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E-Bikes Motor Tech: Learn the terms, Part-2

If you actually know anything about electric motor technology, and you don’t hate me yet? Buckle up…this article is going to be a bumpy ride. I think I have a firm command of the terms that define the more common aspects of electric motors and now…I’ll try to explain some of the more obscure aspects of their design and application (as it pertains to electric bikes).

If you like a lot of pictures to make the explanations more clear? this article may not be for you…here’s an index.

  • Phase/Battery current ratio
  • Swept Magnetic Area / SMA
  • Tangential Magnet Speed / TMS
  • Saturation
  • Magnet Speed per Meter [traveled] / MSpM
  • Back-EMF
  • Inductance
  • Ohm’s Law
  • Delta/Wye

Phase/Battery current ratio

Most common controllers allow the owner to adjust the motor-phase / battery amps current ratio. If you don’t know what that is, the motor-phase current can be set a little higher than the battery current, and 1.5:1 seems to be pretty common.

From madin88, in Austria:

“…High phase amps = good acceleration from zero to mid speed
High battery amps = good acceleration from mid to top speed
For higher efficiency, lower phase amps is better…”

From NeilP, in the UK:

“I would say…use XPD software rather than Lyen / Keywin…then, you can turn block time down to 0.1 seconds”

From gwhy! in the UK:

“The way I have adjusted it in the past is:

Set the battery current to something safe, like 40-50A (depending on the type of battery), set the over-current detection to 0.0s (this is very important), set motor-phase to approx 1.5 times the battery current (1.5:1).

You need to attach a watt meter. On a slightly uphill road (maybe a 2-4% grade), accelerate hard (from a dead stop) up to top speed, or until it stops accelerating but throttle is still at WOT (this may take a while, so you need a longish stretch of road).

Then check the watt-meters’ max current pull. If the max current pulled is less than your set battery current, then…you need to increase the phase current, maybe by 10A. Or, if the set battery current is reached, then reduce phase current by 10A. You keep increasing or decreasing the phase current until the max current pulled from the battery is what you have set it to. Once you have found a battery/phase ratio where the max battery current is always reached? Then this will be the ratio you would use when increasing the battery and phase current together.

If the phase current is too low, then you will never reach full speed or max battery current. My controllers use a ratio of around 1.7:1 for the motors, gearing , my riding style and total weight of my bikes

Another method I have used, is set at a much lower battery limit (maybe 20A), and a phase limit of 30A with the wheel off the ground go WOT and then start applying the brake to slow the driven wheel down (loading the motor). When the wheel is approximately half the max speed, watch the battery current on the watt meter. This should go to your set max battery current, and should stay there as the driven wheel gets slower and slower until the controller cuts out (locked rotor fault protect).

The same applies when you increase or decrease phase current until you see the set max battery current limit hit, always just before the controller cuts out. Each test needs to take around 5-seconds, so it’s a much quicker and easier method of finding the optimum phase setting for the motor. Once you have found your optimum phase ratio you can always turn the battery current down and keep the phase limit at the optimum”

“if you go too high with the phase current, you could pop your controller, if the motor is really chugging and the throttle speed is being dragged down too much. A standard 12-FET controller should be OK for around a safe max of 150A phase current…if I was you, I would play it really safe and do no more than 100A phase current.

So, if your Battery current is 50A, set phase current to no more than 150A (3X battery current) or be extra safe, and use 50A battery and a motor-phase of 100A (2X battery current ). As NeilP said, set the block time down to zero, this will should limit the current as fast as possible if an over-current situation occurs. The optimum settings will just be the least stressful (but still working 100% correct)…”

From AlanB, in the USA:

“…[more] Phase current makes [more] torque, up to magnetic saturation. Then, it still makes [more] torque, but…it increases at a much lower rate [per added amp]. It is best to set max phase current to no more than the point where magnetic saturation begins (and is compatible with wiring and connector capacity), though…you might want to set it slightly higher, but heat is going to be a problem if you run at that level very long. Don’t worry about the ratio, worry about the max current. Calculate the I-squared-R heating and think about your motor dissipating all that heat…

Phase current “squared” makes heat in the motor. So, you will quickly overheat with high phase currents [that are above saturation] for only a modest improvement in torque, because the square grows so fast. The battery current determines the max power, and should be set for battery, BMS, and wiring capacity (or less).

