How to switch and measure 415 v three phase?

I want to switch a 0.33 kW 415VAC three phase motor and also monitor the individual phases. (Yes, I know the dangers - to myself but not necessarily to the Arduino with low voltage inductive spikes etc. I frequently work with 415VAC 3 phase.)

I thought that I could use this setup for each phase with the three respective signal pins connected to the one arduino output pin.

Would this work? With the three control circuits coming into the one signal pin, could there be a crosstalk or any other issues?

Secondly, I would like to stop the motor if one of the phases dropped out through loss of power or failure of one of the triacs. How could I safely monitor the voltage on each phase? I do not want to use relays.

Assuming you use a 3-phase direct on-line contactor as your "isolating" device (ideal for incorporation of the emergency STOP facility) this will/should have "lost-phase" detection due to out-of-balance current sensing via its in-built thermal trips.

Out of interest, why would you want to vary the voltage on a 3-phase motor - as the suggested circuit is intended to do, or are you simply switching it ON and Off. If so why not simply drive the contactor. Yes I know you say you don't want relays, why not !

If it's variable speed you desire then you need to vary the frequency of the drive power and simply chopping the 50Hz mains via triac control will not achieve this.

Edit : This comment was not posted by me, administrators have been advised of system failure

Assuming you use a 3-phase direct on-line contactor ....

I am trying to get away from mechanical components. I find that they are usually the problem in systems controlling AC motors. Either mechnical wear/breakdown or burnt out contacts from arcing is the common cause of failure.

Out of interest, why would you want to vary the voltage on a 3-phase motor

I will be using a digital pin from the Arduino so the motor will be either full on or full off.

I suggest you consider using ready-made SSRs rather than roll your own with opto triacs and power triacs. You can even buy 3-phase SSRs, see e.g. http://www.crydom.com/en/Tech/3P_MC_whitepaper.pdf.

PS - also see the bit in the paper I linked to which explains why you should NOT use zero-crossing SSRs with 3-phase induction motors.

I suggest you consider using ready-made SSRs rather than roll your own with opto triacs and power triacs.

The three phase units cost over $200. A few opto-isolators, triacs, etc cost around $20. I have an application that may need 6 - 8 of these. That few hundred dollars is a fair premium to pay for the SSRs.

PS - also see the bit in the paper I linked to which explains why you should NOT use zero-crossing SSRs with 3-phase induction motors.

Thanks for the heads up. I will look at using non zero crossing units.

Still don't know how to safely monitor that each phase is active.

I found this http://www.ebay.com/itm/DC-to-AC-3-Phase-Solid-State-Relay-SSR-25DA-25A-5-32VDC-380VAC-/360521549005?pt=LH_DefaultDomain_0&hash=item53f0c244cd 3-phase SSR for $25 but I suspect it's zero crossing turn on.

To monitor the voltage on each phase, use a series resistor, bridge rectifier and opto isolator for each one. Make sure the series resistor is rated for the full voltage of each phase. 1 to 2mA peak should be enough current to drive the opto isolator.

Thanks dc42. We have 415VAC here in Australia so that unit won't do but I will have a look for similar units.

Regarding the voltage monitoring; just checking that I've got it right.

The AC voltage connects to the resistor (lowering the current but maintaining the high voltage) and then into the rectifier (which has to be a high voltage unit). The voltage comes out of the rectifier as 240vDC (or slightly greater). Would it matter if the resistor was after the rectifier?

I can only find opto-isolators that take a low voltage to the diode and switch to a high voltage. What I need here, is the reverse: An opto-isolator where the high voltage feeds the diode which is switching the low voltage.

The rectifier does not need to be a high voltage unit provided that the resistor comes before the rectifier, you use a bridge rectifier (not a single diode), and the opto isolator has been connected. The diode in the optoisolator effectively shorts the output of the bridge rectifier, so that the voltage appearing across the rectifier is constrained to a few volts.

If the opto isolator diode goes open circuit, then the rectifier does see the full mains voltage and a low voltage rectifier will suffer avalanche breakdown. But the current will still be limited by the resistor, so this is safe. However, 800V bridge rectifiers are inexpensive, so get a high voltage one if you like.

The resistor needs to be rated at 415VAC (resistors have voltage ratings as well as power ratings), and it will dissipate a fair amount of power. For example, if you use 200K (giving about 2mA RMS current, 3mA peak), then it will dissipate just over 0.8W. You might want to use two 100K 1W resistors in series, to help spread the power dissipation and to allow use of a lower voltage rating.

Thanks dc42. I'm getting there!

The resistor needs to be rated at 415VAC

As I'm putting one of these on each phase, does this only have to be 240VAC? (It doesn't hurt to have overkill but just getting the theory right in my mind).

If you are monitoring the voltage between each live and the common neutral, then 240V AC rating (350V DC) will be sufficient in theory. However, to allow for transients, I suggest you use two 350V resistors in series. Something like 2 x 56K 0.5W 350V metal film.

Could someone kindly send me a wiring diagram or sketch on how to do this as I want my arduino to monitor the output from my boat genset and if it drops a phase sound an alarm

Thanks kindly Tom

The simplest way to monitor the 3 phases with safe isolation is to build yourself an optical sensor: three neon lamps wired phase-to-phase and three optical sensors looking at each neon. QED

My worry here is you are trying to use triacs on an induction motor. Triacs don't switch off till the zero current point, but with an induction motor the zero current point is about the maximum voltage point which is a lot more stress on switch-off than when controlling a resistive load or a universal motor.

Induction motors typically look like massive inductors with a bit of resistance (dependent on the load), so are about the hardest load to switch there is. And of course you can't switch all three phases simulataneously at a zero-crossing, which probably means odd voltage transients at switching.

Normally you use a purpose designed VFD to control a 3-phase motor, or else a simple mechanical contactor/starter unit (which can't do speed control).

Other than monitoring the phases is there any reason not to go with known working technology?

Hold on a minute, we're talking about a 330 watt motor here. Absolute tiddly stuff on 415v 3-phase. Why the writer claims contactors are a source of failure beats me when they have been the industry standard for well over 100 years. At the load currents he's talking about, a suitable or over-rated unit would have to be working for decades to even consider contact failure.

Induction motors can pull huge currents on startup if not used with a soft-start circuit or VFD - 10 to 20 times load current is possible, though it depends a lot on the motor.

MarkT:
My worry here is you are trying to use triacs on an induction motor. Triacs don’t switch off till the zero
current point, but with an induction motor the zero current point is about the maximum voltage point
which is a lot more stress on switch-off than when controlling a resistive load or a universal motor.

Induction motors typically look like massive inductors with a bit of resistance (dependent on the load),
so are about the hardest load to switch there is. And of course you can’t switch all three phases
simulataneously at a zero-crossing, which probably means odd voltage transients at switching.

Normally you use a purpose designed VFD to control a 3-phase motor, or else a simple mechanical
contactor/starter unit (which can’t do speed control).

Other than monitoring the phases is there any reason not to go with known working technology?

jackrae:
Hold on a minute, we’re talking about a 330 watt motor here.
Absolute tiddly stuff on 415v 3-phase.
Why the writer claims contactors are a source of failure beats me when they have been the industry standard for well over 100 years.
At the load currents he’s talking about, a suitable or over-rated unit would have to be working for decades to even consider contact failure.

I think if you look at the post dates, you will find the subject has been hi-jacked and changed to just monitoring a 3phase genset output for phase dropout.
Tom… :slight_smile:

Damn ! fell headlong into an elephant trap

Three light bulbs is the old way of doing what the highjacker wants !