Engineering Blog
Motor Drive Sizing Mistakes: Current, Torque, PWM, and Heating
A motor drive must satisfy startup, acceleration, continuous load, braking, jam, feedback, and thermal conditions. Nominal voltage and no-load speed are not enough to size the electrical or mechanical system.
- Reading Time
- 17 min
- Difficulty
- Intermediate
- Last Updated
- October 3, 2026
Connect the Electrical and Mechanical Models
The basic equations expose the coupling between voltage, speed, current, torque, heat, and shaft power. Their constants and validity range must come from the motor data.
Formula reference
Motor design relationships
V ≈ I R + K_e ωτ ≈ K_t IP_mech = τωP_cu = I_rms²Rη = P_mech / P_elecVariable definitions
- K_e
- back-EMF constant
- K_t
- torque constant
- ω
- mechanical angular speed
- I_rms
- winding RMS current
The Motor Electrical Design Guide develops torque, back EMF, loss, PWM, gearing, and timing fundamentals.
Ten Common Drive-Sizing Mistakes
1. Sizing the driver from no-load current
No-load current says little about startup, acceleration, jam, braking, or stall stress. Driver, wiring, supply, and protection must tolerate the relevant peak and RMS current profile.
2. Treating stall current as a continuous operating point
Stall current estimates the zero-speed electrical limit. Holding it can overheat the winding, brushes, driver, connector, and supply long before a steady state is reached.
3. Ignoring back EMF
Current falls as speed-generated back EMF rises. Torque, speed, supply voltage, winding resistance, and load therefore interact; a fixed current estimate is not valid across the speed range.
4. Equating electrical input power with shaft power
Copper, iron, switching, friction, windage, and mechanical transmission losses separate input watts from output torque times angular speed.
5. Using average PWM voltage as the complete model
Duty-cycle times supply is useful for an ideal average, but current ripple, inductance, decay mode, switching loss, dead time, back EMF, and driver voltage drop affect real behavior.
6. Choosing gear ratio from speed alone
A gearbox trades speed and torque while adding efficiency loss, backlash, reflected inertia, compliance, and load limits. Acceleration and duty cycle matter alongside steady speed.
7. Confusing Kv and Kt conventions
Kv units and line/phase conventions vary, especially for BLDC motors. The ideal reciprocal relationship requires consistent SI definitions and does not include loss.
8. Misreading encoder PPR and CPR
Manufacturers may quote pulses per revolution, cycles, edges, or counts after quadrature decoding. Gear placement and decode mode can multiply the error.
9. Commanding step rate without torque margin
Stepper torque falls with speed as inductance limits current rise. Microstepping improves smoothness and resolution but does not multiply usable positional accuracy or high-speed torque.
10. Ignoring the thermal duty cycle
Repeated acceleration, low-speed high torque, poor airflow, elevated ambient, enclosure heating, and regenerative events can exceed temperature limits even when average mechanical load looks modest.
Design Scenarios
Supply collapses at startup
At zero speed there is no back EMF, so current is limited mainly by winding, driver, wiring, and source resistance. Check supply current limiting and transient impedance.
Motor overheats at low speed
High torque requires current while shaft-mounted cooling is weak. Calculate RMS copper loss over the duty cycle and use thermal data rather than mechanical output alone.
Encoder speed is four times wrong
A controller counted all quadrature edges while software interpreted the data as pulses per channel. Define PPR, CPR, decode mode, and gearbox location explicitly.
BLDC commutation timing is wrong
Mechanical RPM must be converted using pole pairs. Confusing poles with pole pairs doubles the calculated electrical frequency.
Sizing Workflow
- 1. Define load torque and speed across startup, acceleration, steady operation, braking, and jam.
- 2. Include gearbox ratio, efficiency, inertia, backlash, radial load, and mechanical limits.
- 3. Map required motor torque to current and speed to back EMF using consistent constants.
- 4. Calculate stall, peak, continuous, RMS, and regenerative current conditions.
- 5. Check supply sag, wiring drop, driver resistance, switching loss, current limiting, and protection.
- 6. Model PWM ripple, decay mode, switching frequency, acoustic behavior, and control bandwidth.
- 7. Verify winding copper loss, motor temperature, driver temperature, airflow, and duty cycle.
- 8. Validate feedback counts, step rate, BLDC electrical frequency, and operation on measured curves.
Summary
Size a motor system from the complete duty cycle, not a single nominal point. Tie current to torque, voltage to speed and back EMF, RMS current to heating, and feedback timing to the mechanical system. Verify against manufacturer curves and hardware tests.
Support reference
FAQ
Should a motor driver be sized from stall current?
Stall current is an important peak boundary, but driver selection also needs pulse duration, current limiting, RMS current, braking, supply behavior, thermal impedance, and protection.
Why does motor current decrease as speed increases?
Generated back EMF opposes the applied voltage. Less voltage remains across winding resistance and inductance, so current and available torque change with speed.
Is PWM duty cycle equal to motor speed percentage?
No. Duty affects average applied voltage, while speed also depends on load torque, back EMF, winding loss, driver drop, supply sag, friction, and control method.
How are motor torque and current related?
In the linear region, torque is approximately Kt times current. Saturation, commutation, temperature, and controller behavior limit that approximation.
How should gear ratio be selected?
Check required output speed, continuous and peak torque, acceleration, inertia, efficiency, backlash, mechanical ratings, and motor operation across the duty cycle.
What is the difference between mechanical and electrical RPM in a BLDC motor?
Electrical frequency equals mechanical rotational frequency multiplied by pole-pair count. Electrical RPM is therefore mechanical RPM times pole pairs.
Why can a stepper motor miss steps at high speed?
Available torque drops as current has less time to rise. Load inertia, acceleration, resonance, supply voltage, driver mode, and pulse timing can exceed the torque margin.
Does a motor calculator replace a torque-speed curve?
No. Calculator relationships support estimates, while the manufacturer's torque-speed data and system tests capture motor, drive, voltage, and thermal behavior.