Electric motor repair: when to rewind, when to replace, and what the workshop should test
Headcore
In short
Electric motor repair often means rewinding the stator, then testing. In independent trials, good-practice rewinds lost 0.1 efficiency points on average. Replace if repair costs over 60 to 65% of a new motor or the motor is small, and often if old and under about 30 kW. In South Africa, new 0.75 to 375 kW motors must be IE3.
Can an electric motor be repaired, or is it better to replace it?
Most failed induction motors can be repaired, and for larger ones a rewind is often the better choice: the US Department of Energy's 2014 motor guide says "For motor repairs, rewinding is often the best decision."[5]
The guide lists what decides it: the repair price, any efficiency lost in the rewind, the price of a new premium efficiency motor, the motor's size, original efficiency and load, its running hours, the electricity tariff and the payback the plant accepts. Installation labour drops out, because a failed motor comes out and goes back in either way.[5]
When is replacing an electric motor cheaper than rewinding it?
In practice, the US Department of Energy's 2014 motor guide favours replacing small, old, general-purpose motors and rewinding large or special ones, but says to price both options rather than rely only on its rules of thumb.[5]
| Situation | Leans to | Why |
|---|---|---|
| Repair quote above 60 to 65% of the price of a new premium efficiency motor | Replace | Better reliability, a new warranty and lower running costs recover the difference |
| General-purpose motor under 20 to 25 hp (about 15 to 19 kW) | Replace | The repair may cost more than a new premium efficiency motor |
| Motor under 40 hp (about 30 kW) and more than 15 years old | Often replace | Its efficiency is often well below that of a current premium efficiency motor |
| Large motor in a plant that runs around the clock | Rewind | Many continuous process plants repair above 50 hp (about 37 kW) and replace below it |
| Special or definite-purpose motor | Rewind | A new one carries a significant price premium |
| Small premium efficiency motor, out of warranty, that needs a rewind | Replace | Repairing a premium efficiency motor generally carries a 15% cost adder |
| Core damaged in the failure or while stripping | Replace, or restack the core | Damaged lamination insulation cannot be repaired |
| Repeated failures | Find the cause first | Root cause analysis before either option |
Sources:[5]
What efficiency must a new motor have in South Africa?
In South Africa, a new motor within the scope of compulsory specification VC 9113 must have "a minimum efficiency class of IE3 as specified in SANS/IEC 60034-30-1" (clause 4.1), and the manufacturer or importer needs an NRCS letter of authority for each motor type before offering it for sale, import or supply (clauses 2.1.1, 2.1.2 and 3.1).[1]
VC 9113 was published on 3 June 2024 to take effect 12 months later, so from 3 June 2025.[1]
VC 9113 covers 2-, 4-, 6- and 8-pole motors of 0.75 kW to 375 kW rated above 50 V and up to 1,000 V, geared motors included (clause 1.1). It excludes single-speed motors with 10 or more poles, multi-speed motors, motors with mechanical commutators such as DC motors, motors completely integrated into a machine, brake motors whose brake is built into the motor and cannot be removed or separately powered for the efficiency test, motors designed to run wholly immersed in a liquid and smoke extraction motors with a temperature class above 400 °C (clause 1.2).[1]
Does rewinding an electric motor reduce its efficiency?
Not measurably, when the rewind follows good practice. In a 2019 study by EASA and the UK's Association of Electrical and Mechanical Trades (AEMT), an independent laboratory tested ten new premium efficiency and IE3 motors of 30 to 75 kW at full load before and after a rewind: efficiency changed by +0.3 to -0.5 percentage points, an average of -0.1, within the test method's accuracy of ±0.2%.[6]
The 2003 study by the same bodies traces the common claim of a 1 to 5% loss to earlier studies of mostly small motors. In it, motors rewound without specific controls on stripping lost 0.3 to 0.5 points (0.4 on average) once over-greased bearings in two of them were allowed for, while those rewound with controlled stripping and burnout at 360 to 370 °C changed by -0.1 on average.[6]
What protects efficiency is the method: a stator core kept below 370 °C in a controlled burnout oven, a core loss test before and after stripping, and copper of the same or a larger cross-section in the original winding pattern.[7][5]
The US Department of Energy's guide shows the cost of getting it wrong: damaged lamination insulation cannot be repaired, and a change from ten turns to nine raises the starting current by 23%.[5]
What should the workshop test and put in the repair report?
