Fiddling with the Grin motor simulator shows that the Bafang watt ratings don't correspond to anything of any use. I have a theory about this as per item (3) below.
Regarding the motor windings: If you look at specs in the Bafang motor datasheets and catalog "workbook" then you'll find that there is a huge disconnect between the winding suffix (6~15) stamped on the motor after the part number, and the Bafart published voltage+RPM specifications. Perhaps someone here can explain how these correlate? I'm sure EvErYoNe would like to know!
Lastly, if a motor is stamped for example with "FM G312.250.D/V 10" then does this fully define the configuration of the motor? Or could there still be two fundamentally different motors with this same stamp? I ask because Bafang seems to make a big deal about motor voltage ratings, for instance the difference between a 48V motor and a 36/43V motor. Why are the datasheets segregating motors into different voltages and then having to quote n0 values when they could simply have quoted Kv values? Is it because they are afraid to publish Kv and winding T values in the specs? Are they afraid of legal motor power restrictions in the EU? What is going on here?
In the end, it's important to have a good specification for the motor because that will determine how one should spec out the controller current and voltage capabilities, given any particular application/usage scenario. I wanted to order a G311 motor and when I asked for a 13 suffix I got the run-around and yet this is absolutely fundamental. They would rather ask "How big is your tire" and "how fast do you want to go" instead of allowing me to order by rational motor spec. Sigh... It's like having children, when the motor finally arrives in the mail you just don't know what it's going to be.
I'm thinking after looking at a lot of motors and a lot of specs that the following winding turns T (part number suffix) options might make sense in the Bafang world for people who want a top speed of 30KPH with 48V packs and minimal motor currents:
15 for 29" wheels?
13 for 26" wheels?
10 for 24" wheels? (unofficial)
8 for 20" wheels?
6 for 16" wheels?
But this is actually the very information I'd like to clarify.
----- The following analysis is for a newer motor "FM G312.250.D/V 10" using the new part numbering schema -----
SPECIFICATION SHEET BREAKDOWN:
(A) Model Name: H400 - This is the marketing-monkey "model name" for the G312 "model number" motor. See the Bafang catalog aka workbook. Bafang does not publish a catalog, rather a workbook which is actually a fairly nice but incomplete catalog that is missing fully qualified part numbers.
(B) Model No: FM G312.250.D/V - This is the model-number when you are ordering the motor from Bafang, but you also need to specify the winding suffix, for example 8 or 10 or 13. Bafang would rather calculate the siffix for you based upon your battery-voltage, wheel-size and speed(or RPM). This really sucks when you are trying to match a 3rd party controller to a Bafang motor because who knows what you're actually going to get based upon their "calculation".
(C) Wheel Diameter (inch ISO): 26/700C/28 - Bafang uses this to calculate the winding suffix for you. They also recognize a 16/20 inch diameter and a 29 which of course has different specs.
(D) Wheel Circumference (mm): 1900~2250 - This is what Bafang thinks is the circumference of a 26" and 28" regular wheel. For 29" they claim 2330 or 2400. For 20" they claim 1400mm, and for 16/20 they claim 1150~1650. These numbers are not set in stone and fat bikes are different.
(E) Rated Voltage (Vdc): 36/43 - This is the battery pack nominal voltage, assuming that the controller is limited by this as a maximum value, which limits your maximum speed for any given motor T winding. These are probably 2 different motors (T value) as per directly below. This correlates directly with n0 parameter below if you want to calculate Kv.
(F) n0 (RPM): 237/250 - This is the no-load RPM at the above specified motor voltage ie full throttle if your controller does not cap top speed. Two numbers are given, perhaps because two different voltages are quoted in this spec, but these are apparently two different motors because 250*36/43 is not equal to 237! It's close but not the same, therefore I presume the two motors have different windings. The 13T winding for 48V is not mentioned though it may well exist. Can you sense my frustration?
(G) Max Speed (km/h): 25/32 - This is the no-load wheel tire speed at full throttle. It verifiably corresponds simply to KPH = n0 × (D" × 25.4 × π × 60 ÷ 10⁶) where n0 is directly from above. You will typically get 83% of this on the road, though some of the newer motors are more efficient.
