In this new article we analyze the torque and power curves in an electric motor as the RPM changes. Many of you will be surprised at what you discover in the reading!
The electric motor is a reversible machine
As explained in a previous article, when a voltage is applied to a alectric motor a current flows generating a driving torque providing the rotation.
The rotation of the motor creates a dynamo effect, which generates a current opposing to the supply current, that reduces the overall current.
Role of dynamo effect during operation
What happens when we connect the motor to a battery?
Three distinct phases can be identified in the motor operation based on observation time. We call them T(0), T(x) and Tmax.
T(0): at time 0 (zero) and with the motor at a standstill, the battery voltage generates a current that creates torque; the motor is ready to enter rotation and since the dynamo effect is missing (the motor is still stopped), the current I matches the maximum current Imax.
T(x): at time x, the motor is at a generic RPMx, the dynamo effect is involved and reduces the current to a certain value I(x)<Imax. We know that still current flows, driving torque continues to increase RPM.
Tmax: The process described in the previous point has an end, because at a certain number of revolutions the current produced by the dynamo effect balances the supply current and the total current goes zero. The maximum speed RPMmax is reached.
Relation between current consumption and RPM
It may sound crazy, but we have just found out that electric motor has maximum current draw at startup, and virtually no draw at maximum RPMmax.
In truth, a small current draw is left because of the friction of the brushes (if any), the bearings, and the air present between the stator and the rotor. It is true, however, that the electric motor has the lowest current draw at maximum speed.
Relation between driving torque and RPM
The motor torque T (Torque) is directly proportional to the current drawn, so the torque T behaves in the same way as the current I: T is maximum at the start (RPM 0) and zero at RPMmax.
Relation between power and RPM
Finally, the power P (Power) delivered by the motor is proportional to driving torque T when multiplied by RPM.
At the edges of the graph we find that:
T is Tmax when RPM=0: >>> P=0
RPM is RPMmax when T=0: >>> P=0
The Power is zero at zero RPM and RPMmax and achieves the maximum value at mid-rpm, with a symmetrical trend in the graph.