In the resistance starting of a motor, reduced voltage is obtained in the primary-resistance starter by means of resistances that are connected in series with each motor stator lead during the starting period. The voltage drop in the resistors produces a reduced voltage at the motor terminals.
At a definite time after the motor is connected to the line through the resistors, timer contacts close; this short-circuits the starting resistors and applies full voltage to the motor. Typical applications include conveyors, belt-driven equipment, and gear-driven equipment.
How does resistance starting work?
The operation of resistance starting can be summarized as follows:
- Pressing the start push button energizes motor starter coil M and timer coil TR. The motor is initially then started through a resistor in each of the three incoming lines. Part of the line voltage is dropped through the resistors with the motor receiving about 75 to 80 percent of the full line voltage.
- As the motor picks up speed, the motor sees more of the line voltage.
- At a preset time the timer on-delay contacts close to energize contactor C coil. This causes contact C to close, shorting out the resistors and applying full voltage to the motor. The resistors’ value is chosen to provide adequate starting torque while minimizing starting current.
- Improved starting characteristics with some loads can be achieved by the use of several stages of resistance shorting. This type of reduced voltage starting is limited by the amount of heat the resistors can dissipate.
Advantages of resistance starting
- It provides closed transition starting, resulting in smooth starting without any transition high current.
- A higher power factor than auto-transformer starters.
- Absence of line current harmonics.
- It allows the motor to be started while on load.
Disadvantages of resistance starting
- It is usually very bulky and expensive when compared with other methods.
- It also has very high maintenance costs.
- A considerable amount of heat is generated through the resistors when current runs through them.
- The starting frequency is limited.
Sometimes as an alternative to resistor starting, reactor starting is used. This method is mainly used for large motors.