Desertprep
1 kW
Will a 48v motor make a good 48v generator? Put another way, I am trying to find a 48v generator withing using inverters and adapters. Will a motor be efficient? They are a lot lighter than auto alternators.
48v motor make a good 48v generator?
- Thread starter Desertprep
- Start date
olaf-lampe
10 kW
A 48V motor can act as a 48 V generator with the right controller. Infineons for instance can be programmed to a certain regen-voltage. The amount of charge current depends on how fast the motor spins.
-Olaf
-Olaf
Jeremy Harris
100 MW
The voltage output of a permanent magnet motor used as a generator depends on four factors; the Kv (the rpm per volt) for the motor, the motor/generator shaft rpm, the winding resistance of the motor/generator and the current being drawn by the load.
You also need to factor in the difference between the rms (root mean square) voltage from the motor/generator AC waveform and the DC voltage that you will get after rectification.
Here's a worked example for a typical permanent magnet motor:
Kv = 200
rpm = =5000
winding resistance = 0.01 ohms
Current = 20 amps
At 5000 rpm this motor will generate 5000 / 200 = 25 volts with no load. With a load of 20 amps and an internal winding resistance of 0.01 ohms the motor will deliver a terminal voltage of 25 - (20 x 0.01) = 24.8 volts.
Using the above calculations you can work out the voltage you will get from any particular permanent magnet motor when used as a generator at any particular rpm and load.
Jeremy
You also need to factor in the difference between the rms (root mean square) voltage from the motor/generator AC waveform and the DC voltage that you will get after rectification.
Here's a worked example for a typical permanent magnet motor:
Kv = 200
rpm = =5000
winding resistance = 0.01 ohms
Current = 20 amps
At 5000 rpm this motor will generate 5000 / 200 = 25 volts with no load. With a load of 20 amps and an internal winding resistance of 0.01 ohms the motor will deliver a terminal voltage of 25 - (20 x 0.01) = 24.8 volts.
Using the above calculations you can work out the voltage you will get from any particular permanent magnet motor when used as a generator at any particular rpm and load.
Jeremy
Desertprep
1 kW
Cool! I like formulas 
T
T
Jeremy Harris said:
John in CR
100 TW
Great example and explanation Jeremy. Now I know how to choose a motor to attach to a weedeater engine once I know engine rpms for a several hundred watt micro-generator. Can we assume that a high efficiency motor will also be highly efficient as a generator, so we don't have to worry burning up the motor/generator from heat in the windings as long as we don't try to draw too much current?
Jeremy Harris
100 MW
In general terms, yes. Permanent magnet motors and generators follow a law of reciprocity, meaning that a high efficiency motor will also be a high efficiency generator.
Probably the most critical factor is the winding resistance, as this will determine most of the voltage drop under load.
Most ordinary RC permanent magnet motors will make make pretty good generators if run at the right rpm and load.
Jeremy
Probably the most critical factor is the winding resistance, as this will determine most of the voltage drop under load.
Most ordinary RC permanent magnet motors will make make pretty good generators if run at the right rpm and load.
Jeremy
olaf-lampe
10 kW
With a (infineon) controller acting like a step-up/step-down converter it is not that important, which voltage the motor generates.
Am I right, Jeremy?
-Olaf
Am I right, Jeremy?
-Olaf
olaf-lampe said:
I'm also curious about the answer to this. Another ES post claimed that the regen built into the controllers is very inefficient.
I'm wondering if anyone has efficiency numbers for charging via a (infineon or similar) controller?
Compared to say a car alternator (probably not that efficient either)
One reason I can think of that would impact efficiency is that if the regen was implemented primarily for braking, it may be switching at the motor electrical rpm, and that frequency may be too low to make an efficient buck-up converter. However I have no numbers or actual experience. My current controllers don't have regen...
Jeremy Harris
100 MW
Sorry, I missed this earlier.
Regen on controllers works by turning the controller into a boost mode voltage converter (using motor inductance), so the regen voltage can exceed the generated voltage you'd expect to see from any particular RPM. It's reasonably efficient, probably no worse than the efficiency of the controller when driving the motor normally. It does depend on how the controller implements the flyback diode arrangement - if it just uses the FET body diodes then efficiency may be poorer, but if it uses synchronous rectification then it should be pretty good. I'm not sure how the Xie Chang controllers do regen, as I've not played with it much.
Jeremy
Regen on controllers works by turning the controller into a boost mode voltage converter (using motor inductance), so the regen voltage can exceed the generated voltage you'd expect to see from any particular RPM. It's reasonably efficient, probably no worse than the efficiency of the controller when driving the motor normally. It does depend on how the controller implements the flyback diode arrangement - if it just uses the FET body diodes then efficiency may be poorer, but if it uses synchronous rectification then it should be pretty good. I'm not sure how the Xie Chang controllers do regen, as I've not played with it much.
Jeremy
Kingfish
100 MW
John in CR
100 TW
If regen was inefficient wouldn't we see a lot of controllers fried by regen? I haven't heard of a single instance.
Jeremy Harris
100 MW
Kingfish said:
Unfortunately it's not easy to tell from the schematic, because synchronous rectification will mean switching the FETs from the controller firmware, rather than relying on the internal body diodes.
We could add fast-recovery diodes across the FETs to be sure of getting reasonable performance, but as has already been pointed out, we don't see controllers failing or getting hot under regen conditions, so can assume that they're OK for the job.
Jeremy