In practice, the workshop should work to ANSI/EASA AR100 (2025 edition), the recommended practice for the repair of rotating electrical apparatus: test the motor on arrival, record the primary cause of failure, and keep for at least three years a record of nameplate data, tests and measurements before and after repair, winding data and replaced parts, open to the customer on request (clauses 1.3.2 and 1.4).[7]
The EASA and AEMT rewind study, which predates the 2025 edition, found that motors keep their efficiency and reliability when repairers use the methods of AR100 and IEC 60034-23.[7][6]
For a low-voltage motor, the AR100 tests include:[7]
- insulation resistance at 500 V DC for one minute, with a recommended minimum of 5 MΩ at 40 °C for random-wound windings below 1 kV (clause 4.2.1, Tables 4-1 and 4-2);[7]
- winding resistance, with each phase within 2% of the average on a random winding (clause 4.3.1);[7]
- a surge test at twice the rated voltage plus 1,000 V, about 1.8 kV for a 400 V motor, where a single waveform means no fault was found (clause 4.2.5 and Table 4-3);[7]
- a high-potential test of the winding at the voltages of AR100's Table 4-4, applied once and only after acceptable insulation resistance results (clauses 4.4 and 4.4.1.1);[7]
- a core loss or loop test before burnout and after the windings are removed, at a flux within 5% of the first test (clauses 3.1.1 and 4.2.6);[7]
- a no-load run at rated voltage and frequency, recording speed against the nameplate, no-load current, bearing temperatures and vibration, within 0.15 in/s (3.8 mm/s) peak for a resiliently mounted standard motor above 1,200 rpm (clauses 4.5 and 4.5.6).[7]
Who may repair a motor used in a hazardous location?
In South Africa, under the National Code of Practice for Electrical Machinery in Hazardous Locations, a repair facility "shall either be certified under an approved product certification scheme to repair or overhaul specific EPA, or shall submit repaired products for batch testing", and issues a declaration of conformance with each repaired product (Annex A.5).[2]
For surface industries only, clause 4.5 of the code also requires "all new, converted, re-designed or repaired apparatus" for hazardous locations to display an IA certificate number before it goes into service.[2]
The repairer fits a durable label giving its scheme certificate number where it has one, the IA certificate number, its name and the month and year of the repair; it leaves the manufacturer's label on and removes earlier repairers' labels (Annex A.13 and A.14).[2]
The product certificate holder keeps repair records for at least 10 years (Annex A.16), and the code's normative references include SANS 60079-19, on the repair, overhaul and reclamation of Ex equipment.[2]
On site, the Electrical Machinery Regulations allow repairs in a hazardous location only on machinery made dead and kept dead, and require a visual inspection and test at least every two years, or at another interval the chief inspector approves after a risk assessment, signed into a record book (regulation 9(6), (8) and (9)).[3]
What should an electric motor repair quote and warranty cover?
