Understanding KV Rating in Brushless Motors

Introduction

Brushless motors have become the go-to choice for a wide range of applications from drones and RC vehicles to electric bicycles and underwater ROVs. A key specification for any brushless motor is the KV rating. This measures how fast the motor spins per volt applied.

The KV rating along with other factors like stator size and magnet type have a major influence on the power, torque, efficiency, and speed characteristics of a brushless motor. Correctly matching the KV rating to your application and drive system is crucial for achieving optimal performance.

This article provides a comprehensive look at brushless motor KV ratings – what they mean, how they impact motor behavior, and how to select the right KV for your needs.

Key Takeaways

KV rating indicates rpm/volt for an unloaded motor.
Higher KV motors have higher free-spinning speed but less torque and power.
Lower KV means more torque and power but lower maximum rpm.
KV sizing depends on desired top speed vs acceleration and load requirements.
KV, stator size, magnet type all influence power – optimize together.
Gearing, voltage and pole count also impact speed; KV is just one part.

What is KV Rating?

KV stands for “rpm per volt” applied to the motor. Specifically, it is the no-load RPM speed divided by the voltage. For example, a motor with a KV rating of 1000 when powered with 12 volts would spin at approximately 12,000 rpm unloaded.

The KV rating gives the inherent speed constant for the particular motor design. It is determined by the physical construction – number of pole pairs and other factors.

KV = Unloaded rpm / Voltage

So a higher KV indicates a faster unloaded spinning speed per volt. Lower KV equals slower free speed but higher torque generation potential.

Factors that Determine KV

The primary determinants of the KV rating are:

Number of magnet pole pairs in rotor
Number of winding turns in stator
Core material and air gap dimensions

More rotor pole pairs and more winding turns leads to lower KV. The additional poles and windings increase torque generation but limit maximum speed.

KV ratings typically range from around 100 rpm/V for very high torque/lower speed motors up to 3000+ rpm/V for high speed applications where torque is less critical.

How KV Affects Motor Performance

The interaction between the KV rating and the voltage and load determines the actual motor speed, power and efficiency.

Speed

The KV rating indicates the no-load speed at a given voltage. With a fixed voltage supply, a higher KV motor will spin faster when unloaded.

But under load, maximum speed is also limited by the torque required. Motoring torque opposes speed, so a higher KV motor will reach its unloaded speed more quickly but have lower powered top speed.

Torque and Power

Lower KV motors develop higher torque for a given power input, at the expense of lower no-load speed. This makes them better suited for applications requiring high thrust like drones or electric vehicles.

Higher KV equals lower torque but the capability for higher maximum rpm. High KV optimizes for speed rather than torque.

Power is torque multiplied by speed (P = T * ω). A lower KV motor can generate the same power by spinning slower but producing higher torque.

Efficiency

Efficiency varies based on operating conditions and motor design, but often peaks in a mid-range zone between high torque/low speed and high speed/low torque.

Ultra high or low KV motors pushed to their limits may exhibit lower efficiency than a medium KV suited to the voltage and load.

Selecting the Right KV Rating

Choosing the optimal KV rating requires matching the motor characteristics to your operating conditions, including:

Desired top speed – Higher KV for high max rpm, lower KV for slower applications
Acceleration needs – Higher KV allows quicker acceleration to max speed
Drive system voltage – Higher voltage means higher speed for given KV
Typical motor load – Heavier loads need lower KV for more torque
Drive type – Geared or direct drive

Consider the importance of top speed versus torque and acceleration for your application. KV selection involves tradeoffs between speed and torque.

Test different KV options with your drive voltage, gearing and loads to empirically determine the best performance match.

KV Selection Examples

Here are some examples of selecting KV rating for different applications:

Multirotor drones – 2500-3000 KV range allows high power in a compact size. Enables strong lifts and high speeds.
RC cars – 2000-4000 KV range handles rapid acceleration and high speeds needed for racing and maneuvers.
Electric bicycles – Lower KV around 1000-1500 provides excellent torque from a standstill and high efficiency at road speeds.
Underwater ROVs – Low 200-1000 KV paired with gears provide necessary thrust at all speeds despite high drag.
Industrial spindle – 5000+ KV ratings achieve ultra high RPMs needed for milling, grinding, and cutting.

