TL;DR:
- Motor rewinding requires three categories of insulation: slot liners (NMN, NHN, Nomex, PET film, DMD, or fish paper), phase insulation (same materials, cut to shape), and slot wedges (rigid laminate or pressboard).
- Slot liner material choice depends on the motor’s insulation class: B (130°C) uses DMD or PET film; F (155°C) uses NMN; H (180°C) uses NHN or Nomex — matching the material to the nameplate is non-negotiable for safety and service life.
- Small-quantity buying is the norm in motor repair — most technicians need 5–50 kg per order, not pallet loads. Suppliers who cater to this segment offer cut-to-size sheets, mixed-grade kits, and air-express shipping for urgent repair jobs.
- SIDA supplies motor insulation materials in small quantities with no minimum order value, including pre-cut slot liner sheets, phase insulation paper, and slot wedges across all insulation classes B through H.
Motor winding insulation materials are the slot liners, phase barriers, and wedges that prevent electrical contact between the copper winding and the stator core — choose the wrong material or the wrong insulation class, and the motor runs until the insulation fails, which may be hours or years depending on the mismatch. This article explores what materials your rewinding shop actually needs for common motor types, how to select slot liner grades based on insulation class, what phase insulation and wedge materials complete the insulation system, how to read a motor nameplate to match materials correctly, and where to buy small quantities without getting stuck with minimum order requirements designed for large OEMs.
What Insulation Materials Does Motor Rewinding Require?
A complete motor rewinding insulation kit consists of three functional categories of material, each serving a distinct electrical or mechanical role inside the stator slot. Understanding what each category does — and what happens when any one of them is missing or underspecified — is the foundation of a reliable rewind.
Slot Liners: The Primary Groundwall Insulation
The slot liner is a U-shaped or C-shaped sheet inserted into the stator slot before the copper coils are placed. It provides the primary electrical insulation between the energized winding and the grounded stator core. Slot liner failure is the most common cause of rewound motor failure, typically resulting from a liner that was too thin for the voltage class, mechanically damaged during coil insertion, or made from a material with insufficient thermal rating for the motor’s actual operating temperature. Standard slot liner materials include flexible composite laminates — NMN (Nomex/PET film/Nomex), NHN (Nomex/PET film/Nomex, higher temperature), DMD (polyester fleece/PET film/polyester fleece), and single-layer films like PET (Mylar) for lower-temperature applications. SIDA offers NMN insulation paper for Class F motors and NHN insulation paper for Class H applications, both available in sheet form for rewind shops.
Phase Insulation: Separating Coil Groups
Phase insulation is placed between the end-turn coil groups of different phases at each end of the stator. Without adequate phase insulation, the voltage difference between phases — which can reach peak line-to-line voltage — arcs across the air gap between adjacent coil groups, causing a phase-to-phase short circuit. Phase insulation is typically cut from the same flexible composite material as the slot liner, but in narrower strips sized to fit between the coil groups. The key specification difference: phase insulation in the end-turn region operates in ambient air rather than being sandwiched inside a slot, so thermal dissipation is better, but mechanical vibration and movement during motor starting impose fatigue stress that the material must withstand without cracking or delaminating.
Slot Wedges: Mechanical Retention Under Magnetic Stress
Slot wedges are rigid strips inserted at the top of the slot opening to hold the coils and slot liner in place against the powerful magnetic forces generated during motor operation. When a motor starts, the inrush current can reach 6–8 times rated current, producing magnetic forces that try to push the coils out of the slot. A slot wedge that loosens, cracks, or slides out allows coil movement that eventually wears through the slot liner insulation, causing a ground fault. Wedge materials range from rigid laminates (FR4/G10 epoxy glass or phenolic) to high-density pressboard and, for high-temperature applications, Nomex-based rigid laminates. SIDA’s FR4/G10 epoxy glass sheets are commonly machined into slot wedges for medium and large motors, providing the necessary flexural strength and thermal stability.
Slot Liners 101: NMN, NHN, Nomex, PET Film — Which for Your Motor?
The slot liner is where the insulation class meets the shop floor — every rewinding technician needs to know which material corresponds to which temperature rating and why substituting a lower-class material to save cost is never worth the risk.
Class B (130°C): DMD and PET Film
For standard industrial motors with Class B insulation (130°C total temperature, 40°C ambient + 80°C rise + 10°C hot-spot allowance), DMD (Dacron-Mylar-Dacron, more accurately polyester fleece/PET film/polyester fleece composite) is the most widely used slot liner material. The polyester fleece outer layers provide mechanical cushioning and puncture resistance during coil insertion, while the PET film core delivers the dielectric strength. Thicknesses of 0.20–0.35 mm cover most motors up to 600 V. Single-layer PET polyester film in 0.125–0.25 mm thickness is also used for smaller fractional-horsepower motors where the slot liner does not need the mechanical protection of a composite.
