The Motor That Tripped on Acceleration
We sized a servo motor for a vertical axis. The average torque (holding the load) was within the motor’s rated torque. On the bench, it held fine. On the floor, during a fast accel/decel (the axis moved 200 mm in 0.3 s), the motor faulted (overcurrent). The problem: we sized by rated torque, not peak torque. The accel/decel torque (to spin up the load inertia) was 3× the rated torque. The motor’s peak (30-second) rating was 3× rated — but the accel was repeated every cycle (continuous). The motor overheated. We either upsized the motor (higher peak) or reduced the acceleration. We reduced the accel (slowed the move) and the motor stayed within its duty. The mistake was confusing rated (continuous) torque with peak (intermittent) torque.
Servo motor rated vs peak torque separates continuous from intermittent. This article covers the sizing.
The Two Torque Ratings
A servo motor spec lists:
- Rated (continuous) torque (T_rated): The motor can output this torque continuously (without overheating). It’s the thermal limit.
- Peak (intermittent) torque (T_peak): The motor can output this torque briefly (e.g., 30 seconds, or for accel/decel). It’s the magnetic (current) limit. Usually 2–3× the rated.
Step 1: Calculate the RMS Torque
The motor sees a torque profile: accel torque, constant-speed torque, decel torque, dwell. The motor heats up based on the average (RMS) torque. The RMS torque must be under the rated torque:
T_rms = sqrt((T_a² × t_a + T_c² × t_c + T_d² × t_d) / t_cycle)
Where T_a is the accel torque, T_c is the constant torque, T_d is the decel torque, and t is the time at each. If T_rms < T_rated, the motor doesn’t overheat (continuous). If T_rms > T_rated, the motor overheats.
The peak torque (T_a during accel) must be under T_peak (briefly). If the accel torque exceeds T_peak, the motor faults (overcurrent).
The torque rule: Check both. T_rms (average) must be under T_rated (continuous). T_peak (accel) must be under T_peak rating (brief). The motor that tripped on accel had T_rms under rated (fine) but T_peak over the peak rating (faulted). Either upsize the motor (higher peak) or reduce the acceleration.
Step 2: The Accel Torque
The accel torque (to spin up the load) is:
T_a = (J_motor + J_load) × α
Where α is the acceleration (rad/s²). For a high-inertia load (article 141) and fast acceleration, T_a is high. If T_a exceeds T_peak, the motor faults. Slow the accel (lower α) or upsize the motor (more peak).
Step 3: Vertical Load (Holding)
A vertical axis (article 123) has a holding torque (the gravity load). Even at rest, the motor outputs torque (to hold the load). This holding torque adds to the RMS (it’s continuous). For a vertical axis, the holding torque is a big part of the RMS. Size the rated torque for the holding (not just the accel).
Step 4: Duty Cycle
The motor’s rating assumes a duty cycle (e.g., 10 minutes on, 10 minutes off). For a continuous cycle (running 24/7), the motor heats up. Use a motor with margin (T_rms well under T_rated). For intermittent use (a cycle that runs 50% of the time), the motor cools between cycles (more peak is OK).
| Check | Must Be Under |
|---|---|
| RMS (average) torque | Rated (continuous) torque |
| Peak (accel) torque | Peak (intermittent) torque rating |
| At-rest holding (vertical) | Rated torque (continuous) |
| Top speed | Rated speed (or field weakening) |
A Torque Sizing Checklist
- What is the accel torque (T_a)? (J × α?)
- What is the constant torque (T_c)?
- What is the decel torque (T_d)?
- Calculate T_rms? (Under T_rated?)
- Is T_a under T_peak? (Brief?)
- Is there a vertical load? (Holding torque?)
- What is the duty cycle? (Continuous?)
- Is the top speed within rating? (Article 67?)
- Does the motor overheat? (Temp?)
- Does it fault on accel? (T_peak?)
The Bottom Line
Servo motor rated vs peak torque checks both. The motor that tripped on accel had T_rms under rated but T_peak over the peak rating. Calculate T_rms (must be under rated) and T_a (must be under peak). For vertical loads, add the holding torque. For continuous duty, use margin. The motor that ran all shift wasn’t the highest peak — it had the right RMS.
