Synchronous Motor Starting Methods — Complete ECET 2026 EEE Notes

Concept Notes (Deep Explanation + Examples)

A Synchronous Motor is a type of AC motor that runs exactly at synchronous speed, meaning its speed is locked with the frequency of the supply.

🌀 Synchronous speed formula:
$N_s = \frac{120f}{P}$
where

$N_s$ = Synchronous speed (in rpm)
$f$ = Supply frequency (Hz)
$P$ = Number of poles

⚡ Why Synchronous Motor Needs a Starting Method?

Unlike induction motors, a synchronous motor cannot start on its own.
When AC supply is given:

The stator produces a rotating magnetic field (RMF).
The rotor (DC excited) tries to align with this rotating field.
But due to inertia, the rotor cannot immediately catch up → it just oscillates instead of rotating.

Hence, special starting methods are used to bring the motor near synchronous speed before it locks in with the RMF.

⚙️ COMMON STARTING METHODS

🔹 1. Damper Winding Method (Self-Starting Method)

Most practical synchronous motors are “Synchronous Induction Motors” because they have damper (amortisseur) windings on the rotor.

Construction:

Damper winding is similar to the squirrel cage in induction motors.
It’s made of copper bars short-circuited at both ends.

Working:

When the motor is switched ON, AC stator field induces currents in the damper winding.
The rotor starts like an induction motor.
As it reaches near synchronous speed, DC excitation is applied.
The rotor magnetic poles lock with stator RMF, and the motor runs synchronously.

Analogy:
Think of damper windings as the “booster wheels” of a bicycle — they help it start rolling before balancing perfectly.

Practical Example:
Used in large synchronous condensers and industrial synchronous motors where smooth start is essential.

🔹 2. Auxiliary Motor (Pony Motor) Starting

A separate small motor called pony motor or auxiliary motor is mechanically coupled to the synchronous motor shaft.

Steps:

Pony motor brings the synchronous motor up to near synchronous speed.
Then DC excitation is given to the rotor.
Once it synchronizes, the pony motor is disconnected.

Used in:

Large synchronous motors where starting current needs to be limited.
Power stations and steel plants with heavy loads.

Site Example:
Seen in power plant alternator sets — where a small DC motor spins the main machine to speed before synchronization.

🔹 3. Using Variable Frequency Drive (VFD) — Modern Method

In modern systems, VFDs or static frequency converters are used for smooth starting.

The VFD gradually increases supply frequency and voltage.
Rotor speed rises steadily until synchronous speed is reached.
No mechanical stress or high current during start.

Used in:

Smart factories
Electric drives
Renewable systems

🔹 4. Using Slip-Ring Induction Motor as Starter

Here, a slip-ring induction motor is temporarily connected to the synchronous motor.

The slip-ring motor accelerates both rotors together.
Once near synchronous speed, DC is applied, and slip-ring motor is disconnected.

Used in:
Heavy-duty synchronous motors (e.g., compressors, blowers, cement mills).

⚡ Summary Table

Starting Method	Type	Used For	Key Advantage
Damper winding	Self-starting	Medium motors	No extra starter
Pony motor	External	Large motors	Low starting current
VFD	Electronic	Modern drives	Smooth & efficient
Slip-ring motor	Coupled	Heavy-duty	High torque start

🌍 Real-World Example

In a power plant, a 3-phase 11 kV synchronous motor drives a water pump.

Initially, damper windings act as an induction motor.
When it reaches near 1500 rpm (for 4 poles, 50 Hz), DC excitation is applied.
Rotor locks with stator field → runs synchronously.

🔑 ECET Important Points

Synchronous motor cannot start by itself — needs external or auxiliary means.
Damper winding = induction motor behavior during start.
VFD method = modern and efficient.
Once in synchronism → speed remains constant irrespective of load.

3️⃣ ⚙️ Formulas (Plain LaTeX Only)

$N_s = \frac{120f}{P}$
$\text{Synchronous speed (for 50 Hz, 4 poles)} = 1500,rpm$
$\text{Slip} = \frac{N_s - N}{N_s}$
$P = \frac{T \times 2 \pi N}{60}$
$\text{Torque} \propto \sin \delta$

$\text{Induced emf in rotor} = 4.44 f N \Phi$

4️⃣ 🔟 10 MCQs (GATE + ECET Mixed)

A synchronous motor cannot start by itself because:
A) Rotor inertia is low
B) Rotor field is DC
C) Rotor magnetic field cannot rotate initially
D) It runs at synchronous speed
The damper winding in a synchronous motor is used for:
A) Reducing core losses
B) Starting purpose
C) Reducing noise
D) Speed control
The pony motor is connected to the synchronous motor to:
A) Increase torque
B) Bring rotor near synchronous speed
C) Supply excitation
D) Control voltage
During starting of a synchronous motor with damper winding, it acts as:
A) DC motor
B) Synchronous generator
C) Induction motor
D) Stepper motor
Which starting method provides smooth and efficient control?
A) Damper winding
B) Pony motor
C) VFD
D) Slip-ring motor
The main function of DC excitation during start is:
A) To cool rotor
B) To create magnetic field
C) To increase current
D) To reduce voltage
The synchronous speed of a 6-pole, 50 Hz motor is:
A) 1000 rpm
B) 1200 rpm
C) 1500 rpm
D) 750 rpm
Slip in a synchronous motor is:
A) Zero
B) 1
C) Depends on load
D) Varies with current
The damper winding is placed on:
A) Stator
B) Rotor
C) Shaft
D) Frame
The modern starting method of synchronous motor uses:
A) VFD
B) Induction starter
C) Commutator
D) Resistance starter

5️⃣ ✅ Answer Key (WordPress Table Format)

Q.No Answer
1 C
2 B
3 B
4 C
5 C
6 B
7 A
8 A
9 B
10 A

6️⃣ 🧠 MCQ Explanations (Step-by-Step)

1️⃣ (C) — The rotor magnetic field is stationary; it cannot rotate initially with AC field, so self-starting fails.
2️⃣ (B) — Damper winding allows induction motor action at start → used for starting.
3️⃣ (B) — Pony motor brings rotor near synchronous speed before excitation.
4️⃣ (C) — With damper winding active, the motor behaves like an induction motor during start.
5️⃣ (C) — VFD allows frequency and voltage ramp-up smoothly.
6️⃣ (B) — DC excitation produces rotor magnetic field required for synchronization.
7️⃣ (A) — $N_s = 120 \times 50 / 6 = 1000,rpm$ .
8️⃣ (A) — Rotor always runs at synchronous speed → slip = 0.
9️⃣ (B) — Damper winding is on rotor poles.
🔟 (A) — Modern systems use VFDs for efficient electronic starting.

7️⃣ 🎯 Motivation / Why Practice Matters (ECET 2026 EEE)

Synchronous motors are a favorite ECET topic — you’ll find questions on speed formula, starting methods, and damper winding in almost every paper.

Mastering this concept builds your understanding of AC machines, electrical drives, and power systems.

When you revise and practice consistently, you develop a strong “machine sense” — the ability to visualize how a motor behaves in real time, like an engineer in a power station! ⚡

“Consistency turns confusion into clarity — one topic at a time.”

8️⃣ 📲 CTA (Fixed)

Join our ECET 2026 EEE WhatsApp Group for daily quizzes & study notes:
🔗 https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9

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