When buying an IE3 efficient electric motor, most buyers look only at the kW power and the speed. Yet the quantity that really determines whether the motor can drive the load, alongside power, is the torque characteristic. Two motors of the same kW power can behave very differently in terms of starting torque and breakdown torque; one drives the load smoothly on direct-on-line (DOL) starting while the other struggles on start-up. In this guide we cover rated torque (Mn), starting torque (Ma) and breakdown torque (Mk) in IE3 motors, together with the load torque characteristic (constant or variable), and explain how to select the right motor for the load on direct-on-line starting. Our aim is to help you source from stock the IE3 motor that truly matches your load, without being misled by the nameplate power.

IE3 electric motor torque-speed curve and nameplate values

What Is Rated Torque (Mn) and Why Does It Matter?

Rated torque is the torque a motor produces while delivering its nameplate power at its nameplate speed. In practice, rated torque follows from the relationship between power and speed: at the same power, a lower-speed (higher-pole) motor produces a higher rated torque. Therefore a 4-pole (1500 rpm) IE3 motor has a noticeably higher rated torque than a 2-pole (3000 rpm) motor of the same power. If your load demands high torque, reviewing the pole count instead of simply increasing the kW is usually the more correct path. You can find the pole-speed relationship in detail in our articles on 2, 4, 6 pole selection and slip and actual speed.

Reading the IE3 motor nameplate correctly is the first step to understanding rated torque correctly. The kW, speed (rpm) and cosφ values on the nameplate form the basis of torque. For reading the nameplate, our guide on reading the IE3 motor nameplate should be reviewed before ordering.

Starting Torque (Ma/Mn) and Breakdown Torque (Mk/Mn)

Starting torque (Ma) is the torque the motor produces when breaking away from standstill, usually given as a ratio to rated torque (Ma/Mn). Breakdown torque (Mk) is the highest torque the motor can produce; if the motor is pushed beyond this point it stalls and stops. In IE3 motors Ma/Mn is typically in the 2.0–2.3 band and Mk/Mn in the 2.5–3.0 band; however these values vary with power, pole count and design.

On direct-on-line (DOL) starting, the motor is connected to the grid at full voltage and produces its full starting torque. This is the simplest way to bring a load online quickly; however the inrush current rises to 6–8 times the rated current. On DOL, the motor ability to drive the load depends directly on the starting torque: the motor starting torque must be higher than the load resistance torque at that instant. Our article on Design N/H torque classes clarifies torque class selection; on the current side, our comparison starting: star-delta or soft starter is valuable.

Load Torque Characteristic: Constant or Variable?

The key to selecting the right motor is recognising the torque characteristic of the load. Loads such as conveyors, cranes, crushers and mills generally demand constant torque: the load torque stays roughly the same regardless of speed and high torque is needed at start-up. Loads such as pumps and fans have a variable torque (square-law) characteristic: at low speed the load torque is very low and rises rapidly as speed increases. Motor selection differs for these two. Our article on constant torque or variable torque covers the distinction in detail.

Constant and variable load torque characteristic chart IE3 motor

Selecting the Right IE3 Motor by Load on DOL

To select the right IE3 motor on direct-on-line starting, three things must be clarified: the load resistance torque at the moment of start, the load torque characteristic (constant/variable) and the load inertia. At start-up the starting torque produced by the motor must exceed the load resistance torque by a clear margin; otherwise the motor cannot accelerate, the current stays high and the thermal protection trips. Under high-inertia loads the start-up time lengthens; in this case the motor thermal capacity and insulation class become important. For insulation class, see our article on winding and insulation class (F/H).

The service factor (SF) should also not be overlooked in DOL selection; a high SF leaves a margin for short-term overloads. On this, our guide on service factor (SF) and overload capacity shows the way. If the grid voltage is low, the starting torque falls with the square of the voltage and start-up becomes harder; for voltage tolerance our article on voltage tolerance and grid fluctuation is important.

IE3 Efficiency Mandate and the Right Power Class

IE3 is today the mandatory efficiency class in many power ranges in the Turkish and EU markets. When selecting the right motor, both the torque suited to the load and the efficiency class compliant with the regulation must be evaluated together. You can find which power range requires which class from which date in our articles on IE3 and IE4 efficiency mandate and IE3 efficiency class mandate. For the right power and speed combination from stock, our IE3 electric motor stock guide is a practical resource.

