Many believe that the most important characteristic of any electric motor is power. Not everyone has heard of such an important parameter as “torque” and not everyone understands its importance for traction electric drives.
The graph of the dependence of power (P) and torque (M) on revolutions (n), where: Mnom – rated torque (in S1 mode), N * m; Mmax – maximum torque (in S2 mode), N * m; Pnom – rated power (in S1 mode), kW; Pmax – maximum power (in S2 mode), kW; N – rated speed, rpm
From the graph of the dependence of the power of a synchronous electric motor on the speed, it can be seen that its power is not a constant value, but in the operating speed range it grows linearly as the speed increases. To accurately determine the power (P) of the electric motor, the optimal point on the graph is selected as its technical characteristic, which determines the rated power (Pnom) at rated speed (N).
By increasing the rated speed of a synchronous electric motor, it is possible to proportionally increase its rated power, which, in turn, leads to an increase in the specific power.
Chasing power
Most designers and manufacturers of electrical machines try to achieve high power by increasing the rated speed to several thousand or even tens of thousands of revolutions per minute. However, as the speed of the electric machine increases, several unpleasant problems grow like an avalanche: starting with the mechanical balancing of the rotor and ending with high-frequency losses in the stator. The main high-frequency problems are the so-called “skin effect” in the windings, Foucault eddy currents in the core and the speed of the power switches of frequency converters. Overcoming these problems leads to a significant increase in the cost of the electric motor.
Rotating force
For traction electric drives, it is necessary, first of all, to provide the required rotational force, which is also called “moment of force” or “torque” (M). In such systems, the rotational force must reach maximum values from the start.
Unlike asynchronous motors, it is in synchronous electric motors that the rated and maximum torques remain constant throughout the entire operating speed range. Constant rated torque is one of the most important advantages of synchronous motors and is ensured even at minimum power. In the graph, the corresponding torques are indicated by “Mnom” and “Mmax”.
Unfortunately, under real conditions, an increase in the rated speed of the electric motor leads to a decrease in the specific torque (M). If you want to get the minimum weight-dimensions of the electric motor, then you have to choose between the concepts of high power density due to high revs and high specific torque at low or ultra-low speeds.
Converting power to torque
Before the creator of any traction electric drive using a high-speed electric motor, there is a need to convert power into torque, accompanied by a transition from high to low speeds. This transformation is carried out using a mechanical gearbox.
However, the use of a gearbox leads to an inevitable increase in the complexity, weight, dimensions and cost of the electric drive against the background of a decrease in efficiency, reliability and service life. If we are talking about a servo drive, then any mechanical gear, among other things, reduces the positioning accuracy.
Getting rid of the gearbox!
The use of traction synchronous electric motors with a high specific torque can significantly reduce the reduction ratio or completely abandon the mechanical gearbox, providing a transition to a direct drive with all the ensuing advantages.
The specific torque of EM and iEM series synchronous electric motors is 5-10 times higher than that of asynchronous and collector motors of similar power! Such a high torque of EM / iEM electric motors allows either to significantly reduce the reduction ratio, or to completely dispense with a mechanical gearbox!
Thanks to patented technologies, synchronous electric motors of the EM / iEM series are 2-5 times more compact and lighter than other synchronous (including “valve”) and collector motors with the same torque!
Constant power mode
The graph of the dependence of power and torque on revolutions shows that when the nominal speed (N) of the synchronous electric motor is exceeded, its torque (M) begins to decrease. The efficiency also drops. This is because almost any electrical machine is reversible, and can simultaneously operate as an electric motor and as an electric generator. When the rated speed is exceeded, the voltage generated by the electric motor begins to “fight” with the supply voltage from the frequency converter. The more the revolutions increase relative to the nominal, the more the electric motor affects the sources
