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Codemaster
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Small Synchronous Motors

04/04/2018 2:10 PM

I got into a discussion on a heating and plumbing forum about diverter valves and the power consumed by the motor.

The one I have is 3-port, spring return, synchronous motor, stalls at end of travel when energised, which can be for long periods. Some posters queried the power when stalled, and whether that would cause overheating and damage.

Mine is labelled 230/240v AC 50hz motor 5 watt. I guess that is electrical power, as it doesn’t produce any output power when stalled, but in any case it’s clearly not going to do any harm, it will pick up much more heat from the valve body.

I haven’t had much to do with synchronous motors, but I understand the permanent magnet types (as this is) have PF ~ 1.

The query – one of the posters took some measurements on a similar valve, and came up with

“The spec and markings say it is 6W. At 244.3V, with the motor running it takes 31mA with a PF of 0.98 (which surprises me, for a motor) - hence about 7.4W. More surprising, when stalled, both current and PF were completely unchanged, which somewhat offends my understanding of physics!
The DC resistance of the motor was exactly 2.0kΩ”

The apparent impedance when running (or stalled) is = 244.3/0.031 ~ 8kΩ, so as the PF is close to 1, shouldn’t the DC resistance and impedance be the same? Or at least not differ by a factor of 4.

We’re all puzzled, can anybody explain?

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Rixter
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#1

Re: Small Synchronous Motors

04/04/2018 3:26 PM

Maybe the power factor is different when it is running than when it is stalled. When it is stalled, the AC input is basically driving into an inductor.

When it is running, the turning permanent magnet rotor is generating a voltage which would change the impedance seen by the power source.
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Codemaster
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#6
In reply to #1

Re: Small Synchronous Motors

04/05/2018 8:55 AM

Thanks for that, but as per the post, the current and PF measurements were the same, running and stalled.

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RAMConsult
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#2

Re: Small Synchronous Motors

04/04/2018 5:32 PM

There are actually two types of low power/torque motors used in small actuator duty such as a zone valve. Typically they are impedance-protected shaded pole synchronous motors, not higher torque (and more expensive) permanent magnet synchronous motors. Since most zone valves rely on the impedance-protected feature to prevent the motor from overheating when stalled when the valve reaches its end of travel, I wonder which type of motor you have.

The data you measured indicates it's a shaded pole motor (unless the nameplate suggests otherwise). Although it may appear counterintuitive that a stalled motor would have near unity power factor, that's exactly what should happen. Remember if we stall a motor it does work to overcome the stall and overheats until a circuit protective device operates or the coil burns out.

An impedance protected motor is specifically designed with thin wire that overheats very rapidly, and as the wire heats up, its resistance goes up as well, resulting in lower current being drawn. These motors are designed to reach an equilibrium temperature below the rating of the winding insulation so that nothing burns out.

The key to the near unity power factor is that all the current being drawn is going into the I2R losses in heating the motor, and I2R losses are Watts (Real Power).

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JRaef
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#4
In reply to #2

Re: Small Synchronous Motors

04/05/2018 6:56 AM

I agree, that’s what the description fits, an impedance protected motor, often identified somewhere with the letters “ZP”, Z being Impedence, P for Protected.

One thing worth mentioning here, you (OP) indicated that the valve body may be getting hotter than the motor, which I took to mean the fluid is high temperature. In the design and testing standards for ZP motors the motor is allowed to reach no more than 175deg C for 18 days. This makes this type of motorized valve unsuitable for many steam applications without some means of cooling. If it’s a liquid, don’t worry about it but if it is steam, post back and I’ll tell you a trick!

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Codemaster
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#5
In reply to #4

Re: Small Synchronous Motors

04/05/2018 8:39 AM

Thanks for that, and the other posters. I'll post the comments to the other forum.

The valves I have in mind are for domestic heating, so the fluid will be 90°C tops. There is a small air gap, 2-3mm between the base of the actuator and the top of the valve body, but I think it will gain more heat from the surroundings than the ~ 5W from the motor (till it reaches equilibrium of course).

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P.Mason
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#3

Re: Small Synchronous Motors

04/05/2018 4:08 AM

In my diverter valve there are micro switches at either end of the travel and a diode. When the valve reaches the microswitch it switches in the diode and the motor is in effect locked with a half wave dc voltage. The motor then only has half the power in the locked position so should not overheat.
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Codemaster
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#7
In reply to #3

Re: Small Synchronous Motors

04/05/2018 9:06 AM

That sounds like a mid-position valve, so it goes there when both CH and DHW are calling. But when CH only is calling, it motors right over and still stalls (and remains energised) in that position, just like the older 2-position valves.

Some valve types have limit switches to isolate the motor at fully-open or fully-closed position, so the overheating issue doesn't arise. Other types eg two separate 2-port valves for CH and DHW have a limit switch, closed when the valve is fully open, to start the pump and boiler, to avoid dead-heading the system if both valves should fail closed.

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