AC Electronics

[grahamW]

You know you have been working with low-voltage, DC micro-controllers for too long when even a simple AC/mains voltage circuit stumps you. Anyways, can anyone explain how I can add a speed control to the AC fan circuit (circuit1)?

What I am trying to do is have the evaporator fan run at different speeds. I want it to run at full speed when the compressor runs, but through an speed controller when the compressor is off. It is currently wired to run continuously. Would what I've drawn in circuit2 work?

[Cubdriver]

No, because with that direct connection your speed controller would also be feeding the condenser fan and the compressor.  You'd be better off with a relay to switch the fan between the direct feed and the speed controller. 

Something with the relay coil across the compressor so it would come on with the system.  Connect the fan input to the moving contact, the compressor input line (where you currently tap in schematic two - the yellow wires) to the normally open contact and the speed controller to the normally closed contact.

When the system is running, the power to the compressor will close the N.O. relay contacts and connect the fan motor to the full line voltage.  When the system is not running, the relay will drop out and connect the can to the speed controller through the N.C. relay contacts.

-Pat

<edited to clarify after looking at schematic again...>

[rstofer]

Or connect the relay coil to the compressor part of the circuit (after the cold control).  The relay energizes when the compressor is running.

The NO contact (closes with the compressor running) feeds full voltage to the fan.  The NC contact (closes when the compressor is not running) keeps the fan running through the speed control.

Some motors don't like speed controllers.  You should be sure that bit works before you wire in a relay.

[Cubdriver]

Here's a quick sketch of what I mean:



-Pat

<edit to add - rstofer has a very good point too - make sure the fan motor CAN be speed controlled.  If it's a single phase induction motor, it probably can't.>

[grahamW]

Thanks guys. Do I need to tap either side of the compressor or can I install a small in-line load directly after the t-stat and tap either side of this? Wiring would be problematic to do the former.

[Cubdriver]

Thanks guys. Do I need to tap either side of the compressor or can I install a small in-line load directly after the t-stat and tap either side of this? Wiring would be problematic to do the former.

What do you mean by a small in line load?  Something like a dropping resistor that you'd then power the relay from?  If so, I'd recommend against that - you're better off using a line voltage relay with its coil between the t-stat and the neutral line.  The compressor will have a large starting current and a much smaller running current, so the voltage drop across any load placed in line would vary from a high when the compressor initially starts to a lower voltage once it is running.

What kind of motor drives the fan?  It's putting the cart before the horse trying to execute the circuit before determining for certain that the fan speed can be controlled.

-Pat