So far we have looked at direct on line starting and reduced voltage methods of starting, (star-delta and auto-transformer).  In this post we start on starting methods which rely on the use of resistors to limit the current.  Primary resistance starting is concerned with the introduction of resistance into the stator windings.  Rotor resistance, which I will discuss in the next post is concerned with the introduction of resistance in the motor rotor.

Resistors inserted into the primary of the motor increase the overall impedance of the circuit and reduce the starting current.  In effect the voltage drop across the resistors gives a reduced voltage to the motor terminals. While reducing the current, we should also remember that the torque will also be reduced.  than for a star-delta starter (where the voltage is fixed).

In instances where greater control over the current/torque characteristics is required resistors can be stepped into or out of circuits during the starting period. 

During starting the resistors are kept circuit and when full speed is reached are shorted out (effectively connecting the motor directly to the supply).  The values of the resistors are selected depending on the required limits on starting current and torque required. 

One advantage of primary resistance starting over star-delta is that the motor windings are not reconfigured and a six terminal motor is not required.

It is worth noting that during primary resistance starting the voltage across the motor is not constant.  As the motor accelerates the current drawn by the motor will decrease, resulting in a decrease in voltage drop across the resistors and an increase in motor winding voltage.   This increase in voltage and current means that the developed torque increases faster.  It is also possible to use reactors (not resistors) which would again allow more options on the control of voltage and torque.

Power and Control Circuiting

The power circuit is fairly straight forward. Contactor -KM1 closes and the supply is connected to the motor via the starting resistors.  After a time delay, contactor -KM11 closes, shorting out the starting resistors and connecting the motor directly to the supply.

Functioning of the control circuit starts with the pressing of momentary button -S2.  This energises -KM1, which is then latched (13,14) and power is supplied to the motor via the resistors.  After a time delay (-KM1, 68,68) relay -KM11 energises shorting out the resistors.

Pressing the stop button -S1 removes all power and stops the motor.

-F2 is an optional thermal protection for the starting resistors.  This is kept in circuit while the resistors are in circuit by 22,21.  It effectively limits the number of starts allowed, without over heating the resistors.

Characteristics:

• Available starting current:  70%
• Peak starting current:  4.5 In
• Peak starting torque:  50%

Advantages:

• Adjustable Starting Parameters
• No Break in Motor Supply During Starting

Disadvantages:

• Small Reduction in Peak Current
• Resistance Bank Required

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