In short. In steel plants, rolling mills and foundries the melting and rolling loads are far too large to back up with diesel, so the generator’s job is safety and asset protection: furnace coil cooling water, tilt hydraulics, ladle cranes, fume extraction, lubrication and the controls that bring the process to a safe state. Size the set from that subset and its restart sequence, protect it from heat and dust, and test it with the real pumps.
What a generator can and cannot do in a melt shop
A medium-frequency induction furnace of a few tonnes draws one to several megawatts through a converter. An electric arc furnace draws tens of megawatts. Rolling mill main drives are measured in megawatts too. None of these are candidates for standby diesel; the plant stops when the DISCOM supply fails and restarts when it returns. What must not stop is the equipment that keeps people and plant safe while the heat is still there. Molten metal sitting in a furnace whose coil cooling has stopped is a lined crucible with water channels next to it, and the failure modes include coil burnout, refractory damage and, in the worst case, a water and metal explosion. Reddy Generators approaches foundries and steel units in Telangana and Andhra Pradesh with that subset as the starting point and the method in generator sizing applied to it.
| Load | Why it is critical after an outage | Tolerable gap | Supply |
|---|---|---|---|
| Induction furnace coil and converter cooling pumps | Coil and capacitor damage within minutes; explosion risk with molten charge | Seconds; many plants add a header tank or diesel-driven pump as a second path | Generator, first step, with independent backup |
| Hydraulic power packs for furnace tilt | Allows emergency pour or holding at a safe angle | Minutes | Generator, early step |
| Ladle and charging cranes, selected hoists | Move hot metal and empty ladles safely | Minutes | Generator, after cooling |
| Fume extraction and baghouse fans | Ventilation of the melt shop while metal is hot | Minutes | Generator, staged |
| Rolling mill lube oil, roll cooling, descaler controls | Bearing damage, strip stuck in stands | Seconds to minutes | Generator with DC or UPS backup for lube controls |
| Control rooms, PLCs, furnace instrumentation | Visibility and control of the safe-state sequence | None | UPS, fed from generator |
| Emergency lighting, gas detection, fire pumps | Life safety | Seconds | Generator |
Cooling water is the load that cannot wait
Induction furnace coils, converter cabinets and capacitor banks are water cooled, and the manufacturers specify how long cooling may be interrupted. That interval is short, and a well-designed melt shop does not rely on the diesel generator alone to meet it. The usual arrangement is an elevated emergency header tank that gravity-feeds the coil for the generator start interval, or a diesel engine-driven pump that starts independently, with the electric pumps returning on the generator. The generator then carries those pumps as its first load step, which is easy to size and must simply work every time, a matter of maintenance and testing. See single-point-of-failure elimination for the logic of the second path.
Hydraulics and the emergency pour
If the safe-state procedure calls for pouring out a furnace during an extended outage, the tilt hydraulics, the ladle crane and the pouring area lighting must all be live together, in addition to cooling. That simultaneous requirement defines the generator’s second step, and the procedure should be written with the furnace supplier and rehearsed under generator power.
Heat, dust and derating
A generator room beside a melt shop or a rolling mill sees ambient temperatures well above the 40 °C that most ratings assume, together with conductive dust, scale and fume. Output must be derated for the real intake temperature, radiators foul quickly, and air filters need shorter intervals than a clean site. The set should be placed to draw intake air from the coolest, cleanest side available, with ducted discharge away from the shop, and the room or canopy should keep a slight positive pressure with filtered air where the dust is severe. Containerised sets need the same thought, because a steel enclosure does not by itself provide filtration or adequate airflow. Painted enclosures near pickling lines or in coastal plants corrode faster and need a specification to match. See ambient temperature derating and generator room ventilation.
P_available = P_rated × (1 − d_T × (T_ambient − T_ref)) × (1 − d_alt × altitude)
P_rated is the nameplate output, d_T the manufacturer’s derate per degree above the reference temperature T_ref, and d_alt the derate per unit of altitude. A worked example (illustrative): a 500 kVA set with a derate of 1 per cent per 5 °C above 40 °C, installed where the intake air reaches 50 °C, gives roughly 500 × (1 − 0.02), or about 490 kVA, before any dust-related loss of radiator performance; the exact factors must be taken from the engine and alternator data sheets, and radiator fouling can cost far more than the published derate.
Power quality, converters and the rest of the plant
Where a plant chooses to run smaller process loads on generator, the furnace converter and any thyristor drives must be checked as harmonic sources against the generator’s much higher source impedance, because distortion the utility transformer swallowed will appear on the generator bus. Power factor correction capacitors left connected to a lightly loaded generator can push it into leading power factor and unstable voltage, so capacitor banks should be interlocked to disconnect on transfer. Earthing is also plant-specific: the generator neutral arrangement must follow the existing plant earthing scheme and the furnace supplier’s requirements, not a generic diagram. See harmonics and THD and earthing and neutral.
Testing with the real loads
A load bank proves the engine. The safe-state sequence has to be proved with the actual cooling pumps, hydraulics and cranes under a simulated mains failure with the furnace cold, recording the time to restore cooling flow, the voltage dip on the crane start and the state of the UPS-fed controls. Where a plant relies on a single set for this duty, a rental set during overhauls is not optional; see rental power for shutdowns.
Common mistakes
- Trying to size a generator for melting or rolling load. The project stalls on cost, and the safety subset that actually matters is never engineered.
- Relying on the generator as the only backup for coil cooling. A failed start during a melt leaves molten metal beside water channels with no second path.
- Placing the set where it breathes melt shop air. Output falls, radiators choke with scale dust, and the engine overheats on the hottest day, which is when it is needed.
- Leaving power factor correction capacitors connected on transfer. The lightly loaded generator sees a leading power factor and the AVR loses voltage control.
- Testing on a load bank only. The crane and hydraulic sequence has never run on the set, and the first real outage finds the problem.
What to send us
Send the furnace or mill supplier’s cooling and safe-shutdown requirements, the list of safety and asset-protection loads with motor ratings and starting methods, the plant single-line diagram with the earthing arrangement, and the location and ambient conditions proposed for the generator. Photographs of the existing set and its air intake side help us judge derating and filtration. Send your application details and we will propose the load steps and the second cooling path with you.
Questions engineers ask
Can a diesel generator run an induction furnace?
Only very small furnaces, and rarely economically. The converter is a heavy harmonic load and the furnace draws its full rating continuously. In practice the generator protects the furnace through its cooling, hydraulics and controls while the DISCOM supply is restored.
How long can furnace coil cooling be interrupted?
The furnace manufacturer states the limit, and it is short, usually a matter of minutes with a molten charge. Plants add a gravity header tank or an engine-driven pump so that cooling does not depend on the generator start alone.
What size generator does a foundry need?
Typically enough for the cooling pumps, hydraulics, one crane, extraction fans, controls and lighting started in sequence, which for many jobbing foundries falls in the range of a few hundred kVA. The exact figure comes from the safety load list and its restart order.
Why does our generator output drop in summer near the furnace?
Intake air temperature and radiator fouling. Engines lose output above the reference ambient and a radiator coated in scale dust cannot reject the heat, so the set derates or trips on high coolant temperature. Relocating the intake and cleaning the radiator on a shorter interval usually recovers it.
Should the control room be on UPS as well as the generator?
Yes. The furnace instrumentation and the PLCs that run the safe-state sequence cannot tolerate the generator start interval, and without them operators cannot see or control the sequence.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.