The most reliable way to know whether an asynchronous motor can drive a load correctly is to read the motor's speed-torque (M-n) curve. This curve shows how much torque the motor produces at every speed from standstill to rated speed, and when compared with the load's torque demand, it reveals whether the motor can handle the job. Correctly interpreting characteristic points such as starting torque, pull-up (saddle) torque, breakdown (pull-out) torque and rated torque is the foundation of correct motor selection. In this article we cover the asynchronous motor's M-n curve, the importance of breakdown torque, the intersection with the load torque characteristic, torque classes (Design N/H) and the risk of stalling under overload.

Asynchronous motor speed-torque M-n curve and characteristic torque points

What Is the Speed-Torque (M-n) Curve?

The speed-torque curve of an asynchronous motor is drawn with speed (from 0 to synchronous speed) on the horizontal axis and the produced torque on the vertical axis. Starting from the torque produced at standstill (speed = 0), the torque changes as speed increases and reaches the rated operating point. There are four critical points on this curve:

  • Starting torque (Ma): The torque produced at zero speed, when the motor is still stationary. This torque must exceed the load torque for the load to move from standstill.
  • Pull-up (saddle) torque (Ms): The lowest point of the curve in the mid-speed region after start. If the load torque exceeds the motor torque here, the motor cannot accelerate.
  • Breakdown (pull-out) torque (Mk): The maximum torque the motor can produce. Beyond this point, if the load increases the motor stalls, i.e. its speed drops rapidly and it stops.
  • Rated torque (Mn): The torque the motor can continuously produce at the rated power and speed on the nameplate.

To understand the speed-torque curve, you also need to know the concept of slip and actual speed; find it in our slip and actual speed article, and the relationship between rated and starting torque in our IE3 motor rated and starting torque article.

Breakdown (Pull-Out) Torque: The Motor's Upper Limit

Breakdown torque (Mk) is the maximum torque the motor can produce and is usually between 2 and 3.5 times the rated torque (Mk/Mn ratio). This point sets the ceiling of the motor's load capacity. If the load torque exceeds the breakdown torque:

  • The motor speed drops rapidly (it stalls).
  • The current surges and the motor overheats.
  • If the thermal protection does not trip, the winding can burn.

Therefore, when selecting a motor, sudden torque impacts and transient overloads of the load must be accounted for, and the breakdown torque must be above the load torque with an adequate safety margin. You can review behaviour under overload and service factor in our service factor and overload capacity article, and the extra stresses under impact load in our impact load, flywheel and inertia article.

Intersection with the Load Torque Characteristic

The motor's M-n curve alone is not enough; the load also has its own torque-speed characteristic. The motor operates stably at the point where its own curve intersects the load curve. Load characteristics typically fall into three groups:

  • Constant torque load: demands the same torque regardless of speed (conveyor, crane, extruder, positive-displacement pump).
  • Variable torque (quadratic) load: torque rises with the square of speed (centrifugal pump, fan, blower).
  • Constant power load: high torque at low speed, low torque at high speed (winding machines, some machine tools).

Correct motor selection requires the motor's M-n curve to stay above the load curve at every point; especially in the starting and pull-up region the motor torque must exceed the load torque so the motor can accelerate. We detail the effect of constant and variable torque loads on motor selection in our constant torque or variable torque article. You can find the power calculation for quadratic loads such as pumps and fans in our power calculation for pump, fan and conveyor article.

Intersection of the motor speed-torque curve with constant and variable torque load characteristics

Torque Classes: Design N and Design H

The IEC 60034-12 standard classifies asynchronous motors into torque classes by their starting and breakdown torque values. The two most common classes:

  • Design N (Normal torque): has standard starting torque, suitable for most pumps, fans and general industrial applications. The M-n curve has a typical asynchronous motor profile.
  • Design H (High torque): has higher starting torque; preferred for loads requiring high starting torque such as conveyors, crushers, cranes and extruders.

Choosing the wrong torque class results in the motor being unable to handle the load or in buying an unnecessarily large motor. We cover torque class selection comprehensively in our torque classes (Design N/H) article. You can see the effect of pole count on efficiency and torque in our efficiency and pole count article.

Relationship with Starting Current and Starting Method

As the motor starts from standstill, it produces high starting torque while simultaneously drawing high starting current (5-8 times the rated current). This starting current stresses the supply and switchgear. On large motors, starting methods such as star-delta or soft starter are used to reduce starting current; however, these methods also reduce starting torque. Therefore the M-n curve must be considered together with the starting method. We cover the origin of starting current in our LRA and starting current article, and starting method selection in our star-delta or soft starter article. You can review the starting current issue when running on a generator in our motor selection on generator-powered sites article.

Stall Risk Under Overload and Correct Selection

A motor can run briefly above its rated torque; but if the load continuously approaches the breakdown torque, both thermal and mechanical danger arise. In correct selection:

  • The load's highest (peak) torque demand must be below the motor's breakdown torque with an adequate safety margin.
  • In the starting and pull-up region, the motor torque must be kept above the load torque.
  • At the continuous operating point the motor should be around its rated torque and not held in continuous overload.

