Emergency/Backup DC Motors

[eanton22]

I have a situation where because of the distance from the battery to the motor the starting current is an issue.

I have two motors

2HP, FLA=14, 125V DC
7.5HP, FLA=53.3, 125V DC

First off, is there a code requirement that limits the starting current say between 300% to 500% of FLA? Also, is there a time requirement? I am speaking of emergency/backup motors?

Emmanuel

 

[rcwilson]

I am not aware of a code requriement for DC voltage drop or starting time for DC motors.

Usually, DC motors for critical service have a DC motor starter with one or two steps of starting resistors. The resistors are sized to limit the armature inrush current. Too high a current and the motor burns up or the upstream breaker trips. (Overloads do not trip, they alarm only.) If the resistor step is too large there may not be enough current and torque to get the load rolling. Time delay relays or current relays are used to cut the resistors out of the circuit. Starting time is usually less than 7 seconds for emergency lube oil pump applicaitons.

Talk to yor motor control supplier. They may have a DC starter design off the shelf.