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Lathe motor won’t turn over on solar

Sunburnt

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Apr 6, 2020
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I have a hobbyist lathe with a single-phase 115v motor of 3/4 HP. This motor turns on normally when I am connected to grid power, and VERY occasionally when connected to solar. But most of the time on solar the motor does not spin up. I’m wondering what I can do to allow it to start on solar.

My solar system, when running on batteries alone, can produce at least 70 amps output for several minutes at least (I have tested this with an electric heater and the battery monitor shows 70 amps coming out of the batteries when the heater is on). I wouldn’t think the 3/4 HP lathe motor would take anything like that for startup amperage, but perhaps it is possible.

Could adding some additional start capacitors to the motor cicuit help in this situation?
 
Is your inverter pure sine wave or modified sine wave?
What does your battery bank consist of? Can it provide enough amps?
 
We don't have enough info to even start to help.

What kind of inverter are you using? What is it's rating? Is it a modified sign wave or pure sign wave?
What is the voltage of your battery bank?
Does your lathe have a VFD for speed control or is the motor hooked directly to the AC motor?
 
Pure sine wave magnum energy inverter. Batteries are 4 parralelied simplified 3.5’s

as mentioned above it can deliver at least 70 amps
 
Startups can go way over the rated power, like 10x and even 20x and beyond. I have a big lathe with a 25hp motor that normally draws around 40 amps. When my mains breaker blew out I put a recording ammeter on it and it showed the first ten or so seconds peaking between 250 and 350 amps. Had to upgrade my entire mains over that oversight. 70 does sound high for a 3/4, but then again a lathe takes a lot of torque to get it moving.

-----EDIT-------, yeah, you are definitely hitting that sweet spot! Practically a guarantee you are exceeding 70amp spikes and you won't register that on a standard meter unless it can capture data in microseconds. BTW, not sure if they make soft-starters that small but that would be an obvious remedy. They probs want a mint for one though so maybe that money could go to more power gear. Not sure about caps. I'm running a 40hp rotary phase converter with a lot of caps but they did nothing to stop my mains from melting on a 25hp high current start.


locked rotor.PNG
 
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But most of the time on solar the motor does not spin up.
And the Magnum inverter doesn't display an error or complain when this happens?
I would think it would throw a low battery fault and beep loudly.

4 parralelied simplified 3.5
Hmmm, not sure what this is.
??
 
Check for loose or corroded connections through the entire DC side of the system. Or a weak cell.
 
Pure sine wave magnum energy inverter. Batteries are 4 parralelied simplified 3.5’s

as mentioned above it can deliver at least 70 amps

Just as a clarification. The 70 amps on the battery monitor is *not* what is on the AC side. Assuming your batteries are 12 Volt (and no losses), the AC side would only be at around 7 amps.

Watts in = Watts out
DC Side: 70A*12V=840W​
AC Side: 840W/120V=7Amps​

According to the chart above you need 13 or 14 amps on the AC side for running and a lot more for start-up.
 
If the motor can start “occasionally” I would honk you have too small battery wire, and too small an inverter.

3/4HP is a HUGE 120v load.
One way to get it started would be to spin it up before you turn it on.
I wouldn’t use my hands, but perhaps put a rubber roller on a battery drill to get it spinning before powering on...
 
Startups can go way over the rated power, like 10x and even 20x and beyond. I have a big lathe with a 25hp motor that normally draws around 40 amps. When my mains breaker blew out I put a recording ammeter on it and it showed the first ten or so seconds peaking between 250 and 350 amps. Had to upgrade my entire mains over that oversight. 70 does sound high for a 3/4, but then again a lathe takes a lot of torque to get it moving.

-----EDIT-------, yeah, you are definitely hitting that sweet spot! Practically a guarantee you are exceeding 70amp spikes and you won't register that on a standard meter unless it can capture data in microseconds. BTW, not sure if they make soft-starters that small but that would be an obvious remedy. They probs want a mint for one though so maybe that money could go to more power gear. Not sure about caps. I'm running a 40hp rotary phase converter with a lot of caps but they did nothing to stop my mains from melting on a 25hp high current start.


View attachment 10438
That is an interesting chart. Where did you get it?
 
FilterGuy, your comments were most interesting. If I am understanding you correctly you are saying my inrush or startup current is only around 7 amps AC? That should be no problem for the solar system to handle. Also, @Remote Power Mania this motor is a single phase.
 
