The Induction Motor Torque-Speed Curve: Starting, Breakdown and Rated Torque

There is a single graph that best describes the character of an induction motor: the torque-speed curve. This curve shows how much torque the motor can produce at every speed from standstill to rated speed, and at a single glance it reveals whether the motor can lift a load, at which speed it will reach equilibrium and how hard it will struggle at start-up. Every engineer who selects a motor must know how the load torque and the motor torque intersect, because this intersection determines the actual point at which the motor will operate. At DRG Motor, when we design our IE3, IE4 and IE5 class AC induction motors, we carefully address every region of the torque-speed curve. In this article we explain in detail every characteristic point of the curve, from the locked-rotor (starting) torque to the pull-up torque, from the breakdown torque to the rated torque, along with the relationship of torque to slip and the concept of the design class.

What Is the Torque-Speed Curve?

The torque-speed curve is the graph that shows how the torque produced as the motor shaft turns changes with the speed. The horizontal axis carries the speed (or slip), and the vertical axis carries the torque. The curve begins at the starting torque when the motor is at standstill (zero speed), reaches its highest value at a certain point and then drops rapidly toward the rated speed.

The shape of this curve depends on the rotor design, the bar geometry and the electrical parameters of the motor. Two motors of the same power can have completely different torque-speed characteristics; it is precisely this difference that makes a motor suitable or unsuitable for a given application.

Synchronous Speed and Slip

The rotor of an induction motor can never fully reach the synchronous speed of the rotating magnetic field; there is always a slip between them. Torque is produced only thanks to this slip. The closer the rotor approaches the synchronous speed, the smaller the slip becomes and the lower the torque produced. The concept of slip in an induction motor is the key to understanding the torque-speed curve.

Characteristic Torque Points

There are four critical torque points on the curve. The table below summarizes these points and their typical values relative to the rated torque.

These values vary with the motor design, but the general behavior is similar in every induction motor: a high starting torque, a slight dip in the middle, then a peaking breakdown torque and finally stable operation at the rated point.

Starting (Locked-Rotor) Torque

The torque the motor produces while at standstill, that is, while the shaft is not yet turning, is the starting torque. This moment is when slip is at its highest (100 percent). The starting torque must be sufficient to set the load in motion from standstill. The higher the initial resistance of the load, the greater the required starting torque.

Its Relationship with Starting Current

The current drawn at the moment of starting is usually 5 to 7 times the rated current. This high current is the price of high starting torque. To protect the grid and the motor from this current, soft starting (soft starter) methods are used; however, soft starting also reduces the starting torque, and this balance must be established carefully.

Pull-Up Torque

After the motor begins to accelerate, there is a region between the starting point and the breakdown point where the torque drops somewhat. The lowest torque value in this region is called the pull-up torque. If the load torque exceeds the motor torque at this point, the motor cannot continue to accelerate and stalls. For this reason, pull-up torque is of critical importance in hard-starting loads.

Breakdown Torque

Breakdown torque is the maximum torque the motor can produce and is the peak point of the curve. Beyond this point, if the load continues to increase, the motor torque can no longer keep up, the speed drops rapidly and the motor stalls (breaks down). The breakdown torque shows how much the motor can withstand sudden load shocks and temporary overloads.

The Importance of Breakdown Torque

A motor with a high breakdown torque has a stronger reserve against unexpected load increases. For example, when a large piece enters a crusher or a sudden load is applied to a conveyor, the motor can keep running without stalling thanks to the breakdown torque. At DRG Motor, we treat high breakdown torque as a priority in our motors intended for heavy-duty applications.

Rated Torque

Rated torque is the torque the motor continuously produces at the power and speed values written on its nameplate. In normal operation the motor runs near this point, with a small slip. Rated torque is found by dividing the power by the angular speed; this fundamental relationship is addressed in detail in our article on the power, torque and speed relationship.

The Relationship of Slip to Torque

Near the rated point, in the low-slip region, torque and slip change almost linearly: as the load increases the slip increases, and as the slip increases the torque increases. In this way the motor adjusts itself to the load. However, after the breakdown point this relationship reverses, and more slip means less torque.

