Induction motor start up.

[davelectronic]

So sometimes you come up against an idea, but for some technical reasons you can't move forward with an idea or project you would like to do. This is something I have visited a couple of times in the past, but I've not managed to find an answer to this problem.

For as long as I can remember I have wanted to put together an airbrush compressor, but the silent type using refrigeration compressors. The problems in the past involved frequent stop starts where heat in the motor windings and or PTC prevent restart after short periods. To some extent I know I can mitigate this by using a large air receiver. This would offer more litres or air, and allow longer cooling periods. I've considered forced air cooling to speed up the cooling process. Possibly Only cooling via relay fan turn on once the compressor has reached final upper pressure.

Recently I discovered VFD's but as it's a single phase type compressor, it's not really practical because most VFD are single phase to three phase drives. And also recently discovered a soft start circuit for induction motors of the single phase variant. But the circuit I found, the designer hadn't tried it, so I'm a bit sceptical it will work, or is even a viable option. I should mention the compressor would be in an unloaded state after reaching final pressure and blow down. So the compressor motor wouldn't be under back pressure on start up. I have added the circuit of this soft start type below, it shows components list, but doesn't list the SCR. I know soft start effects start up torque, but shouldn't be a problem for an unloaded compressor.

So I'm looking for experienced opinions on this scenario of a soft start circuit to control a single phase split phase induction motor with start and run windings. Any thoughts appreciated on this.
Thanks for reading.

https://www.homemade-circuits.com/motor-soft-start-circuit-for/

[Gyro]

Hmm, I'm not sure how an induction motor is going to take to having rectified AC pushed through it - my guess is 'not well at all'. I can see it working ok on a resistive load but that's about all.

I note the disclaimer at the bottom of the article...

Warning: The circuit has not been tested or verified practically, and the effects are unknown. Initially try the circuit by using a 200watt bulb. The bulb should brighten up slowly in comparison to when it's connected directly to mains.

[Andy Watson]

It might work with a universal motor; don't use it with an induction motor.

The control is asymmetric. D1 provides full power for the negative half of the cycle whereas the power for the positive cycle is ramped-up by control of SCR1 - there's going to be a huge DC component during the ramping-up phase. Should it reach its full-power state, without popping breakers/fuses, the positive cycle still requires an average of 50V to be lost from the cycle such that SCR1 will trigger. This will present a significant DC component to the motor - it will end in tears!

[Benta]

The question is what you hope to achieve with a soft start. Soft start is not a speed control, it's just that: a soft start.
It won't do anything for your thermal issues.

[davelectronic]

Thanks for clarifying this circuit, ok yes I get it, similar somewhat to a dimmer circuit. I've no idea why the designer would think it usable on a fridge induction motor. Soft start might cut the inrush current there by reducing heat in the start winding. Probably no good for a PTC relay start control. It was just an idea to stop the motor heating up, so the Relay and start winding would start up some what frequently. I have no need for speed control of the compressor. Anything that will aid an unloaded start up when the pressure switch cuts back in.

[davelectronic]

So I know the circuit above is a flop... But would cooling the compressor from underneath, there by cooling the oil and hopefully cooling inside the compressor. In addition to this, would cooling the PTC allow more frequent start ups. The idea being both underneath fan and fan on the PTC come on after the tank is full, and the compressor has blown down. As when it comes to start up again, it's cool enough to do so. Does this seem a viable option ? I wouldn't want to cool the PTC until the compressor stopped, once the air receiver is full. And the duration of the off period ( 5 to 10 minutes ) forced air cooling the compressor from below ( cool oil cool compressor ) And cool PTC. Can you see this being a workable solution to frequent stop starts. Really would appreciate your thoughts on this.

I'm thinking of trying this, but wanted to put the idea to you, having more experience than me. I really try to exaust all avenue's before giving up on an idea. But something I might revisit in the future, should another idea come up to this issue.
Thanks again for reading, any thoughts appreciated.

[richard.cs]

If started unloaded the run winding shouldn't be overly stressed with lots of starts, but the start winding might strongly object.

I would probably start by abandoning the PTC control of the start winding and controlling it from a timer. My experience is that the PTC* drives the start winding for tens of seconds, long after the motor is up to speed. I would keep the PTC in circuit as a backup to protect the start winding from a failed control circuit, but use a timer to only power the start winding for a second or two.

*The ones I have seen are not really a PTC thermistor, but a thermal switch heated by the passage of the start winding current. They then latch until some tens of seconds after power is removed, presumably there's a secondary heater in there that does the latching thermally.

