Posted January 1, 20232 yr Hi there, My brother I law wants to go off grid on his farm in the Northern Cape. Apart from normal household usage, he has 2 pieces of equipment with a 7.5kW induction motor each. Will he be able to run one of these at a time on a 12 kW inverter? Or will the startup current trip the inverter? Or will he need 2 x 12 kW inverters?
January 1, 20232 yr I personally think that the starting current of any of those motors may trip the inverter, try and get a soft starter for the motors.
January 1, 20232 yr 51 minutes ago, Corneel said: Hi there, My brother I law wants to go off grid on his farm in the Northern Cape. Apart from normal household usage, he has 2 pieces of equipment with a 7.5kW induction motor each. Will he be able to run one of these at a time on a 12 kW inverter? Or will the startup current trip the inverter? Or will he need 2 x 12 kW inverters? My call would be yes he can. Just ensure you start those motors via a STAR-DELTA or other starter to reduce the start current. Why? If I can start a 0.75kW with my brick 1.5kW inverter there will be no problem. Take note the 12kW inverter can provide 24kW I guess during starting. Easy solution. Try it with 1 inverter while planning to buy 2. If it works then no need for the 2nd unit. Major saving than incurring the cost up front. It is not just the motor size to consider. What type of machine is started and does it have a high load to start turning. Edited January 1, 20232 yr by Scorp007
January 1, 20232 yr 6 minutes ago, Scorp007 said: My call would be yes he can. Just ensure you start those motors via a STAR-DELTA or other starter to reduce the start current. Why? If I can start a 0.75kW with my brick 1.5kW inverter there will be no problem. Take note the 12kW inverter can provide 24kW I guess during starting. Easy solution. Try it with 1 inverter while planning to buy 2. If it works then no need for the 2nd unit. Major saving than incurring the cost up front. It is not just the motor size to consider. What type of machine is started and does it have a high load to start turning. Yes, Star/Delta if the motor is 3-phase will do the trick, but if single phase motor the starting current can be as high as 6 times the running current. Depending on the inverter if it can stand overload for I would say 5 to 10 seconds.
January 1, 20232 yr 11 minutes ago, Antonio de Sa said: Yes, Star/Delta if the motor is 3-phase will do the trick, but if single phase motor the starting current can be as high as 6 times the running current. Depending on the inverter if it can stand overload for I would say 5 to 10 seconds. In decades in the electrical field I doubt if you actually get this size single phase motor. You will hardly be able to start it on a 60A utility connection. And what would the start current be when you apply 6 times running current if single phase? Edited January 1, 20232 yr by Scorp007
January 1, 20232 yr 11 minutes ago, Scorp007 said: In decades in the electrical field I doubt if you actually get this size single phase motor. You will hardly be able to start it on a 60A utility connection. That makes two of us, yes you can get a 4 pole 7.5 Kw single phase motor. 7.5kW 4P 230V B35T 132M IE1 WEG SINGLE PHASE MOTOR - Zest WEG ABB also makes them. Edited January 1, 20232 yr by Antonio de Sa
January 1, 20232 yr Author 26 minutes ago, Antonio de Sa said: That makes two of us, yes you can get a 4 pole 7.5 Kw single phase motor. 7.5kW 4P 230V B35T 132M IE1 WEG SINGLE PHASE MOTOR - Zest WEG ABB also makes them. Wow, I did not know you get such big single phase motors. His are definitely 3 phase.
January 1, 20232 yr 21 minutes ago, Antonio de Sa said: That makes two of us, yes you can get a 4 pole 7.5 Kw single phase motor. 7.5kW 4P 230V B35T 132M IE1 WEG SINGLE PHASE MOTOR - Zest WEG ABB also makes them. Thanks for the link. Having a normal running current of 40A and perhaps a start current of 240A it would never be a good idea to run it on single phase. The munics would for sure not like the idea of using it on their network. Imagine applying for a 250A single phase connection. Decades ago IIRC SANS 142 had a limitation on the size of single phase motor you could run direct on line. That was long before the 1ph to 3ph convertors. Lets get @Corneel to answer if single phase.
January 1, 20232 yr Just now, Corneel said: Wow, I did not know you get such big single phase motors. His are definitely 3 phase. So the problem is solved
January 1, 20232 yr Author 52 minutes ago, Scorp007 said: My call would be yes he can. Just ensure you start those motors via a STAR-DELTA or other starter to reduce the start current. Why? If I can start a 0.75kW with my brick 1.5kW inverter there will be no problem. Take note the 12kW inverter can provide 24kW I guess during starting. Easy solution. Try it with 1 inverter while planning to buy 2. If it works then no need for the 2nd unit. Major saving than incurring the cost up front. It is not just the motor size to consider. What type of machine is started and does it have a high load to start turning. Tx, I will research the Star Delta a bit more, or see what a Soft Starter cost.
January 1, 20232 yr 3 minutes ago, Corneel said: Tx, I will research the Star Delta a bit more, or see what a Soft Starter cost. I'm not in plants anymore so the soft starters could cost less than a star-delta starter. Good contactors have a good price tag on them but I still think more reliable and sustain power surges much better.
