Electric Motor Cooling: Housings and Jackets
Short answer: Electric motor cooling housings and jackets are usually aluminum castings or extrusions that carry a spiral or axial coolant channel around the stator. Design targets are typically 3–8 L/min coolant flow, 1–2 bar working pressure, wall thickness of 3–6 mm over the channel, and a machined bore held to ±0.02 mm with the jacket's sealing faces flat within 0.05 mm. BQUQ machines these jackets on CNC centers to ±0.005 mm where the interface demands it, produces them alongside stamped and extruded heat sinks in one ISO9001 Dongguan factory, and returns quotes in 12 working hours.
Why motor cooling is a mechanical problem, not just a thermal one
Most thermal discussions about electric motors start with watts to dissipate and end with a coolant flow rate. That is the easy half. The hard half is that the cooling jacket is also the motor's structural spine. It locates the stator, carries bearing loads through the end bells, defines the air gap concentricity, and seals a fluid that will find any porosity, any incomplete weld, and any gasket face that is not flat.
A jacket that removes heat beautifully but distorts the stator bore by 0.08 mm will cost you more in noise, vibration, and efficiency than it ever saved in copper temperature. So the design conversation has to run on two tracks at once: thermal resistance and mechanical integrity.
Three cooling architectures dominate:
- Frame-and-fin (air cooled). Extruded aluminum housing with external fins, no coolant. Simple, cheap, and adequate for low-duty intermittent motors.
- Water jacket (liquid cooled). A channel cast, extruded, or machined into the housing wall, sealed by an outer sleeve or a bolted cover.
- End-winding and rotor cooling. Often a supplementary path — spray, oil, or a dedicated heat sink on the inverter module bolted to the same housing.
The rest of this article focuses on the liquid jacket, because that is where most of the manufacturing risk lives.
What does a motor cooling jacket actually look like?
There are four common constructions, and the choice drives your tooling cost, your leak-test strategy, and your unit price.
Cast-in channel jackets
A die caster or sand caster forms the channel as part of the housing. Wall thickness over the channel is typically 3–5 mm. This is the cheapest route at volume but the most sensitive to porosity. Expect to pressure test 100% of units, and expect to scrap or impregnate a percentage of them.
Extruded jacket with machined end caps
The channel runs axially as a straight extrusion, and the end caps or a welded return manifold create the U-turns. Extrusion gives you excellent dimensional consistency and low tooling cost, but the turning geometry is limited — you cannot get a true helical path without secondary machining.
Machined-from-billet jackets
For prototypes, low volume, and high-performance motors, the channel is milled directly into a billet and closed with a bonded or bolted cover plate. This gives complete freedom in channel geometry, but it is the most expensive per part and generates a lot of chip. It is also where CNC tolerance capability matters most, because the cover plate sealing face and the stator bore are machined in the same setup.
Helical tube or spiral insert jackets
A formed tube or a machined spiral is bonded or press-fitted into the housing. Good thermal contact requires either a shrink fit or a filled epoxy with known thermal conductivity.
| Construction | Typical wall over channel | Tooling cost | Best volume band | Leak risk |
|---|---|---|---|---|
| Cast-in channel | 3–5 mm | High | 5,000+ units/yr | Medium–high (porosity) |
| Extrusion + end caps | 3–4 mm | Low | 1,000–50,000 units/yr | Medium (joint faces) |
| Machined billet + cover | 2–4 mm | Very low | 1–2,000 units/yr | Low–medium (gasket face) |
| Tube / spiral insert | 1.5–3 mm | Low–medium | 500–20,000 units/yr | Low (tube is seamless) |
How do you size the coolant channel?
Channel sizing is a trade between heat transfer coefficient, pressure drop, and pump power. Narrow channels raise velocity and improve the convective coefficient, but pressure drop rises roughly with the square of velocity and pumping power with the cube.
Practical starting points for a 5–30 kW traction or industrial motor:
- Channel cross-section: 8–20 mm² for small motors, 30–80 mm² for larger frames.
- Flow rate: 3–8 L/min is a common band; high-performance motors push 10–20 L/min.
- Velocity target: 1–3 m/s in the channel. Below 1 m/s you risk poor heat transfer and settling; above 3 m/s pressure drop becomes painful.
- Pressure drop budget: keep the jacket under 0.3–0.5 bar at design flow so the system pump is not oversized.
- Coolant temperature rise: 3–8 °C across the jacket is typical for a well-designed unit.
