Rolling mills are a "motor-killer" grade of load
A steel rolling mill is a high-torque, shock-loaded duty. Billets and coils are squeezed between rolls at high force and variable speed, and the drive must handle repeated starting, reversing and speed changes without stalling or overheating. So a mill drive is chosen on starting torque, overload capacity and speed range, not just rated power.
Which motor for which stand?
| Rolling mill duty | Recommended motor | Why |
|---|---|---|
| Continuous speed-controlled main drive | Z4 DC motor | Precise, smooth speed regulation over a wide range, high torque at low speed |
| Roughing stands, high starting torque | YR / YRKK wound-rotor | Rotor-resistive starting gives high starting torque with low starting current |
| Simpler constant-speed auxiliaries | YKK / YKS cage | Rugged, low maintenance |
| Energy-saving variable-speed retrofit | TYPKK permanent-magnet | High efficiency, wide-speed direct drive |
| Mill auxiliaries (rollers, shears) | ZZJ mill-auxiliary DC | Built for mill duty and frequent starting |
How to size a mill drive
The dominant factors are torque and duty, not just power:
- Required torque — work it out from the rolling force and roll radius, then add a margin for shock and acceleration.
- Speed range — a mill often runs far below and above base speed; DC or vector drives cover a wide range, while a fixed-speed cage motor needs a gearbox or VFD.
- Duty cycle — a continuous finishing mill and a reversing roughing mill have very different thermal demands.
- Overload — expect short-duration overloads (e.g. 1.5–2× rated) when a billet is gripped; the motor and its cooling must survive that.
- Ambient heat — near a furnace the motor sees a high ambient temperature; use F/H insulation and adequate cooling (YKS water-air, or separate fan cooling).
An order-of-magnitude guide
As a rule of thumb, a small bar-mill stand might take a 55–160 kW DC or wound-rotor motor, while a heavy plate or large-section stand can run 500–1000 kW+ at 6 kV. Always confirm against the mill builder's rolling-force and speed data rather than guessing.
Wound-rotor vs DC vs PMSM
Wound-rotor (slip-ring) motors give high starting torque with rotor-resistive starting, ideal for roughing stands. DC motors deliver class-leading low-speed torque and precise control for finishing stands. PMSM drives are the high-efficiency, low-maintenance option for new and retrofit variable-speed lines. The right choice depends on the stand, the speed range, and whether it is a new build or a retrofit.
Frequently asked questions
What voltage do rolling-mill motors use? Low-voltage mills use 380/690 V; heavier stands use 3 kV, 6 kV or 10 kV. Export builds are wound for the destination supply.
Do I need soft start or a VFD? For wound-rotor and large cage motors, yes — a rotor starter, soft start or VFD protects the motor and the network from high starting current.
Why does my mill motor overheat? Usually from overload, poor ventilation near the furnace, dust on the cooling ducts, or a mismatch between the duty cycle and the motor's cooling class.
Can you supply a motor for a specific mill? Yes. Send us the stand power, speed, voltage, torque and ambient conditions, and we will match a Z4, YRKK or TYPKK drive and suggest the cooling.