FilterGuy, your comments were most interesting. If I am understanding you correctly you are saying my inrush or startup current is only around 7 amps AC? That should be no problem for the solar system to handle. Also, @Remote Power Mania this motor is a single phase.
Not quite. I was only pointing out that 70 amps on the 12 volt DC side equates to only 7 amps on the 120V AC side.

Going by the chart that @Remote Power Mania posted. 7 amps is not even enough to run the motor at full load, let alone get it started.
What does the nameplate on the motor or lathe say the current rating should be. I am guessing it requires either a 15 amp or 20 amp circuit.

Have you measured what the DC amperage is while the motor is trying to start? (Warning: Don't let it stay in the 'trying-but-failing' state for more than a few moments. You can burn out the motor that way.)

Here is some more math for you:
.75Hp is ~560Watts

on the DC side that is 47 Amps On the AC side that is 4.7 amps.

When I am trying to size a circuit with a large motor, the rule of thumb I use is to assume an inrush of 4 times the run time current. That is 2240W or is 188 amps on the DC and 18.8 amps on the AC. What size is your inverter?
 
FilterGuy, your comments were most interesting. If I am understanding you correctly you are saying my inrush or startup current is only around 7 amps AC? That should be no problem for the solar system to handle. Also, @Remote Power Mania this motor is a single phase.
Regarding 1phase, that was my assumption based on the chart, showing 120v single phase at 74 amps (locked rotor). I think the website listed everything as 3ph, but 120v 3ph is not really a thing afik. Either way, single phase will draw the most current of the two scenarios.
 
My idea for adding some start caps is based on a 3-phase rotary converter. There, you need to start turning a 3-phase motor from single-phase input, and to do that you can store a charge in start caps and then discharge that after powering up 2 of the 3 phases. That provides the rotary force to begin the motor turning, and once turning it can keep going by itself. So, @Supervstech that would theoretically replace the rubber roller you mentioned, or a starting cord.
 
Wow, pulling 70amps at 48v and its still not starting?
And no complaints/warnings/beeps from the 4400w inverter?

This seems odd, there has to be another tidbit of info we are missing.
 
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If there are beeps, I can't hear them. But I am auditorily challenged and the lathe is in another room from the inverter over 40 feet away.
 
Not quite. I was only pointing out that 70 amps on the 12 volt DC side equates to only 7 amps on the 120V AC side.

Going by the chart that @Remote Power Mania posted. 7 amps is not even enough to run the motor at full load, let alone get it started.
What does the nameplate on the motor or lathe say the current rating should be. I am guessing it requires either a 15 amp or 20 amp circuit.

Have you measured what the DC amperage is while the motor is trying to start? (Warning: Don't let it stay in the 'trying-but-failing' state for more than a few moments. You can burn out the motor that way.)

Here is some more math for you:
.75Hp is ~560Watts

on the DC side that is 47 Amps On the AC side that is 4.7 amps.

When I am trying to size a circuit with a large motor, the rule of thumb I use is to assume an inrush of 4 times the run time current. That is 2240W or is 188 amps on the DC and 18.8 amps on the AC. What size is your inverter?
You bring up a good point converting this problem over to watts. If the 120v AC locked rotor rating for a 3/4Hp motor is 74Amps, then that total power is going to be 8.8Kw. Now bringing this locked rotor power over to the DC end, you will be looking at 8.8Kw/48v or 8800/48 =183Amps. I didn't catch his system voltage so if rather than 48v he has 12v we see 8800/12 =733Amps!

I realize that a 3/4 motor pulls nowhere near that, but at a non start ie locked rotor those number do match up with the chart.
 
My inverter is
Magnum Energy MS4448PAE MS-PAE Series 4400W 48VDC
OK.... 48 volts changes my equations. I was using 12 volts.

.75Hp is ~560Watts
On the 48V DC side that is 11.7 Amps On the AC side that is 4.7 amps.
When I am trying to size a circuit with a large motor, the rule of thumb I use is to assume an inrush of 4 times the run time current. That is 2240W or is 47 amps on the 48V DC and 18.8 amps on the AC.

I agree with @MisterSandals..... It seems like we are missing something.

I would go through and check the connections all the way from the battery to the lathe. Is it possible there is a bad connection someplace? A resistive connection can seem perfectly fine under no/low load but have a huge voltage drop under high load.

What is the make/model of the lathe? Is it variable speed? Does it use a VFD?
 
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