[amyk]

A "soft start" is the opposite of your goal here, and in fact it'll heat up the start winding more if it takes longer to reach running speed. You want a quick burst of current that spins up the motor as quickly as possible.

The heat of starting up the motor itself in the windings is negligible.

If you want to be able to cycle it more frequently, replace the PTC with a potential or current start relay, like what refrigeration compressors used to use before PTCs and the era of cost optimisation.

[ConKbot]

Are you sure you want a HVAC compressor for an airbrush? They generally make minimal attempt to keep the oil contained. I have seen people use them for shop air, by using a filter, and running the drain from that back to the inlet, but a basic filter isn't going to leave the air oil free. If you aren't using it for an airbrush, and just meant an airbrush compressor style unit, then disregard this concern.

[davelectronic]

Thank you for the replys, I didn't realize you could use a current relay in place of a unit that has a PTC. I just assumed if it's got a PTC it wouldn't start any other way. So that's something I didn't know, yes I did know some lite oil trace might be in the air stream, but thought a water trap would catch it. I'm interested in the fridge type compressor because there used a lot in commercial type airbrush compressors like the manufacturer Bambi. Although they do state there compressors have a duty cycle. But if I use a large enough air receiver that shouldn't be a problem.

The other issue is noise, I currently have a compressor of the silent type, it's a diaphragm type motor unit, although it's on a noise damping surface, it's still about 70 dB when running. Although it does fill it's 24 litre tank quite quickly. I'm in a 2 bedroom apartment, so noise is an issue, or at least me not making to much of it that is.

[amyk]

It just needs the start winding connected momentarily, long enough to spin up the motor but not long enough that it overheats and burns out. You can test them manually like this too:



A PTC is somewhat universal because it's a timed start, but a current relay needs to be sized correctly to the start and run currents of the motor.

[davelectronic]

A "soft start" is the opposite of your goal here, and in fact it'll heat up the start winding more if it takes longer to reach running speed. You want a quick burst of current that spins up the motor as quickly as possible.

The heat of starting up the motor itself in the windings is negligible.

If you want to be able to cycle it more frequently, replace the PTC with a potential or current start relay, like what refrigeration compressors used to use before PTCs and the era of cost optimisation.

If the PTC is still in circuit, how can a timer work ? I'm not sure how a timer would work with the compressor. Either way if the PTC and start winding haven't cooled enough, the relay just clicks and trips out with out starting the motor. Just another thought, my fridge is quite old now and runs for extended periods with out turning off if it's iced up. Could I just run the compressor continuously but divert the air flow via a pressure switch (solianoid) So the air is routed just out in free air. Until tank pressure drops, then the valve (2 way) diverts the flow back to the tank again. And the same again once the tank reaches pressure.

[davelectronic]

In the video he starts the compressor with momontery bring the start winding into circuit. I'm assuming a current start relay does the same thing. But a PTC starter clicks and can trip out as the PTC hasn't recoved to it's starting temperature. So some thing that can momentarily contact the start winding, just to start up then disconnect once up and running. Would a current relay trip out the same way a PTC does if started to soon ? I hazard a guess that it would. Thank you again for all your replys.

PS. My current compressor, but still to noisy, I would like to convert it to two refrigeration compressors. Then it would be virtually silent.

[drussell]

If the PTC is still in circuit, how can a timer work ? I'm not sure how a timer would work with the compressor. Either way if the PTC and start winding haven't cooled enough, the relay just clicks and trips out with out starting the motor.

The timer is going to cut off the current to the start winding much more quickly than waiting for the PTC to heat up and shut it off, so the PTC doesn't ever need any time to "cool" as it never gets up to the heated high-resistance point in the first place (unless your timer circuit fails and sticks on, in which case it still would come into play and act as a failsafe to keep the start winding from burning out.)

[davelectronic]

Oh ok, now I get it. Thanks for explaining that, so a timer needs to be microcontroller type of thing ? I've never used a micro before. It's the software to program it I find daunting. Or maybe there's an analogue solution, but that would probably be to slow, although I don't know for sure.

[amyk]

If the PTC is still in circuit, how can a timer work ? I'm not sure how a timer would work with the compressor. Either way if the PTC and start winding haven't cooled enough, the relay just clicks and trips out with out starting the motor.

That's the overload protector, which is usually integrated in the same unit as the PTC.

As I mentioned before, the heat in the start winding is negligible on a normal start.

You're overthinking this. Just replace the PTC start relay with a suitably rated current relay, and add an overload protector if the relay doesn't have one built in.

[davelectronic]

Yes I will give the current relay a try, with a 24 litre air receiver the duty cycle should be ok. Thank you for all the help, hopefully I can get a positive result.