Bunn Relay Purpose?

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DJR
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#1: Post by DJR »

I was wondering if anyone would explain why there is an old fashioned relay in my Bunn G3? I'm getting rid of the bag switch, but can't figure out why they put that relay there?

Thanks very much.

dan

DavidMLewis
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#2: Post by DavidMLewis »

I don't have direct experience with the Bunn, so take my words with a grain of salt. A relay is often used in an application where the thing you are controlling draws more current than a switch will comfortably handle. For instance, that is why your headlights are not controlled directly by the headlight switch, but the switch controls the headlight relay and that controls the current for the headlight. A related reason is that certain kinds of loads, bulbs and motors among them, can draw a very large amount of current when they are first starting up, and that inrush current will burn out a switch fairly quickly unless the switch is made very large or the load is isolated with something like a relay. I'm guessing that this is what is going on.

Best,
David

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DJR (original poster)
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#3: Post by DJR (original poster) »

You might be right, but I still don't get it exactly. There isn't a transformer or two voltages, and the relay is just a switch. The motor isn't huge: 1.5 HP, it has a capacitor for starting it, so I guess unless someone advises me otherwise, that I'll try controlling it with a heavy duty momentary toggle switch.

Thanks,

dan

OldNuc
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#4: Post by OldNuc »

What is connected to the relay coil and what is connected to the relay contacts? There should be numbers next to the pins and possibly a drawing on the side. 85 and 86 are coil and 30, 87, 87a are contacts for example. A 1.5hp capacitor start and/or run motor is going to usually have a relay as the switch will not last long under that starting current.
To directly switch that load will require a 30A to 35A rated switch.

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civ
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#5: Post by civ »

Hello Dan:
DJR wrote:You might be right ...
He certainly is.
That's basically what relays do.
They 'relay' a command.

Now, in the case of an electric motor, relays are also used for other purposes such as energising the start capacitor and windings while er ... starting.
Gives the motor the extra torque needed to get things moving, which in a coffee grinder is sort of crucial.

My 220V Cimbali Jr. Max has two capacitors (I believe the 110V versions have four) and both a start and command relays.
DJR wrote: ... unless someone advises me otherwise, that I'll try controlling it ...
You risk burning out the start windings.
They must only be energised for a spacified time, not more.
There's a reason why the relay is there.

I'm uploading the circuit diagramme the people at DRM kindly sent me.
Check out how it is connected and what the relay does when you flip the switch.


Image courtesy of DRM Elettronica Sas

Cheers,

CIV

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DJR (original poster)
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#6: Post by DJR (original poster) »

Thanks everyone! I'm convinced. I'll put the relay inline the way it was before starting the motor.

This has been really helpful.

Dan

ira
Team HB
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#7: Post by ira »

In the Bunn or at least in my G3 it's just a latch. If you press the switch that the bag holds down and press the rocker switch down, the relay latches the motor on till you press the rocker switch up or remove the bag. If you put in a switch with enough rating, the relay has no need to be there. On mine, the bag switch is gone and it's been rewired so the motor runs when you press the switch down and stops when you let go.

The schematic is on-line in the manual number 41545.0000D on the Bunn web site and you can see that it works like I say.

Ira