# RMS current calculation

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#### Troll

I am looking at predicting the RMS current of a DC motor at a given cycling frequency. It's not so straight forward to me because the current is not linear, but always changing. The motor will rotate about 20 sec in CW direction, brake and hold its position, rotate about 20 sec in CCW direction, and then brake and hold its position. Without using any type of model or recording the current traces, would I be able to calculate this. I'm sure someone had to compute something similar to this in the past. If so, is there a generic equation I can use for this? The given information I have is the parameters of the motor.

#### PhilCorso

Responding to Troll's query... The following procedure will provide a good approximation:

1) There are 4 "ON" operations per cycle. Label them A, B, C, and D.

2) Determine the current for each operation. Label them Ia, Ib, Ic, and Id.

3) Determine the duration of each operation. Label them Ta, Tb, Tc, and Td.

4) Determine the total time for the complete cycle. Label it Tt. If there are no "OFF" periods then Tt should = Ta+Tb+Tc+Td.

5) Irms = sqrt [ ( Ia^2xTa+Ib^2xTb+Ic^2xTc+Id^2xTd ) / Tt ]

Regards,
Phil Corso, PE
Boca Raton, FL
[[email protected]] ([email protected]) {[email protected]}