How to read the characteristic curves?

Characteristic curve

In the first Cartesian dial are shown the Torque, in [Nm] (X axis), the rotation speed, in [Rpm] (Y1 axis), the current consumption in [A] (Y2 axis), the efficiency (Y3 axis).

- For completeness, only in this graph, is represented also the output power curve in[W] (Y4 axis). In the technical data sheets the output power curve is not present.

Below are listed the reference parameters and is explained how to read the characteristic curve.

Continuous functioning area S1  
Nominal tension Vn [V] Supply tension
No load speed n0 [Rpm] Motor's speed with no load applied
No load current I0 [A] Current absorbed by the motor with no load applied
Nominal speed ηN [Rpm] Minimum speed to which the motor can run continuously
Nominal torque MN
[Nm ]
Maximum torque to which the motor can run continuously
Nominal current In [A] Maximum current to which the motor can run continuously
Stall torque Ms [Nm] Torque with blocked rotor
Stall current Is [A] Current with blocked rotor
Maximum power Pmax [W] The maximum power the motor can supply
Maximum efficiency ηmax Maximum value of the relation between produced power and supplied power (C*ω)/ (V*I) that is [Nm*rad *s^(-1)]/ [V*A].

How to read the characteristic curve

To each Torque value corresponds a value of current and speed. Every point, corresponding to a specific load is located on an ideal, vertical line starting from the torque value on the “x” axis.

In the chart is then defined an area called “continuous working area” which encloses all the working condition up to the line defining the nominal values.

The nominal working load is the last one, in ascending order of applied load, to which the motor can work continuously without being damaged.

The nominal values written in the technical data sheets are indicative values and to homologate a product it’s necessary to run a life test.

About the life of a Dc motor or a DC gear motor, there are no parameter to define with precision how long can run a motor under a specific load and with a certain duty cycle.

Every application has in fact their peculiarities, like mechanical friction, peaking, special environmental conditions, which can condition also strongly the brushes life.

Characteristic curves tolerances

Unless otherwise specified, it must be considered as tolerance for the speed, the area between two lines, parallel to the speed line, drawn from the values of +/- 10% of the no load speed.

For the current, it must be considered a tolerance of +/- 10% of each current value.

These tolerances are applied when the motor is “cold" (20°C).