Mechanical Assist to start AC Induction Motor? - Page 1
Forum section: electronics / beginners

author: Krisby
author_rank: Contributor
timestamp: January 01, 2021, 04:10:35 am
content: Hello All,Living off-grid, I have a 12 volt DC power system, with a 2,000 Watt inverter to run AC tools. The inverter can surge to outputting 4,000W when needed to start AC Induction motors, and I think it may well have a "soft-start" capability function as well. However, It cannot start up the 2.2HP electric motor on my new direct-drive air compressor. The connection between the inverter and battery is by short and thick copper cables, so it is not an issue on that side of things. I also tried connecting a whacking big capacitor bank to the battery output, to ensure that full battery current and voltage was getting to the inverter. The air compressor would start when connected to a 10,000 Watt inverter, but I don't want to have to lay out a lot of money just to get a motor to start.So, my question is whether or not I could give some mechanical assistance to the compressor motor to get it initially spinning, thereby overcoming the inertia of the motor and compressor pump mass as well as the surge of electrical power inherent in starting an induction motor. The compressor unit is of the silent type, and has a cylinder on each end of the motor. There is a fan connected to the motor shaft on either side of the unit, and so I was thinking that I could add a pulley to either end of the shaft, so that I could wind up a length of cord on the pulley, and then give it a pull, like on the old lawn mowers and outboard motors. There is no compressed air in the tank when starting, so there is no problem of load in that regard.Will my idea work? When running, in theory the motor will draw a MAX of 1,650 Watts from the 2,000 Watt inverter (750W X 2.2Hp), so there shouldn't be a problem in operating the compressor if I can get it started.

author: helius
author_rank: Super Contributor
timestamp: January 01, 2021, 04:21:03 am
content: 1650 W but how many VA? Motors have apparent power requirements greater than their real power consumption.Also, what is the configuration of the induction motor? At 2.2 hp it's probably not shaded pole, but could be permanent split capacitor or just a run capacitor type. There should be a diagram showing the winding phases.If you can increase the capacitance on the start winding, you may be able to make it start. The change in phase angle created by a start capacitor creates a larger torque at a lower speed, similar to using a gear reduction. This helps to overcome the inertia during start. Does your motor have a centrifugal switch that could be used to change capacitors?

author: viperidae
author_rank: Frequent Contributor
timestamp: January 01, 2021, 07:58:50 am
content: Your motor will probably max out the inverter at full load, assuming a power factor of 0.8. I wouldn't be surprised if it required 10kva to start. A stalled induction motor is just a massive inductor, drawing huge amounts of current. Quite out of phase with the voltage, but your inverter still needs to supply the current.I don't know how much you can fix that with a power factor correction capacitor.Being a compressor with a tank, I assume it turns on and off automatically to maintain the tank pressure. How do you suppose to manually start it every time?

author: elekorsi
author_rank: Regular Contributor
timestamp: January 01, 2021, 11:56:34 am
content: Those 1650W that you calculated is mechanical power on the motor shaft. Power rating of the motor (2,2HP) is measured on the shaft.You should look at the current rating of the motor, which would be i assume for your motor somewhere around 10A at 230V, so probably around 2300kVA.I would suggest bigger inverter or smaller compressor. You should test it before, as there can still be a problem with the inrush current...Another sollution would be a 3ph motor + VFD, so you can make a controll start and reduce the inrush current.Even if you start this compressor on this 2000W inverter it will probably trip the inverter when full, as the current draw will be close to the rated one.

author: richard.cs
author_rank: Super Contributor
timestamp: January 01, 2021, 12:17:52 pm
content: I agree with other posters that at 2.2 hp mechanical output the motor is just too large to run from that inverter. Once started it might just squeeze under 2 kW input but it won't be under 2 kVA so the inverter will be unhappy. Other than a larger inverter or smaller compressor you have a few other options:Mechanically uncouple one of the cylinders to reduce the mechanical load and therefore the power input.If the motor is capacitor-run it is effectively a 2-phase motor, and could be run from a VFD, giving both slow ramp up and also a reduced running speed for reduced power consumption.Dig into the inverter and modify it to create a "compressor running mode" where it outputs 20% lesser voltage and frequency (advanced electronics project).If you really want to try it with mechanically starting assistance try spinning it up with a drill or similar and switching it on, I suspect that the inverter will still barf but it's a quick test that tells you whether the mechanically-assisted start is worth looking at at all.

author: bdunham7
author_rank: Super Contributor
timestamp: January 01, 2021, 12:29:49 pm
content: I think what you have falls way short of what you need to run that motor.We would need exact models and performance data to do any real calculations, but assuming this is a typical, genuine 2.2HP induction motor with at least a capacitor start and 85% efficiency running, you will likely need at least 1930wattsto just run it at rated load. (746 x 2.2/.85). And that 85% efficiency would be very, very good, so probably more than 2000 watts in real life. Now your inverter is most likely limited by volt-amps, not watts. That means if the power factor is less than 1.00, your available watts are less by the same amount. Power factor, best case for a motor like that, is probably 0.85. So now you are at 2270 volt-amps in the very best case. And that is just the running best case.Starting an induction motor can easily require 5X the current it takes to run it, with a very low power factor. If it is limited by output current, as most are, your inverter can put out less than 2X the running current. You're way short. You probably need at least a 5kW inverter with 10kW surge to reliably start motors of that size.

author: elekorsi
author_rank: Regular Contributor
timestamp: January 01, 2021, 12:35:19 pm
content: QuoteI doubt that this will work. I asume that the position of pistons is 180° apart, so there is no counterweigt as they balance each other. Disconnecting one could result in a spectacullar process of self dissasembly...

