Home > Why Does an EC Motor Feel Stiff or Jerky?

Why does an EC motor feel stiff or jerky?

When you rotate an EC motor or EC fan by hand, you may notice that it feels stiff or moves in small steps. Compared to a conventional AC motor, an EC motor does not rotate as freely. During start-up, the motor may also move briefly in a jerky manner or run slightly less smoothly at very low speeds.

This often raises the question: is the motor faulty?

In most cases, the answer is no. This behaviour is a normal characteristic of an EC motor.

If you’re new to EC technology, we recommend reading our article “What is an EC fan?“. It explains how EC fans work and why they are increasingly used in industrial ventilation systems.

Why does an EC motor feel stiff?

EC motors are permanent magnet motors (PM motors). Unlike many conventional AC motors, the rotor contains permanent magnets. As a result, a magnetic field is present even when no power is applied.

When you turn the motor shaft or fan impeller by hand, the rotor moves through these magnetic fields. This creates noticeable magnetic resistance, causing the motor to move in small steps instead of spinning freely.

So, if an EC motor feels stiff when turned by hand, this is usually a characteristic of the motor design rather than an indication of a fault.

Would you like to learn more about the differences between both motor technologies? Read our article “EC motor vs AC motor fan“.

Why does an EC motor move jerky during start-up?

When power is applied, the motor electronics first determine the exact position of the rotor relative to the magnetic field. During this process, the motor may move briefly in a jerky manner before accelerating smoothly.

At very low speeds, an EC motor may also run slightly less evenly than a conventional AC motor. This is also completely normal and typical of permanent magnet motor technology.

Differences between an AC motor and an EC motor

Feature	AC Motor	EC Motor
Manual rotation	Spins freely	Feels stiff and moves in small steps
Magnetic resistance	Hardly noticeable	Clearly noticeable due to permanent magnets
Start-up behaviour	Usually smooth	May move briefly in a jerky manner
Rotor	Without permanent magnets	With permanent magnets

Besides these operational differences, EC fans also offer significant advantages in terms of energy efficiency, speed control and service life. Learn more in our article “Energy Savings with EC Fans.”

When is a stiff EC motor not normal?

Although a stiff-feeling EC motor is usually perfectly normal, there are situations where further inspection is recommended. For example, if:

the motor does not start after power is applied;
the impeller rubs against surrounding components;
scraping, grinding or clicking noises are present;
the motor reports an error or fault;
excessive bearing play or bearing damage is detected.

If you’re unsure whether your motor is operating as expected, we’re happy to help you assess the situation.

Looking for the right EC fan?

Are you looking for an energy-efficient fan for your application? Explore our range of EC fans.

Not sure whether an EC fan is the right choice? Our article “Applications of EC Fans” explains where EC fans are commonly used and why they are often the preferred solution.

Conclusion

If an EC motor feels stiff when turned by hand or moves briefly in a jerky manner during start-up, this is usually exactly what you should expect.

Because the rotor contains permanent magnets, a magnetic field is present even when the motor is not energised. This causes an EC motor to behave differently from a conventional AC motor. It is a normal characteristic of permanent magnet motor technology and not an indication that the motor is faulty.

Do you have questions about the operation of an EC motor or would you like advice on selecting the right fan for your application? Feel free to contact us. Our technical specialists are happy to help.