Combining KV with Other Motor Parameters

While KV determines the rpm/volt constant, actual motor performance depends on additional factors:

Stator size – Larger motors generate higher torque and power for given KV.
Magnet type – Stronger magnets improve torque and efficiency across KV range.
Number of turns – More turns increase torque and lower KV.
Pole pairs – More poles increase torque and lower KV.
Voltage – Higher voltage increases rpm.
Gearing – Gears convert between speed and torque.
Controller – Better controllers improve efficiency and precision.

Optimize KV in conjunction with motor size, construction, voltage, gearing and control to match your specific load requirements.

Power, Torque, and Current Relationships

The relationships between power, torque, speed and current in a brushless motor further illustrate the impact of KV:

Power (P) = Torque (T) x Speed (ω)

Torque (T) = Kt x Current (I) (Kt = torque constant)

Speed (ω) = KV x Voltage (V)

Current (I) ∝ Torque

For a given power, increasing current and torque (lower KV) allows lower rpm, while decreasing current and torque (higher KV) requires higher rpm.

Optimizing KV for Efficiency

Efficiency is highest in the motor’s mid-range between low speed/high torque and high speed/low torque.

Very high or very low KV selections pushed to extremes of speed or torque will exhibit lower efficiency.

Choose a KV that keeps your typical operating rpm and load in the optimal efficiency band. Measure power consumption over your required speed/load range to determine the most efficient KV.

Causes of Variation in KV Ratings

The published KV is based on a no-load test result that assumes 100% efficiency. Real world conditions introduce variances:

Friction – Bearing and windage losses reduce rpm and increase current draw.
Magnet weakening – Partial demagnetization at very high speeds lowers KV.
Voltage sag – Battery voltage droop under load decreases rpm.
Temperature – Motor heating changes copper resistance and magnet strength.
Efficiency – Motor losses make input voltage ≠ output voltage.

Published KV values should be considered a starting point. Verify performance under your actual operating conditions.

Using Gear Reduction to Adjust KV

Gearing between the motor and load provides a way to convert between speed and torque. This effectively shifts the operating point for a given KV rating.

Higher gear reduction ratios allow:

Operating a higher KV motor at lower RPMs
Increasing torque at reduced speeds
Improving efficiency when load speeds differ from optimal motor speed

But gear losses can offset gains if poorly matched to the speed-torque profile. Direct drive is often best when the natural KV speed range already aligns with the application.

KV Rating Standards

While there is no single established standard, KV values are typically specified under these assumed conditions:

Unloaded motor
Efficiency = 100%
7.4V or 11.1V power supply
Phase current ≤ 10% of rated current
Ambient temperature 25°C

Precise measurement requires eliminating friction and windage losses while minimizing current draw. Follow motor data sheet guidelines for obtaining the KV rating.

Conclusion

The KV rating provides valuable insight into the inherent speed and torque characteristics of a brushless motor. Engineers can apply their understanding of how KV impacts performance to select motors optimized for their particular application requirements.

Combining the right KV with the proper drive voltage, gearing, and electronic control is key to maximizing efficiency and achieving design goals. As brushless motor use continues expanding into new industries, KV rating will remain a crucial specification for matching these motors to their intended function.

FAQ

What does motor KV measure?

KV measures the no-load RPM speed per volt applied to the motor. It indicates the free-spinning speed constant.

Does higher KV mean more power?

Not necessarily. Higher KV yields higher unloaded speed but less torque. Lower KV provides more torque and the same power at lower RPMs.

How does KV affect torque?

Lower KV increases torque for given power input, while higher KV reduces torque in favor of higher no-load speed.

What motor KV should I use?

Choose a KV based on your desired top speed, acceleration needs, voltage, loads and whether using gears. Test multiple KV options.

Why does KV vary from published specs?

Friction, voltage drop, magnet weakening, heating and losses make real-world KV differ from ideal no-load ratings.

Should I choose high or low KV for efficiency?

Medium KV that keeps your typical operating point in the motor’s mid-range tends to give highest efficiency.

Can gearing change the KV rating?

Gear reduction lowers speed and increases torque, effectively shifting the operating point for a given KV rating.

How is motor KV measured?

KV is measured under no-load with minimal current draw and friction losses to determine the rpm/volt ratio.

What causes lower KV ratings?

More pole pairs and more winding turns increase torque generation but limit maximum rpm, resulting in lower KV.