Operating Point: Intersection with the Load Torque

The point at which the motor will actually operate is where the motor torque curve and the load torque curve intersect. At this intersection point the motor torque and the load torque are equalized and the speed becomes stable. If the load increases, the intersection point shifts to a higher slip and the motor slows down slightly; if the load decreases, the motor speeds up.

Stable and Unstable Operating Regions

The region to the right of the breakdown point (the rated side) is stable: when the load increases, the torque also increases and re-establishes the balance. The region to the left of the breakdown point is unstable; continuous operation of the motor is not possible in this region, and the motor either accelerates into the stable region or stalls.

The Match of Load Type with the Curve

Different loads exhibit different torque-speed characteristics. A conveyor demands a constant torque independent of speed; a fan demands a torque that increases with the square of the speed. The torque curve of the motor must exceed the torque curve of the load across the entire speed range so that the motor can accelerate the load smoothly.

The Concept of NEMA Design Classes

Induction motors are divided into design classes according to their torque-speed characteristics. These classes define different combinations of starting torque, starting current and slip. While one class offers high starting torque and low starting current, another has high slip and is therefore suitable for shock loads.

How Is the Design Class Selected?

In general-purpose applications, a design with medium starting torque and low slip is preferred. For applications requiring high starting torque such as conveyors and crushers, designs with high starting torque are suitable; for shock loads such as presses and shears, designs with high slip are appropriate. The design class directly determines the shape of the curve.

The Effect of Rotor Design on the Curve

The shape and depth of the rotor bars largely determine the character of the torque-speed curve. Deep-bar or double-cage rotors provide high torque and low current at start-up. The choice of rotor type fundamentally changes the starting behavior of the motor. The comparison of squirrel-cage and wound-rotor induction motors addresses this subject in depth.

Torque Control in Wound-Rotor Motors

In wound-rotor motors, the torque-speed curve can be shifted by adding resistances to the rotor circuit. In this way, high torque and low current are obtained at start-up. This feature provides a great advantage in applications requiring high starting torque, such as cranes and lifting.

Managing the Curve with a Frequency Inverter

A frequency inverter can move the torque-speed curve to the desired speed by changing the frequency and voltage applied to the motor. In this way, high torque is maintained at every speed and the starting current is limited. Energy saving with a frequency inverter offers a great opportunity not only in terms of efficiency but also in terms of torque control.

Starting Torque in Compressor Applications

Compressors demand high starting torque because they start from standstill under pressure. If the starting torque of the motor is insufficient, the compressor cannot get moving. Our article on compressor motor starting torque examines this demanding starting condition in detail.

The Curve in Crane and Lifting Applications

In lifting applications the load begins to be lifted at full load from standstill, which requires high starting torque. In the selection of a crane and lifting motor, the starting region of the torque-speed curve plays a decisive role.

Voltage Variation Affecting the Curve

The motor torque is proportional to the square of the applied voltage. If the voltage drops by 10 percent, the breakdown torque decreases by approximately 19 percent. For this reason, at low voltage the motor behaves much weaker than expected at start-up and under overload. The subject of voltage, frequency tolerance and derating explains this sensitivity.

Pole Count and the Position of the Curve

The pole count of the motor determines the synchronous speed and therefore the horizontal position of the torque-speed curve. A two-pole motor reaches its rated point at high speed, while an eight-pole motor reaches it at low speed. The relationship of pole count and speed determines at which speed the curve is positioned.

Torque Reserve and Reliability

The difference between the rated torque and the breakdown torque is the torque reserve of the motor. The larger this reserve, the more resistant the motor is to sudden load increases. A correct motor selection considers not only the rated torque but also a sufficient torque reserve.

The Practical Benefit of Reading the Curve

Being able to read the torque-speed curve shows an engineer in advance how the motor will behave in the field. Will it stall at start-up, will it stop under a sudden load, at which speed will it reach equilibrium; all these questions find answers on the curve. For this reason, the curve is the visual map of motor selection.

