Calculation
RV = U/Imax − RA
At standstill there is no back EMF, so only armature and starting resistance limit inrush current.
Busch 2006, Abschnitt 8.2.3.3
DC motor starting resistor
At standstill there is no back EMF, so only armature and starting resistance limit inrush current.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Required starting resistance RV — Additional series resistance sized only for the considered starting condition.
- Starting armature voltage U — Actual DC voltage applied across armature plus starting resistor at standstill.
- Allowed initial current Imax — Maximum permissible armature current immediately after switching, from motor, converter and protection limits.
- Armature-circuit resistance RA — Resistance of armature winding, brushes and permanent connection path at the relevant temperature.
Example
U=230 V, Imax=80 A and RA=0.5 Ω give RV=230/80−0.5=2.375 Ω.
Assumptions and limits
Motor stationary, zero back EMF and constant resistances; start duration, heating, resistor stages and converter control are excluded.
Understand DC motor starting resistor
This calculator sizes added series resistance that limits armature current at standstill and explains why inrush is high before back EMF exists.
What does this quantity describe?
At n=0 rotational back EMF E is zero, so immediately after switching I=U/(RA+RV). Rearranging gives RV=U/Imax−RA.
Formula and variables
RV = U/Imax − RA
Istart = U/(RA+RV)RV = U/Imax − RA
| Symbol / input | Meaning |
|---|---|
| Required starting resistance RV | Additional series resistance sized only for the considered starting condition. |
| Starting armature voltage U | Actual DC voltage applied across armature plus starting resistor at standstill. |
| Allowed initial current Imax | Maximum permissible armature current immediately after switching, from motor, converter and protection limits. |
| Armature-circuit resistance RA | Resistance of armature winding, brushes and permanent connection path at the relevant temperature. |
Choose the inputs correctly
U is actual starting armature voltage. Imax must come from motor, brush, converter and protection limits. RA includes armature winding, brushes and permanent connection path at the relevant temperature.
How to use the calculator
First establish a documented permissible initial current, determine cold or hot armature resistance for the adverse case, and calculate RV. Then size pulse energy, power and switching stages separately.
Worked example
230 V, maximum 80 A and RA=0.5 Ω require RV=2.375 Ω.
Understand the result and units
RV applies only at initial standstill. As speed rises, back EMF develops; leaving the resistor inserted would unnecessarily reduce torque, efficiency and final speed.
Use V, A and Ω; mixed resistance units are converted internally.
Typical applications
Teaching and preliminary sizing of classical resistor starters for DC machines; modern controlled converters usually limit current electronically.
Assumptions, limits and common mistakes
No thermal or time rating, staging or motor acceleration. Inductance, supply resistance, brush drop and converter control are not separately modelled.
Common mistake: Do not automatically use rated current as permissible short-time inrush. If RV<0, RA already limits current below Imax; a negative physical resistor is impossible.
Frequently asked questions
What is “DC motor starting resistor” used for?
Teaching and preliminary sizing of classical resistor starters for DC machines; modern controlled converters usually limit current electronically.
Where do the input values come from?
U is actual starting armature voltage. Imax must come from motor, brush, converter and protection limits. RA includes armature winding, brushes and permanent connection path at the relevant temperature.
What does the result not cover?
No thermal or time rating, staging or motor acceleration. Inductance, supply resistance, brush drop and converter control are not separately modelled.