Series versus parallel paths
A single continuous spiral gives the most uniform stator temperature and the highest pressure drop. Parallel paths reduce pressure drop but invite maldistribution — one branch can starve if its resistance is even slightly higher. If you need parallel paths, keep them symmetric and mirror-imaged, and verify with a flow test rather than assuming.
Where the heat actually enters
The dominant path is stator lamination to housing bore. That interface is a contact resistance problem, not a coolant problem. A 0.05 mm air gap at the stator-housing interface can add more thermal resistance than the entire coolant path. This is why housing bore tolerance, roundness, and surface finish deserve as much attention as channel geometry — and why the machining spec matters more than most thermal models suggest.
Which aluminum alloy should the housing use?
Alloy selection is driven by casting method, corrosion environment, and whether you need to weld.
| Alloy | Typical use | Thermal conductivity (W/m·K) | Machinability | Notes |
|---|---|---|---|---|
| A380 / ADC12 | Die-cast housings | ~96–100 | Good | Best castability, moderate conductivity |
| A356 / A356-T6 | Cast jackets, structural | ~150–160 | Fair | Heat treatable, good strength |
| 6061-T6 | Machined billet jackets | ~167 | Excellent | The default for CNC housings |
| 6063-T5 | Extruded jacket bodies | ~200 | Excellent | Best extrusion surface, lower strength |
| 1050 / 1100 | High-conductivity liners | ~230 | Fair | Soft, used where conductivity dominates |
Two practical notes. First, anodizing the coolant-wetted surfaces is usually a bad idea — a porous anodic layer in contact with glycol coolant can degrade over time. Keep wetted passages bare or use a conversion coating specified for coolant contact. Second, if the housing is also a heat sink for the inverter, alloy choice becomes a compromise between castability and conductivity, and you may be better off with a separate extruded or skived heat sink bolted to a cast frame.
Tolerances that decide whether the motor runs quietly
The cooling jacket is a precision part. These are the features that matter, with typical values:
| Feature | Typical tolerance | Why it matters |
|---|---|---|
| Stator bore diameter | ±0.02 mm (down to ±0.005 mm on request) | Air gap uniformity, contact resistance |
| Bore roundness / cylindricity | 0.02–0.03 mm | Cogging torque, NVH |
| Bore surface finish | Ra 1.6 µm or better | Interference fit consistency |
| End face perpendicularity to bore | 0.03–0.05 mm | Bearing alignment |
| Seal groove flatness | 0.05 mm | Leak path |
| Coolant port position | ±0.1 mm | Hose routing, assembly |
| Mounting hole pattern | ±0.05 mm | Frame integration |
If you are machining from billet, bore and seal groove should be cut in the same operation, or at minimum on the same machine without re-chucking. Re-chucking between operations is the single most common source of concentricity complaints we see in incoming inspection.
Leak testing: what specification should you write?
Write the leak test into the drawing, not into a separate verbal agreement. A workable specification:
1. Pressure test: 1.5× maximum working pressure, held 60 seconds, no visible leakage or pressure decay beyond a defined threshold.
2. Helium leak test for high-voltage or sealed-system motors: reject rate threshold typically 1×10⁻⁵ mbar·L/s or tighter depending on the application.
3. Flow test: measure pressure drop at design flow and confirm it falls within a band, e.g. 0.25 ± 0.05 bar at 6 L/min.
4. Burst test: on a sample basis, confirm the jacket holds 3–4× working pressure without rupture.
Also specify the test fluid and whether the part is shipped dry or wet. Residual test water in an aluminum channel is a corrosion seed.
How does the jacket integrate with the rest of the thermal stack?
A motor is a system, and the jacket is one node. The inverter, the DC-link capacitors, and the end windings all need paths. In practice, most of our customers end up with a hybrid:
- A machined or cast jacket as the primary stator cooling path.
- An extruded or skived heat sink on the inverter module, often sharing the same coolant loop.
- Thermal interface material between the power module and its heat sink, selected for the expected temperature and mounting pressure.
The interface material choice has an outsized effect. A poorly chosen gap filler can add 0.3–0.5 °C·cm²/W, which on a 200 W module is several degrees of junction temperature. It is worth reading our comparison of thermal interface options for power electronics before locking the stack.
For higher-power-density designs where the coolant loop itself is being reconsidered, immersion cooling approaches change the housing requirements entirely — no sealed channel, but different material compatibility and much tighter flatness requirements on the wetted surfaces.
Manufacturing route: casting plus machining, or all-machined?
A realistic cost picture for a mid-size motor jacket (roughly 150–200 mm bore):
- Die cast + CNC finish: lowest unit cost above ~5,000 units/yr, but tooling runs into tens of thousands of dollars and lead times are long.