In practice, a quote you can compare separates the strip-and-inspect fee, the rewind (wire, insulation class, varnish), bearings, shaft and housing work, balancing, tests and transport; the US Department of Energy's guide lists class F or better insulation and brazed rather than crimped connections among its best practices.[5][7]
No typical rewind cost can be named for a given rating, the guide says, because design, failure mode, rewind practice, materials and regional costs vary.[5]
Where the Consumer Protection Act applies, including to a juristic person below the asset or turnover threshold the Minister sets (section 5(2)(b)), a workshop holding your motor for a repair above the prescribed value may charge only for work you authorised after an estimate or, in a recorded form, after declining an estimate or setting a maximum. It may charge for the estimate (stripping included) only at a price you approved first, and may not exceed the estimate without your approval (section 15).[4]
Under section 57 of the Act, every new or reconditioned part installed during a repair, and the labour to install it, is under warranty for three months from installation or any longer period the supplier gives in writing; the warranty is void after misuse and does not cover ordinary wear and tear.[4]
A company at or above the threshold is outside the Act (section 5(2)(b)), so get the warranty period, cover and conditions in writing with the quote.[4]
What to put in your request
- A photo of the nameplate: kW, voltage, frequency, speed, frame, mounting, IE class, insulation class, serial number.[7][1]
- How it failed, hours run, earlier rewinds and the duty: VSD or direct-on-line start, starts per hour, dust, moisture or heat.[5][7]
- A strip-and-quote: the price of the assessment stated up front, a written estimate before any work, and the core loss test result before the repair is approved.[4][5]
- An itemised quote: rewind (copper of the same or larger cross-section, class F or better insulation, varnish), bearings, shaft and housings, balancing, tests, transport.[7][5]
- A test report: insulation resistance, winding resistance, surge, high-potential, core loss before and after stripping, no-load current and speed, vibration.[7]
- Burnout in a temperature-controlled oven, with the stator core kept below 370 °C.[7][5]
- A price and lead time for a new IE3 motor with the same frame, mounting and shaft, to compare with the rewind.[1][5]
- For a hazardous-location motor: the repairer's scheme certificate or a batch test, a declaration of conformance, a repair label and, in a surface industry, the IA certificate number.[2]
- The date the motor must be back in service, and the warranty in writing: period, parts and labour covered, conditions.[4]
Questions buyers ask
How much does it cost to rebuild an electric motor?
It depends on the motor, the failure and the region, so there is no typical price. Get a strip-and-quote with the core loss result and a price for a new IE3 motor of the same frame; the US Department of Energy's guide says to buy new when the repair exceeds 60 to 65% of the new motor's price.[5]
What are the most common causes of electric motor failure?
Historically, bearing failures have been the largest cause, says the US Department of Energy's guide, from overloading by a motor mismatched to its load or from running on unbalanced voltage. The guide's list also includes contamination by moisture, oil, chemicals or dust, single phasing, overloading, normal insulation ageing, misalignment and vibration, and poor maintenance. AR100 has the workshop record the primary cause.[5][7]
How many times can a motor be rewound?
The EASA and AEMT 2003 study found no significant loss from repeat rewinds done to good practice: five motors of 75 to 150 kW rewound two or three times changed efficiency by -0.1 percentage points on average, and two 5.5 kW motors put through the burnout oven three times and rewound once gained 0.5 points. A damaged core is different: lamination insulation cannot be repaired, and the lost efficiency returns only with a major repair such as restacking.[6][5]
Sources
- Department of Trade, Industry and Competition (Government Gazette 50748), Compulsory Specification for Efficiency Requirements of Electric Motors (VC 9113), General Notice 2542 of 2024, 3 June 2024.
- Department of Employment and Labour (Government Gazette 46750), National Code of Practice for Electrical Machinery in Hazardous Locations: regulatory requirements for explosion-protected apparatus, Notice 2398 of 2022, 19 August 2022.
- Department of Labour (Government Gazette 34154), Electrical Machinery Regulations, 2011 (GN R.250), 25 March 2011.
- Government Gazette 32186, Consumer Protection Act 68 of 2008, 29 April 2009.
- US Department of Energy, Advanced Manufacturing Office, Premium Efficiency Motor Selection and Application Guide: A Handbook for Industry (DOE/GO-102014-4107), February 2014.
- EASA and the Association of Electrical and Mechanical Trades (AEMT), The Effect of Repair/Rewinding on Premium Efficiency/IE3 Motors (2019 rewind study, with the 2003 study), January 2021.
- EASA (approved as an American National Standard), ANSI/EASA AR100-2025: Recommended Practice for the Repair of Rotating Electrical Apparatus, 2025.