Motor NotesConditions for the Maximum Flow of Current in a Motor

In applications using a motor, when determining the capabilities of the motor driver IC and the power supply specifications, one must know in advance what the maximum current flowing in the motor will be. The maximum current under specified steady-state conditions and the like can be learned from the motor specifications, but in actual use, apart from constant forward and reverse rotation, there are also transient operating conditions, such as at startup and when stopping (braking). In this article, the conditions under which the maximum current flows in a motor are discussed.

Conditions Under Which the Maximum Current Flows in a Brushed DC Motor

Under what conditions, or in what state, does the maximum current flow during operation of a brushed DC motor?

During motor operation, the largest current flows immediately after an instantaneous change from a state in which the motor is rotating at maximum speed with no load in one direction to rotation in the opposite direction. The equivalent circuit of a brushed DC motor is shown in Fig. 1; Fig. 2 shows the equivalent circuits immediately before and immediately after reversing the polarity of the power supply when the brushed DC motor is rotating at the maximum rotation rate.

A brushed DC motor can be represented as the equivalent of a series-connected circuit consisting of resistance, inductance, and motor generated voltage. When the motor is rotating in a certain direction at the maximum rotation rate with no load, the motor generated voltage is maximum; this value is Ecmax (see Fig. 2-(a)). In order to change from this state to rotate in the opposite direction (rotation reversal), the power supply is connected to the motor with the polarity reversed (see Fig. 2-(b)).

At this time, the supply voltage and the generated voltage are added and are applied to the series motor circuit of the resistance and inductance. This will result in maximum current flow during motor operation. The changes in current at this time are as indicated by the blue line in Fig. 3.

The vertical broken black line indicates the timing with which the power supply is reversed; at that time, the motor current rises suddenly. If the maximum current is Ipk (broken blue line), then considered in DC terms, a current Ipk = (Ea+Ecmax)/R should flow; but due to the inductance, the time constant affects the current rise, and so the current rise, while steep, has a certain slope. Hence the actual peak current is not as high as the maximum value indicated by the equation.

When the motor power supply is reversed, current flows in the opposite direction, and a torque acts to cause rotation in the direction opposite the rotation up to that point, so that the rotation rate falls. The motor generated voltage is proportional to the rotation rate, and so the generated voltage also falls, and the current flowing in the motor decreases (in the forward-rotation region, differentiated by the broken red line at which the angular velocity is zero-crossed).

Thus the maximum current in a brushed motor occurs after a certain time (the time constant) after switching to reverse-direction rotation from rotation at the highest rotation rate.