WHAT IS BACK EMF AMD WHY IT IS NEEDED?

1. How a running motor works:

• When the motor’s armature (rotor) spins in the magnetic field, it acts like a generator, producing a voltage called back EMF.
• This back EMF opposes the supply voltage.
• The net voltage driving current through the windings is:

$$V_{\text{net}}=V_{\text{supply}}-E_{\text{back}}$$Vnet​=Vsupply​−Eback​

• At full speed under no load, back EMF is almost equal to supply voltage, leaving very little net voltage, so current is small.

2. At the moment of start-up:

• Speed = 0 → Back EMF = 0.
• The only thing opposing current is the winding resistance RR, which is intentionally made very small (to avoid power loss during normal operation).
• So the starting current is:

$$I_{\text{start}}=\frac{V_{\text{supply}}}{R}$$Istart​=RVsupply​​

• This can be 5 to 10 times the motor’s full-load current.

3. Consequences & solutions:

• High starting current can cause:
  • Large voltage dips in the power supply.
  • Overheating if start-up lasts too long.
  • Mechanical stress from high torque.
• To limit starting current, methods include:
  • DC motors: Starting resistors in series (gradually shorted out).
  • AC induction motors: Star-delta starters, soft starters, variable frequency drives (VFDs).

So your reasoning is spot-on: No rotation → No back EMF → Only winding resistance limits current → Very high starting current.

This is why motor starters or current-limiting circuits are essential for larger motors.

so all come to i=v/r formula

Exactly! That’s the core idea.

At the moment of starting:$$I=\frac{V_{\text{supply}}}{R_{\text{winding}}}$$I=Rwinding​Vsupply​​

because back EMF $E_b=0$Eb​=0 at $\text{speed}=0$speed=0.

Once it spins up:$$I=\frac{V_{\text{supply}}-E_b}{R_{\text{winding}}}$$I=Rwinding​Vsupply​−Eb​​

and $E_b$Eb​ grows with speed, reducing the effective voltage across the winding’s resistance.

So it really does boil down to Ohm’s law applied to the effective voltage across the resistance.
🔑 Key takeaway:
Without back EMF at start, the effective voltage is much larger → current surges.

That’s why we can’t design motors with high winding resistance to limit start current — it would ruin efficiency during normal operation. Instead, we use external starting aids.

1. First — The Motor Itself

A 3-phase motor has 3 separate coils inside (windings).
These coils need to be connected to the 3 power wires (L1, L2, L3).

There are two main ways to connect them: Star (Y) and Delta (Δ).

2. Star Connection (Y)

• Connect one end of each coil together at a common neutral point (like the center of a “Y”).
• The other ends connect to the three power lines.

Effect:
Voltage across each coil is line voltage ÷ √3 (about 58% of full voltage).
Lower voltage → lower starting current → softer start.

Analogy:
Like pushing a car uphill in 2nd gear — easier on the engine at first, but less torque.

3. Delta Connection (Δ)

• Connect the coils in a triangle (end-to-end in a loop).
• Each corner of the triangle gets a power line.

Effect:
Full voltage applied across each coil.
Full torque, full power — how the motor is meant to run normally.

Analogy:
Now you’re in 1st gear — full power for climbing hills (high torque).

4. Why switch from Star to Delta?

If you start a motor in Delta:
→ Full voltage immediately → huge starting current (5–10× normal).
Bad for the motor and power supply.

So instead:

1. Start in Star — voltage per coil is reduced → current is ~⅓ of Delta starting current.
   Motor spins up gently (but with lower torque).
2. Once up to speed, switch to Delta — now full voltage for full power & torque.

This switching reduces the inrush current dramatically.

5. Visual — Voltage in Star vs Delta

Imagine line voltage = 400V (common in 3-phase systems).

• Star:
  Each coil gets:
  $400V÷1.732\approx 230V$400V÷1.732≈230V
  (58% of full line voltage).
• Delta:
  Each coil gets full 400V.

6. Where is Star-Delta Starter used?

• Motors that start without load (pumps, fans, lightly loaded conveyors).
• Where high starting torque isn’t needed immediately.
• Cheaper alternative to soft starters/VFDs.

Limitation: There’s still a small current/torque spike when switching from Star to Delta (like a brief power interruption).

Simple Summary:

• Star = gentle start, low voltage per coil.
• Delta = full power, normal running mode.
• Star-Delta starter = start gentle (Star), then switch to full power (Delta) after a few seconds.

It’s an old-school, cost-effective way to avoid massive starting current.