hvac control and ammeter

Dec 19, 2007 6 Replies

Hi all,



I'm working on designing a thermostat that controls the heating and cooling unit in my apartment, and which in turn I can monitor and control via the computer. The current thermostat is a Honeywell T6169 line voltage (120VAC) unit. The sticker on the back shows the following ampere ratings:



Full load: 8A Locked rotor: 48A Resistive: 13A


48 amps seems like quite a bit! In fact, the circuit breaker for the unit (lights in the room are on the same circuit as well) is rated at
15A.

I'd like to monitor the current that the unit draws. Typically I would use a small resistor in series, but at these currents even a small resistor would burn. How is this typically done without starting a fire? I do not have a fireplace to accomodate :)



I'm also planning on using a relay (Potter & Brumfield K10P-11D15-12, rated for 15A, 1/3HP, 120VAC) to control the unit. Is this sufficient? Since the motor is an inductive part, are bleeder circuits of some sort typically included to gracefully remove current, or is the relay designed to handle a little spark?



A couple side questions: What does AFL stand for in the context 8 AFL @ 120VAC? How is the horsepower specification used, for instance the 1/3HP for my relay? Does it mean it can power a 1/3HP motor?



Cheers, Chris


For current measurement You can use an Hall effect sensor like those produced by LEM. Its only necessary to pass a single mains wire (phase) into a hole... bye delo

ha scritto nel messaggio news: snipped-for-privacy@w40g2000hsb.googlegroups.com...

Yes, but that breaker does not trip instantaneously, there is a delay. The relay/switch whatever in the tstat supplies that current instantaneously.

Don't bother trying to monitor it, it can vary greatly depending on operating conditions like start-up counter-torque from whatever the motor is driving.

Only if your motor loading is < 1/3HP...

The relay may even require that inductive spark to preserve its contact material, don't worry about it.

Amperes Full Load

Duh

Don't do it. If this were your own home, I'd still suggest that you not mess around with the wiring. But as you are likely to burn down the homes of other renters as well as your own, this is all the more a bad idea.

AC current is typically measured using something called a current transformer. They have a side benefit of providing isolation between the high voltage circuit being monitored and the monitoring equipment connected to their secondary (which I can already visualize bursting into flames).

It appears to be OK.

AFL = Amps Full Load

If a device has a horsepower rating, that indicates that it is suitable for use with a motor up to that size.

Paul Hovnanian paul@hovnanian.com ----------------------------------------------------------------------- Procrastinators: The leaders for tomorrow.

Usually the rotor is not locked on a motor :-)

If you do that for more than a few seconds three things can happen. Either the circuit breaker pops, or the thermoswitch begins to cycle, or smoke billows out of the motor.

You can use a clamp meter. Contactless. You open the clamp and then close it around one of the motor wires (not both). Harborfreight has meters like that for little money, don't know how good they are.

That relay sound a bit wimpy for that purpose. At least for my taste.

WRT horses I've seen rather bizarre statements for motors that sure sounded like the marketing guys had gussied them up a bit.

Regards, Joerg http://www.analogconsultants.com/

Who decided that the HVAC was to be connected on the same wiring circuit as domestic lighting? GPO's and lighting should be on individual circuits as should HVAC. Having HVAC on the same circuit as lighting is simply a disaster waiting to happen.

would

sufficient?

This is the tstat

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...and you're not going to improve on that. The *real* problem is that the landlord has you locked out of adjusting it and you want to bypass it. Isn't that right?

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