Understanding DC Motor Starters: Types, Working, and Importance

A DC motor starter is a crucial electrical device designed to safely start DC motors by limiting the dangerous high starting current that occurs when a motor begins operation. Without proper starting control, DC motors can experience currents 10-15 times their normal operating current, leading to equipment damage and safety hazards.

Why DC Motors Need Starters: The Problem of High Starting Current

When a DC motor starts from rest, it faces very high starting current. Let us derive why this starting current in DC motors is high, starting from basic operational voltage equation of a dc motor:

E = E_b + I_aR_a

Where:

• E = Applied voltage
• E_b = Back EMF (electromotive force)
• I_a = Armature current
• R_a = Armature resistance

Starting Current Calculation:
From above equation: E - E_b = I_aR_a: Thus, I_a = (E - E_b) / R_a

At startup, back EMF (E_b) = 0, so: I_a = E/R_a

Critical Point: Since armature resistance is typically very low (around 0.5Ω in practical DC machines), the starting current becomes dangerously high.

Consequences of Excessive Starting Current

High starting current creates three major problems:

1. Equipment Damage: Fuses may blow, and armature windings or commutator brushes can be permanently damaged
2. Mechanical Stress: Excessive starting torque (proportional to armature current) creates dangerous centrifugal forces that can damage windings
3. Power System Issues: Other connected loads experience voltage drops due to high current draw

When Direct Starting is Acceptable

Small DC motors (typically under 1 HP) can often start directly because they:

• Have low rotor inertia
• Reach operating speed quickly
• Build back EMF rapidly, naturally limiting current

When Starters are Required

Large DC motors require starters due to:

• High rotor inertia causing slow acceleration
• Prolonged high current periods
• Greater potential for damage

Understanding DC Motor Starters: Types and Working Principles

The primary function of any DC motor starter is to limit the high starting current to a safe and manageable level. This is achieved by introducing external resistance into the armature circuit during the starting phase. As the motor gains speed and its back EMF increases, this external resistance is gradually cut out, eventually allowing the motor to operate at its full rated voltage.

There are various types of DC motor starters, including manual starters (like 3-point and 4-point starters), automatic starters, and modern electronic starters like thyristor controllers. The most common types for shunt wound and compound wound DC motors are the 3-point and 4-point starters.

1. Three-Point Starter (3-Point Starter)

Figure: Three Point Starter Diagram

The 3 point starter is a widely used type for shunt and compound DC motors. Its internal wiring is designed to protect the motor from both overcurrent during starting and loss of supply voltage.

Working Principle:

1. Starting Position: When the DC motor needs to be started, the operator gradually moves a lever (or handle) to the right. As the lever makes contact with the first stud (e.g., "point 1"), the shunt field winding gets directly connected across the supply voltage. Simultaneously, the armature winding is connected in series with the full starting resistance (R1 to R5 in a typical diagram). This maximum series resistance significantly limits the initial armature current.
2. Gradual Resistance Reduction: As the lever is moved further to the right, the external resistance is progressively cut out from the armature circuit. This allows the armature current to gradually increase as the motor accelerates and the back EMF builds up.
3. Run Position: When the lever reaches the final position (e.g., "position 6"), all the starting resistance is removed from the armature circuit, and the armature is directly connected across the supply. At this point, an electromagnet, often called the "No-Voltage Release (NVR) coil" or "Hold-on Coil" (labeled 'E' in the diagram), holds the lever in the "run" position.
4. No-Voltage Release (NVR) Protection: The NVR coil is connected in series with the shunt field winding. If the supply voltage drops significantly or fails entirely, the current through the shunt field (and thus the NVR coil) decreases, weakening its magnetic pull. This causes the spring-loaded lever to return to the "OFF" position, disconnecting the motor from the supply and preventing it from restarting unexpectedly when the voltage returns (which could cause a high starting current again).
5. Overload Protection: A 3-point starter typically includes an "Overcurrent Release Electromagnet" (labeled 'D' in the diagram). If the motor draws current exceeding a predefined safe limit (e.g., due to an overload), this electromagnet gets activated. It short-circuits the NVR coil 'E', causing the lever to be released and the motor to shut down, protecting it from damage.

Consideration: A minor drawback of the 3-point starter is that the starter resistance is in series with the shunt field winding during the starting phase. While the starter resistance is usually small compared to the shunt field resistance, it can cause a slight decrease in the shunt field current, potentially affecting the starting characteristics. Some designs incorporate a brass or copper arc to ensure the field winding remains directly connected to the supply independently of the starter resistance, as seen in some 4-point starter configurations.

Key Features:

• External resistance: Added in series with armature during starting
• Gradual resistance reduction: Manual lever progressively removes resistance
• No-voltage coil (NVC): Electromagnet 'E' holds lever in run position
• Overcurrent protection: Release coil 'D' provides overload protection

Limitation: Starting resistance affects field current slightly, though negligible due to high shunt resistance.

2. Four-Point Starter (4-Point Starter)

Figure: Four Point Starter Diagram

The main difference between a 3 point starter and a 4 point starter lies in the connection of the No-Voltage Release (NVR) coil.

