High KV vS Low KV Motors [A Complete Guide]

Last Updated on November 17, 2023 by Jaxon Mike

When selecting a motor for an application, one of the most important specifications to consider is the KV rating. KV stands for “RPM per volt” and indicates the relationship between the motor’s input voltage and its no-load speed. Motors are commonly categorized as either high KV or low KV, with significant differences in their performance and suitability for various uses.

In this article, we will provide a complete overview of KV rating and the differences between high KV and low KV motors. We will cover what exactly KV means, how it impacts motor behavior, and the pros and cons of both high and low KV motors. We will also discuss the factors that determine KV rating and how to select the right KV motor based on the requirements of your application.

Whether you are choosing a motor for a drone, electric vehicle, robotics project, or other application, understanding KV rating is crucial for getting the ideal performance. This guide will help you make sense of the KV specification so you can confidently select the best motor for your needs.

Here is an in-depth comparison table between high KV and low KV motors:

Parameter	High KV Motors	Low KV Motors
Typical KV range	500 – 1000 KV	50 – 300 KV
Maximum no-load speed	Very high, 20,000+ RPM for small motors	Low, less than 5000 RPM typically
Voltage range	Requires 10V+, performs best at 20V+	Operates well down to 3-6V
Torque output	Very low torque, especially at lower RPMs	Extremely high torque production
Power density	Higher power per size and weight	Lower power density due to larger size
Acceleration	Very fast acceleration due to low inertia	Slow and sluggish acceleration
Load capacity	Low load capacity, cannot drive high loads without gearing	High load capacity, ideal for direct drive systems
Speed regulation	Difficult to maintain stable speed under varying loads	Excellent speed regulation under changing loads
Current draw	Lower current draw at unloaded high RPMs	Much higher current required for high torque
Efficiency	Peak efficiency near maximum RPM	Most efficient at mid-range RPM under load
Duty cycle	Low duty cycle recommended due to high wear at max speeds	High duty cycle capable with less wear issues
Complexity	Requires advanced control electronics for best performance	Much simpler electronics usually sufficient
Cost	Generally low cost	Higher cost due to larger construction
Applications	Drones, RC vehicles, spindles, pumps	EVs, industrial robots, underwater ROVs, mixers
PRICE	CHECK LATEST PRICE	CHECK LATEST PRICE

What is KV Rating?

KV rating stands for “RPM per volt” and indicates the relationship between input voltage and no-load motor speed. It is a specification usually provided by motor manufacturers to help characterize motor performance.

The KV rating is calculated by dividing the motor’s unloaded RPM by the voltage applied. For example, if a motor spins at 5000 RPM when 12V is applied, its KV rating is 5000/12 = 417 KV. This means for every volt applied to the motor, it will spin at 417 RPM without any load attached.

The lower the KV rating, the slower the motor will spin per volt applied. Higher KV equates to higher RPM per volt. KV rating is primarily useful for determining the maximum unloaded speed of a motor when running on a particular voltage source. It provides a quick way to estimate performance across different voltage and load conditions.

How KV Rating Works

To understand KV, it is important to first realize that the speed of a motor is proportional to the voltage applied. Without any load, increasing the voltage will make the motor spin faster.

KV rating describes this relationship between voltage and speed. Motots with higher KV values will spin proportionally faster for the same voltage compared to a lower KV motor.

For example:

• A 500 KV motor powered with 10V will spin at 500 * 10 = 5000 RPM unloaded
• A 250 KV motor powered with 10V will spin at 250 * 10 = 2500 RPM unloaded

Even though both experience 10V, the 500 KV motor spins twice as fast as it has higher KV.

When load is applied to a motor, it will spin slower due to torque requirements. But the KV relationship remains the same. Higher KV means faster unloaded RPM per volt.

KV rating is usually specified for a motor using its maximum rated voltage, but the proportional relationship holds true for lower voltages too.

Factors that Determine KV

The KV rating of a motor depends on several design factors:

• Number of Turns in Windings – Motors with fewer turns in their copper windings exhibit higher KV. The lower number of turns results in lower electrical resistance and inductance, allowing higher RPM per volt applied.
• Motor Size – For same windings, smaller motors have higher KV compared to physically larger motors. The smaller rotor has lower inertia allowing it to spin faster.
• Magnet Strength – Higher strength magnets in the rotor result in higher KV. Stronger magnetic field provides more torque for the same voltage input.
• Core Material – Cores made from lamination steels generally result in higher KV compared to iron cores. The laminated core has lower losses.
• Mechanical Design – Bearing friction, precision balancing, and minimized air gap between rotor and stator can improve KV.

