At standstill, the number that matters is winding loss: P = I²R. With phase A off and phase B held at 150–200 mA, phase B produces continuous heat while preserving torque at the commanded 90° boundary. A year-long dwell is not inherently damaging when winding temperature, insulation temperature, magnet temperature, bearing condition, and driver ratings remain within their specified limits.
Failed holding strategies
Several common changes address the symptom without controlling its real cause. This is heat, not logic: elapsed time becomes secondary after the motor reaches thermal equilibrium, but temperature remains decisive.
| Attempted fix | Why it fails | Better test |
|---|---|---|
| Remove all phase current | Eliminates copper loss but also removes electromagnetic holding torque. Detent torque alone may not preserve position against load torque, vibration, or shock. | Compare unpowered detent torque with the maximum external disturbing torque. |
| Increase holding current for more margin | Heating rises with the square of current. Additional torque may be unnecessary while winding and magnet temperature increase. | Find the lowest current that maintains the required position margin. |
| Treat the 90° boundary as proof of safety | The stable electrical position improves holding behavior but does not establish an acceptable winding temperature or bearing life. | Measure temperature in the actual enclosure and mounting arrangement. |
| Judge safety from current alone | The same current produces different losses in motors with different winding resistance, cooling paths, and ambient conditions. | Use the winding resistance and thermal limits from the motor documentation. |
Current, loss, and thermal equilibrium
For one energized phase, approximate winding copper loss as Pphase = Iphase² × Rphase. At 150 mA, the loss is 0.0225 × Rphase watts. At 200 mA, it is 0.0400 × Rphase watts. The resistance must be entered in ohms. Raising current from 150 mA to 200 mA multiplies winding loss by about 1.78, even though current increases by only one third.
Phase A contributes no deliberate copper loss while it is off. Phase B converts electrical power into heat continuously. Winding resistance rises as the copper warms, and the final temperature depends on the driver’s current regulation, motor construction, mounting surface, enclosure airflow, ambient temperature, and nearby heat sources.
Temperature normally rises toward a steady value rather than increasing without limit. If the steady-state value remains below every applicable motor limit, continuous energization does not create a special electrical failure merely because it lasts a year. A stalled stepper is a normal operating state only when its continuous-current and temperature limits cover that state.
Long-standstill damage mechanisms
Excess winding temperature accelerates insulation aging. Excess rotor temperature can reduce permanent-magnet strength, particularly if current, ambient temperature, or an external heat source pushes the magnetic material beyond its allowable operating region. The motor datasheet or manufacturer must supply those temperature limits; current magnitude alone cannot establish them.
Bearings create a separate time-dependent risk. Long inactivity can allow lubricant to age, migrate, separate, or become resinous. That can increase starting torque even when the energized winding remained cool. The relevant limits come from the motor’s storage-life, lubricant, and bearing documentation rather than from the phase-current rating.
The energized phase creates a magnetic field that favors the held rotor position. It does not remove mechanical aging, contamination, corrosion, shock, or externally imposed shaft load. For equipment expected to sit untouched for a year or longer, storage environment and bearing specifications belong in the same review as electrical heating.
Quantities that decide acceptability
| Quantity | Limit or decision | Where to read or measure it |
|---|---|---|
| Phase current | 150–200 mA in the described hold state | Driver phase-current measurement or current-regulator configuration |
| Phase resistance | Required for I²R loss |
Motor datasheet or a resistance measurement corrected for winding temperature |
| Winding temperature | Below the motor’s continuous insulation limit | Motor documentation and a temperature test at worst-case ambient |
| Rotor or magnet temperature | Below the stated magnetic-material limit | Motor documentation; infer cautiously from a validated thermal test |
| Driver dissipation | Within the driver’s continuous standstill rating | Driver thermal data and measured enclosure temperature |
| Holding torque | Greater than the maximum disturbing torque with engineering margin | Load analysis and a controlled disturbance test |
| Bearing storage condition | Compatible with a year-long stationary interval | Bearing, lubricant, or motor storage documentation |
Confirm what the driver reports as current. DC supply current is generally not interchangeable with regulated phase current because switching conversion and winding recirculation change the relationship. Measure the energized winding current with a method suitable for the driver waveform, or use the manufacturer’s defined phase-current setting.