- Sand cast + CNC finish: viable from a few hundred units, moderate tooling, more machining stock to remove.
- Extrusion + CNC end features: excellent for cylindrical jackets with axial channels, low tooling, fast turnaround.
- Billet machined: no tooling, fastest to first article, highest unit cost and material waste.
The machining step is where tolerance is won or lost regardless of route. If your supplier casts the housing but cannot hold the bore and seal groove in one setup, the casting quality will not save you. Our guide to CNC milling of heat sinks and thermal housings covers the setup and fixturing practices that keep these features concentric.
BQUQ runs four production lines in one Dongguan factory — CNC machining, metal stamping, custom springs, and heat sink production — so a motor program that needs a machined jacket, a stamped mounting bracket, and an extruded inverter heat sink can be sourced as one package rather than three. That matters for tolerance stack-up, because the bracket hole pattern and the jacket mounting bosses end up on the same drawing revision.
What to put on the drawing
A jacket drawing that gets quoted accurately and built correctly usually contains:
- Alloy and temper, plus casting method if fixed
- Coolant channel cross-section, path, and total wetted volume
- Wall thickness over the channel, minimum
- Bore diameter, roundness, cylindricity, and surface finish
- Seal groove dimensions and flatness
- Port thread specification and position
- Leak test pressure, duration, and acceptance threshold
- Flow test flow rate and pressure drop band
- Surface treatment, with wetted areas explicitly excluded from anodizing
- Marking and traceability requirements
Ambiguity in any of these lines is where quotes diverge and where first articles fail. If you send a model without a leak spec, expect three suppliers to quote three different test regimes.
Sourcing and lead time
For prototyping and low-to-mid volume, flexible MOQ matters more than unit price. A jacket program typically starts with 1–5 machined prototypes, moves to a small pilot batch, then transitions to casting once the design is frozen and volume justifies tooling.
Quotes from BQUQ come back within 12 working hours for CNC-machined jackets and heat sinks, based on a complete drawing or STEP file with the tolerance and test specification attached. Send files to sc@bquq.com or reach the engineering team on WhatsApp at +86 13713157787.
Frequently Asked Questions
Q: What coolant flow rate should a motor water jacket be designed for?
A: For most industrial and traction motors in the 5–30 kW range, 3–8 L/min is a practical design band, giving 1–3 m/s channel velocity and a 3–8 °C coolant temperature rise. Higher-power-density motors may need 10–20 L/min. Size the channel so pressure drop stays under roughly 0.3–0.5 bar at design flow, otherwise pump power and system cost climb quickly.
Q: Should the cooling jacket be cast or machined from billet?
A: Cast when annual volume exceeds roughly 5,000 units and the channel geometry suits a mold — unit cost is lowest but tooling is significant. Machine from billet for prototypes, low volume, and complex channel paths, where there is no tooling cost and tolerances are tightest. Extrusion plus CNC end features sits between the two and suits cylindrical jackets with axial channels.
Q: Why does my motor housing leak at the seal groove?
A: Usually because the seal groove face is not flat enough or the groove depth varies around the circumference. Specify flatness within 0.05 mm and machine the groove in the same setup as the bore. Also check that the gasket or O-ring compression is within the manufacturer's recommended range — over-compression causes extrusion and premature failure just as often as under-compression.
Q: Can the motor housing double as the inverter heat sink?
A: Sometimes, but it is a compromise. Cast housings have lower thermal conductivity (roughly 96–160 W/m·K) than extruded or skived aluminum (up to ~200 W/m·K), and the inverter mounting face adds another interface. For modules above roughly 100–150 W, a separate extruded or skived heat sink sharing the coolant loop usually performs better and is easier to service.
Q: What tolerance can be held on a machined motor cooling jacket?
A: BQUQ holds ±0.005 mm on critical CNC features where the interface requires it, with stator bores typically specified at ±0.02 mm and roundness at 0.02–0.03 mm. The practical limit is set by fixturing and by whether bore, seal groove, and mounting faces can be cut in one setup. Re-chucking between operations is the most common cause of concentricity problems.
Related Resources
- About BQUQ and our Dongguan manufacturing footprint: /about/
- Heat sink product range, including liquid-cooled and extruded profiles: /heat-sinks/
- CNC-machined thermal housings and jackets: /cnc-machined-heat-sinks/
- Industry trends in electrification and thermal management: /industry-dynamics/
- Full technical article library: /bquq-blog/
- Frequently asked questions on tolerances, finishes, and lead times: /faq/
- Case studies and sourcing examples: /case/
- Request a quote in 12 working hours: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