Manufacturers ultimately select an appropriate KV rating by optimizing these design factors for the target application and operating conditions.

High KV Motors

High KV motors are designed to spin very fast when unloaded. Their RPM per volt applied is much higher compared to low KV equivalents. Some key details about high KV motors:

• Typical KV Range – About 500 KV to 1000 KV generally qualifies as high KV for most hobby motors. Ratings above 1000 KV are considered ultra-high KV.
• Speed – When unloaded, these motors can reach very high speeds, often over 20,000+ RPM for small brushed motors. Their maximum speed is largely limited by mechanical integrity.
• Voltage – Best suited for higher input voltages, typically over 10V. This allows them to reach their high potential RPM range.
• Torque – Relatively low torque output, especially at lower speeds. High KV trades off torque for maximum attainable speed.

Now that we’ve covered the basics of high KV motors, let’s look at their key benefits and drawbacks.

Benefits of High KV Motors

• Achieve very high RPM – Ideal when maximum speed is required. Allows smaller motors to reach insane speeds.
• Maintain speed with lower voltage – High KV means motor will spin relatively fast even as voltage drops. Helpful for maintaining performance in battery-powered devices as charge depletes.
• Higher power density – Generate more power for their size compared to low KV equivalents. Makes them suitable for applications with weight and space constraints.
• Rapid acceleration – Get up to speed very quickly due to low inertia. Great for applications requiring fast starts/stops.
• Efficiency at high RPM – Tend to have highest efficiency near max RPM in unloaded state. Losses are lower at higher speeds.

Drawbacks of High KV Motors

• Low torque output – Does not produce much torque, especially at lower RPMs. Cannot drive high load applications without gear reductions.
• Inefficient under load at low RPM – Will draw high current and get hot if operated at moderate/high loads below its high RPM range.
• Requires higher voltage – Needs at least 10V+ to reach advantageous RPM range. Less than 10V input will result in poor performance.
• Speed regulation difficult – Maintaining stable speed under varying load conditions is challenging. Needs complex control electronics.
• High wear and tear at max RPM – Bearings, brushes, and commutator experience very heavy wear when spun for long durations at max unloaded speeds.

Applications of High KV Motors

Here are some common applications that benefit from the advantages of high KV motors:

• Multirotor drones – The motors spin propellers at high RPM to generate required lift and maneuverability. High KV allows large props to spin fast on lower voltage batteries.
• RC vehicles – Quick acceleration and high top speeds are achieved using high KV small motors. They provide strong powerband performance.
• Spindles – Spinning grinding wheels, pumps and other components often requires reaching 10,000+ RPM. High KV motors are well-suited.
• Turbochargers – Turbos can use high KV outrunner motors for fast compressor shaft speeds to develop boost pressure.
• CNC routers – When driving small bits at very high velocities, high KV motors maintain constant speed under varying load.

Selecting the right KV rating is key to getting the best performance for these applications. Excessively high KV will result in poor low RPM torque. Too low KV will limit maximum speed potential.

Low KV Motors

On the opposite end of the spectrum from high KV are low KV motors. These have a much lower RPM per volt ratio. Some key attributes of low KV motors:

• Typical KV Range – Usually under 300 KV, some as low as 50 KV or less. Very low KV ratings under 100 are common.
• Speed – Produce relatively low RPM, often less than 5000 RPM when unloaded. Maximum speed is severely limited.
• Voltage – Typically work well with voltages under 10V. Can still generate decent torque from sources below 6V.
• Torque – Provide very high torque output, especially right off the line at low RPMs. Ideal for driving heavy load applications.

Benefits of Low KV Motors

• High torque output – Excellent low-end torque essential for devices that need to work against heavy loads.
• Efficient under load at lower RPM – Optimized to provide optimal torque efficiency within typical operating RPM range.
• Stable speed regulation – Maintains consistent RPMs even as loading conditions change. Requires less complex electronics.
• Lower current draw – Less tendency to overheat and trigger overcurrent protection compared to high KV equivalents.
• Operates well at lower voltages – Can be effectively driven down to 3V-6V input sources.