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 01, 2021, 12:44:42 pm
content: I think the key word is "direct drive air compressor". They have a very heavy start under pressure. Perhaps, then, it would be better to open the cylinder of the pump to avoid air pressure.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2021, 01:14:18 pm
content: Quote from: elekorsi on January 01, 2021, 12:35:19 pmI agree. The compressor will start more easily with the tank depressurised though.

author: CaptDon
author_rank: Super Contributor
timestamp: January 01, 2021, 02:31:22 pm
content: Why did you multiply 2hp X 750 watts?That would be 100% efficiencywhich we surely know doesn't exist. C'mon man, think it through. Thatmotor may be at absolute best 80% efficient. Now, what about inrush....I bet if you hooked that motor to the mains and used a current probeand oscilloscope you would find inrush of 30 to 40 amps for maybe upto .5 seconds and then tapering back to the load current. A variablefrequency drive VFD may be your only answer and if the VFD has asizeable capacitor bank for the link voltage it may trip your inverter a coupleof times trying to get the capacitors charged. All of this leads to oneundeniable truth, the fact that most motor circuits have to use time-delayfusible devices to handle inrush of 5 to 10 times the normal run current.

author: Krisby
author_rank: Contributor
timestamp: January 02, 2021, 12:51:08 am
content: Wow, so many responses! Most helpful! Thank you-all....OK, I see the problem more fully now, even if I got the air compressor up and running by an initial mechanical assistance, the load will increase as the tank pressure is building, and is likely to exceed the capability of the 2000W inverter. Even if the compressor managed to fill the tank to automatic shut-off, the start-up problem will be encountered every time air usage triggers the auto-start top-up function.So, a larger inverter is the solution.My other idea would be to get a smaller compressor, which may be more economical than upgrading to a good quality and higher output inverter.But, then I found a brand of "silent" compressors (Chicago Air Hush) which builds up their product's capacities by adding multiples of their basic 1.1Hp 2 cylinder direct drive units. So, their 2.2Hp 50L tank compressors have two individually switchable 1.1Hp compressor units on top of the tank, meaning that I could probably start and operate the compressor with just one of the 1.1HP motors, and if I had more electrical power on hand, I could operate the compressor with both motors engaged. Effectively, I would have the option of a small or large compressor at the flick of two switches! URL below:https://sydneytools.com.au/product/chicago-hush50-aluminium-silenced-50l-air-compressor

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: January 02, 2021, 02:43:25 am
content: In addition to above, you should verify the capability of your inverter. Those tall numbers on the device label are often more marketing than science.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 02, 2021, 03:56:32 am
content: Quote from: Krisby on January 01, 2021, 04:10:35 amAt first I thought you were describing the unloader valve being faulty but since it is new and the compressor does indeed run with enough input grunt, prolly isn't that.What you might like to think about doing is adding relief valve solenoids in key places to lower the compression for a few seconds or better when RPM is detected.

author: Krisby
author_rank: Contributor
timestamp: January 02, 2021, 04:20:11 am
content: But, if I am starting the compressor with an unpressurised tank, then surely compression isn't an issue.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 02, 2021, 04:28:13 am
content: Quote from: Krisby on January 02, 2021, 04:20:11 amI was wondering that, if you just cheat and put a valve on the hose, would it start? Trouble is, that won't work if the compressor wants to restart at 90psi.Can you try an empty tank start with the hose open? Does it run?

author: Krisby
author_rank: Contributor
timestamp: January 02, 2021, 05:31:07 am
content: No, there is only a buzzing sound from the motor when the tank is in a depressurized state and the tank's outlet gate valve open to the atmosphere.

author: Krisby
author_rank: Contributor
timestamp: January 02, 2021, 05:52:46 am
content: It has been recommended above that I would need an inverter of at least 5000W/10,000W surge capability to start the 2.2Hp air compressor. But, I'm thinking if I had the Chicago twin motor compressor mentioned above, I could run it on a 3500W inverter. Here's my thinking: If I build an electrical circuit that could delay the start of one motor, so that the inverter only has to start a single 1.1HP motor, and then the combined motors would probably not exceed the power demand from the inverter, right up to the auto-switch off at full tank pressure. With the motors shut off, the electrical circuit would reset, so that when lower tank pressure activates the auto-switch to commence refilling the tank, only one motor would be initially engaged. I was thinking perhaps of using a 555 timer IC, with an external capacitor to provide the delay function. A bleed resistor would discharge the Cap when the motors are shut off, and so the cycle would repeat. I suppose I would have to use a TRIAC to control the AC power, or, maybe a relay, that would be activated by the 555 timer chip (maybe via a Mosfet?).

author: Zero999
author_rank: Super Contributor
timestamp: January 02, 2021, 01:15:38 pm
content: Is it easy to change the motor?The obvious solution would be to swap it for a 12V motor, but I doubt you'll find one which is compatible and you'll need to alter the wiring so the pressure switch, operates a big contactor, as it won't be able to pass enough current to drive the motor directly.Failing that, how about a three phase motor, run of a VFD (Variable Frequency Drive)? It will have a much lower starting current, which will be much easer on the inverter. Again the pressure switch will have to be connected to enable/disable the VFD, rather than switching the motor directly.

author: BrianHG
author_rank: Super Contributor
timestamp: January 02, 2021, 03:23:30 pm
content: Is your inverter a 'Pure True Sinewave', or a 'Modified Sinewave / Sinusodal' AC output. The phony modified sinewave inverters, even the really high powered ones have trouble starting many AC induction motors. The motors tend to buzz much more rather than get enough startup torque. Unfortunately, the only fix is a different compressor, or a true sinewave inverter. At least the price of the true sinewave units have radically dropped in price in the past few years and are readily available at Amazon of all places...