DC motor starter design

Reference notes

Use Next → on the narrator above to walk through why DC motors need starters and how the classical stepped resistance design works.

The starting problem

At standstill, the motor's back-EMF E_a = K_a·Φ·ω is zero (ω = 0). From V_t = E_a + I_a·R_a, the armature current at startup is:

R_a is small (~5 % per-unit). For a 230 V motor with R_a = 0.5 Ω, I_start = 460 A — typically 10–20 × rated current. Catastrophic for brushes, windings, couplings, and the supply line. DC motors must NOT be started by simply closing a contactor on full voltage; they need a starter.

Stepped armature-resistance starter — the classical solution

Insert an external resistance R_start in series with the armature at startup, sized so that:

where I_max is typically 1.5–2 × rated current (a safe inrush). The motor begins to accelerate. As it speeds up, back-EMF E_a builds:

So I_a drops naturally as the motor speeds up. Just before I_a falls below a chosen minimum I_min (say 1.2 × rated), cut out a step of resistance. The current jumps back up to I_max, motor accelerates more, current drops to I_min, cut another step. Repeat until R_start = 0 and the motor is on full voltage with R_a only. The armature current is kept bouncing between I_min and I_max throughout the start — always within safe limits.

3-point starter

The classical electromechanical starter design has three terminals (hence "3-point"):

• L (line) — connected to the supply +
• A (armature) — connected to one end of the armature
• F (field) — connected to one end of the field winding

A spring-loaded lever sweeps across a row of contacts, gradually shorting out the resistance steps as it moves. Two protective coils:

• No-volt release (NVR): an electromagnet that holds the lever in the "run" position. If the supply fails or is disconnected, the NVR drops out and the lever springs back to the OFF position — so when supply returns, the operator has to deliberately re-start. Prevents uncontrolled restart on supply restoration.
• Overload release (OLR): another electromagnet that monitors armature current. If I_a exceeds a threshold, the OLR short-circuits the NVR coil, dropping the lever and disconnecting the motor.

Weakness of 3-point design: the NVR coil is in series with the shunt field winding. If the field-current regulator (used for speed control above base speed) reduces I_f, the NVR weakens too and may drop out spuriously. A 4-point starter cures this by giving the NVR a separate path.

4-point starter

Add a 4th terminal (N) that connects the NVR coil directly across the supply, independently of the field circuit. Now field-current regulation doesn't affect the NVR. Standard for motors that use field-weakening speed control.

Modern equivalent — current-limited converter

Today's DC drives don't use stepped resistors. A thyristor or IGBT converter on the armature has a current-limit loop that holds I_a at a programmed maximum (typically 1.5 × rated) by reducing V_t. As back-EMF builds, the converter raises V_t to maintain the current setpoint. Continuous current control instead of stepped — smoother, no wasted I²R in external resistors, no moving contacts. The classical starter is now mostly a historical and pedagogical artefact.

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