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Home / Technical Articles / Motor Starting via Chokes or Resistors

Reducing the voltage and starting current

The series-connected chokes (Figure 1) or resistors (Figure 2) reduce the voltage at the motor and hence also the starting current. The starting torque is reduced by around the square of the current.

Motor Starting via Chokes or Resistors
Motor Starting via Chokes or Resistors

Starting via chokes

At rest the motor impedance is small. Most of the supply voltage drops across the series-connected chokes.

The breakaway torque of the motor is therefore strongly reduced.

With increasing speed, the voltage across the motor increases because of the fall of the current consumption and the vectorial voltage distribution between the motor and the reactance connected in series.

Motor starting via series-connected chokes
Figure 1 – Motor starting via series-connected chokes

Hence the motor torque also increases. After the motor start-up, the chokes are shorted by the time-delayed main contactor K1M and the starting contactor K2M is dropped out.

Starting via resistors

The basic circuit diagram is the basically the same as described above, with only difference that the chokes are replaced by lower-cost resistors.

Motor starting via series-connected resistors
Figure 2 – Motor starting via series-connected resistors

With this method, the starting current can only be slightly reduced, as the motor torque falls with the square of the voltage and the voltage across the motor, other than with starting via chokes, only increases slightly with increasing speed. It is more advantageous to reduce the series-resistance during starting in steps.

This reduces the voltage across the resistor and increases that across the motor. The expenditure on hardware is thereby significantly larger.

A simpler solution are enclosed electrolytic resistors with a negative temperature coefficient. Their ohmic resistance decreases automatically during starting because of heating by the starting current.

Reference: Low voltage switchgear and control gear application guide – A technical reference handbook for electrical engineers by NHP

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. dogan
    Aug 16, 2014

    now you explained the biggest problem of the World and why we consumed 50% more energy and why we can avoid this by using inverters for example by cars or climatization. we can cut the energy by 50% by increasing oilprices. another problem is the electrical motor dont use a mechanical vitess gear. you could use it but it would work only 4 months so itisnt a solution. THIS IS THE BIGGEST PROBLEM BY ELECTROTECHNİK ANT IT MAKES ALSO VOLTAGE DROP BY DYNAMIC LOADS for example by trains. so if you solve this problem you could also dimension the cables 6 times thinner or could use at home 6 times bigger motors. for example by air cleaners. it is a much bigger problem and easier to solve then by wind generators etc. first patents were 1967 and then 1985 under the name motor start and vectorial control by Siemens. vectorial control means the gear could replaced by electronics so dc motor could replaced by ac motor. i do that since 1981 best electronics for that you can find by freescale microchip infineon atmel.

  2. Nitesh Mishra
    Mar 07, 2014

    Thanks for the information. Can you please give me the working philosophy for starter panel? How do we calculate the value of resistors, contactors ? No of step required? Also can you put some light on field trimmer resistors and what impact does it make?

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