What Is a T-N Curve and Why Does It Matter?
The torque-speed (T-N) curve is the single most important performance specification for a servo motor. It defines the complete operating envelope — every combination of torque and speed the motor can sustain — and it determines whether a motor is suitable for a given application without overloading or overheating.
Yet despite its importance, many engineers select servo motors based solely on rated torque and rated speed, ignoring the shape of the curve between those two points. This guide explains how the T-N curve is generated on a test bench, what each region of the curve means, and how to spot quality issues from curve shape alone.
Clean T-N curve diagram with three labeled regions: Continuous Duty Zone (flat torque), Field Weakening Region (hyperbolic decline), and Peak Torque Zone (dashed line above continuous)
The Three Regions of a Servo Motor T-N Curve
Region 1: Constant Torque Zone (0 to Base Speed)
From zero speed up to the motor’s base speed (also called rated speed or corner speed), a well-designed servo motor delivers constant rated torque. In this region:
- Output torque = rated torque (flat, horizontal line on the curve)
- Output power increases linearly with speed
- The motor operates within its current limit
- Field orientation is fully optimized — no flux weakening applied
The flatness of this region is a quality indicator. A curve that droops before base speed indicates either inverter current limiting, thermal limitations at lower speeds, or cogging torque interference.
Region 2: Field Weakening Zone (Base Speed to Maximum Speed)
Above base speed, the motor’s back-EMF reaches the inverter voltage limit. The inverter can no longer supply full current, so it reduces excitation (flux) to allow higher speed — at the cost of reduced torque. In the field weakening zone:
- Output torque decreases as speed increases (hyperbolic curve)
- Output power remains approximately constant (constant power region)
- The ratio of maximum speed to base speed is called the speed ratio or flux weakening range
A typical high-performance servo motor achieves a speed ratio of 3:1 à 5:1 (par ex., base speed 3,000 RPM, maximum speed 9,000–15,000 rpm). A well-shaped field weakening curve should be smooth and hyperbolic — not stepped or irregular.
Region 3: Peak Torque Zone (Dashed Line Above Continuous)
Above the continuous torque line, the motor can deliver higher torque for short durations — typically 10–30 seconds — before thermal limits are reached. The peak torque zone is bounded by:
- Peak torque at zero speed (typically 2–3× rated torque for permanent magnet servo motors)
- A dashed curve above the continuous T-N curve extending to maximum speed
The ratio of peak torque to continuous torque (peak-to-continuous ratio or overload factor) is critical for applications with high-inertia loads, impulsive loading, or frequent acceleration-deceleration cycles.
Two T-N curves overlaid — one with clean field weakening, one with irregular/stepped field weakening — highlighting the quality difference
How T-N Curves Are Generated on a Test Bench
Test Setup
Generating a T-N curve requires a four-quadrant servo test bench with the following configuration:
- Motor under test (MUT) coupled to load dynamometer via torque-speed sensor and flexible coupling
- Load dynamometer operating in speed-control mode — it sets and maintains the shaft speed while the MUT produces torque
- Torque-speed sensor between MUT and dyno — measures actual output torque and speed in real time
- Power analyzer on MUT electrical terminals — measures input power simultaneously
- Test software — commands the dyno to step through speed points from 0 to maximum rpm
Test Execution: Speed Sweep Method
The most common method for T-N curve generation is the speed sweep:
- Set MUT drive to rated current (couple) limite
- Command load dyno to ramp speed from near-zero to maximum speed at a controlled rate (typically 20–100 rpm/s)
- Record torque, vitesse, tension, actuel, and power at each step
- Software plots torque (Y-axis) vs. vitesse (X-axis) automatically
- Repeat for peak current limit to generate the peak torque curve
The sweep rate must be slow enough for thermal conditions to stabilize at each point, but fast enough that temperature drift doesn’t distort the overall curve. Automated test software manages this balance with configurable dwell times at each speed step.
Continuous vs. Peak Curve Generation
Two curves are generated in sequence:
- Continuous curve: MUT operated at rated current for sufficient time to reach thermal equilibrium at each speed point. This is the S1 duty (continuous operation) curve per IEC 60034.
- Peak curve: MUT operated at peak current for a short duration (10–30 seconds) at each speed point, then allowed to cool. This captures the intermittent capability per S2 or S3 duty cycles.
What the T-N Curve Tells You About Motor Quality
Issue 1: Torque Droop Before Base Speed
What you see: Torque falls below rated value before reaching base speed.
What it means: Inverter under-rated for the motor, or winding resistance too high causing current limiting at low speed.