Class F (155°C): NMN — The Workhorse
Class F is the most common insulation class for modern industrial motors, and NMN (Nomex/PET film/Nomex, also called NPN or 6641 DMD-F in Chinese standards) is the dominant slot liner material. The Nomex outer layers provide thermal stability up to 220°C — far exceeding the PET core’s 155°C limit — which means the composite’s overall temperature rating is governed by the PET film layer. NMN in 0.20–0.35 mm thickness covers the majority of Class F motor rewinds from 1 HP to 500 HP. The Nomex outer layers also provide superior puncture resistance compared to DMD, which matters when inserting pre-formed coils into semi-closed slots. For a detailed comparison of composite materials, see our guide on NHN insulation paper performance factors for motors.
Class H (180°C): NHN and Pure Nomex
Class H motors — typically found in steel mills, foundries, marine applications, and traction motors — require slot liner materials rated for 180°C continuous operation. NHN (Nomex/Polyimide film/Nomex) replaces the PET film core with polyimide (Kapton-type) film, raising the composite temperature rating to match the Nomex outer layers. Pure Nomex paper (unlaminated, Type 410 or Type 414) in 0.25–0.50 mm thickness is also used where the application does not require the additional dielectric strength of a film layer. These materials cost 3–5 times more than NMN, but using NMN in a Class H motor results in thermal degradation of the PET core within months, leading to dielectric failure — a cost equation that heavily favors the correct material. SIDA’s aramid insulation paper provides a cost-effective alternative to branded Nomex with equivalent thermal and electrical performance.
Phase Insulation and Wedges: What You Need
Slot liners get the most attention, but phase insulation and slot wedges complete the insulation system — and neglecting either one leads to failures that are just as catastrophic as a liner breakdown.
Phase Insulation: Material and Cutting Guide
Phase insulation is functionally identical to slot liner material — it must meet the same insulation class — but the practical requirements differ. Phase insulation pieces are typically cut to shape by the rewinding technician on the shop floor using scissors or a guillotine cutter, so the material must cut cleanly without delamination at the edges. NMN and NHN composites with well-bonded layers perform best; poorly laminated material frays at the cut edge, creating a path for partial discharge. Phase insulation thickness is usually 0.20–0.30 mm — slightly thinner than the slot liner because the dielectric stress between phases in the end-turn region is lower than the groundwall stress inside the slot. Standard sheet sizes of 1,000 × 1,000 mm or 1,000 × 2,000 mm are convenient for rewind shops cutting custom shapes for each motor. For Class B and F motors, DMD insulation paper in 0.25 mm works well for phase barriers.
Slot Wedges: Rigid Materials and Sizing
Slot wedges are the unsung workhorses of a motor rewind. The material must have sufficient flexural strength to resist the magnetic slot-wedging forces — typically 150–300 MPa flexural strength for medium and large motors — while being thin enough (usually 0.5–3.0 mm) to fit into the slot opening without reducing the copper fill area. Common wedge materials include: epoxy glass laminate (FR4 or G10 grade) for general-purpose motors up to Class F; phenolic laminate for older motor designs and cost-sensitive applications; and Nomex-based rigid laminate for Class H motors. Machined or sheared strips 6–15 mm wide, cut to the stator stack length plus 5–10 mm overhang at each end, are the standard format. Wedges should fit snugly — a loose wedge is worse than no wedge because it vibrates, generating abrasive wear against the coil insulation.
The Complete Insulation Kit Concept
An efficient rewind shop stocks pre-cut or easily trimmable insulation materials that cover 80% of the motors they service. A basic shop kit might include: one roll each of 0.25 mm DMD, 0.25 mm NMN, and 0.30 mm NHN (all 1,000 mm wide) for slot liners and phase barriers; one sheet each of 1.0 mm, 1.5 mm, and 2.0 mm epoxy glass laminate for slot wedges; and a roll of 0.15 mm PET film for coil wrapping and lead insulation. With this kit, a technician can handle most rewinds from fractional HP through 200 HP across Classes B, F, and H. SIDA supplies all of these materials in small quantities — see our complete motor insulation product lineup including motor insulation paper options for a detailed breakdown by application.
