DC Motor vs AC Motor With VFD: Replacing a Rolling Mill DC Motor

When a rolling mill DC motor fails, the drive decides the replacement, not the motor. If the DC motor’s frame and commutator are sound and the thyristor DC drive still has spares and service support, rewind the motor or fit a like-for-like DC mill motor, because both keep the torque, reversal and overload behaviour the stand was designed around. Convert to an AC motor with a VFD when DC drive spares are gone or the mill duty has changed, and treat the conversion as a drive-system project rather than a motor swap.

TL;DR

• A failed DC mill motor has three exits: rewind it, fit a new DC motor, or convert to an AC motor with a variable frequency drive (VFD).

• New India Electricals Ltd (NIEL) recommends keeping DC wherever the motor and its DC drive are a matched, working system, and converting mainly when DC drive spares are unobtainable or the duty has changed.

• Never size the AC replacement from the DC nameplate. Many 800-series motors plated at 230 VDC actually run at 460 or 500 VDC, roughly doubling their speed and power.

• An AC mill duty motor can match the AIST 800-series foundation dimensions, but it cannot use a split frame, so the whole motor goes to the workshop for repair.

• TMEIC published efficiency gains of 3.2 to 4.4 percentage points for AC mill duty motors over equivalent DC motors at rated conditions.

DC motor vs AC motor with VFD: which should replace a failed rolling mill DC motor?

Rewind or replace like-for-like when the DC drive is healthy and supported. Convert to an AC motor with a VFD when the DC drive is obsolete or the duty has changed. The decision also depends on which motor failed: a main stand motor carries reversal and regeneration duty that an auxiliary table motor does not.

TMEIC’s AC mill duty motor in frames 806 to 818 was designed to replace DC mill duty motors, and TMEIC describes it as ideally suited to table motors. A main mill stand with frequent reversals and heavy regenerative braking demands a full drive study before any conversion.

Why rolling mills ran on DC: shunt-wound motors and armature voltage speed control

DC motor speed control is the reason rolling mills chose DC. Varying armature voltage gives constant torque from standstill to base speed, and weakening the field extends speed at constant power. A thyristor converter does this with no AC frequency conversion, which made the DC mill motor the default for reversing and multi-stand duty for decades.

A mill duty DC motor is a heavy-duty DC machine built to the AISE (now AIST) 800-series dimensional and rating standard for steel mill drives, cranes and table service. AIST Standard No. 1, last revised in September 1968, defines frames 802 to 818 and 620 to 624. It lists series, compound and shunt-wound ratings. According to TMEIC, the shunt-wound ratings are the ones most applicable when an AC induction motor replaces the DC machine.

NIEL’s mill duty DC motors are built for this duty:

• Range and duty: 1.5 to 970 kW in frames 802 to 824, rated S1, S4 and S5, conforming to AISE technical report no. 1-1991 and IPSS 1-03-002-94.

• Speed designs: full speed, half speed and quarter speed versions, designed for 3-phase, 6-pulse thyristor converters.

• Overload construction: a fully laminated magnetic circuit permits a very high rate of change of current. Compensating windings are preferred from frame 812 upward, and NIEL’s DC specification states that a fully compensated machine enhances overload and field weakening capability.

When keeping the DC motor and DC drive beats converting

Keep DC when the existing DC machine and its drive are a matched, working system. In that case continued maintenance and rewinding is often more economical than conversion, and the mill keeps the reversal and shock-load tolerance that mill and crane duty demands.

A DC mill motor that has run for decades on a healthy thyristor drive has a proven torque envelope on that stand. Converting means re-proving that envelope with new equipment, so AC plus VFD is not automatically the better call.

DC also still wins new projects on merit. On a hot strip and cold rolling mill project in Nepal, NIEL supplied two 750 kW DC motors and 31 further DC motors of assorted ratings. The same order included 1,000 kW and 1,100 kW 690 V AC motors, so the mill chose DC or AC per duty, not by default.

NIEL’s mill duty DC motors drop onto existing foundations. Mounting dimensions can be matched to replace 600-series motors to AISE or Russian standards, with a replaceable shaft and a 1:9.6 (AISE) or 1:10 (IPSS) tapered double shaft extension. Split yokes are preferred from frame 808 upward. The full range is on our DC motors page, including laminated yoke and solid yoke designs.