For correct sizing and load ratio, our load ratio and correct sizing article; for the thermal effect of frequent start-stop, our starts-per-hour limit article; and for winding temperature protection, our PT100 and thermistor article are good guides. For a general motor selection map, see our electric motor types and purchasing map article.

How Is Rated Torque Calculated?

A motor's rated torque is derived from its rated power and rated speed: torque is found by dividing power by the angular speed (derived from speed). In practice, at the same power a low-speed (high-pole) motor produces higher rated torque than a high-speed motor. For example, a 4-pole (1500 rpm) motor of the same kW has roughly twice the rated torque of a 2-pole (3000 rpm) motor. That is why a low-speed motor is preferred in applications requiring high torque (conveyor, crusher, mixer), and a high-speed motor in applications requiring high speed (pump, fan, blower). You can review the effect of pole count on torque and efficiency in our efficiency and pole count article, low-speed high-pole motors in our 6 and 8 pole low-speed motor article, and power understanding and ordering in our HP or kW article.

The Pull-Up (Saddle) Torque Trap

The most frequently overlooked point on the M-n curve is the pull-up (saddle) torque. As the motor increases speed after start, it passes through a minimum point in the mid-region of the curve. If the load torque exceeds the motor torque exactly at this point, the motor "sticks" at that speed, cannot reach rated speed and overheats by drawing high current. This is seen especially in high-inertia loads (large fans, centrifuges, mills) because the motor stays at low speed, i.e. in the saddle region, for a long time. In correct selection, the pull-up torque must also be kept above the load torque. We address the starting difficulty under high inertia in our impact load, flywheel and inertia article, the effect of starting current on heating during long starts in our LRA and starting current article, and the thermal limit under frequent starting in our starts-per-hour limit article.

The Torque Curve and Operation with a VFD

In a motor running on a frequency drive (VFD), the M-n curve is not fixed; the drive adjusts the output frequency and voltage so that the motor produces similar torque at different speeds. This is a great advantage especially in lifting and conveyor applications requiring high torque at low speed. However, a motor running continuously at low speed on a VFD cools less; therefore forced cooling or derating may be needed. We detail VFD and asynchronous motor compatibility in our VFD with asynchronous motor article, drive selection for constant and variable torque loads in our constant torque or variable torque article, and cooling and harmonic heating at low speed in our VFD harmonics and bearing current article.

Consequences of Wrong Torque Selection

Motor selection without considering the M-n curve and load characteristic leads to two types of error. First, under-selecting the motor: the starting or pull-up torque falls below the load torque, the motor cannot handle the load or reach rated speed, and burns by drawing continuous excessive current. Second, over-selecting the motor: an oversized motor is bought even though the load is light, the motor runs at a low load ratio, efficiency and power factor drop, and both initial investment and energy cost rise. Correct selection requires evaluating the load analysis and the M-n curve together. For correct sizing, our load ratio and correct sizing article; for motor selection steps our electric motor types and purchasing map article; and for matching the right motor to the nameplate our nameplate matching article are recommended.

Frequently Asked Questions

What is breakdown (pull-out) torque and why is it important?

Breakdown torque is the maximum torque an asynchronous motor can produce, usually 2 to 3.5 times the rated torque. If the load torque exceeds this value, the motor speed drops rapidly, it stops and overheats by drawing excessive current. That is why, when selecting a motor, the load's peak torque demand must be below the breakdown torque with an adequate safety margin.

What is the difference between a Design N and Design H motor?

Design N has standard starting torque and suits pumps, fans and general industrial applications. Design H produces higher starting torque; it is preferred in applications that lift heavy loads from standstill such as conveyors, crushers, cranes and extruders. The correct class should be chosen according to the load's starting torque demand.

Why are the load curve and motor curve evaluated together?

The motor operates stably at the point where its own M-n curve intersects the load's torque-speed curve. The motor torque must exceed the load torque, especially in the starting and pull-up region, so the motor can accelerate and reach the rated point. If the two curves are not evaluated together, the motor cannot handle the load or is selected unnecessarily large.

Get a Quote

We supply asynchronous motors with the right torque class (Design N/H) and the correct breakdown torque, matched to your load's torque-speed characteristic. For load analysis, M-n curve interpretation and correct motor selection, contact us at +90 (532) 345 49 86 or via our contact page. For asynchronous motor selection, see our 2, 4, 6 pole selection guide and our electric motors blog.

Purchasing and Selection Checklist

  • Have you determined the load's starting (standstill) torque demand?
  • Have you defined the load's torque-speed characteristic (constant/variable/constant power)?
  • Have you calculated the load's peak (highest) torque demand?
  • Have you verified the motor's starting torque exceeds the load torque?
  • Have you checked the motor's breakdown torque is safely above the load's peak torque?
  • Have you chosen the correct torque class (Design N / Design H)?
  • Have you accounted for how much the starting method reduces the starting torque?
  • Have you confirmed the continuous operating point is around the rated torque?
  • Have you set winding temperature protection and the thermal relay?