AGV Drive Motor Sizing | Continuous vs Peak Torque Example

Key takeaways

• A drive motor has to satisfy two limits: the peak torque needed for short events (acceleration, ramps, restart) and the continuous torque that the motor can deliver without overheating, checked with the RMS torque of the duty cycle.
• Always size from the wheel force backwards: wheel force, wheel torque, then motor torque through the gear ratio and efficiency.
• The cycle RMS can look comfortable while a sustained worst case does not. In the example below a 5% ramp climbed at constant speed needs almost twice the motor's rated torque. A motor is only suitable for that if the climb is short enough for its thermal limits.
• The gear ratio changes torque, speed and RMS together. Re-run the numbers for each candidate ratio.

Step 1: wheel force

For the driven wheels, the total traction force is the sum of rolling resistance, gradient force and the force for acceleration (air drag is negligible at AGV speeds):

• Rolling resistance: F_roll = C_rr × m × g
• Gradient force: F_grade = m × g × sin(θ)
• Acceleration force: F_acc = m × a

Here m is the loaded vehicle mass, g = 9.81 m/s² and C_rr is the rolling resistance coefficient. For hard wheels on smooth floors it is typically in the range of 0.01 to 0.03, depending on wheel material, floor and load; use a measured value for your wheel and floor if you have one. The tractive force and wheel load guide covers this step in more detail, including traction limits.

Step 2: wheel torque, then motor torque

With n driven wheels of radius r, the torque at each wheel is T_wheel = F / n × r. Through a gearbox with ratio i and efficiency η:

• Driving: T_motor = T_wheel / (i × η)
• Braking (negative wheel torque): T_motor = T_wheel × η / i

Motor speed is n_motor = v / r × i (in rad/s), which must stay below the motor's rated or maximum speed at the voltage of your battery.

Step 3: peak torque

Identify the worst short event: full payload, maximum acceleration, possibly on a gradient. The motor and drive must be able to supply that torque for as long as it lasts. Check the motor's peak (or intermittent) rating, the drive's current limit and the permitted duration.

Step 4: RMS torque over the duty cycle

Motor heating follows the square of the current, which is roughly the square of the torque, so continuous capability is checked with the root-mean-square torque over one complete cycle:

T_rms = √( Σ ( T_i² × t_i ) / Σ t_i )

Include every segment, including standstill time, which adds time to the denominator and little to the numerator. Compare T_rms with the motor's continuous torque rating, with a margin (a 20 to 30% margin is common practice) and with the ambient temperature in mind.

Worked example

This is an illustration with assumed values, not a product specification.

Forces. F_roll = 0.02 × 1,200 × 9.81 = 235 N. During acceleration the total force is 235 + 1,200 × 0.5 = 835 N. During deceleration it is 235 − 600 = −365 N (braking).

Motor torque for a gear ratio of 20. Per wheel the force is half the total, and T_motor = (F / 2 × 0.1) / (20 × 0.9) while driving:

RMS torque = √( (2.32²×2 + 0.65²×20 + 0.82²×2 + 0) / 60 ) = 0.59 N·m.

Speed and power. The wheel turns at 95 rpm at 1 m/s, so the motor runs at about 1,910 rpm with a ratio of 20. Peak mechanical power is about 460 W and cruise power about 130 W.

Comparing with a motor. Take a 48 V servo motor with a rated torque of 1.27 N·m, a peak torque of 2.54 N·m and a rated speed of 3,000 rpm, such as the JY Robot 48V DC servo motor. At a ratio of 20 the peak requirement (2.32 N·m) is inside the 2.54 N·m peak rating and the cycle RMS (0.59 N·m) is well inside the 1.27 N·m rated torque. At a ratio of 15 the peak requirement would be 3.09 N·m, above the peak rating, which shows why the ratio must be re-checked.

The case the average hides: a ramp

Now suppose the same vehicle must climb a 5% gradient at 1 m/s for a longer time. The gradient force is 1,200 × 9.81 × sin(2.86°) = 588 N, so with rolling resistance the motor torque at a ratio of 20 is about 2.29 N·m. That is 1.8 times the rated torque. The motor can deliver it as a short peak, but not continuously, even though the cycle RMS was only 0.59 N·m. Whenever the vehicle has a sustained segment such as a ramp, a long haul with a heavy load or a cold-store floor with high rolling resistance, evaluate that segment separately against the continuous rating.

Step 5: check the rest of the drivetrain

• Gearbox. Rated and peak output torque, backlash, and the permitted input speed. See the planetary gearbox selection guide.
• Motor and drive. Voltage, peak current limit and thermal derating at your ambient temperature. See the DC servo motor selection guide.
• Wheel and traction. The wheel must transmit the force without slipping: compare the required force with friction times the load on the driven wheels.
• Inertia. Reflected inertia through the gear ratio affects acceleration torque and control stability.
• Test. Confirm the result with a thermal measurement on a prototype under the real duty cycle.

If you would rather start from an integrated unit, our integrated servo wheel guide and the drive-steering unit selection guide show how the load, speed and duty cycle map to module series. You can send your vehicle mass, speed, acceleration, gradient and duty cycle to our engineers for a recommendation.

FAQ

Why use RMS torque instead of the average?

Heat in the windings depends on current squared. The average hides short high-torque segments, while RMS weights them according to their effect on temperature.

How much safety margin should I leave?

A 20 to 30% margin on the continuous rating is common practice, but the right value depends on ambient temperature, cooling, duty cycle uncertainty and how critical the application is. Validate with a test.

Does the gear ratio change the RMS?

Yes. A higher ratio reduces the motor torque in proportion but raises the motor speed. Check both the torque and the speed limits for each ratio.

What if the motor is slightly too small for the peak?

Use a higher ratio, a larger motor or limit acceleration in the controller. Do not rely on the drive's current limit to hide an undersized motor, because the vehicle will then not reach the expected acceleration or gradient capability.