How to Match Insulation Class to Motor Nameplate (B, F, H)
The motor nameplate is the one document that overrides every assumption, every rule of thumb, and every “this is what we usually use” habit in the rewinding shop. Reading it correctly — and understanding what each insulation class means in terms of material temperature limits — is the difference between a rewind that lasts 15 years and one that fails at commissioning.
Reading the Nameplate: Insulation Class, Temperature Rise, and Ambient
The nameplate typically lists the insulation class as a letter (B, F, or H) and the rated temperature rise in degrees Celsius, often expressed as “INS CL F” or “RIS 80°C.” The total hot-spot temperature is the sum: ambient temperature (typically 40°C, unless the motor is specified for a hotter environment) plus the rated temperature rise plus a hot-spot allowance (typically 10°C for random-wound motors, 5°C for form-wound). A Class F motor with an 80°C rise operates at 40 + 80 + 10 = 130°C hot-spot, well within the 155°C limit of Class F materials. If the nameplate says “INS CL F, RIS 105°C,” however, the hot-spot reaches 155°C, which is at the limit — and any material substitution must be upward (Class H) not downward.
Temperature Limits: What Each Class Actually Means
IEC 60085 and NEMA MG 1 define the maximum total temperature for each insulation class: Class A = 105°C (obsolete for new motors), Class B = 130°C, Class F = 155°C, Class H = 180°C, Class N = 200°C, and Class R = 220°C. The rule in rewinding is simple: every insulation material inside the motor — slot liners, phase barriers, wedges, lead wire insulation, varnish, and tying cord — must meet or exceed the nameplate insulation class. A Class F motor rewound with Class B slot liners will fail, regardless of how carefully the rest of the job is done, because the slot liner is the weakest thermal link. For H-class motors, SIDA supplies H-Class 180°C insulation paper specifically engineered for high-temperature motor winding applications.
When to Upgrade the Insulation Class During Rewind
It is standard and recommended practice to upgrade the insulation class by one level during a rewind — rewinding a Class B motor with Class F materials, or a Class F motor with Class H materials. The upgrade cost is typically 10–20% in materials for the slot liners and phase barriers but extends the rewind’s service life significantly because it provides thermal headroom against overloads, voltage imbalance, frequent starting, and other real-world stresses that push the motor beyond its nameplate conditions. The copper wire does not need to change for an insulation class upgrade — standard magnet wire with a modified polyester or polyester-imide enamel is suitable for Class F, and polyamide-imide enameled wire for Class H.
Where to Buy Small Quantities of Motor Insulation Materials
Motor repair shops rarely order by the pallet. The typical rewind shop places orders of $200–$1,500 at a time, for cut sheets, partial rolls, and mixed-grade assortments. Finding suppliers who serve this segment — rather than directing small buyers to distributors with 2–3x markups — is one of the persistent challenges of the motor repair industry.
Small-Quantity Supply Models: What to Look For
A supplier that genuinely serves the motor repair segment offers: no minimum order quantity (or a minimum below $200), cut-to-size sheets rather than full rolls only, mixed-grade ordering (one purchase order covering DMD, NMN, NHN, wedges, and PET film), air-express shipping options for urgent repair jobs (3–7 day delivery to most countries), and technical support that understands motor insulation as distinct from transformer insulation. Suppliers who only sell full pallets, who require separate orders for each material grade, or whose minimum order is $2,000+ are structured for OEM production volumes and will frustrate a rewind shop’s purchasing patterns. SIDA’s small-quantity insulation buying guide explains how to navigate these options.
Direct-from-Manufacturer vs Local Distributor
Buying direct from the manufacturer typically offers 20–40% lower per-unit cost compared to buying through a local electrical distributor, but the trade-off is longer lead times (2–3 weeks for sea freight from Asia vs 1–3 days from a local distributor) and the need to plan ahead rather than buying reactively. Most successful rewind shops use a hybrid model: maintain a 3–6 month stock of fast-moving items (standard DMD and NMN in common thicknesses) ordered direct from the manufacturer at lower cost, and buy urgent or one-off items from local distributors at a premium when speed overrides price. For first-time buyers evaluating a direct-from-manufacturer supplier, requesting a pre-shipment sample kit of material swatches and spec sheets costs little and confirms quality before committing to an initial order.
Shipping Small Orders: Air Express Makes It Work
The economics of shipping insulation materials internationally have shifted in favor of small buyers over the past decade. Air express services (DHL, FedEx, UPS) can deliver 20–50 kg of sheet materials from China to North America or Europe in 3–5 business days for approximately $8–$12 per kg — adding $400–$600 to a 50 kg order. When compared against the cost of downtime for a $50,000 industrial motor waiting on insulation materials, that freight cost becomes negligible. For non-urgent replenishment orders, sea freight LCL (less-than-container-load) services deliver 100–300 kg for $2–$4 per kg with 25–35 day transit, making it practical to maintain stock levels through regular direct orders.
