Q: What is the maximum motor output?

Many factors affect overall performance, some of which we can change. If we focus specifically on motors and no other influences, then the list becomes much smaller and easier to understand.

Electrical Power (Pe) is equal to Current (I, Amps) * Potential (V, Volts), and is provided by the Controller and Battery. Mechanical Power (Pm) is equal to Torque (τ) * Angular Velocity (ω) and is the output created by Pe passing through the Motor. Generally, Current (I, Amps) creates Torque, and Potential (V, Volts, from Battery) creates speed. The physical factors of the Motor manipulates Power and Output ingeniously. They are often described as "6-Turn 3-Phase BLDC Motor, 28mm 1000W". But what does this mean?

Let's walk through the elements that effect Motor Output.

For BLDC Hub Motors:

  • Rotor Diameter: Larger creates more Torque, less more Speed. Generally, hub motors come in two diameter sizes: 8-inch and 4-inch. Believe it or not this has everything to do with the availability of cheap iron pipe that is used in motor fabrication to create "back iron". In brief, a circuit is "a roughly circular line, route, or movement that starts and finishes at the same place". This applies both electrical and magnetic circuits. With BLDC motors, "back iron" completes one leg of the magnetic circuit, with iron having magnetic conducting qualities analogous to copper electrical conducting qualities. The cheapest source of circular hollow iron is 8-inch iron pipe. And that is why the lion's share of hub motors are just over 8 inches in diameter world-wide, that – and Chinese manufacturing.

Kinaye's MXUS 3K Turbo Direct Drive Motor

  • Rotor Width: Larger creates more Torque, smaller more Speed. Typical sizes are 28mm and 45mm. Larger sizes also handle heat and load-spikes better, although are heavier and more expensive.
  • Number of Turns (T) or Windings as some call it: Turns directly affects the Speed verses Torque Calculation, like gearing between the Chainring and the Sprocket. More Turns lowers the Speed though increases the Torque.
  • Slot-Fill: A measure in Percent (%), sometimes also given as the Strand-count in a winding cross-section relating to filling the Slot between the Teeth; it is rarely used as a specification because all quality motor manufacturers will adjust the number of strands of a single turn to optimize best possible complete filling of available Slot space. Less fill equals less efficiency at converting Electrical Power into Mechanical Power.
  • Number of Teeth: Each Turn is wrapped around a Tooth (also called a Slot). The number of Teeth directly affects the Speed verses Torque Calculation: More Teeth creates more Torque at the cost of Speed.
  • Number of Pole Pairs (PP): Magnets have polarity; a North and a South Pole. These are oriented perpendicular to the Teeth. A rise in thickness creates better Torque and Efficiency at the cost of Speed. The strength of the Magnets (T, Tesla) increases Torque, Efficiency, and Cost as it rises.
  • Shaft Size: The size of the Axle. All hub motors use a hollow shaft to penetrate the center of the motor to deliver the Phase Wires. Most axles are 14mm in diameter with 10mm flats, and typically have 13-AWG phase wires. At Kinaye, we use 16mm Diameter axles for increased strength, and for allowing 10-AWG Phase Wires for 3-fold increased current capacity.
  • Stator Material: Stators are an assembly of thin die-cut sheets of special soft silicon-alloy termed “electrical steel” which has unique magnetic properties suitable for use in motors, having high permeability and low loss (low heat-producing) characteristics. The quality of the alloy, the number of sheets, and their thickness all contribute to the efficiency and reaction of the magnetic field flux. These layers are captured by a framework that is welded to the axle, and can be either structural steel or aluminum: Steel is strong, though heavy, yet can absorb quite a bit of heat. On the other hand, forged aluminum is equally as strong, lighter, and able to purge heat more quickly. The MXUS 3K Turbo motors all use aluminum framework, plus more sheets of thin electrical steel for enhanced performance.