Action: Check inverter current rating vs. motor peak current rating; check winding resistance and insulation.
Issue 2: Unstable Field Weakening (Stepped or Oscillating)
What you see: Torque curve is not smooth above base speed — it steps, oscillates, or has discontinuities.
What it means: Inverter field-weakening algorithm is poorly tuned, or the motor’s d-axis inductance is outside the inverter’s compensation range.
Action: Re-tune inverter flux-weakening parameters; verify inductance values match inverter specification.
Issue 3: Peak Torque Lower Than Specification
What you see: Peak torque curve is closer to continuous curve than specified (low overload factor).
What it means: Magnets partially demagnetized, or actual peak current limited by inverter protection.
Action: Verify magnet magnetization; check inverter peak current setting and derating policy.
Issue 4: Maximum Speed Not Reached
What you see: Motor cannot reach rated maximum speed even at light load.
What it means: Back-EMF at maximum speed exceeds inverter voltage, or current at maximum speed exceeds inverter capability.
Action: Verify motor back-EMF constant (Ke) vs. inverter voltage rating.
Four small T-N curve sketches showing each defect pattern (droop, stepped field weakening, low peak, speed limited) with red annotation arrows
T-N Curve Parameters: Quick Reference
| Paramètre | Definition | Typical Range (Servomoteur) |
|---|---|---|
| Couple nominal (TN) | Continuous torque at rated speed, S1 duty | 0.1 – 500 N·m |
| Peak torque (Tculminer) | Maximum torque for 10–30 s at rated speed | 2–3 × TN |
| Base speed (nbase) | Speed at end of constant-torque region | 1,000 – 6,000 RPM |
| Maximum speed (nmaximum) | Maximum operating speed at minimum load | 3,000 – 20,000 RPM |
| Speed ratio | nmaximum / nbase | 2:1 – 5:1 |
| Puissance nominale (PN) | TN × nbase × 2π/60 | 0.1 – 500 kw |
| Stall torque (Tstall) | Torque at zero speed (continuous) | ≈ TN (PMSM) or higher |
T-N Curve Testing Standards
The T-N curve test for servo motors follows:
- GB/T 30549-2014 — Permanent-Magnet AC Servo Motors: General Technical Conditions (Chine)
- CEI 60034-12 — Starting performance of single-speed three-phase cage induction motors
- CEI 60034-1 — Rating and performance (duty cycles S1–S10)
Foire aux questions
What is the difference between a T-N curve and a torque-speed characteristic?
They are the same thing — “Courbe T-N” is the common Chinese engineering term (T for torque/转矩, N for speed/转速), while “torque-speed characteristic” ou “torque-speed curve” is the more common English terminology. Both describe the relationship between output torque and rotational speed across the motor’s operating range.
Why does the continuous T-N curve droop at very low speeds?
At very low speeds (below 5–10% of rated speed), some servo motors show a slight torque droop due to increased friction at low speed, cogging torque interference, and reduced inverter current control accuracy at very low electrical frequencies. This low-speed droop is normal for most motors and does not affect applications operating above 10% of rated speed.
How long does T-N curve testing take?
A continuous T-N curve sweep with adequate stabilization time at each point takes 2–4 hours for a servo motor in the 1–30 kW range. The peak torque curve adds another 1–2 hours. Automated test software can run these tests overnight with no operator supervision required.
Can the T-N curve be used for motor sizing?
Yes — the T-N curve is the primary tool for motor sizing in servo applications. The operating cycle (torque vs. time profile) of the application is plotted on the motor’s T-N curve. If all operating points fall within the continuous duty zone, the motor is correctly sized. If some points fall in the peak zone, the RMS torque must be calculated to verify the motor won’t overheat over a complete cycle.
What is the effect of ambient temperature on the T-N curve?
Higher ambient temperature reduces the motor’s continuous torque capability (the continuous T-N curve shifts downward) because the thermal budget for winding heating is reduced. Most manufacturers specify T-N curves at 25°C or 40°C ambient. For installations in hot environments (Middle East, tropical markets), derating factors must be applied.
Conclusion
The T-N curve is far more than a marketing specification — it is a quality fingerprint of the motor and its drive system. Generating it correctly on a calibrated test bench, and knowing how to read its shape, gives engineers the information they need to select motors with confidence and catch problems before they reach production.
EconoTest servo motor test benches generate full T-N curves automatically, with continuous and peak curves plotted in real time and exported in standard report formats.
→ Talk to our engineers about T-N curve testing for your servo motor range.