The Mathematical Background of the Curve

The shape of the torque-speed curve arises from the resistance and reactance values in the equivalent circuit of the motor. As the rotor resistance increases, the point at which the breakdown torque occurs shifts toward a higher slip; that is, the motor produces higher torque near standstill. The rotor reactance, on the other hand, determines the magnitude of the breakdown torque. The balance of these two parameters is the most powerful tool in the designer's hand. By changing the bar geometry and material, the designer can practically reshape the curve. This is precisely why motors of the same power can exhibit very different starting and breakdown behaviors.

Constant-Torque and Variable-Torque Loads Meeting the Curve

Just as a load's torque demand changes with speed, its intersection with the motor curve is determined accordingly. In constant-torque loads the load line is horizontal and intersects the motor curve at a single point. In variable-torque loads, for example fans and pumps, the load line rises with the square of the speed; in this case, because the load torque is low at start-up, the motor accelerates easily, but as it approaches rated speed the load increases rapidly. Knowing the character of the load makes it possible to see in advance at which point the motor will reach equilibrium.

The Relationship Between Temperature and Torque

If the motor runs in the high-slip region for a long time, heat accumulates in the rotor bars and the winding. High-slip operation, that is, producing high torque well below the rated speed, stresses the motor thermally. For this reason, staying for a long time in the unstable region of the torque-speed curve is dangerous both mechanically and thermally. Torque production always comes at the cost of slip, and this slip turns into heat. While using the motor's torque reserve, the thermal limits must also be observed; otherwise high torque demand rapidly raises the winding temperature.

Starting Time and the Effect of the Curve

It takes a certain amount of time for the motor to bring a load from standstill to rated speed. This time depends on the magnitude of the difference between the motor torque and the load torque. The more the motor torque exceeds the load torque at each point of the curve, the faster the acceleration. When the two curves approach each other, the acceleration slows down; if the gap narrows in the pull-up region, the start lengthens. A long start means a high current is drawn for a longer time and therefore more heating. For this reason, the torque curve running sufficiently above the load curve is essential for a fast and safe start.

The Reflection of the Starting Method on the Curve

The method by which the motor is started directly changes the effective torque-speed curve. In direct-on-line starting the motor uses its full curve and produces the highest starting torque, but the starting current is also the highest. In star-delta starting, because the voltage drops, the torque curve shifts downward; the starting torque falls to about one third. For this reason, the star-delta method is only suitable for applications that start with a light load. Soft starters, on the other hand, smooth the curve by gradually increasing the voltage; they sacrifice starting torque while limiting the current. The correct method must be chosen according to the load's starting torque demand; otherwise the motor cannot move the load and hangs at high current.

Multiple Motors Operating on the Same Curve

In some applications, multiple motors drive the same load together. In this case the torque-speed curves of all the motors must be compatible; otherwise the load can fall onto the motor with the steeper curve. For equal torque sharing, it is important that the slip characteristics of the motors be close to one another. At DRG Motor, we carefully evaluate the compatibility of the motors' torque curves in multi-drive systems.

The Place of the Curve in Maintenance and Monitoring

A motor's starting behavior that weakens over time is often a harbinger of cracks in the rotor bars or high-resistance connections. A start that takes longer than expected can indicate that the motor's torque production has dropped. For this reason, monitoring the starting time is a valuable tool in the early diagnosis of invisible rotor faults. The torque-speed curve guides not only during the selection stage but also in interpreting the health of the motor throughout its life.

The Right Torque Characteristic with DRG Motor

The torque-speed curve summarizes the soul of an induction motor: its power at start-up, its endurance in the middle, its reserve at the peak and its stability at the rated point. At DRG Motor, we design our IE3, IE4 and IE5 class AC induction motors with a torque characteristic suited to the load profile of each application. The right torque curve means the motor lifts the load smoothly, operates stably and has a long life. To determine the motor with the most suitable torque-speed characteristic for your application, you can contact the DRG Motor engineering team, and if you wish, you can discover the right solution for your application by reviewing our industrial electric motors range.