When does DC to AC motor conversion pay for itself?

DC to AC motor conversion pays when DC drive spares have become unobtainable, or when the application has changed enough that the original DC sizing no longer fits. The plant then faces new capital anyway, and spending it on an AC motor and VFD removes the commutator from the maintenance plan.

ABB’s modernisation at Kanthal’s rolling mill in Hallstahammar, Sweden, shows the obsolescence trigger. A DC motor and drive had run there for nearly seven decades, with low efficiency and no available spare parts. Kanthal ran a feasibility study of three alternatives before choosing a full AC drive system, commissioned in 2021.

The scope shows what conversion really means: ABB supplied the AC drive and motor, transformer, control system, a new 70-tonne gearbox, foundation work and commissioning. ABB reports total harmonic distortion of 3 to 4% after the upgrade, against 30% before.

Efficiency is the other half of the case. TMEIC’s paper presented at AISTech 2008 gives these rated-condition efficiencies for its DC and AC mill duty motors:

TMEIC adds that DC motors at half or quarter speed fall well below these figures, and that savings more than double where a DC generator set, not a static converter, feeds the motor.

The efficiency rules also change on conversion. IEC 60034-30-1:2025, the standard that defines IE efficiency classes, does not apply to motors with mechanical commutators such as DC motors. A DC mill motor therefore carries no IE class, while its AC replacement does, and India has moved the compliance floor to IE3 for most new LT motor purchases in the mandated kW and pole bands.

Why the DC nameplate will not size your AC replacement motor

The DC nameplate often does not describe how the motor is applied. AIST Standard No. 1 rates 800-series motors at 230 VDC, but many were run at 460 or 500 VDC. Because DC speed and power scale with armature voltage, a motor plated at 230 VDC on a 500 VDC drive is delivering roughly double its plated speed and power.

TMEIC’s ratings show how large the gap is:

An AC motor bought to the 230 VDC nameplate on a 500 VDC application would be undersized by about half. Measure the real duty cycle first.

Overload needs the same care. TMEIC notes that an induction motor may have no overload capability at top speed unless overload is specified across the whole speed range. It also notes that frames 802 to 818 generally lacked compensating windings, so 200% armature current produced less than 200% torque.

Inertia catches buyers too. TMEIC reports its rugged AC mill duty rotors carry higher inertia than its equivalent DC motors, so check acceleration time on a reversing table.

Will an AC motor fit an 800-series DC mill motor foundation?

Yes, if it is built to the AIST 800-series mounting dimensions. TMEIC’s AC mill duty frames 806 to 818 match AIST Standard No. 1 shaft height and foot dimensions, with a maximum deviation of 0.07 inch after metric conversion. The motor lands on the existing foundation, but the frame, shaft and cabling differ from the DC machine.

Plan for four differences:

1. No split frame. A DC mill motor frame splits horizontally so the armature lifts straight out. An AC stator has too many leads crossing the split for that to be practical, so the whole AC motor goes to a motor workshop for repair.

2. No tapered shaft. TMEIC’s AC mill duty shafts are keyed without a taper or locking nut. Where the old DC motor used a tapered coupling, the coupling must be adapted.

3. New cabling. A DC mill motor needs two armature leads and two field leads. The AC motor needs a new 3-phase cable terminated in its terminal box.

4. Bearing protection. TMEIC fits a grounding brush at the non-drive end against shaft currents from inverter switching, and insulated brackets on both bearings from frame 808 upward.

A like-for-like NIEL mill duty DC motor in frames 802 to 824 avoids all four changes.

DC drive vs AC drive: what changes in the panel, the motor and the supply?

A DC drive is a thyristor converter feeding the armature and field. An AC drive is a VFD feeding a 3-phase motor. For rolling mills, the AC drive must be regenerative or share a common DC bus, and the motor must be built for converter duty, not a standard line-fed induction motor.

On the DC side, NIEL supplies DC drives for the modernisation of old plants. They deliver armature output up to 1,380 V in two-quadrant and 1,235 V in four-quadrant operation, with master-slave control. Replacing only the converter keeps a healthy DC motor in service and removes the obsolete electronics.