All BLDC motors use 3-phase wiring. The size of the wire dictates the amount of current flow possible. However - ALL WIRES ARE RESISTIVE. The only way to change this equation is:

  • Shorten the Wires: As a rule, all wires on an Electric Vehicle should be made short as possible to reduce voltage drop, weight, and conserve valuable materials.
  • Increase the Wire Diameter: If we can't reduce the length, then increase the wire diameter to reduce the overall resistance of the circuit and thereby increase current flow. Of all things possible, this is the least expensive, easiest fix to obtain, and has by far the greatest impact in all aspects. Trim the Phase Wires at the Motor, and connect them to upsized (125% or larger) multistrand conductors and run them right up to the controller.
  • Reduce the number of Connections: Every connection and splice within a circuit causes voltage drop and also becomes a source for potential arcing; when contacts arc, they create carbon deposits over time which become resistive and eventually lead to failure.
  • Change the Wire Architecture: Generally phase wires are multistrands of smaller wires which allow for bending and dynamic motion without breaking. Wire that overheats become brittle and will fail. Tight bends in wire also create resistance and if in dynamic motion will fail. Smaller-counts of multistrands increases chance of fatigue. The best solution for wires that must be flexible are to employ silicone noodle wire which have many strands of very fine wire. In addition, it is a good idea to upsize the wire to increase durability (plus it reduces resistance).
  • Pull larger wires through the Axle: Common Hub motors use inferior 13-AWG Phase Wires, though it is possible to upgrade to 12-AWG by opening up the motor and attempt a very difficult modification. We at Kinaye solved this problem by upsizing the Axles on our MXUS-3k Turbo product line to employ 10-AWG Phase Wire.

We know that connectors are a source of problems, and the best way to overcome them is to use high-quality high-capacity premium gold-plated connectors. The construction of a good connector is made of high-purity 99.98% Oxygen-Free Copper, then Nickel-Plated, followed by Gold-Plate. Do not bother with Silver or Tin-Plates as they will oxidize, creating a thin film of resistance. Barrel- and Blade connectors are the best. Avoid spring-like contacts such as Anderson Power Poles (APP) for high-current if placed in-line with dynamic motion; however, APPs are excellent if used in static locations and mounted against a framework or case. Again – we stress using oversized overrated connectors to avoid problems.

  • Keep it cool: Motors perform better in colder weather, whereas they decrease with heat. As long as the limits are not breached, they should in theory last for a very long time. However, if the motor temperature exceeds 80°C/180°F then the Rare Earth Magnets will begin to degrade and the performance will falter. To prevent overheating, reduce the power going to the motor, and/or reduce the load; easiest to just slow down.
  • Let it breath: It's also possible to vent the motor to allow for excessive heat to escape. A somewhat common practice is to remove the side covers, drill 2 or more holes near the perimeter about 1/16 to 1/8 inch diameter, and reinstall. This is enough to allow both venting and draining. Do not bother trying to affix vanes or propellers; none of that works effectively and only fosters more problems. Larger vent holes leads to accumulation of dirt and debris. If the motor is getting that hot, spend a bit more coin and upsize the motor.
  • Keep it dry: A particular problem in the rainy Pacific Northwest is constant moisture, and unvented motors will eventually fail prematurely due to accumulation which leads to corrosion and shorting. The best way to solve this problem is to vent the hub covers (2 or 3 holes each side should do it) and remove the stator in order to seal it with two coats of Electrical Insulating Varnish like the type used on Starter Motors.

Understanding these factors are the meat and potatoes of motor design and utility, and are crucial in the mathematics of performance, especially for calculators and simulators.

  • Km: Motor Constant, Km = τ/√P where P, Power is equal to the Resistive Loss. Motor Efficiency plays a big role here.
  • Kv: Motor Velocity Constant, equals unloaded motor speed given as RPM/Volt
  • Kt: Motor Torque Constant, equals Torque/Phase Amps, τ/Ia or 60/(2π * Kv)

Measured in Watts and Kilowatts (W, kW) this is a consumer rating of typical power output set by the motor manufacturer. Commonly, motors are rated as 350, 500, 750, 1000 Watt or 1 kW, 3 kW, 5 kW… and so on. The actual motor output varies widely based upon usage, environment, and politics. In the technical sense the power rating of a motor or kit is at best imprecise because we are able to affect the performance greatly by many other factors; it is more of marketing classification number and less of a performance statistic. Instead - look at the detailed motor specifications, plug the values into calculators and simulators to model the intended extremes, and evaluate if the results and price are affordable.

Summary:

Therefore to answer the question properly, the maximum motor output is governed by the quality of voltage and current supplied, environmental conditions, and load over Time. If we know the Km or Kt and Kv, then we can predict unloaded motor velocity which is useful when selecting rim and tire. The key factors though are stator width, rotor diameter, phase wire diameter through the axle, the number of Turns, and even the stator construction.

Good Hunting