4 Point Starter — Construction, Working Principle & Comparison with 3 Point Starter
What is a 4 Point Starter?
A 4 point starter is a protective device used to start DC shunt motors and compound wound DC motors safely. It limits the high inrush current during starting by inserting external resistance in series with the armature circuit. The name "4 point" comes from its four external connection terminals — L (Line), A (Armature), F (Field), and N (No Voltage Coil).
The key difference from a 3 point starter is that the No Voltage Coil (NVC) is connected independently across the supply through the fourth terminal 'N', rather than being in series with the field winding. This makes the holding coil current independent of field current variations.
Why is a Starter Needed for DC Motors?
When a DC motor is at standstill, the back EMF (Eb) is zero. The armature current at starting is given by:
Since armature resistance Ra is very small (typically 0.5–1.0 Ω), the starting current can be 15–20 times the full-load current. This excessive current can damage the armature winding, commutator, and brushes. A starter inserts external resistance to limit this current to a safe value (1.5–2 times full-load current).
Construction of 4 Point Starter
A 4 point starter consists of the following components:
- Starting Resistance: A series of resistors divided into sections (studs marked OFF, 1, 2, 3, 4, 5, RUN) that are gradually cut out as the motor accelerates.
- Handle (Starter Arm): A manually operated lever that moves over the studs, progressively removing resistance from the circuit.
- No Voltage Coil (NVC): An electromagnet that holds the handle in the RUN position. Connected independently across supply through terminal N with a protective resistance R in series.
- Overload Release (OLR): A protective coil that short-circuits the NVC when current exceeds safe limits, releasing the handle back to OFF position.
- Four Terminals: L (Line), A (Armature), F (Field), and N (No Voltage Coil).
4 Point Starter Circuit Diagram
Working Principle
The operation of a 4 point starter follows these steps:
- Step 1: With the handle at OFF position, no current flows. The motor is disconnected from supply.
- Step 2: When the handle is moved to stud 1, the full starting resistance is inserted in series with the armature. The line current divides into three independent paths:
- Through starting resistance (R1 + R2 + R3...) → Armature
- Through field winding F → produces magnetic flux
- Through NVC + protective resistance R (via terminal N) → holds the handle
- Step 3: As the motor gains speed, back EMF builds up, reducing armature current. The handle is moved to successive studs (2, 3, 4, 5), cutting out resistance sections.
- Step 4: At the RUN position, all starting resistance is bypassed. The motor runs at full speed with the NVC electromagnet holding the handle firmly.
Critical Point: Since the NVC is connected independently across the supply (not through the field circuit), any change in field rheostat setting does not affect the NVC holding force. This is the fundamental advantage over the 3 point starter.
Starting Current Formula
The starting current with external resistance Rext in the circuit:
Where V = supply voltage, Ra = armature resistance, Rext = external starting resistance.
Starting torque: T = K × φ × Ia
4 Point Starter vs 3 Point Starter
Advantages and Disadvantages
Advantages:
- Field rheostat can be used for full-range speed control without affecting starter operation
- NVC holding force remains constant regardless of field current changes
- Suitable for both DC shunt and compound wound motors
- Provides reliable overload and no-voltage protection
- Simple construction and easy maintenance
Disadvantages:
- Does not provide inherent overload protection through NVC (unlike 3 point starter) — requires separate OLR
- If supply fails and returns, motor may restart without operator knowledge (NVC re-energises independently)
- Manual operation — not suitable for frequent starting/stopping or remote operation
- Starting resistance dissipates energy as heat during starting period
Applications
- DC shunt motors requiring speed control above rated speed (field weakening)
- Compound wound DC motors in industrial drives
- Machine tools — lathes, milling machines, drilling machines
- Printing presses and paper mills requiring variable speed operation
- Crane and hoist applications where compound motors are used
Frequently Asked Questions
1. Why is a 4 point starter preferred over a 3 point starter?
A 4 point starter is preferred when speed control by field weakening is required. In a 3 point starter, reducing field current also weakens the NVC, which may release the handle and disconnect the motor. The 4 point starter eliminates this problem by connecting the NVC independently across the supply.
2. What happens if the No Voltage Coil fails in a 4 point starter?
If the NVC fails (open circuit or supply interruption), the spring force pulls the handle back to the OFF position, disconnecting the motor from supply. This provides no-voltage protection — preventing the motor from restarting unexpectedly when supply is restored.
3. Can a 4 point starter be used with a DC series motor?
No. A 4 point starter is designed for DC shunt and compound motors only. DC series motors use a different type of starter (two-point starter) because the field winding carries the full armature current and cannot be separated.
4. What is the function of protective resistance R in the NVC circuit?
The protective resistance R (connected in series with NVC through terminal N) limits the current through the No Voltage Coil to a safe value. It ensures the NVC draws just enough current to produce sufficient electromagnetic force to hold the handle, without overheating.
5. How does overload protection work in a 4 point starter?
The Overload Release (OLR) coil is connected in series with the armature circuit. When armature current exceeds the safe limit, the OLR energises and short-circuits the NVC. This de-energises the holding coil, and the spring pulls the handle back to OFF, disconnecting the motor.
Great explanation of the 4 Point Starter! 🔹 It's very clear how the No Voltage Coil ensures safe operation of DC motors by preventing high starting currents and protecting the armature. For engineers implementing such motor control systems, integrating reliable solutions from a trusted HT and LT Panel Manufacturer ensures efficient, safe, and smooth operation of both high and low voltage circuits in industrial and commercial setups.
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