Rectifying Output of a 12V-120V Inverter

[Jeremy Samuels]

One of the unique challenges of building an electric go cart is the
motor. Most 1+ HP motors are one or three phase, which means an
expensive VFD for speed control. There are almost none in the 12-36
volt range, and they are usually very expensive. A surplus shop has a
120VDC treadmill motor, which would allow a much simpler and cheaper
chopper drive. The power is enough at ~1 HP, but I couldn't find a
reasonable (price) 2KW DC-DC converter. A 2KW 12VDC-120VAC inverter,
built for cars, is under $70. The model is very cheap, but I'm not
very concerned about the AC waveform. Is it possible to rectify and
filter the 120VAC output and use that for the motor? I'm assuming the
120VDC rating is an average, because treadmills run off house voltage,
and average is more convenient when calculating power. Would there be
issues with the power factor of a bridge rectifier/filter caps?


Thanks

 

[Tim Williams]

A 2KW 12VDC-120VAC inverter,
built for cars, is under $70. The model is very cheap, but I'm not
very concerned about the AC waveform. Is it possible to rectify and
filter the 120VAC output and use that for the motor?

Psst, take a peek inside. I'm betting it has a 160VDC supply and an
H-bridge output to drive the waveform either in a "magic sinewave" (see Don
Lancaster) pattern, or a boring square-wave-with-dead-time deal. Whatever
it does, just hack off the stuff after the 160V supply and have fun. Heck,
you could even salvage the H bridge assembly to drive your motor at variable
speed (PWM) and reverse!

Tim

 

[Rich Grise]

One of the unique challenges of building an electric go cart is the
motor. Most 1+ HP motors are one or three phase, which means an
expensive VFD for speed control. There are almost none in the 12-36
volt range, and they are usually very expensive. A surplus shop has a
120VDC treadmill motor, which would allow a much simpler and cheaper
chopper drive. The power is enough at ~1 HP, but I couldn't find a
reasonable (price) 2KW DC-DC converter. A 2KW 12VDC-120VAC inverter,
built for cars, is under $70. The model is very cheap, but I'm not
very concerned about the AC waveform. Is it possible to rectify and
filter the 120VAC output and use that for the motor? I'm assuming the
120VDC rating is an average, because treadmills run off house voltage,
and average is more convenient when calculating power. Would there be
issues with the power factor of a bridge rectifier/filter caps?

Are you prepared to deal with 166 amps at 12VDC? If you have a 120 VDC
motor, why not use 120V worth of batteries? Then you could lift the
PWM right out of the treadmill along with the motor.

Good Luck!
Rich

 

[Jeremy Samuels]

2 KW is a surge limit, current will be limited by a PIC, which will
provide the PWM signal. A lead acid deep cycle marine/rv battery will
provide quite a bit of current. Running two batteries in parallel is
also possible, or even two battery/inverters with their outputs
paralleled. The only time that much current is drawn is during heavy
acceleration, which can be limited by the PIC in addition to current
limiting.

Now getting a power IGBT that can handle 16 amps (worse possible case,
extreme acceleration with no limiting), 10 amps normal at 120
volts+transient spikes may be a little expensive, especially if they
decide to blow up. I'm thinking of using a 400 volt IGBT with a MOV on
G-S, as they will need to be driven hard to reduce switching losses. I
was thinking around 5-10khz, but I can easily see how annoying a 10khz
sound would be for extended perioeds of time.