Drawbacks of Low KV Motors

• Low maximum RPM – Upper speed range is severely limited. Cannot reach speeds over 5000 RPM typically.
• Sluggish acceleration – Slow to get up to speed due to high rotor inertia. Not suitable for applications needing quick starts/stops.
• Higher weight – Require larger cores and magnets to produce torque. Adds bulk and weight.
• Losses increase at higher RPM – Operate far from peak efficiency range when run near maximum speed.
• Requires gear reductions – Often need mechanical gears to trade torque for higher speed. Adds complexity.

Applications of Low KV Motors

Here are some example uses where a low KV motor would be well-suited:

• Electric vehicles – Providing strong torque off the line without instant overheating. Lower RPM range matches wheel speeds.
• Industrial robots – Precise movement of heavy arms and components requires high torque, not speed.
• Underwater ROVs – Thick, slow spinning propellers are ideal for producing heavy thrust with limited battery power.
• Stirrers/mixers – Blending viscous fluids doesn’t require high RPM, just steady torque.
• Mills/lathes – Spinning cutting tools against metals and composites needs high torques at moderate RPMs.

Choosing the right KV for these applications prevents issues with insufficient torque generation. An overly high KV motor will not work optimally.

So in summary, low KV and high KV motors have clear trade-offs. Selecting the proper KV rating depends primarily on the speed and torque requirements of the application. Next we’ll go over some key considerations when choosing between high and low KV.

Choosing Between High and Low KV Motors

Selecting the right KV motor involves evaluating four main factors:

Speed and Torque Requirements

The most important factor is understanding the speed and torque needed for your application. High KV favors high speed, low KV favors high torque.

For example, drones need to spin propellers at very high RPM so high KV is best. But robots need slow, powerful movement so low KV is better.

Voltage Source

The voltage driving the motor determines available power. High KV needs 10V+ to reach useful RPM while low KV works well under 6V.

Match KV to your voltage source. If using a 22V lithium pack, consider a higher KV around 1000-1200 KV. For a 6V lead acid source, 50-100 KV may be appropriate.

Load and Inertia

Heavier loads require more torque to move and accelerate. Systems with low friction or low mass loads can utilize higher KV.

Also consider inertia – high rotor inertia makes acceleration difficult, favoring lower KV. Low system inertia allows utilizing higher KV.

Duty Cycle and Frequency of Use

Frequent start/stop cycles and long high RPM operation causes more wear. For high duty cycle applications, consider slightly lower KV to reduce long term impacts of wear.

By evaluating these factors, you can narrow down the optimal KV range for your application’s requirements and operating constraints.

FAQs

What is a good KV rating for an electric skateboard motor?

150-300 KV motors are typically used for electric skateboard drives. This provides good low-end torque to accelerate the rider, while still allowing 25+ mph top speeds. Higher KV would result in poor hill climbing torque.

If I gear down a high KV motor, will it have more torque?

Yes, adding a gear reducer will increase torque output at the expense of maximum speed. This allows using a high KV motor optimized for high speed in a application needing more torque.

Can I run a motor on lower voltage than its rated max voltage?

Yes, but the motor will have proportionally lower RPM. A 500 KV motor powered with 20V instead of 50V will spin at half its unloaded speed. Torque output will also be lower.

What happens if KV rating is too high for an application?

Excessively high KV can cause poor torque output, sluggish acceleration, overheating issues, and motor damage over time. Always use a KV rating suitable for your operating voltage and load requirements.

Will connecting motors in parallel increase total KV?

No, KV will remain the same as a single motor. Connecting identical motors in parallel will simply increase total available current and power, not change the KV relationship.

Conclusion

KV rating is a crucial specification to evaluate when selecting a motor. It describes the relationship between input voltage and no-load motor speed. High KV motors are designed for maximum speed while low KV equivalents trade speed for higher torque output.

Choosing between high and low KV requires understanding your speed, torque, voltage, load, and duty cycle requirements. High KV motors operate best on 10V+ sources driving low-inertia loads needing quick acceleration and very high RPM operation. Low KV motors provide excellent torque at low RPMs for high load, low voltage applications needing high duty cycle operation.

By matching the KV rating to your specific needs and constraints, you can ensure optimal motor performance. Understanding the nuances between high and low KV ratings will give you the knowledge to confidently choose the right motor for your next project.