Controlled Acceleration Ramp: Use a suitable current, PWM-duty, or speed-reference ramp based on the driver architecture.
- Implement a Startup Timeout: If the motor fails to reach the required motion or speed within a defined time, treat the event as a startup fault.10
- Add Stall Detection: Use Hall sensors or an encoder where direct motion feedback is available. In sensorless systems, startup and stall detection depend on the specific driver architecture and may use current behavior, commutation timing, rotor-position estimation, observer methods, and startup timeout until reliable back-EMF or estimated-speed information becomes available.
- Define a Smart Retry Strategy: If a fault occurs, implement a delay and limit the number of retries before entering a persistent fault state to prevent thermal accumulation.
| Startup Parameter | Too Low / Too Short | Target Design Window | Too High / Too Long |
|---|---|---|---|
| Current Limit | Cannot overcome breakaway load | Reliable acceleration with controlled peak | Excess electrical/thermal stress |
| Ramp Profile | Long low-speed dwell or stall | Controlled transition to operating speed | Electrical/mechanical shock |
| Startup Timeout | Nuisance fault trips | Separates normal acceleration from stall | Excess heating during failed start |
| Retry Delay | Thermal accumulation | Allows appropriate recovery/cooling | Unnecessarily long system downtime |
Conclusion
The correct engineering question is not "What is the lowest safe current limit?" but "What current-time profile can reliably accelerate the real load without exceeding the limits of the motor, driver, and power source?" For coreless brushless motors, startup protection therefore requires balancing the torque needed for reliable acceleration with the electrical and thermal limits of the complete system.
The final startup profile should be validated under worst-case conditions, including maximum expected load, minimum supply voltage, highest expected ambient temperature, and realistic start-stop frequency. This system-level validation helps improve startup reliability while reducing unnecessary electrical and thermal stress over long-term operation.
FAQ
Q1: Why is startup current high in a coreless brushless motor?
At startup, motor speed is zero, so back EMF is also near zero. Without back EMF to oppose the supply voltage, current can rise rapidly, limited mainly by the winding's resistance, inductance, and the driver's control.
Q2: Does lowering the current limit always protect the motor?
No. If the limit is set too low to generate enough torque to overcome the load's friction, preload, and inertia, the motor may stall. A prolonged stall can cause significant heating, so a current limit that prevents successful acceleration may not be safer.
Q3: How should stall protection work during startup?
Direct motion sensors such as Hall sensors or encoders can verify rotor movement. In sensorless systems, detection depends on the driver architecture and may require current monitoring, commutation timing, rotor-position estimation, and a startup timeout because reliable back-EMF information is not normally available at or near zero speed.
Q4: Can I use the stall current from a datasheet as the startup current limit?
No. Stall current is a specified zero-speed current value under defined datasheet conditions. It should not automatically be used as the driver's startup-current limit or as a continuous allowable current. The actual startup limit must be evaluated together with the winding, driver capability, load, acceleration time, and thermal conditions.
Q5: Why can a motor start on a bench power supply but fail with a battery?
The two power sources can have very different transient behavior. A battery system may be affected by BMS over-current protection, pack impedance, wiring and connector resistance, and state of charge. These factors can cause enough bus-voltage sag during startup to trigger driver UVLO or a controller reset even when the battery's nominal voltage and current rating appear sufficient.
The reverse can also occur if a bench power supply enters its own current-limiting mode during startup.
JKONGMOTOR, “How Does Back EMF Affect a BLDC Motor?” Explains that back EMF increases with motor speed and is absent at zero speed, allowing current to rise rapidly during startup unless limited by the motor winding and drive. ↩
Michigan State University, “Back Emf – ISP209: The Mystery of the Physical World.” Explains the relationship between rotational speed and back EMF, including the zero-speed condition where back EMF is zero. ↩
Wikipedia, “Brushed DC Electric Motor.” Describes the stall condition in which motor speed and mechanical output power fall to zero while electrical current can remain high. The same zero-speed power relationship is used here as a general motor principle. ↩
MOSRAC, “Motor Winding: Types, Differences, Benefits, & Examples You Can Learn From.” Describes how winding configuration can affect electrical characteristics such as resistance, voltage and current requirements, and torque-related performance. This is used as a general winding-design principle rather than evidence for a specific BDCL1525 winding version. ↩
Wevolver, “Motor Controller: Types, Design Considerations, Control Strategies, and Selection for Engineers.” Reviews different motor-controller architectures and control strategies, including PWM, torque/current control, and speed control, showing that different control approaches act on different variables and can produce different dynamic behavior. ↩
Linear Motion Tips, “Why RMS Torque Is Important for Motor Sizing.” Explains why peak demand alone is insufficient for thermal sizing and why load magnitude, duration, dwell periods, and the full duty cycle must be considered. The same RMS approach can also be applied to current when evaluating repeated operating cycles. ↩
Astrodyne TDI, “Understanding Max Output Ratings in Power Supplies.” Distinguishes continuous rated output from short-duration peak capability, supporting the need to evaluate transient power demand separately from nominal or continuous ratings. ↩
ABB, “Power-Quality Disturbances and Their Effect on VSD Performance.” Shows that voltage sag can reduce a drive's DC-link voltage sufficiently to trigger an undervoltage trip, fault state, or shutdown. The exact behavior of auxiliary control electronics and controller reset thresholds depends on the specific drive architecture. ↩
Rockwell Automation, “Drives Engineering Handbook.” States that drive sizing and protection must account for both the magnitude and duration of current demand, and that motor and drive overload capabilities must both be considered. This supports setting the allowable transient current within the most restrictive applicable component limit. ↩
Blue Jay, “Motor Protection Relay Function – 9 Things You Must Know.” Describes startup-timeout protection in which a protection relay acts when motor starting time exceeds a configured limit under specified fault conditions. The exact startup-success criterion may be based on time, current, motion, or speed depending on the controller and protection architecture. ↩
