The Thermal Protector Was the Only Unusual Line on the Drawing

Build to this drawing

A North American appliance OEM sent a complete drawing and a short instruction: build this. Everything on it was standard except a motor thermal protector set to trip at 95C. In short, they needed a manufacturer rather than a design partner.

That one line changed how we read the rest of the sheet. The protector was wired into the windings themselves, not clipped to the outside of the motor.

What the rest of the sheet asked for

Otherwise the fan was modest. A tangential blower moving 23 m3/h against 8 Pa, on 115 VAC at 60 Hz.

• First, an overall length of 158mm, with a 108mm impeller.
• Next, a housing only 50mm high, set by the chassis it goes into.
• Meanwhile a 25 dB(A) ceiling, which is very quiet for a moving part.
• Finally, an ambient range of -25C to +60C.

What those two numbers together imply

Firstly, 25 dB(A) says people will be near this appliance and will notice it. So the product sits in an occupied room rather than a plant room.

The 95C protector says the fan lives inside an enclosure that can get hot. Put together, they describe something like a heated display cabinet or a fireplace insert: quiet, close to users, and warm inside.

Consequently the safety case matters as much as the airflow. Certification under an appliance standard was clearly in the customer plan.

Where the protector sits, and why there

Here the drawing matched our existing 30mm cross-flow platform closely, so this was verification work rather than new product development. Meanwhile the motor thermal protector was the one item that needed thinking about.

What a motor thermal protector actually does

A motor thermal protector is a temperature-sensitive switch buried in the winding. When the winding reaches its set point, it opens and the motor stops.

Above all, the point is where it sits. A motor thermal protector on the outside of the housing reads case temperature, which lags the winding badly. Consequently by the time the case is hot, the winding insulation may already have taken damage.

Therefore embedding it in the windings means it reads the thing that actually fails. That is why the customer specified the location and not just the trip point.

Why 95C and not higher

At first glance 95C sounds low for a motor. But the figure comes from the insulation class, not from comfort.

Set too high, a motor thermal protector protects the paperwork and not the motor. Set too low, it nuisance-trips on a warm day and the appliance looks faulty. The number has to sit between normal hot running and the point where insulation degrades.

Why the shaded pole motor stayed

Admittedly we could have offered a brushless DC motor with electronic control, which would run more efficiently.

However the customer system runs the fan at one speed and has no digital communication to talk to. So the efficiency gain would arrive with a controls change nobody asked for, on a build-to-print job.

Worth stating plainly: a shaded pole motor cannot be speed-controlled by a variable frequency drive. If you ever expect to add speed control, that decision has to be made now rather than later.

Technical Specifications

Two numbers on this sheet do the real work: 25 dB(A), and the 95C trip point on the protector.

Parameter	Value
Model	LWCA-3090UN-06
Voltage	115 V
Frequency	60 Hz
Motor Type	AC Shaded Pole Motor
Input Power	6.3 W
Speed	2400 RPM (±10%)
Maximum Airflow	23 m³/h / 13.5 CFM
Maximum Static Pressure	8 Pa
Noise Level	25 dB(A)
Overall Dimensions (L x D x H)	158 x 68 x 50 mm
Impeller Dimensions (Ø x L)	30 x 108 mm
Bearing Type	Ball Bearing
Insulation Class	Class B / F / H Available
Operating Temperature	-25°C to +60°C
Safety Feature	Integrated 95°C Thermal Protector

Technical Documentation

The following documents were central to the project’s successful execution. They represent the initial product family data and the final, approved engineering deliverable.

Meanwhile the LWCA-30 series product datasheet provides performance parameters and dimensional data for the full family of blowers, which procurement managers use for initial component selection.

View the LWCA-3090UN-06 document (PDF)

The final approved engineering drawing, LWCA-3090UN-06-00, contains precise mechanical dimensions, material specifications, and custom technical requirements like the 95°C thermal protector, essential for mechanical design engineers integrating the component.

Specifying thermal protection yourself

If you are writing a motor thermal protector into your own fan specification, four things decide whether it works.

• Say where the protector goes, not just its rating. A motor thermal protector in the windings and one clipped to the housing behave completely differently. The winding version reacts to the actual failure mode; the external one reacts late.
• Choose auto-reset or manual deliberately. An auto-reset protector restarts the fan once it cools, which can cycle a fault indefinitely without anyone noticing. A manual type forces a service call. Which you want depends on whether silent recovery or visible failure is safer in your product.
• Set the trip point from the insulation class. Look up the class of the winding insulation and work back. Guessing a round number invites either nuisance trips or a protector that never saves anything.
• Check it survives your certification. Appliance safety standards care about what happens under locked rotor and fault conditions. A protector is often what makes that test pass, so it belongs in the specification you submit, not in a later change note.

See our cross-flow fans range, the AC cross-flow fans section, or cross-flow impellers. Appliance safety requirements are published by UL Solutions.

Questions about thermal protectors

Auto-reset or manual reset, which should I specify?

It depends on what a repeat trip means in your product. Auto-reset keeps the appliance working after a one-off overheat, but it can hide a developing fault by cycling quietly for months. Manual reset makes the failure visible and forces someone to look at it.

Does a motor thermal protector replace a fuse?

No, they cover different faults. A fuse responds to overcurrent, typically a short. A thermal protector responds to temperature, which is what you get from a blocked inlet, a stalled rotor or a hot enclosure. Most appliance designs need both.

Why not just fit a protector with a higher trip point?

Because the trip point is set by the winding insulation, not by convenience. Raising it lets the winding run hotter before anything intervenes, which shortens motor life and can defeat the purpose of fitting protection at all.

Can a shaded pole motor be speed-controlled?

Not with a variable frequency drive. Shaded pole motors are not built for frequency control and a drive will damage one. Voltage reduction gives a limited speed range with a torque penalty, but if real speed control matters, choose a different motor type from the start.