What do controller and motor ratings actually mean

[NRG]

Yep! Its PWM...

 

[trex]

I still don't think that's right because the timing of the pulses is critical, If you just had an average 18v at half power, there would be no pulses, so the motor won't turn. See what happens when your hall sensors go up the creek, so that the controller can't get its pulse timing right.

d8veh is right. Voltage is irrelevant. It's the change in magnetic flux that makes the motor turn. The change in current intensity causes change in the flux. If the current is static, the flux remains static, one or more of your FETs have died. The voltage would be either 36V or 0V but no current would flow. If it did, your battery would die shortly.
When the upper rail FET connects the phase wire to 36V, you do not have 36V on the winding but somewhere between 18V and 36V depending on the flux at the time. If the flux is near its maximal, you'll get near 36V, if it's near the middle, you get 18V. Of couse, current intensity is highest when voltage is 18V and nil at zero or 36V. That makes the voltage trace lags the current trace by 90 degrees.
Similarly, when the lower rail FET is switched on, voltage on the phase wire will vary from 18V to near zero. Current is highest when voltage is 18V. The alternating of this current causes spurious harmonics limiting the efficiency of the motor.
Wurly's scope trace shows clearly the peak voltage (as seen at the phase wire) oscillates between 36V and 0V.

 

[banbury frank]

Hi What about how many fets and the volts and amps they handle we use 12 fet and 15 fet and 18 fet we only use 48 volts so a fully charged battery is 54 volts so cheap 63 volt caps are no good

To handle the AMPS Also the capacitors are very important so as not to add more peaks

Frank

 

[Marctwo]

d8veh is right. Voltage is irrelevant. It's the change in magnetic flux that makes the motor turn. The change in current intensity causes change in the flux. If the current is static, the flux remains static, one or more of your FETs have died. The voltage would be either 36V or 0V but no current would flow. If it did, your battery would die shortly.

Isn't that what the 3 phases are for? What happens when a controller like the e-crazyman is using over 100% power and delivering a solid phases of voltage without pusles.

 

[NRG]

Voltage isn't 'irrelevant'!

Jeremy on the +100% setting:

100% controller program setting:


Motor speed is controlled by varying the motor voltage using something called pulse width modulation (PWM). PWM is just switching the battery voltage on and off very quickly (around 15,000 times per second for a typical ebike controller). The voltage at the motor is proportional to the ratio of the PWM on time to the PWM off time. So, if the ratio was 50% (on for half the time, off for half the time) then the voltage at the motor, and the motor speed, will be 50%.

There are two potential problems with PWM:

1 - At 0% (zero throttle) the controller isn't switching on or off at all, it is just sat switched off, in effect, so the PWM circuitry has to detect zero throttle and stop switching altogether. This is easy enough for the controller to do, as it can just internally cut the power, in effect.

2 - At 100% (full throttle) the PWM also has to stop, but in this case it has to switch fully on. This is harder to do, because you want a nice smooth transition from nearly full throttle (say, 99%) where the controller is switching power on 99% of the time and off 1% of the time, to the point where it's 100% on. Equally, you want a nice smooth transition back from this as the throttle is closed. This switch from 99% to 100% is termed the switch from PWM to block commutation. Block commutation essentially means that the three phase drive to the motor is now in "blocks" of solid applied voltage, rather than PWM sliced.

One slight snag with this transition to block commutation is that switching the PWM off (and allowing full voltage) has the effect of advancing the timing of the three phase waveform that drives the motor. In essence, PWM adds a tiny delay to the three phase motor voltage when it's operating and block commutation makes the motor run slightly faster (incrementally) than the small change in applied voltage would suggest.

When you select greater than 100% in the programming software for a Xiechang controller, then what happens is that the throttle position where this switch from PWM to block commutation occurs moves down a bit. This can mean that the controller can switch to block commutation at a slightly lower throttle setting and the timing of voltage applied to the motor over the remaining part of the throttle to full can be advanced. If the timing is over-advanced then the motor will draw more current for very little extra speed. This is very motor dependent, some big direct drive hub motors with a high inductance respond fairly well to this timing advance, others respond rather badly.

Because this setting is rather crude in the way it operates, it can produce variable results. On smaller, lower inductance, motors settings above 100% rarely give good results. If you want to play around with this then the best way to do it is to hook up a meter and measure the supply current to the controller at various programmed maximum speed settings. Measuring the no-load current (and listening to the noise the motor makes at the transition from 99% PWM to block commutation) will give a good idea as to how effective any setting is. If you go too far you will see the no-load current spike up over the last tiny bit of throttle movement, which is a good indication that you need to reduce the % maximum a bit.

 

[Marctwo]

I think there's a bit of confusion about driving the motor and driving the rotor.

The rotor is obviously driven by magnetic force but the rotor is only part of the motor along with the coils, etc. The motor as a unit is driven by voltage.

 

[Old_Dave]

And it all ends up as the RMS value (Root Mean Square) that's being applied.