Starters and Motor Control Circuits
Overview
Starters and Motor Control Circuits is a core chapter in ITI Electrician Semester 3 (NCVT). It covers methods of starting DC and AC motors to limit high inrush starting current, types of starters (DOL, Star-Delta, Auto-transformer, Rotor Resistance), control circuit components like contactors, overload relays, push buttons, and interlocking. Understanding these circuits is essential for industrial motor protection and safe operation in electrical installations.
Typed Notes
Direct On Line (DOL) Starter
DOL starter is the simplest and cheapest type of starter. It connects the motor directly to the full supply voltage. Starting current is 6 to 8 times the full-load current, causing voltage dip in the supply system. Hence DOL is used only for small motors up to 5 HP (3.7 kW). It consists of a contactor, overload relay, and push buttons (Start/Stop). When Start is pressed, the contactor coil energises, main contacts close, and a NO auxiliary contact provides self-holding (latching). Stop button breaks the control circuit, de-energising the contactor.
Star-Delta Starter
Star-Delta starter is the most commonly used reduced-voltage starter for squirrel cage induction motors above 5 HP. During starting, stator windings are connected in Star (Y), reducing voltage across each winding to 1/√3 (57.7%) of line voltage. Starting current and torque both reduce to 1/3 of DOL values. After motor reaches about 75-80% of rated speed, a timer (time-delay relay) switches connections to Delta (Δ) for normal running. The motor must have 6 terminals accessible. Starting torque is low, so it is not suitable for high-torque loads like compressors.
Auto-Transformer and Rotor Resistance Starters
Auto-Transformer Starter uses a three-phase auto-transformer with taps at 40%, 60%, or 75% of supply voltage to reduce starting voltage. Starting current from supply is reduced by the square of the transformation ratio. It is used for large squirrel cage motors where star-delta is not suitable. Rotor Resistance Starter (also called Slip Ring Starter) is used exclusively for Slip Ring (Wound Rotor) Induction Motors. External resistance is inserted in the rotor circuit through slip rings, which increases starting torque while reducing starting current. Resistance is gradually cut out as motor accelerates to full speed.
Control Circuit Components and Interlocking
Key components in motor control circuits: (1) Push Buttons — NO (Normally Open) for Start, NC (Normally Closed) for Stop. (2) Contactor — main contacts carry motor current; auxiliary contacts used for self-holding and interlocking. (3) Overload Relay (OLR) — bimetallic type; NC contact in control circuit opens on overload, tripping the contactor. (4) Timer Relay — used in Star-Delta starter to switch from Star to Delta after set time (typically 5-10 seconds). Interlocking prevents Star and Delta contactors from closing simultaneously, avoiding short circuit. Electrical interlock uses NC auxiliary contact of one contactor in the coil circuit of the other.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Why Starters are Needed
- Starting current 5-7× full load
- Limits current, protects winding
- Overload + no-volt protection
5 HP motor bina starter line voltage drop karta hai — lights dim.
02DOL Starter
- Direct-on-line, full voltage
- Up to ~5 HP suitable
- Simple, cheap
1 HP monoblock pump DOL par — simple aur reliable.
03Star-Delta Starter
- Start in star (voltage ÷√3)
- Run in delta
- 6-lead motor needed
15 HP motor star-delta par — starting current 1/3 ho jaata hai.
04Control Circuit Elements
- Push buttons (NO/NC)
- Contactor coil, OLR
- Interlocking, timer
Star-delta timer 5-10 s baad delta par shift karta hai.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01DOL (Direct On Line) starter is used for motors up to 5 HP; it connects motor directly to supply causing 6-8 times full-load current at start.
- 02Star-Delta starter reduces starting current to 1/3 of DOL starting current; used for motors above 5 HP with both ends of windings accessible (6 terminals).
- 03Auto-transformer starter uses a tapped transformer (40%, 60%, 75% taps) to reduce voltage during starting; suitable for large motors where star-delta is not applicable.
- 04Overload relay (OLR) protects motor from sustained overcurrent; bimetallic strip type is most common in starters — it trips on thermal overload.
- 05Contactor is an electromagnetically operated switch used in motor control circuits; it has main contacts (for power) and auxiliary contacts (for control circuit interlocking).
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Why is a starter necessary for a three-phase induction motor?
What is the function of the self-holding (latching) contact in a DOL starter?
State two advantages and two disadvantages of a Star-Delta starter.
What is the purpose of the overload relay in a motor starter, and how does a bimetallic overload relay work?
Explain why interlocking is essential in a Star-Delta starter.
Starters limit high inrush current during motor starting. DOL is simplest (up to 5 HP); Star-Delta reduces current to 1/3 (above 5 HP, 6 terminals needed); Auto-transformer suits large squirrel cage motors; Rotor Resistance starter suits slip ring motors with maximum starting torque. Control circuits use contactors, OLR, push buttons, and timers. Interlocking is essential in Star-Delta starters to prevent short circuits.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.In a Star-Delta starter, the starting current is reduced to what fraction of the DOL starting current?
Q2.Which type of starter is used for a Slip Ring (Wound Rotor) Induction Motor?
Q3.What is the function of the NO auxiliary contact connected in parallel with the Start push button in a DOL starter?
Q4.A DOL starter is generally used for motors up to which rating?
Mark karo — dashboard par progress dikhega.