Sizing Resistors

[ControlConcepts]

How do you go about sizing resistors for a DC motor starter 2 point accelerating, I have a 2Hp and a 5Hp motor. Are there predetermined resistor sizes for different number of steps or is there some formula to determine the size resistor?

 

[ScottyUK]

Depends on winding resistance, voltage , maximum (or minimum) acceptable current.

Minimum current limit will be set by starting torque.
Maximum current limit will be from one or more of the following: the capacity of the DC source; the thermal rating of the motor, in which case acceleration time also matters; mechanical strength of the motor / load / coupling / etc.

Resistors tend to be short-term rated. Time ratings are dependent on how fast your load accelerates, which in turn governs how fast the motor back EMF rises.

If you post some figures for max / min current and voltage, I will send you a worked example with your figures.

 

[jbartos]

Suggestion: Reference:
Direct Current Machines
By M. G. Say, E. Openshaw Taylor
Paperback / Financal Times Management / July, 1986