Frequently Asked Questions
Can I use the same slot liner material for all motors in my shop?
Using a single material — typically 0.25 mm NMN — as your default slot liner for all rewinds is a common and practical approach for shops that handle primarily Class B and F motors. NMN rated for Class F (155°C) safely covers Class B motors with thermal margin to spare, and it is mechanically robust enough for most insertion methods. However, any Class H motor (nameplate INS CL H) must have its slot liners upgraded to NHN or Nomex — the NMN default will not survive at 180°C. A practical shop policy is: default to NMN, stock a small quantity of NHN for Class H jobs, and keep a roll of DMD for cost-sensitive Class B rewind quotes where every dollar of material cost counts.
What thickness of slot liner should I use for a given motor voltage?
For motors up to 600 V (the vast majority of industrial motors), a slot liner thickness of 0.20–0.25 mm is standard for motors up to 50 HP, and 0.30–0.35 mm for motors 50–200 HP. For 690 V motors (common in European and marine applications), step up to 0.30–0.35 mm minimum regardless of horsepower. For medium-voltage motors (2.3 kV–6.6 kV), slot liner design is an engineering exercise involving partial discharge analysis, multiple layers, and semi-conductive coatings — this is not a rewind-shop judgment call and the OEM’s original insulation specification should be followed exactly.
How do I store insulation materials to maintain warranty validity?
Store flexible composite materials (NMN, NHN, DMD) in their original sealed packaging in a dry, climate-controlled area at 15–30°C and below 60% relative humidity. Exposure to high humidity causes the aramid paper layers to absorb moisture, which can cause delamination during coil insertion and reduce dielectric strength. Rigid laminates (FR4, phenolic) should be stored flat, not on edge, to prevent warping. Most manufacturers, including SIDA, specify a shelf life of 12–24 months from date of manufacture for flexible insulation materials when stored under recommended conditions. Material that has been exposed to moisture can often be reconditioned by drying at 50–60°C for 4–8 hours, but consult the supplier’s technical guidance before drying.
Summary
Reliable motor rewinding starts with matching the right insulation material to the motor’s insulation class — Class B (130°C) for DMD or PET film, Class F (155°C) for NMN, and Class H (180°C) for NHN or Nomex. A complete insulation kit for any rewind consists of three elements: a slot liner for groundwall insulation, phase barrier pieces cut from the same-grade material, and rigid slot wedges to retain the coils against magnetic forces. Reading the motor nameplate and upgrading the insulation class by one level during rewind (B → F, F → H) is standard practice that extends service life for a modest material cost increase. For the small and mid-sized orders that define the motor repair industry, suppliers offering cut-to-size sheets, mixed-grade ordering, low minimums, and air-express shipping make it practical to buy direct rather than through high-markup distributors.
SIDA supplies motor winding insulation materials — NMN, NHN, DMD, Nomex/aramid paper, PET film, fish paper, and rigid laminates for slot wedges — in small quantities with no minimum order value, cut-to-size options, and air-express delivery worldwide. Contact us for a motor insulation sample kit or a quotation for your next rewind job.
Contact SIDA for motor winding insulation materials:
📞 +86-15958243831
📧 jessie.feng@sidanm.com
💬 WhatsApp: https://wa.me/8615958243831
🌐 sidanm.com
References
- IEC 60085:2007. Electrical insulation — Thermal evaluation and designation. International Electrotechnical Commission. https://webstore.iec.ch/publication/162
- NEMA MG 1-2021. Motors and Generators. National Electrical Manufacturers Association. https://www.nema.org/standards/view/motors-and-generators
- IEC 60034-1:2022. Rotating electrical machines — Part 1: Rating and performance. International Electrotechnical Commission. https://webstore.iec.ch/publication/121
- IEEE Std 43-2013. IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery. IEEE. https://standards.ieee.org/standard/43-2013.html
- Stone, G. C., Culbert, I., Boulter, E. A., & Dhirani, H. (2014). Electrical Insulation for Rotating Machines: Design, Evaluation, Aging, Testing, and Repair (2nd ed.). Wiley-IEEE Press. ISBN: 978-1118057063.
- EASA AR100-2020. Recommended Practice for the Repair of Rotating Electrical Apparatus. Electrical Apparatus Service Association. https://easa.com/standards
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