How does Motor RPM effect amp draw?

Question

In my opinion it does not. However, I am working with pre-existing electrical drawing which specifies a 380v 3 phase electrical system. There are two motors which are both 3/4HP, however, one motor is 1740rpm and the other is 870rpm. The circuit protection device for the 870rpm motor is rated at twice the amperage of the 1740rpm motor. I think this is not nececessary as I can calulate the amp draw of the motor using the HP and Voltage of the system. Can anyone shed some light on the situation?

Answer 1

The current for 3/4 Hp 1740 motor at 380v, 3-phase supply is almost equal to that of a 3/4 HP 840 rpm motor 380v, 3-phase.You may not have to indicate  the running circuit protection.Therefore, the circuit breaker protection should be the same.

Answer 2

"I can calculate the amp draw of the motor using the HP and Voltage of the system"

Keep in mind that the starting current can be significantly higher than the running current, you might investigate what kind of loads the two motors are driving and how often they are started. Perhaps the 870rpm motor is starting a higher inertia load than the 1740rpm one.

[EDIT] Well if the mechanical loads are the same, then I would tend to agree with you that it seems it is not necessary to have the circuit protection device for the slower motor rated twice the amperage as the one for the faster motor.

Another thought that occurs to me,  perhaps there was once a larger motor installed where the 3/4hp 870rpm one is now, or the original installer thought that a larger motor might be needed and decided to be prepared for that possibility.

The circuit protection device is there to protect the wiring, mainly from short circuits, if the motor over load protection is properly sized for the motor,  having a larger than needed supply circuit does little harm.