How to Choose the Right Voltage for Brushless DC Motors in AGVs and AMRs

A brushless DC motor voltage choice is not a standalone decision. It is a system decision that starts with the battery, runs through the controller, and ends with how long a machine runs on a charge. Engineers who pick a voltage first and check the rest of the system later usually end up reworking the design.

Every AGV or AMR platform has its own mix of battery type, load, speed target, and duty cycle. That mix decides which motor voltage makes sense. There is no single voltage that works best across all mobile robots, so the goal here is a process, not a fixed answer.

A practical way to think about it: battery-powered operation sets the ceiling, motor efficiency and runtime set the priorities, and load, speed, and controller compatibility set the constraints. Voltage is simply the number that has to satisfy all four at once.

Why Motor Voltage Matters in AGVs and AMRs

In AGV and AMR design, the voltage number itself is not what matters. What matters is the effect voltage has on current, heat, wiring size, and overall system efficiency. That effect comes from one simple relationship:

Power ≈ Voltage × Current

If the power output is the same, increasing the voltage decreases the current that the system must carry. Thinner wiring, smaller connectors, and lower resistance losses usually result in lower current. This is a great benefit in many designs.

It doesn’t necessarily mean that the higher the voltage, the better, though. If the rest of the platform, battery, controller, wiring and connectors can’t support a higher bldc motor voltage, then it won’t help the bldc motor voltage. If the battery pack is limited to 24V, the only way to get a 48V motor is to build a new battery pack and/or converter.

That is why it is never a good idea to select a voltage from a spec sheet by itself. It must be selected in conjunction with the battery pack, wiring harness, and controller—they are all three parts of a single decision, not four.

Start With the AGV or AMR Battery System

Motor voltage selection should begin with the battery, not the brushless DC motor catalog. Before comparing voltage options, confirm the following:

• Nominal battery voltage of the platform, such as 24V, 36V, or 48V nominal packs
• Full operating voltage range, from fully charged down to low-charge cutoff
• Battery chemistry and pack configuration, since series cell count sets the achievable voltage
• Input voltage range accepted by the motor controller or driver

The motor’s maximum and minimum operating voltage is determined by the battery’s nominal voltage and its actual range. A brushless motor designed for one specific voltage will not perform or run as well or as hot if the pack drops too low under load. The lesson is simple: Design the electrical system first, then choose a motor that works in the system.

It’s also important to watch how the pack performs in real conditions, rather than its rated nominal value. It is possible for a lithium pack to be significantly higher than nominal voltage immediately after charging and significantly lower than nominal voltage at the end of a shift. The brushless motor/controller combination will only work well at the nominal voltage and will have trouble at both the high and low ends of that curve.

How 24V, 36V, 48V and Other Motor Voltages Compare

Once the battery system is defined, it becomes easier to compare voltage classes on their real trade-offs rather than on marketing claims.

These are general trends and not rules. The following table provides key comparisons from which actual advantages and disadvantages will vary according to the motor, controller, and battery pack used, and should therefore be treated as a starting point rather than a final specification for a comparison.

Usually, the decision hinges on the platform’s task. A smaller indoor AMR with a lighter payload might focus on simplicity and availability, and it would be 24V. A higher-voltage system could be more advantageous for a heavier load transported longer distances in a warehouse application, as lower current may simplify wiring and thermal design. There is no single right or wrong answer; either alternative will work for a different operating profile.

Match Motor Voltage With Power and Torque Requirements

Voltage alone does not describe whether a brushless motor fits an application. Power, torque, and speed have to line up with the actual mechanical job. Before finalizing a voltage, check the load side of the system:

• Total load, including payload, chassis weight, and any towed weight
• Wheel diameter, since it directly affects the torque needed at the motor shaft.
• Required torque at startup, on ramps, and during acceleration.
• Target travel speed and how quickly the platform needs to reach it.
• Gear ratio, if a gearbox sits between the motor and the wheel.
• Duty cycle, meaning how much of the run time the motor spends under load.

Acceleration and stopping behavior is of particular interest for AGVs and AMRs, as their speed is rarely constant. This is often repeated start/stop/acceleration/stopping to dock and/or avoid obstacles and to position accurately. These times can be when the most torque is required, compared to cruising speed, and if the motor is sized for cruising only, it may not have enough margin if the platform is required to move at a high speed.

Next, choose a voltage class that matches the battery system, ensuring the motor can provide the rated power, peak torque, and top speed the platform requires. A brushless DC motor may be electrically sized correctly to the battery, but mechanically undersized to perform the required task.

Consider Motor Controller Compatibility

A brushless DC motor always works as a package. It must be used with a brushless motor controller, and you should treat this combination as an integrated unit, not two separate components.

• Controller input voltage range, including how it behaves near the battery’s low-charge cutoff.
• The controller’s continuous and peak current capacity.
• Compatibility with the motor’s feedback method, such as Hall sensors or encoders.
• Control method supported, including trapezoidal, sinusoidal, or field-oriented control.
• Protection features, such as overcurrent, overvoltage, and thermal cutoffs.

Matching the voltage rating between motor and controller is only the first check. A brushless motor controller with the correct voltage rating can still limit motor performance if its current capability is too low, since sustained or peak current, not voltage, is often what actually caps torque and acceleration. This happens when a brushless motor and controller pair are rated for the correct voltage but do not feature current limits, control methods, or feedback requirements that are suitable for each other. List them together, don’t list them separately.

How Motor Voltage Affects Battery Runtime and System Efficiency

Additionally, the voltage selection has an impact on runtime, but not in the same linear way as “the higher the voltage, the longer the runtime.”

A higher-voltage system draws less current from a motor for a given output. A lower value will decrease resistive losses in wiring and connectors, which increase by the square of the current. That’s really helpful, especially when running long cables or a constant load on platforms.

But overall system efficiency depends on more than voltage. Motor design, switching losses in the controllers, gearbox efficiencies, and the actual load profile also play a role. If the controller or gearbox causes losses in the system, a well-matched 24V system can beat a poorly matched 48V system. Use voltage as one of many levers, not the only dial that sets run times.

When planning a run time, it is beneficial to view total energy consumed per hour of use, not just voltage. Amp-hours or watt-hours of battery capacity, plus the actual duty cycle, provide a much more accurate estimate of run time than voltage classification.

The other side of this equation is heat. The resulting current, if not mitigated by a higher voltage option, must dissipate somewhere, and in the case of a BLDC system, it is dissipated as heat in the windings, controller power stage, and wiring. Thermal margin is just as important as the voltage level when platforms are not only operating, but running near continuously.

Conclusion

In reality, selecting brushless motor voltage is based on a simple principle: First the battery system, then the motor, then the controller, and then runtime and thermal performance. The motor specification should NOT be the sole basis for the final voltage; the entire system should determine it.