Motor Current When Applying a Torque Load

Motor Current When Applying a Torque Load

Equivalent Circuit of DC Brush Motor Connected to a Power Supply

Figure 1. Equivalent Circuit of DC Brush Motor Connected to a Power Supply

Question: When applying a torque load to a DC brush motor by connecting the motor to the driver output side, rotation will become difficult, decreasing rotational speed (RPM). At this point doesn’t the output current increase?

When no torque load is applied, the brush DC motor will rotate at maximum speed. Applying a torque load will cause the speed to decrease in proportion to the size of the load. In addition, motor current is at its lowest point with no torque load, and increases in proportion to the size of the load, while rotational speed reaches its maximum at zero.

An equivalent circuit of a DC power supply connected to a brush DC motor is shown in Figure 1 below.

Equivalent Circuit of DC Brush Motor Connected to a Power Supply

Figure 1. Equivalent Circuit of DC Brush Motor Connected to a Power Supply

Ea: Supply Voltage

Ia: Motor Current

R: Equivalent Motor Resistance

L: Equivalent Motor Inductance

Ec: Generated Motor Voltage

Motor Current When Applying a Torque Load to the Motor

 

With equation 4 above, the graph of the relationship of the current with respect to the torque from equation 3 is shown in Figure 2.

Brush DC Motor T-N/T-I Characteristics

Figure 2. Brush DC Motor T-N/T-I Characteristics

Maximum rotational speed is achieved with no torque load to the motor, but when the torque load increases the rotational speed will drop in proportion to the torque load. Maximum load torque is reached at the point the rotational speed becomes zero. In addition, motor current increases in proportion to load torque, reaching its maximum value when the rotational speed becomes zero as well.

In actual motors, however, a small load torque will be present, even under no load conditions (i.e. due to friction), so the current will never reach zero at maximum rotational speed. (This is represented by the solid border in the above graph. The dotted border indicates zero load torque under ideal conditions.) Increasing the supply voltage will cause the T-N characteristics to travel to the upper right, approaching maximum rotational speed and load torque. The maximum current value will be reached under the T-I characteristics as well.

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