Two suppliers quote a 170 KV motor. Both units fit the arm and accept the intended battery voltage. One is lighter and costs less. Can it replace the other?
Not on that information. Matching KV is a useful first filter, but it is not an interchangeability specification. It leaves out how much torque the motor can sustain, how much heat it produces at the required load, and whether the installed motor can get rid of that heat.
For procurement, the useful question is not "Are these both 170 KV?" It is "Can this complete motor, propeller and ESC combination meet our operating points without exceeding its limits?" That question usually exposes the missing data quite quickly.
What the same KV actually tells you
KV is a speed constant, commonly expressed in revolutions per minute per volt. It describes a speed-to-voltage relationship, not kilograms of thrust and not an allowable wattage. No-load speed is only an approximation to the loaded operating speed; voltage drops, losses and propeller torque still matter.
There is an important nuance here. With consistent electrical definitions and units, speed constant and torque constant are related. Therefore, it would also be wrong to say that KV has no connection to torque. The missing distinction is between torque per specified motor current and the current and torque a motor can safely sustain. Matching the first does not establish the second. Maxon's explanation of motor constants sets out that relationship.
Do not use a torque-constant conversion and multiply it by a battery-current figure without checking the definitions. Phase current, DC input current, RMS current and peak current are not interchangeable. Commutation conventions also affect the quoted constants, as explained in maxon's comparison of block and sinusoidal commutation. Ask suppliers to identify how their values were measured before comparing them.
If you are still deciding the speed range, start with the drone motor KV selection guide. Once the target KV is settled, the comparison below becomes more useful than another discussion of low KV versus high KV.
Why the heavier motor might help, and why it might not
Different active dimensions, magnetic circuits, winding arrangements and thermal paths can produce the same nominal KV. A larger motor may be able to carry more continuous torque, but outside diameter and mass alone do not prove it. Some of that mass may be housing, shaft or mounting hardware rather than active material.
Resistance is worth asking about because resistive heating rises with the square of current for a given resistance. But the number needs a definition. Is it a phase value or a line-to-line value? Was it measured cold, at what temperature, and with the leads included? A lower number from a different measurement convention is not necessarily an improvement.
Even a properly comparable cold-resistance figure is not a complete efficiency map. At the intended propeller speed, other losses and the ESC matter too. It is possible to select a larger motor with more thermal headroom and still add more mass than the mission needs. Conversely, a light motor that looks attractive on the bill of materials may not have enough continuous capability for a long, heavily loaded hover.
The right trade-off depends on where the aircraft spends time. A brief climb and a twenty-minute hover place different demands on the same propulsion unit. Compare the motor at those operating points instead of deciding from the largest current or power number on the front of the datasheet.
Turn the quotation into a comparison you can use
Send both suppliers the same load case and use one comparison sheet. Blank cells are useful: they identify what remains unproven. Do not fill them with assumptions just to make the quotations look complete.
| Both quotations say... | What is still unanswered? | Evidence to request |
|---|---|---|
| 170 KV | Are the test method and acceptable variation comparable? | KV measurement method, conditions and production acceptance range |
| Suitable for the same battery | Can the combination operate at full-charge voltage and the required loaded voltage? | Motor and ESC limits, speed limits and tests across the intended voltage range |
| A similar maximum power | For how long, at which speed, and with what cooling? | Continuous and time-limited ratings with temperature limits and test conditions |
| A resistance value | Are the electrical boundary and temperature the same? | Phase or line-to-line definition, measurement temperature and lead inclusion |
| Enough maximum thrust | What power and temperature occur at the actual working thrust? | Results with the exact propeller, ESC setup and operating duration |
| The same mounting pattern | Does the rest of the interface fit and stay clear? | Current drawing, screw-insertion limit, rotating envelope, shaft and cable-exit details |
Be especially cautious with a maximum-power rating that has no duration. "2,000 W" could describe electrical input during a short test rather than continuous shaft output. Until the supplier identifies the boundary, duration and thermal conditions, it cannot be used as a continuous-duty approval.
Compare the propeller combination at your working load
Suppose an aircraft design calls for 3 kgf of static thrust per motor at a particular test condition. This is an illustrative comparison point, not a TORGATEK performance claim. Ask for both candidate systems at that thrust using the exact specified propeller and a clearly stated voltage, not simply at 50% throttle.
Record input power at the same measurement boundary, motor speed, temperature measurement location and run duration. Also record the ESC model, firmware and settings. If one result comes from a brief sweep and the other from a sustained run, do not treat their ending temperatures as equivalent thermal evidence.
Where possible, test both candidates on the same rig with the same instrumentation. State ambient conditions and whether the installation represents the actual aircraft. Static test results are useful for screening, but they do not by themselves establish forward-flight efficiency, installed cooling or flight endurance.
The thrust-test data checklist covers how to check those boundaries. For the motor comparison, keep the decision simple: does each candidate meet the required load for the required duration, and what are the measured power, temperature and mass consequences?
Cooling is part of the rating
A motor does not retain every test-bench rating regardless of its installation. An adapter plate can obstruct airflow; a cowling can change it; a different speed profile can change self-ventilation. Maxon's technical note on ventilated motors shows why cooling-dependent ratings need their operating conditions. Its numerical examples concern maxon motors, not UAV qualification limits.
For a UAV, ask how the candidate was cooled during the published test and how temperature was measured. A housing temperature and a winding temperature are different quantities. A brief case reading cannot substitute for an approved winding-temperature limit.
Then validate the intended installation and mission envelope. A successful unloaded spin proves very little about a long hover. Equally, a test with a large external fan should not be presented as proof for a sheltered installation unless that airflow is representative. If the evidence is missing, keep the motor listed as a candidate rather than an approved substitute.
A replacement needs mechanical and controller checks too
Before exchanging samples, compare shaft dimensions, propeller fastening, motor height, rotating clearances, lead length and connector ratings. Check the complete mass on a consistent basis: bare motor, motor with leads, and motor with adapters are not the same item.
Use the actual mounting stack to verify screw insertion and usable thread engagement. The separate motor mounting-screw guide explains why a thinner plate can make an existing screw too long. A matching bolt pattern is not a reason to reuse the old hardware without checking.
The ESC also needs review. Motors with the same KV can differ in pole count, inductance and control behaviour. Confirm the controller's electrical-speed limits and manufacturer-approved settings for the combination, then test starting, acceleration and the required load range in a properly restrained setup. Do not assume that an ESC tune validated on one motor transfers unchanged to another.
Make the substitution decision on evidence
A useful purchasing outcome can be "not enough information yet." That is more defensible than accepting a cheaper motor because three catalogue labels match. Keep the exact sample configuration, test report and approved drawing tied to the purchase specification so a later revision is not silently substituted.
For TORGATEK's multi-KV multirotor motor options, request the data for the exact winding and propeller combination under consideration. For a custom propulsion enquiry, include the working thrust or torque, voltage range, duty cycle, cooling arrangement and mechanical envelope. Send those requirements with your comparison sheet; the missing rows will show what needs to be measured before either motor is approved.
