The Consequences of Lower Voltage
Toys should not kill.
Ebikes are in effect "toys" and so if some kid accidentally electrocutes himself with their ebike they should not die from it. It only takes about 50 DC volts to kill someone.
So I really want to design this AC Induction motor to be able to operate at the very low voltage of 48 volts. This is half of the standard line voltage and I found this link that discusses it:
http://www.motorsanddrives.com/cowern/motorterms12.html
When electric motors are subjected to voltages, below the nameplate rating, some of the characteristics will change slightly and others will change more dramatically. A basic point is, to drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of power from the power line. The amount of power the motor draws is roughly related to the voltage times current (amps). Thus, when voltage gets low, the current must get higher to provide the same amount of power. The fact that current gets higher is not alarming unless it exceeds the nameplate current rating of the motor. When amps go above the nameplate rating, it is safe to assume that the buildup of heat within the motor will become damaging if it is left unchecked. If a motor is lightly loaded and the voltage drops, the current will increase in roughly the same proportion that the voltage decreases.
For example, a 10% voltage decrease would cause a 10% amperage increase. This would not be damaging if the motor current stays below the nameplate value. However, if a motor is heavily loaded and a voltage reduction occurs, the current would go up from a fairly high value to a new value which might be in excess of the full load rated amps. This could be damaging. It can be safely said that low voltage in itself is not a problem unless the motor amperage is pushed beyond the nameplate rating.
Aside from the possibility of over-temperature and shortened life created by low voltage, some other important items need to be understood. The first is that the starting torque, pull-up torque, and pull-out torque of induction motors, all change based on the applied voltage squared . Thus, a 10% reduction from nameplate voltage (100% to 90%, 230 volts to 207 volts) would reduce the starting torque, pull-up torque, and pull-out torque by a factor of .9 x .9. The resulting values would be 81% of the full voltage values. At 80% voltage, the result would be .8 x .8, or a value of 64% of the full voltage value.
Most of the amperage ratings I've seen for these AC motors are around 5 amps or less. I know that in order to obtain my desired 1000 watt input power with 48 volts that I will need about 20 amps of current. So somehow I need to be able to change the windings to be able to handle roughly four times the current.
It will be interesting...
The only positive in all this is that I've been looking at the way that the variable frequency drive works and apparently they PURPOSELY lower the voltage during starting so as to protect the motor. So I have a feeling that the net result is going to be something where you gain some things in some areas of the powerband and lose in others.
My guess is that if you are faced with a large load and your throttle is declaring a frequency desire for the motor that is producing a lot of "slip" that it will draw a lot of current. If you had a feedback (closed loop) system that would do a "throttle pulldown" to a lower frequency you could protect the motor from overheating itself with excess current. (sort of the Armature Current Limiting concept)
The AC controller chip has ramping which is sort of a blind way to try to get a current feedback loop. Ramping just means that the "normal" acceleration is going to be limited to a certain rate. Ideally a current sensor feedback is better than ramping. (though ramping might be good enough)
The "bottom line" is that low voltage will mean higher currents and so that's a central thing to be focused on....