Heating zone valve sychronous motors question

[MathWizard]

For example the common honeywell zone-valve for a furnace, runs off 24VAC, and has a coil resistane of 20ohms or so.

When these motors hit the end of their travel, and the motor can't push any harder, and it stops, does it just sit there, with the 24V AC still applied, or is there any internal circuitry that bypasses the motor when it stop's, or, is it just built to take it?

If there's no locking mechanism, and the value is active-open, then I guess the power has to be kept applied, to keep it open? Otherwise it would close, is that it?

[Benta]

For this kind of stuff (actuators, automotive power windows etc.), the normal procedure is just monitoring the motor current. Should it rise sharply, this indicates that an end stop has been hit. The controller will then depower or brake the motor.

[MathWizard]

Is the controller built into the motor? Mine is a 4-wire type, my understanding is the switch wires are just for telling the furnace pump/and/or burner to start.

In physics I can sort of remember the most basic motors, but IDK, I'm imaging a few thing's, I'll have to crack a book on both the physic's and EE of these soon enough.

[Zero999]

Small actuator motors are often designed to be able to run continuously stalled, without overheating. The winding will have a lot of leakage inductance, which limits the current to a safe level.

[Gyro]

Normally (at least in the UK) zone valves use the synchronous motor to drive the valve in one direction and then spring return in the other when power is removed. Simple induction synchronous motors are pretty inefficient and there isn't a very high stall current (their coil inductance doesn't change that much). Traditional UK ones are normally 240V motors.

In the case of 3 way valves, they usually rely on the return spring for one position, have a microswitch to switch in a series resistor to reduce motor torque to stall it when it reaches the mid position, and then uses full motor torque to reach the third position.

The motors are very low torque and geared down so yes, you could say that they're just built to take it (impedance protected), although you do get the occasional failure - over here, spare motors are available from plumbers' merchants and DIY stores.

[floobydust]

Old 1950's (American) zone valves had a gate valve and limit switches at both ends, with an oddball thermostat SPDT but a middle-off for a few degrees of deadband. When these jam, the current gets very high, many amps at 24VAC and would blow fuses.

Newer zone valves are spring-return and V8043 measures 200mH and 21R (plus around 2nF in parallel) and I consider them "impedance protected". A locked rotor does not change the current much because it's mostly inductance-limited (75Z at 60Hz) so no danger to the Class II wiring and 24V transformer.

These zone valves get stuck, I had a building maintenance job and I would push the manual lever back and forth many times to unjam them and then they work fine for the heating season. Changing them is a bear, draining the system and plumbing mess.

Note the inductance is large and trouble for triacs, easy to make 600V spikes and a need for quadrant IV operation. For my designs I use a snubber and Spice model it including wiring and transformer inductance.

[MathWizard]

So when it's powered and looks like it's stopped, it's still using energy to balance the forces trying to close it, so the AC is still whipping around all the coil(s) and giving it just enough of a bump to keep the shaft apparently static.

[floobydust]

Yes, the motor reaches the end-of-travel stop and sits there, on. They have low torque like a clock motor so the gears do not get stripped.

These zone valve motors also come in fail-safe, so they need power to close (not open). All summer, the motor is energized and the valve closed. There are normally-open or closed varieties.

What are your 4 wires about? It's usually just two.

[jmelson]

For example the common honeywell zone-valve for a furnace, runs off 24VAC, and has a coil resistane of 20ohms or so.

When these motors hit the end of their travel, and the motor can't push any harder, and it stops, does it just sit there, with the 24V AC still applied, or is there any internal circuitry that bypasses the motor when it stop's, or, is it just built to take it?

If there's no locking mechanism, and the value is active-open, then I guess the power has to be kept applied, to keep it open? Otherwise it would close, is that it?

Yes, that's all correct.  The motors are impedance-limited, meaning the current will not go much higher when the rotor becomes locked.
These motors are the same type used in wall clocks from 50 years ago, they draw just a Watt or so.  The DC resistance may be 20 Ohms, but the inductance limits the current to way less than the DC resistance would indicate.

The thermostat feeds power to the motor when heat is called for, the motor opens the zone valve and turns on the circulator pump, which turns on the gas valve.

Jon

[jmelson]

What are your 4 wires about? It's usually just two.

Two wires energize the motor.  When the valve is fully open, it trips a microswitch that turns on the circulator pump.  That's where the other 2 wires come in.  The relay there also turnns on the gas valve.  Or, at least, that is one of the common systems.

Jon

[MathWizard]

A lot of them have 5-wires aswell, I kept forgetting to write down my Honeywell model, but it's all working, just wondering.

[gcewing]

Big Clive made a video about one of these recently:

[wizard69]

Your best bet is to get the art number for the valve in question and go looking for the data sheet.  The goal of course is to find out how a specific valve works.

I have little experience with water systems and the hot air systems  were very old.    However I have worked with a lot of industrial furnace controls and systems, it really pays to have the data sheets on hand.