Understanding Direct-on-Line (DOL) Starting for Motors up to 7.5 kW
- MTS DNC ENERGY CONSULTANTS LIMITED
- Nov 9
- 2 min read
When it comes to electric motor control, one of the simplest and most commonly used starting methods is the Direct-on-Line (DOL) starter. But did you know that DOL starting is generally suitable only for motors up to 7.5 kW?
Let’s explore why that limit exists and when DOL starting makes sense.
⚙️ What Is a Direct-on-Line Starter?
A DOL starter connects the motor terminals directly to the power supply. This means the motor receives full line voltage instantly, allowing it to start immediately at full torque.
This simplicity makes DOL starters:
Easy to install
Cost-effective
Reliable for small motors
However, this method also causes a very high inrush current, typically 6–8 times the motor’s full-load current.
⚡ Why the 7.5 kW Limit?
Because of that large inrush current, DOL starting can cause:
Voltage dips on the electrical supply network
Mechanical stress on couplings and shafts
Potential interference with other equipment
To avoid these issues, DOL starting is typically limited to motors rated up to 7.5 kW (depending on local utility regulations and the installation’s supply capacity).
Above this power range, the high starting current can become excessive for the supply network — and that’s where reduced-voltage starting methods such as:
Star-Delta starting
Autotransformer starting
Soft starters or Variable Frequency Drives (VFDs)are preferred.
🔧 When to Use DOL Starting
DOL starting is ideal when:
The motor is small (≤ 7.5 kW)
The electrical network can tolerate high starting currents
The driven load can withstand full starting torque (e.g. pumps, small fans, conveyors)
It’s especially suitable in industrial and building services applications where simplicity and cost efficiency are important.
🧠 Key Takeaway
Direct-on-Line starting is best suited for smaller motors — generally up to 7.5 kW — where the supply network and mechanical system can handle the sudden start.
For larger motors, always consider alternative starting methods that provide smoother acceleration, protect your equipment, and maintain supply stability.
📍Need Help Selecting the Right Motor Starting Method?
At Nexus M&E Design, we provide mechanical and electrical design services — including motor selection, starter configuration, and control panel integration — tailored to your project’s specific requirements.
👉 Contact us today to discuss your next project.
Disclaimer: The content shared in these posts is intended for informational purposes only and should not be interpreted as design advice, specifications, or a calculation template. For professional guidance or design services, please contact us through our contact form.
#NexusM&E