January 1, 20232 yr 13 minutes ago, Corneel said: Tx, I will research the Star Delta a bit more, or see what a Soft Starter cost. Ok so the motor is 3 Phase 7.5 Kw a Star/Delta would do. I have one question? is your 12 Kw inverter single or three phase? If single phase you can look at a VSD, you can get one that converts single phase to 3 phases Amazon.com: ATO 7.5 hp 5.5 kW VFD, 24 A 220V Single Phase to 3 Phase Variable Frequency Drive, RS485 Enabled : Tools & Home Improvement Edited January 1, 20232 yr by Antonio de Sa
January 1, 20232 yr Author 1 hour ago, Antonio de Sa said: Ok so the motor is 3 Phase 7.5 Kw a Star/Delta would do. I have one question? is your 12 Kw inverter single or three phase? If single phase you can look at a VSD, you can get one that converts single phase to 3 phases Amazon.com: ATO 7.5 hp 5.5 kW VFD, 24 A 220V Single Phase to 3 Phase Variable Frequency Drive, RS485 Enabled : Tools & Home Improvement We are still in the planning stages, so the plan is to use a Deye 3 phase inverter.
January 2, 20232 yr Direct On Line (DOL) starting of a huge motor like that is brutal. Definitely use some sort of soft starter, preferably a VSD. If starting on battery alone, you will need a monster battery as well.
January 2, 20232 yr 7 hours ago, Coulomb said: preferably a VSD. This is the way. You could even go for a single phase inverter like the Sunsynk 16kW then.
January 2, 20232 yr 15 minutes ago, P1000 said: This is the way. You could even go for a single phase inverter like the Sunsynk 16kW then. yes, agree with you @P1000 and @Coulomb best option would be a VSD, however, there is someone in this thread that says that with (DOL) is Ok. That in my opinion will most definitely fry the 12 Kw inverter.
January 2, 20232 yr 3 minutes ago, Antonio de Sa said: there is someone in this thread that says that with (DOL) is Ok. That in my opinion will most definitely fry the 12 Kw inverter. The inverter overload protection should prevent it from getting fried, but I don't think there is a way that you could start a 7.5kW induction motor with 2x12kW Sunsynks. Induction motors are not forgiving, and the (5 to 7)x rule will still apply, and the batteries also need to be able to cater for the surge. VSD is by far the best solution, soft starters are not very kind to inverters.
January 2, 20232 yr Author 1 hour ago, P1000 said: The inverter overload protection should prevent it from getting fried, but I don't think there is a way that you could start a 7.5kW induction motor with 2x12kW Sunsynks. Induction motors are not forgiving, and the (5 to 7)x rule will still apply, and the batteries also need to be able to cater for the surge. VSD is by far the best solution, soft starters are not very kind to inverters. Tx @P1000, will go for the VSD then. Just for my education - why is the soft starter not kind to an inverter? Also, what size VFD is required?
January 2, 20232 yr 25 minutes ago, Corneel said: Tx @P1000, will go for the VSD then. Just for my education - why is the soft starter not kind to an inverter? I it has to do with the Harmonics, not 100% sure Also, what size VFD is required? You can do your search on the net, but I think that take the full load Amps from the motor plate and X 2 that should do it, but then again, I think you should contact any VSD supplier they will most definitely be able to calculate the right VSD for your application.
January 2, 20232 yr 23 hours ago, Scorp007 said: What type of machine is started and does it have a high load to start turning. Very good point. A pump started against a shut head do not need much but a fan will draw up to 8x.
January 2, 20232 yr 47 minutes ago, Peter V said: Very good point. A pump started against a shut head do not need much but a fan will draw up to 8x. In my years in factories I had 1 type of machine that always had me worrying to get it started in summer power failures and that were 2 force draft fans on the 4 boilers chimney. While in star and even with a long time setting they were just not up to speed and as it went over to delta the increase in current caused it to trip on thermal overload. The other unit was a chipper to cut all off cuts of pine wood as the fuel for the boiler. The chipper had one very heavy flywheel to which the blades were attached. Edited January 2, 20232 yr by Scorp007
January 2, 20232 yr 18 minutes ago, Scorp007 said: In my years in factories I had 1 type of machine that always had me worrying to get it started in summer power failures and that were 2 force draft fans on the 4 boilers chimney. While in star and even with a long time setting they were just not up to speed and as it went over to delta the increase in current caused it to trip on thermal overload. The other unit was a chipper to cut all off cuts of pine wood as the fuel for the boiler. The chipper had one very heavy flywheel to which the blades were attached. @Scorp007 Coincidence, the last 35 year of my working life was around boiler/turbine control systems. I've design, install, commissioning and optimized all kinds, from old chain grade to 200 MW PF fired boilers, including a bagasse fired boiler in a sugar mill. But the best control system I've seen it's in a power station in Botswana, what we call "Operator proof", one push button. all sequence control. Designed by a German company, I'm not going to mention the name. The most difficult to control was the bagasse fired boiler almost impossible to control the furnace pressure.
January 2, 20232 yr Hi. Yes you can. My workshop run just on solar. All motors we installed vsd for starting. No problems.
January 2, 20232 yr 6 hours ago, Antonio de Sa said: I it has to do with the Harmonics, not 100% sure Soft starters typically chop the supply voltage to limit startup current. This will negatively impact the inverter electronics that try to create a perfect sine wave. Depending on the quality of the VFD (filters), harmonics may still be an issue. I would suggest measuring total harmonic distortion continiously.
January 2, 20232 yr Author 2 hours ago, Peter V said: Very good point. A pump started against a shut head do not need much but a fan will draw up to 8x. I missed this, the equipment are: 7.5 kW hammer mill 7.5 kW feed mixer (none of them have a flywheel and they will be started "empty") 3 kW submersible borehole pump 3 kW cold room compressor 2 x 1.5 kW submersible borehole pumps All of them are on manual switches so it is possible to make sure that only one of them runs at any time.
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