# Will I have a problem running a Harbor Freight 7 x 12 mini lathe from a 2000 watt inverter



## TQA222 (Oct 31, 2018)

I  live on a boat mostly at anchor and had been planning to set up a small workshop but someone has suggested that the electronic speed control might not like the type of wave form an inverter produces.

I also have a Honda 1000 suitcase generator but would prefer to run it from the inverter if I can as I like the peace and quiet.


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## homebrewed (Nov 1, 2018)

I found an old schematic of the 7x12 motor controller on 7x12MiniLathe@groups.io, and it looks like the AC input is converted to DC with a rectifier/capacitor, then converted to a PWM signal that's routed to the motor.  IF the current controller is substantially the same, the inverter waveform shouldn't be a problem.  But if it were me I would buy the inverter from someone who has a "nice" return policy....


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## TQA222 (Nov 1, 2018)

After being reduced to RTFM for the Honda generator it produces a 'pure' sine wave output so the genny should run it just fine.

The inverter manual is unclear on the topic of wave form which suggests that it is modified [ rough ] sine wave.


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## GL (Nov 1, 2018)

Just a random thought with minimal information. If the lathe is converting A/C to D/C , but you are starting from D/C and converting to A/C, is there a way to stay D/C and avoid switching back and forth?  Obvious issues would be voltage differences, available wiring, and how/where to connect into the correct point on the machine. Would want to be able to plug it into either source depending on what power is available.


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## markba633csi (Nov 2, 2018)

If the motor controller is an older SCR type then no, you can't use a non-sine (modified sine or rectangle wave) inverter. A more modern PWM/mosfet controller may be OK however, as it rectifies and filters the incoming power first as Homebrewed mentioned in post #2; if your motor controller has one or two large capacitors and a bridge rectifier on the board you most likely have the latter style and are fine- post a picture if you are not sure
Mark


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## TQA222 (Nov 2, 2018)

OK If I have understood correctly all I have read the Harbor Freight 7 x 12 uses is a 350 watt 12 volt DC motor.

The electrical system takes 110 volts AC and converts that to 12 V DC that is fed into a PWN speed controller. If that is the case it seems unlikely that the electrical stuff involved in taking 110 and converting it to 12 volts should not be very fussy about the wave form it is fed. It might draw the line at square wave but my inverter produces a modified sine wave which should be OK. 

If the magic smoke escapes from the speed control system I can buy a 12 volt 40 amp speed controller from Banggood [ a whole 7,76 $ US ] and run that directly off my battery bank. 

Panic over and my thanks to everyone who has helped with info.


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## TQA222 (Nov 2, 2018)

*OK Hold the presses* I might have got the above very wrong. It might be that the speed controller is not a simple PWN but a much more complex thing using MOSFETS and the motor runs on up to *100 volts DC*. If so YIKES


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## homebrewed (Nov 2, 2018)

So I did think of a reason you may have problems with an inverter that generates square waves.  The peak voltage will be different for an inverter vs. pure sine wave power, for the same RMS voltage.  What this means is that the rectified voltage from an inverter could be lower, so your lathe motor may act under-powered compared to running off a sine wave @ 120VRMS.  

The details:  the peak voltage of a 120VRMS sine wave is 1.4*120 = 170V (approximately), while an inverter outputting a square wave will have the SAME peak and RMS voltages.  120/170 = .71, so the lathe motor will "see" 30% less voltage when driven by the inverter.

You may be able to get an inverter that outputs a waveform which better replicates a sine wave.  That would lessen the penalty of using an inverter compared to the generator approach.


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## TQA222 (Nov 4, 2018)

I have been in contact with a gentlemen who repairs mini lathe speed control boards by email. He tells me that provided my lathe has the later type of control board which it should as I am buying new from Harbor Freight then it should run OK from an inverter producing a modified square wave, although he did suggest adding a 30 mfd oil filled capacitor rated at least 250 ac fitted as close to the control board as possible. 

It should also run OK from a generator which produces a pure sine wave although he did have a caveat related to the impedance.


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## homebrewed (Nov 5, 2018)

I'm skeptical about the 30uf capacitor suggestion.  If the inverter is outputting a square wave the cap will likely draw a pretty large transient current, which could (eventually) damage the inverter.  Here are some numbers to think about....

If the capacitor is driven by a 60Hz sine wave @ 120VRMS it will consume a peak current of 1.9 amps.  Not too bad, right?  But if the capacitor is driven by a square wave that changes 240V in 1 millisecond (pretty slow), it will take a peak current of about 7.2 amps to charge the capacitor from -120 to +120V or vice-versa.  If the inverter waveform switches in .1 millisecond, the peak current will be 72 amps!  It gets worse from there.

I think the idea is to produce a brute-force filter that tries to coerce the inverter waveform to more closely resemble a sine wave -- but it will likely be pretty hard on the inverter.  I bet the warranty would not cover failure due to that kind of abuse.

Here is the math:  current through a capacitor = C*dV/dT, where dV/dT is the change in voltage divided by the change in time.  C is the capacitance in Farads.  So if the inverter output switches from -120V to +120V in 1 millisecond, dV/dT = 240/.001, or 240,000 volts/second.  Multiply by 3x10^-5 Farads and you get 7.2 amps of peak current.

I'd suggest trying the inverter w/o a capacitor and see how it works.  If you MUST filter the inverter's output, use an LC (inductor-capacitor) low pass filter.  The series inductance will isolate the inverter from the capacitor and the inverter will be much happier for it.  

Or just use your generator and bypass all this.


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