Right Sizing: Neither Too Large Nor Too Small

Choosing an oversized motor just so it drives the load increases both the initial investment and the efficiency loss at part load; choosing too small leads to start-up stress and overheating. The right path is to calculate the load torque and start-up requirement and size the motor accordingly. For right sizing, our articles on motor load ratio and right sizing and, for power calculation, motor power calculation are core references. If soft starting is needed instead of DOL under high inertia, also consider the option of VFD with an asynchronous motor.

Torque Curve: The Intersection of Motor and Load Curves

The technical essence of correct motor selection lies in how the motor torque-speed curve intersects the load torque-speed curve. As the motor accelerates from zero to rated speed, it produces a certain torque at each speed; this is the motor torque curve. The load also shows a certain resistance torque at each speed; this is the load torque curve. As long as the motor torque at each speed is higher than the load resistance torque at that speed, the motor keeps accelerating. If the two curves intersect in the start-up region, the motor stalls at that point and cannot reach rated speed.

Therefore looking only at the rated point is not enough; you must be sure the motor can “pull” the load across the whole range from start-up to rated speed. Especially in applications starting under load (a loaded conveyor, a crusher), it matters that the motor leaves sufficient torque margin throughout start-up and acceleration. This curve behaviour of an IE3 motor varies with pole and design; lower-speed (higher-pole) motors generally offer a higher torque margin. For pole comparison, our article on efficiency and pole count (2, 4, 6, 8) clarifies the right pole selection.

Breakdown Torque and Short-Term Overload

Breakdown torque (Mk) is the motor “limit of endurance”. If a sudden load increase during operation (for example, a large piece entering a crusher) momentarily stresses the motor, the motor can meet this load up to the breakdown torque; beyond this point the motor stalls and stops. A high breakdown torque provides a safety margin against short-term overloads. Therefore a motor with a high breakdown-torque ratio is preferred under impact and variable loads. The service factor offers a similar safety margin; together they determine the motor real-world durability.

In an IE3 motor, the short-term overload capacity depends on both the breakdown torque and the thermal capacity. If the overload lasts long, the protection circuit shuts the motor down; therefore the protection devices must be set correctly. For protection selection, our article on thermal, relay and fuse selection shows the way. In applications with frequent start-stop, duty type also becomes important; our guide on duty type (S1-S6) selection explains the difference between continuous and intermittent operation. To see how an IE3 motor fits different applications at the same power, our article on use areas of IE3 motors gives an idea.

For all your IE3 motor needs you can review our efficient electric motors product group and source the motor in the right torque class through HEM Motor. In applications needing low output speed with a reducer, our worm gear reducers group offers a complementary solution.

Frequently Asked Questions

Why do IE3 motors of the same kW produce different starting torque?

Starting torque depends, alongside power, on the pole count and motor design (rotor bar shape, torque class). At the same kW, a lower-speed (higher-pole) motor produces a higher rated torque; the rotor design also changes the starting torque. So two motors of the same power can behave differently on start-up.

Which loads cause problems on direct-on-line (DOL) starting?

Loads with high inertia and high starting-torque demand (large mills, crushers, heavy conveyors) cause long start-up times and high current on DOL. For these loads the motor starting torque must be sufficient and the thermal capacity selected appropriately; if needed, soft starting with a soft starter or VFD is preferred.

What should I do if I do not know the load torque characteristic?

Knowing the type of machine you are driving (pump, fan, conveyor, crusher, etc.) is usually enough: pumps and fans have variable torque, conveyors and crushers have constant torque. If you share the machine type and operating conditions with us, we can clarify the IE3 motor suited to the load together.

Get a Quote

Contact us to source the IE3 motor that truly matches your load, in the right power and the right torque class. Share the machine you drive, the required speed and the operating conditions; let us determine the motor suitable for direct-on-line starting. Phone: +90 (532) 345 49 86 — or request a fast quote via our contact page.

Purchasing and Selection Checklist

  • Are the driven machine and the load torque characteristic (constant/variable) identified?
  • Is the required starting torque higher than the load resistance torque?
  • Is the pole count (speed) selected according to the load torque need?
  • Are the load inertia and start-up time suitable for DOL?
  • Are the service factor and insulation class (F/H) checked?
  • Is the IE3 efficiency class compliant with the regulation in the power range?
  • Is the motor sized correctly (neither too large nor too small)?
  • Are the nameplate data (kW, rpm, cosφ) verified before ordering?