Kv, Kt, and the number of poles: these are the three key figures that describe an electric motor
Two motors of the same size can behave very differently: one might provide strong initial acceleration but quickly reach its maximum speed, while the other might accelerate less but achieve a higher top speed. The difference lies in two constants, the Kv and the Kt, and in the number of poles. This guide explains these concepts, along with the calculations that are useful in everyday use.
The essentials
- The Kv rating indicates how many revolutions per minute (RPM) the motor spins per volt, when it's running without any load.
- The Kt value indicates the amount of torque the motor produces per ampere. It is the inverse of the Kv value, differing by a constant factor.
- A high Kv value provides speed, while a high Kt value provides torque. You can't have both at the same time with a single motor.
- The number of poles determines the frequency of the current that the controller must produce.
Kv: Revolutions per minute per volt
The Kv rating is measured in revolutions per minute per volt. A motor with a Kv of 15 will, under no-load conditions and without losses, rotate at 15 revolutions per minute for each volt applied. Therefore, with a fully charged 84V battery, it would ideally rotate at approximately 15 × 84 = 1260 revolutions per minute.
It's a maximum speed: under load, the voltage drop in the battery, cables, and windings, as well as friction, reduces it. This explains why a higher voltage battery makes a scooter faster with the same motor, and why we talk about field weakening to exceed this limit.
From rotation to scooter speed
For a hub motor, the wheel rotates at the speed of the motor. The speed in kilometers per hour is calculated as follows:
Speed (km/h) = RPM × Wheel circumference (m) × 60 ÷ 1000
Example: 1,260 revolutions per minute on a wheel with a circumference of 0.8 meters results in 1,260 × 0.8 × 60 ÷ 1,000 ≈ 60 km/h, in theory and without load. The actual speed will be lower.
Kt: Torque per ampere
The Kt value is measured in Newton-meters per ampere. A motor with a Kt of 0.6 produces 0.6 N·m for each ampere of phase current. With 100 amps, it produces 60 N·m.
Kv and Kt describe the same thing from two different perspectives: the strength of the magnets and the number of windings. With consistent units, they are related by a simple equation:
Kt ≈ 9.55 ÷ Kv (Kt in N·m/A, Kv in revolutions per minute per volt)
With a Kv of 15, we find a Kt of approximately 0.64 N·m/A. According to the manufacturers' measurement conventions (voltage between phases or per phase, peak or effective value), this relationship can vary by a factor of approximately 1.2 to 1.7: use this as a rough estimate, not as an exact value.
High Speed or High Torque: the same motor, but with different winding configurations
For the same motor casing and the same magnets, you can wind the stator with more turns of thinner wire, or fewer turns of thicker wire.
| More windings (High Torque) | Fewer windings (High Speed) | |
|---|---|---|
| Kv | Lower: Less speed per volt | Higher: More speed per volt |
| Kt | Higher voltage: more torque per ampere | Lower: Less torque per amp |
| Current draw for the same torque | Lower | Higher |
| This refers to what the controller and battery are required to deliver | Less current | More current |
At the same size, both motor windings can produce comparable power: one achieves this with higher voltage and lower current, while the other does the opposite. The best choice depends on the battery voltage, what the controller and battery can provide in terms of current, and the intended use: acceleration and hill climbing, or top speed. This is the logic behind the High Speed and High Torque versions of our electric scooter motors.
The poles: electrical frequency
A motor has a certain number of magnetic pole pairs, meaning pairs of north and south magnets on the rotor. For each mechanical revolution, the current must complete as many cycles as there are pole pairs.
fréquence électrique (Hz) = tours par minute ÷ 60 × nombre de paires de pôles
Hub motors for scooters have a large number of poles. This is what gives them high torque at low speeds without the need for a reduction gear. However, the controller must produce a high frequency at high speeds, which requires fast control and a sufficient switching frequency.
Software settings, such as VESC Tool, often display the speed in electric revolutions per minute (ERPM): this is the mechanical speed multiplied by the number of pole pairs.
Here's what to consider when choosing a motor:
- The maximum speed depends on the Kv rating and the voltage: Kv multiplied by the voltage, and then the wheel size.
- The torque depends on the Kt value and the phase current that the controller can deliver.
- The heat generated depends on the current: a motor with a low Kt value requires more current to produce the same torque, and therefore generates more heat at the same torque level, assuming it's not physically larger.
- The controller must monitor the phase current and electrical frequency.
Frequently Asked Questions
How do you measure the Kv rating of your motor?
Run the motor without a load, with the wheel raised, at a known voltage, and measure its rotational speed. The Kv value is the ratio of these two measurements. Adjustable controllers like the VESC also determine this value during motor detection, based on the magnetic flux.
Does a higher Kv rating always make an electric scooter faster?
It increases the maximum speed, but requires more current to achieve the same torque. If the battery or controller cannot supply the necessary current, the machine will be slower when accelerating and may even shut down. The performance gain is only real if the entire system is properly sized.
Why do hub motors have so many poles?
Because they drive the wheel directly, without a reduction. A high number of poles provides strong torque at low speeds in a flat, large-diameter motor, which is exactly what's needed in a wheel.