Holding-current qualification procedure
- Record the exact state: phase A off, phase B energized, commanded position on the 90° boundary, applied load, enclosure condition, and ambient-temperature range.
- Obtain the motor’s phase resistance, continuous phase-current rating, winding-temperature limit, magnet-temperature limit, storage requirements, and bearing or lubricant restrictions. Obtain the driver’s continuous-current and thermal limits as well.
- Calculate phase-B copper loss with
Pphase = Iphase² × Rphaseat both 150 mA and 200 mA. Use the higher current for the thermal qualification unless the controller physically prevents that state. - Command the real hold mode and verify that phase A is actually off. Check for unintended current, periodic refresh pulses, or control-state changes that alter total heating.
- Run a thermal-soak test in the worst credible ambient, enclosure, mounting, and neighboring-equipment condition. Continue until temperatures stop rising materially; a short bench test in open air does not qualify an enclosed installation.
- Compare measured temperatures with the motor and driver limits. Apply the project’s required measurement uncertainty and design margin rather than operating on the published boundary.
- Apply the largest credible disturbing torque, vibration, and supply variation. Reduce current only while the rotor continues to hold position with the required margin.
- After the dwell test, command motion in both directions and check starting torque, current regulation, noise, missed motion, and repeatability at the reference position.
Verification and recurring pitfalls
A passing result requires both thermal and positional evidence. Log ambient temperature, motor case temperature, driver temperature, commanded current, measured phase current, and the time at which temperature stabilizes. Case temperature is not automatically winding or rotor temperature; use the manufacturer’s thermal model or test guidance when internal limits must be inferred.
Repeat the test at 200 mA if software can select any value in the stated range. The 150 mA result cannot qualify 200 mA because the calculated copper loss is about 78% higher. Also test the least favorable shaft load direction, since a stable electrical boundary does not guarantee equal mechanical margin in every installation.
Check recovery after a representative long dwell. Rising breakaway torque points toward bearing or lubricant behavior; excessive case temperature points toward electrical or environmental heating; lost position points toward inadequate holding-torque margin or an uncommanded loss of current. Separate these failure modes before changing the current setting.
FAQ
How do I calculate stepper motor heat at standstill?
For the single energized phase, use Pphase = Iphase² × Rphase. The described range produces 0.0225 × Rphase watts at 150 mA and 0.0400 × Rphase watts at 200 mA.
How do I know whether 200 mA can remain on for a year?
Run the real motor and driver to thermal equilibrium at worst-case ambient and compare winding, magnet, case, and driver temperatures with their published limits. Then review the motor’s bearing, lubricant, and storage-life requirements for the stationary interval.
How do I keep position with one stepper phase off?
Energize the other phase at the lowest current that provides adequate holding torque at the 90° boundary. Verify it against maximum load torque, vibration, shock, supply variation, and current-regulation tolerance.
How do I verify the driver is really supplying 150–200 mA?
Read the driver’s defined phase-current setting or measure winding current with instrumentation suitable for its switched waveform. Supply current is not a substitute for phase current unless the driver documentation explicitly defines that relationship.
When should I stop testing and contact official support?
Stop if the motor or driver approaches a published temperature limit, the required internal temperature cannot be derived from the case measurement, or the storage and lubricant limits are missing. Do not continue an energized year-long hold when insulation, magnet, or bearing limits remain unresolved. Escalate to the motor and driver manufacturers’ official technical support with current, resistance, ambient, mounting, enclosure, temperature-rise, load, and dwell-test records.