On the AC side, ABB’s metals application guide sets the rolling mill requirements:

• Regenerative braking: reversing mills generate high braking energy, and ABB drives feed it back to the line supply.

• Common DC bus: multidrives let energy regenerated by one section in braking be used directly by another section, without drawing from the supply.

• Direct Torque Control (DTC): ABB’s DTC updates its motor model every 25 microseconds and controls speed and torque without a shaft encoder.

NIEL supplies the ACS 800 up to 1,900 kW, including multidrives on a common DC bus. We also supply medium voltage AC drives rated as suitable for retrofit of existing motors.

The motor changes as much as the drive. NIEL’s HT motor specification uses insulated bearings in all inverter-driven motors and offers shaft grounding brushes for VFD applications. A totally enclosed fan-cooled motor loses airflow at low speed, so constant-torque table and stand duty needs forced ventilation. We cover that effect in detail in VFD panels: where drives save energy and what they do to your supply.

The rolling line then attacks the new AC motor. We rank the causes we see on rerolling mills:

1. Scale and dust ingress through seals and bearing housings.

2. Water and moisture ingress near cooling beds.

3. Bearing failure from misalignment and vibration under duty-cycle operation.

4. Over-fluxing from voltage and frequency excursions on weak site supply.

5. Phase imbalance on buses carrying large drive loads.

The fix is a real seal inspection schedule rather than a higher IP number on the datasheet. Our rerolling mill duty motors use heavier bearings, enhanced sealing and higher-temperature impregnation for exactly this environment.

Rewinding a DC motor: commutator, carbon brushes and when it is worth it

DC motor rewinding is worth it when the frame, magnetic circuit and shaft are sound and the commutator can be restored. It is not worth it when the failure cause is unknown. The commutator is the most technically complicated part of a DC machine, the most prone to failure, and the part that needs periodic machining, so its condition usually decides the case.

NIEL’s DC motor specification covers the parts that decide rewind life:

• Commutator: arch-bound construction using high-conductivity, hard-drawn, silver-bearing copper segments, built in-house in a dedicated commutator facility.

• Brush gear: constant-pressure brush holders remove the need to adjust spring pressure as carbon brushes wear. The complete brush gear assembly can be rocked and clamped to set the magnetic neutral axis.

• Insulation: vacuum pressure impregnation of the complete armature. Mill duty motors use Class H insulation with Class F temperature rise limits, giving thermal margin for repeated overloads.

• Access: brushes on mill duty motors sit at a 45 degree position for maintenance access, except on laminated yoke versions.

Test before you rewind and again before you energise: our guide to megger testing a motor before you energise it covers insulation resistance and PI ratio, and Class F insulation with Class B temperature rise explains the thermal margin.

The repair-or-replace logic for AC machines is in why an ageing motor costs more than a new one.

What to collect before deciding: a DC mill motor replacement checklist

1. Nameplate and photographs. Record frame, kW, base and top speed, armature and field voltage and current, and enclosure. Photograph the plate while it is readable.

2. Actual application rating. Measure armature voltage and current, field current and speed on the real duty cycle, including rms load and peak overload. The nameplate is not the application.

3. Duty and position. State whether the motor drives a main stand, a reversing stand, a roller table, a shear or a crane, and how often it reverses.

4. Failure report. Identify what failed (armature, commutator, field, bearings or frame) and why, so the fix does not repeat it.

5. DC drive status. Record converter make and model, spares availability and service support. This single item usually decides between keeping DC and converting.

6. Mechanical interface. Record foot dimensions, shaft height, shaft taper or key, coupling, double shaft extension and inertia limits.

7. Supply and panel. Note transformer capacity, existing capacitor banks and fault level. Conversion adds VFD harmonics, which our HT capacitor bank guide addresses.

8. Site conditions. Record ambient at the motor, scale, water spray and supply voltage variation.

9. Downtime limit. State the date the stand must be back in production. A rewind or like-for-like DC motor is a motor job. Conversion is a drive project.

Send us the nameplate, the drive details and what failed, and New India Electricals Ltd will compare a rewind, a like-for-like DC mill motor and an AC conversion for your stand. You can reach the team through our contact page.

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