Starting is a critical part of how electric motors work—but not all motors start the same way.
Can both asynchronous and synchronous motors start by themselves?
No, they can’t. Asynchronous (induction) motors can start on their own when power is applied, making them very common in industrial machines. Synchronous motors, however, can't start automatically because their rotor and stator magnetic fields must be synchronized before they can produce torque. Without special help, they just sit there.
Let’s take a closer look at why this happens.
Why can't synchronous motors start by themselves?
It all comes down to timing—and a little thing called inertia.
Why can't synchronous motors self-start?
When power is applied to a synchronous motor, the stator instantly creates a rotating magnetic field. But the rotor doesn’t move right away—it’s stuck at zero because of inertia. Since the rotor isn’t rotating at the same speed as the magnetic field, no torque is created. The motor just hums, but doesn’t move.
🧠 Understanding the sync issue
When I first learned about this, it reminded me of trying to jump onto a spinning merry-go-round. If you don’t match its speed, you’re just going to stumble. That’s what happens inside a synchronous motor—the rotor needs to “jump in” at just the right speed to stay in sync. But it can’t do that alone.
Let’s break it down in simpler terms:
| Part | What Happens When Power is Applied |
|---|---|
| Stator | Generates a fast rotating magnetic field instantly |
| Rotor | Stays still at first due to its weight/inertia |
| Result | No torque is generated because rotor and stator are out of sync |
Without synchronization, nothing happens. That’s why we need to give the rotor a push.
So, how do synchronous motors actually start?
It takes a little outside help to get these motors going.
How do synchronous motors start if they can't self-start?
There are three main ways:
- Auxiliary motor starting: A separate motor gets the synchronous motor up to speed.
- Variable frequency drive (VFD): Slows down the starting speed so the rotor can catch up.
- Asynchronous starting: Uses special windings in the rotor to start like an induction motor.
🔧 Method 1: Auxiliary Motor Starting
This one's old-school. We use another motor—usually an asynchronous one—with the same number of poles to bring the synchronous motor close to its running speed. Once it’s spinning fast enough, we switch off the helper and energize the synchronous motor.
But honestly? It’s kind of a hassle. The setup is complex and not very efficient. That’s why this method is slowly going out of fashion.
⚙️ Method 2: VFD (Variable Frequency Drive) Starting
Now this is the modern way to do it.
A VFD lowers the frequency of the electricity supplied to the motor. This slower “start” gives the rotor time to catch up and sync up. Then, the frequency is gradually increased to reach full speed. It’s smooth, safe, and efficient.
Why I like this method? It’s super reliable and VFD technology is widely available now—even affordable.
🔄 Method 3: Asynchronous Starting
Here’s a clever trick: you build extra windings into the rotor that act like an induction motor at startup. This lets the synchronous motor start up like an asynchronous one. Once it's up to speed, you energize the rotor’s excitation system and boom—it syncs.
Quick heads-up: During startup, you need to short-circuit the excitation winding through a resistor. It’s a safety thing.
결론
Asynchronous motors can start on their own—easy. But synchronous motors? They need a little help from the outside to get up to speed.