Working Principle:

1. Separate NVR Coil Connection: In a 4 point starter, the NVR coil (Hold-on coil) is not connected in series with the shunt field winding. Instead, it is connected directly across the main supply lines, in series with a current-limiting resistance (R_h).
2. Independent Field Circuit: As the starter lever moves, the field winding gets directly connected to the supply independently of the starting resistance and the NVR coil circuit. This ensures that the current through the shunt field winding remains constant, regardless of the position of the starter lever or any fluctuations in the NVR coil circuit.
3. Reliable Hold-on Coil: This independent connection of the NVR coil ensures that its electromagnetic pull remains consistent and sufficient to hold the lever in the "run" position, even if the shunt field current is adjusted (e.g., using a field rheostat for the purpose of operating the motor above rated speed by weakening the field) or fluctuates. This prevents the lever from unnecessarily returning to the "OFF" position due to variations in field current.
4. Applications: A 4-point starter is particularly advantageous in applications where the DC motor's speed needs to be adjusted by varying the field current. Overload protection is also typically included, similar to the 3-point starter.

Key Advantages Over 3-Point:

• Independent no-voltage coil: Not connected in series with field winding
• Brass arc connection: Ensures field winding gets direct supply connection
• Current limiting resistance (R_h): Protects hold-on coil
• Field rheostat compatibility: Allows field current adjustment without affecting starter operation

Why Choose 4-Point Over 3-Point:

• Field current changes don't affect hold-on coil current
• Prevents unwanted motor shutdown during field adjustment
• Essential for variable speed applications

3. Two-Point Starter (Series Motor Starter)

Figure: DC Series Motor Starter Diagram

The DC series motor starter, also known as a 2 point starter, is specifically designed for DC series motors. Its construction is simpler due to the inherent characteristics of a series motor.

Construction and Working:

1. Basic Design: As shown in the figure, a 2 point starter consists of a main switch, a starting resistance, and a "No-Load Release (NLR) coil" or "Hold-on coil."
2. Starting Process: To start the motor, the start arm is manually moved towards the right. This connects the maximum starting resistance in series with the armature and series field winding.
3. Resistance Cut-out: As the start arm continues to move right, the resistance is gradually cut out from the circuit.
4. Run Position: When the arm reaches the "run" position, all the starting resistance is bypassed, and the motor operates directly across the supply. The "No-Load Release (NLR) coil" holds the start arm in this position.
5. No-Load Protection: The NLR coil is usually connected in series with the motor's main current path or across a portion of it. If the load on a DC series motor becomes very low or is entirely removed, the motor speed can rise to dangerously high levels (as torque is proportional to the square of the current and flux is proportional to current, thus current drops at no-load). The decrease in current activates the NLR coil, releasing the start arm and disconnecting the motor. It also functions as a low-voltage release, ensuring the motor stops if the supply voltage is lost.

Simple Design Features:

• Basic construction: Start arm moves resistance in/out of circuit
• No field complications: Series connection simplifies design
• No-load release coil: Holds arm in run position
• Voltage loss protection: Releases arm when supply voltage fails

[Also Read: Electric Braking of DC Motors]

How to Choose the Right DC Motor Starter

Selection Criteria

Motor Type	Recommended Starter	Key Reason
Small DC Motors (<1 HP)	Direct Starting	Quick acceleration, low inertia
DC Shunt Motors	3-Point Starter	Standard protection, cost-effective
DC Compound Motors	3-Point or 4-Point	Depends on speed control needs
Variable Speed Applications	4-Point Starter	Field adjustment capability
DC Series Motors	2-Point Starter	Simple, series-specific design

Modern Alternatives

Thyristor-Based Starters:

• Electronic control for smooth starting
• Better current limiting
• Reduced maintenance
• Higher initial cost but improved reliability

Installation and Safety Considerations

Proper Installation Guidelines

1. Correct sizing: Match starter rating to motor specifications
2. Environmental protection: Use appropriate enclosure ratings (IP ratings)
3. Proper grounding: Ensure all metallic parts are grounded
4. Ventilation: Provide adequate cooling for starter resistors

Safety Features to Verify

• Overcurrent protection: Properly calibrated release coils
• No-voltage protection: Functional hold-on coils
• Emergency stops: Accessible manual disconnection
• Proper labeling: Clear operational instructions

Maintenance and Troubleshooting

Regular Maintenance Tasks

Monthly Inspections:

• Check contact surfaces for wear or burning
• Verify smooth lever operation
• Test no-voltage coil functionality
• Inspect resistance elements for overheating

Annual Maintenance:

• Complete electrical testing of all circuits
• Resistance measurement verification
• Contact replacement if necessary
• Calibration of protection settings

Common Problems and Solutions

Problem	Likely Cause	Solution
Motor won't start	Faulty no-voltage coil	Test and replace coil
Excessive sparking	Worn contacts	Clean or replace contacts
Starter doesn't hold	Weak electromagnet	Check coil connections/voltage
Overheating	Incorrect sizing	Verify starter-motor compatibility

Frequently Asked Questions

Q: Can I use a 3-point starter for any DC motor?

A: No, 3-point starters are designed for shunt and compound motors. Series motors require 2-point starters.

Q: What happens if I start a large DC motor without a starter?

A: High starting current can damage the motor, blow fuses, and cause voltage drops affecting other equipment.

Q: How do I know if my starter needs replacement?

A: Signs include frequent contact burning, inability to hold run position, or inconsistent starting performance.

Q: Are electronic starters better than manual starters?

A: Electronic starters offer smoother control and reduced maintenance but cost more initially. Choose based on application requirements and budget.

Conclusion

DC motor starters are essential safety devices that protect motors, electrical systems, and connected equipment from dangerous starting currents. Understanding the differences between 3-point, 4-point, and 2-point starters helps ensure proper selection for specific applications.

For reliable motor operation, always:

• Choose the appropriate starter type for your motor
• Follow manufacturer installation guidelines
• Implement regular maintenance schedules
• Monitor starter performance for early problem detection

Need help selecting the right DC motor starter? Consider factors like motor type, power rating, speed control requirements, and environmental conditions to make the best choice for your application.