Help designing a circuit to control 120VAC motor

[vjlavista]

Hi everyone, This is my first time here and I thought your youtube videos have awesome explanation. I was wondering if you could help me out with this.

This is the information I have regarding a single phase motor that will run at 120VAC. I would like to know how to calculate the current the motor would source from the supply. I have limited experience dealing with motors but this is interesting to know nonetheless.
Here are the details

1 off assembly part number: P1-008-XXX-XXX-XXX-C03
P1 axial piston water pump @ 0.8l/min
0.37kw, single phase, 4 pole, 71 frame, foot/flange mount in permanent cap style electric motor
Bell housing
Coupling
Relief, check and unload C03 valve assembly


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It is 120 volt of course and would have a current draw similar to a ½ hp capacitor motor that we would source locally.
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How to know what value it sources from supply? Please let me know. I am trying to control this motor from a Data acquisition card that would control a relay which switches on/off the motor. Any help would be greatly appreciated. Calculations, circuit setup, diagrams, explanation... etc.. Thanks!


V
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[Chriskarr]

"0.37kw"

This means it pulls 370W.

To find the amount of current (in a DC circuit) based on wattage, one must divide the wattage by the input voltage, so your motor pulls a bit over three amps. Also, since said motor is running AC, you might want to give it a fair berth, in the form of an amp or two extra on the relay, due to the motor's current pulled being greater under load.

[SparkyProjects]

Should help a bit
http://sawdustmaking.com/ELECTRIC%20MOT ... otors.html

Looking at the chart, the half HP motor would draw 6.5-4.6 depending on efficiency (read the text above the chart), those ratings are 'full load current'.
How often would the relay operate ?
Motors are inductive, and will have a high inrush current, this could wear down relay contacts quickly if duty cycle was short, so make sure you rate it higher (i would go for at least double rating), you could use spark suppression capacitor/resistor system.
Alternately think about solid state relays or triac control.

[vjlavista]

Thank you guys for your reply. Here is what I have right now. (not the motor yet, but I hope this logic would work).

I have a DAQ card that can trigger an OP-AMP which has 24V DC on its rails. The Op-AMP triggers a 24V DV relay and the output of the relay is also 24V DC. Now, Can I use this 24V output from the relay to power another solid state relay to run the 120VAC motor? Can someone show me diagrams or point me toward more videos that show such connections?

ThankS!

V

[vjlavista]

And, when you say it draws about 6-7Amps, That current is drawn from the AC supply right? and not the supply that powers the relay.

[SparkyProjects]

Solid state relays are often powered by 5v, though i think there are 12 volt versions.
What i would do is source the solid state relay first and see what operating voltage it is.
Then get a voltage regulator that has an output to match, the input of 24 volts is converted to the right voltage for the relay.

Though i would ask what the output voltage from the DAQ card is, if it's 5 volts, it may be able to power the SSR directly, just check the currents, SSR's usually operate on a much lower current than relays.

Edit: Just checked the SSR's on my old disco decks, they show an input of 3 - 18 volts DC

[vjlavista]

Thank you so much sparky!

The reason I asked this is because this is my current setup. I have a PCB done and everything. Now, if I would have known that SSR can be powered by 5V supply DC to operate a 120VAC motor, Then I wouldn't have put an op-amp to power a 24V dc relay in between.

For now though, Is there any way I could use the 24V dc from the mech relay be used to power another relay to run the AC motor? PLease let me know. Also, if you wouldn't mind letting me know part numbers and manufacturers in the US, that would be great. THanks a ton!

V

[SparkyProjects]

I think i would tap into the coil of the relay, take that through a voltage regulator, and onto the SSR directly.
Otherwise you would need to get a supply to the contacts and switch that into the SSR.

As for US suppliers, i am not familiar with them, but a search for 'solid state relays' should bring up a few results, i would guess your favourite electronics suppliers would have them.

[SitkaGrown]

This will probably be no help ,but I just acquired a few plug-in photo switches that are used to switch on street lights .
The ratings are : 105-305v,50-60HZ 1000W,1800VA
These things use a tiny photo-cell on the circuit board to switch a fair amount of power.
Could these be used (adapted) to be a relay ?
I've been trying to think of things to use them for besides turning my yard lights on.

[SparkyProjects]

This will probably be no help ,but I just acquired a few plug-in photo switches that are used to switch on street lights .
The ratings are : 105-305v,50-60HZ 1000W,1800VA
These things use a tiny photo-cell on the circuit board to switch a fair amount of power.
Could these be used (adapted) to be a relay ?
I've been trying to think of things to use them for besides turning my yard lights on.

Not really, as they have a delay inside (i used to service the streetlights round the estate i worked on)
The way they work hasn't changed a lot, except for the addition of a circuit board to reduce the current through the photocell.
Basically the old circuit was he photocell in series with a wire heater wrapped around a bimetalic strip, when light falls on the cell, the strip would bend and break the contacts (when you first power these up, the light comes on, then goes off a few seconds later).
When it gets dark, the strip cools and closes the circuit.
The photocell circuit is powered from the mains, so is not isolated electrically, you'll find 3 pins, live, neutral and switched live.

[vjlavista]

Thanks Sparky! Would you mind enlightening me on the terminology, Hot wire and Neutral.... Can u give examples of circuits? I think I know what they mean, but just wanted be to sure...

[SparkyProjects]

Not sure which circuit you wanted
One is for the SSR control circuit, the 'from existing' would be connected to the coil terminals of the relay you already have, this makes the contacts redundant, but is a better method,

The other for those interested is the basic photocell for streetlights, later versions simply replace the photocell with a circuit board that includes a smaller cell.

[vjlavista]

Thanks! The motor has 3 wires right; Hot, neutral and ground. But, why have you shown the motor down and not connected to the hot? I know it doesn't matter, because the SSR would create a short when triggered. Where would I connect the ground?

[SparkyProjects]

The ground of the motor would be connected to mains earth

The SSR is basically a switch, all switches should be in the hot side, you can add a mains isolation switch if you wish where it says 'mains supply', this can be single or double pole.

Be sure that the 3 wires from the motor are what you say they are, the earth should be the one connected to the frame of the motor.