(H) Rated Power (W): 250 - This is NOT the motor power in watts. See item (3) below for details.
(I) nT (RPM): 205/215 - I believe this is motor RPM at at maximum torque and therefore under load. It is at the RPM where the motor exhibits best performance. It is somewhat consistently about 85% of n0 across different models.
(J) Max Torque (Nm): 45 - I believe this may be the torque measured at the nT parameter RPM, where the motor exhibits best performance. If so then it is not sustainable in regard to eventual overheating.
(K) Efficiency (%): ≥ 80 - This correlates motor no-load speed with motor loaded speed at any given voltage. The "slippage" is due to losses.
STAMPED PART NUMBER BREAKDOWN:
In this example consider: FM G312.250.D/V 10
(1) FM means it is a Front hub Motor. RM would be a Rear hub Motor. MM would be a Mid drive Motor.
(2) G312 is the model number. According to the Bafang Workbook the corresponding marketing-monkey model name is H400.
(3) 250 means that the motor is appropriate for applications which legally limit maximum power to 250W. The continuous rated power of the motor is often higher and the hill climbing power of the motor is always much higher. I don't think it has anything to do with the continuous power dissipation capacity of the motor because different motors having similar form factor but very different surface area are often rated the same.
(4) D/V means that a Disc-brake can be retrofitted to the motor. D would mean the motor comes with a brake Disc. V means that no brakes can be attached to the motor but that V-brakes can be used on the rim. R means the motor comes with a mounting spline for Shimano Roller-brakes. DM means the motor comes with an integrated drum for Drum-brakes.
(5) 10 means the motor has a 10T winding. Windings from 6T through 15T are commonly available but the most common ones are 8, 10 and 13. You'll probably want 8T for small diameter wheels or low voltage batteries and 13T for large wheels or high voltage batteries. This ties into the motor specification numbers for Wheel Diameter, Battery Voltage, Motor Current, RPM and Speed. It has nothing directly to do with maximum available torque nor maximum available power nor maximum available motor speed, so long as your controller is able to deliver any voltage at any current. The T winding parameter simply determines the ratio of Voltage to Current which the motor needs to achieve any particular level of performance.
RPM Speed Voltage Current Torque Turns Power:
• Motors are specified as having a number of turns T in their windings.
An 8T motor has a higher speed than a 12T motor at the same voltage.
A 12T motor has a higher torque than an 8T motor at the same current.
RPM/V and Nm/A are quoted with no load on the motor and the loaded values will be about 20% less due to efficiency losses.
• Motor speed & torque vs voltage & current:
Motor torque = Kt × Current through motor
Motor speed = Kv × Voltage across the motor
Kt × Kv = 60 ÷ 2 ÷ π = 9.55
∴ 8rpm/V = 1.2Nm/A
• Calculations if you know actual number of turns:
Turns × Kv = Constant
Turns ÷ Kt = Constant
Kv(new) = Kv(old) × Turns(old) ÷ Turns(new)
Kt(new) = Kt(old) × Turns(new) ÷ Turns(old)
Rdc(new) = Rdc(old) × Turns²(new) ÷ Turns²(old)
The above direct current resistance calculation assumes a constant coil cross-section.
• Power vs torque and speed:
KW = Nm × RPM ÷ (60 ÷ 2 ÷ π) = Nm × RPM ÷ 9.55
• Tire speed vs motor RPM:
KPH = RPM × (D" × 25.4 × π × 60 ÷ 10⁶)
RPM = KPH ÷ (D" × 25.4 × π × 60 ÷ 10⁶)
KPH = RPM × 0.1245 for 26"
RPM = KPH ÷ 0.1245 for 26"
And so I'm pretty parched after having tried to buy an FM G311.250.V/D 13 motor and getting the run-around regarding "how fast do you want to go" and "how big are your wheels" and "what is your battery voltage" and all that crap. My answer is "suffix 13" and so I still don't have a motor and I'm still trying to buy it. 80% of all expended human effort is usually wasted so once again, ignorance is bliss.