I set the phase current to control the front wheel lift (if nothing else limits it first). No point in throwing the front wheel skyward too quickly, that’s just wasted torque….”

Swept Magnetic Area / SMA

I don’t know the proper term for this, so…I just invented this term, and I’ll use it until I stumble across the correct term (if you are a professional motor-design engineer, email me. Please understand that, I will still ignore you, but…at least then I can say that real engineers emailed me).

I needed a way to compare two similar hubmotors, which both use the same common off-the shelf lamination, which results in a 205mm diameter stator. I already covered my “rogues gallery” of hot rod hubmotors in this article on large Direct Drive (DD) hubbies.


An outrunner DD hubmotor. The circumference of the stator, times the width of the electromagnet-faces on the stator…equals the Swept Magnetic Area  / SMA

So here’s a direct drive hubmotor list (off the top of my head) to help explain what I mean. All of these motors have a 205mm diameter stator, and the numbers listed below are the width of their stators.

28mm, MXUS 1000W

35mm, Edge 1500W (click here to read about this motor)

45mm, MXUS 3000W (click here to read about this motor)

50mm, QS 205/50H V3

The wider the stator, the more copper mass (in wire) is wrapped around the stator-teeth, which results in a greater ability to use higher amps without overheating. However, even if they are all being fed the same exact amount of volts and amps, the larger magnetic area that results from a wider rotor magnet (which interacts with the stator electro-magnets) will result in more wheel-torque.

(edit: a larger diameter motor, like the QS 273, adds a lot more weight, and also shortens the available spoke length)


Both of these DD hubmotors have the common 205mm diameter stators. One has a width of 28mm on the magnets and stator tooth-faces, and the other is 50mm.

I’ve already listed the width of the magnets, so…next comes the circumference. Of course, circumference would be “the diameter times Pi”, which for our purposes can be rounded-off to 3.14 (for the OCD among us, 3.14159265 is more exact). This means that 205mm X 3.14 = 643.7mm, and…this is the circumference of the magnetic “air gap” where the electrical power is converted to rotary movement, and produces torque.

This common motor-stator circumference multiplied by the width of the various stators, will give us the active magnetic area in squared millimeters (I know there are small gaps between the electromagnets, but they are fairly equal between all of these hubs, so their area is negligible in these calculations, when comparing one motor to the other).

There are 100 square millimeters in one square centimeter, so moving the decimal point over two spaces will result in the motors’ useable magnetically-active area being listed in square centimeters.

The rule of thumb has been that;…if your motor is getting hot under your loads, you need a bigger motor. If it’s running very cool, you could probably get by with a lighter and less expensive motor. If it only gets warm, then…it’s “just right”

But now? this calculation can show you the ratio of how much more torque (or less torque) a given motor has, compared to another…IF…the permanent-magnet strength and input watts to the stator are the same.

180.2, MXUS 1000W V2

225.2, Edge 1500W

289.6, MXUS 3000W V2

321.8, QS 205/50H V3

Tangential Magnet Speed / TMS

This is a real engineering term, not to be confused with the rest of the bullshit terms I used in this this article.

In casual conversation, “motor speed” is often referring to the RPM’s (Revolutions Per Minute). As a result, motor design engineers had to invent a phrase that specifies how fast the permanent  magnets in the rotor are passing by the electro-magnets  in the stator (which can be turned on and off as needed, in order to make the damn thing spin).

I try to find the proper term for anything that I feel is important enough to write about, and TMS is one of those instances where I accidentally stumbled across something when reading a lot of obscure technical papers on motor design. (*most of which I still don’t understand. Edit: change “most” to all).

Source: electricbike


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