Technical DataCharacteristics of DC Motors

Basic Characteristics of DC Motors

Generally, the T-N and T-I Characteristics of DC Motors can be expressed by two straight lines as shown below when the supply voltage is constant.
Basic performance is that when torque (load) becomes higher then speed becomes lower and current becomes higher. On the other hand, when torque (load) becomes lower then speed becomes higher and current becomes lower.

And when supply voltage is changed, T-N, T-I curves move parallel.
DC motors have a rated speed and rated current. Please note these parameters. If the motor is used higher than them, commutator and brush might ware out then shorter lifetime or get a trouble due to overheating.
In addition, performance of DC motors change by operation temperature.

If the motor operated under higher temperature, winding resistance gets higher and magnet performance gets lower. Finally, DC motor has lower starting torque and higher no-load speed

Rated voltage[ V ]

A voltage is required to run a motor at the rated load and rated speed. (A voltage when applied to between DC motor terminals.) The rated voltage to use must be ± 10% of the marked voltage. (Please contact us for use outside this range.)

No-load speed【min-1

Speed per minute when a motor is run at the rated voltage under no load.

No-load current【mA】

A value of current when a motor is run at the rated voltage under no load.

Starting torque【mNm】

Maximum current that flows at the rated voltage when a DC motor is started or locked.

Starting current【mA】

Maximum current that flows when the rated voltage is applied.


The DC motor output can be calculated by the following equation:

Maximum efficiency【%】

The maximum value of the input-to-output ratio.

Terminal resistance【Ω】

Resistance between winding/brush and commutator/terminal or lead wire. A value of resistance measured between terminals of a DC motor. In the case of the graphite brush, the resistance changes according to current.

Rotor moment of inertia【gcm2

Moment of inertia of the mass of the rotor.

Rotor inductance【mH】

An approximate value measured between terminals of a DC motor with a measuring frequency of 1 kHz.

Mechanical time constant【ms】

A constant (tm) that indicates a time required by the DC motor from rising from the rest state to reaching 63.2% of the no-load speed.

Torque constant【mNm/A】

Torque that occurs when current is 1A.