VFD Small AC Pump control

Discussion in 'The Projects Forum' started by croydoncorgi, Oct 25, 2011.

  1. croydoncorgi

    Thread Starter New Member

    Oct 25, 2011
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    Hi
    I'm new on here - so if this topic has been well-covered but not found by my topic search, please send me gentle pointers!

    I'm a heating engineer and have a minor problem which potentially affects a large number of high-efficiency condensing boilers (aka furnaces - 2 nations divided by common language!). To work efficiently, these boilers need relatively tight control of the delta-T between the Flow and Return. Too big damages the boiler (thermal stress) but too little - ie. Return temperature 'catching up' to the Flow temperature - means that the boiler won't condense. The Return temperature needs to be below about 56C degrees and the delta-T less than 20 C degrees. (There are situations, eg. when the Flow setpoint for some reason needs to be above 75 degrees, when the ideal is not achievable!)

    The boiler pump needs to slow down as the Return temperature rises, so that the maximum delta-T is achieved. This is particularly vital if the boiler is connected to a header: if the 'shunt' pump runs too fast, water whizzes between the boiler-side connections on the header and straight back to the boiler, so the Return temperature is virtually the same as the Flow.

    Expensive boilers have modulating pumps which usually have 3 operating speeds. The boiler controller selects the optimum pump speed to match the total heat demand, so that the Flow / Return delta-T is maximised but the demand is still satisfied.

    Cheaper boilers have a fixed-speed or manually adjustable pump which matches the average demand. However, there are often situations (especially low demand) when the pump is running too fast and the boiler efficiency suffers.

    I'm looking for a way to mate an existing single-phase AC-motor pump (up to 300W - ie. VERY small) to a VFD with servo temperature control, with NTC sensors on the Flow and Return connections of the boiler. The process would be:
    - on initial start up, pump runs at full speed.
    - as the Return temperature rises, the pump speed decreases progressively until a predetermined temperature difference between Flow and Return temperatures is maintained.
    - in the event that the Flow temperature rises, the pump speed increases to maintain the target delta-T.
    - in the event that the Flow temperature keeps rising so that the desired delta-T cannot be achieved, the pump should run at full speed.

    Obviously (??), a 'large' VFD designed for a motor of 1kW or larger would be too expensive. But really small ones seem not to exist as stand-alone products. I am aware that solar-thermal systems include pump speed control (so that the system flow is matched to the temperature rise in the panels achieved by the current sunshine intensity) but these are integrated into the solar controllers. Also ,they may not in fact use variable frequency...?

    The point of all this is to maximise the output efficiency of the boiler. Getting the delta-T right can improve this by several percent, first because the delta-T between the water and the heat-exchanger surfaces will be greater (basic physics = more heat transferred) and secondly because one the boiler is in condensing mode, an extra 2 or so percent of efficiency will result. With the price of fuels going up, we need every percentage point we can get!
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    A VFD is only able to run a three phase motor. You would have to change your motor also if you have a single phase motor. The starting winding for a single phase motor is what disallows the use of a VFD on a single phase motor.

    A lot of newer appliances and furnaces are changing to BLDC motors so they can be speed controlled for differing loads/conditions. But that is an expensive fix also.

    Could you use a 'three-port control valve' to do what you need? http://www.filter.lt/extensions/filter/brochures/216-8627.pdf
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    as mentioned, you can't hook up a VFD to a single phase motor. you may be looking for a new motor as well.

    ...

    Sure they do.
    look up the Yaskawa GPD205-10P7
    There are many others; I have seen some from allen bradley and siemens I believe as well.

    On suggestion I would make is (if you decide to get a 3ph motor & VFD) that you get a drive with PID algorithm. Trying to do the PID in a PLC (I assume you would use PLC, sounds like industrial application) and then transfer it to the VFD is tough to impossible.
     
  4. croydoncorgi

    Thread Starter New Member

    Oct 25, 2011
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    Didn't realise the limitation to 3-phase motors.

    After posting yesterday, I called a contact at the company that actually makes the existing pump - and also makes what he referred to as pumps with 'EC' motors as opposed to AC or DC. They have a couple of different models featuring variable speed using single phase motors with and without permanent magnets. He called them 'EC' motors, as opposed to AC and DC. Apparently, they use electronics to chop the 50 (60) Hz single-phase supply, not create variable frequency. Unfortunately, neither are suitable for what I want: the sensing is all done internally, using RPM and coil current, and there's no external control input.

    What I was hoping to do was make an external controller that would drive the existing motor at varying speed, not change the pump. To make anything else fit available dimensions would be very tricky!

    Knowing that solar pump stations use the same (or very similar) pump and somehow achieve variable speed leads me to the conclusion that it IS possible to do it - but it must be with a 'chopper', not variable frequency.

    Circuit ideas, anyone? (I believe there's likely to be issues with the starting capacitor already in the motor....)
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    This is a half-guess but I highly suspect by "EC" he meant "electronically Commutated" - a new term that I've never heard of to describe a BLDC (Brushless DC) AKA PMAC (permanent magnet AC). Which is basically a 3phase AC motor which uses unidirectional pulses of DC instead bidirectional pulses of DC (AC) - hope that's not too confusing. basically I think he tried to sell you something real, but masquerading as snake oil. All it is, is a variable speed (3phase DC) motor.

    You can keep chasing the single phase variable speed (*induction) pipe dream if you want, but something similar to this is probably what you will end up buying. I'm not saying it's not possible, but I've never seen it, and I've looked, long and hard.

    *you can get variable speed out a single phase universal motor, but their uses (as far as I know) are limited to things like blenders and dremel tools.

    If your solar pump stations have variable speed single phase motors, they might have more than one set of windings, for multiple speeds.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    The motors they use for "chopper" (PWM) control are DC type. So I would look for a suitable DC motor with a PWM motor controller.

    Also what about a small AC 3-phase controller such as this which starts at $99. Then you just need a suitable small 3-phase motor.
     
  7. croydoncorgi

    Thread Starter New Member

    Oct 25, 2011
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    Aha!

    Found it!

    See http://net.grundfos.com/doc/webnet/hvacoem/pdf/Grundfos_HVAC_OEM_solar.pdf

    Now all I've got to do is find someone prepared to sell me a couple and then develop a circuit (or maybe something like a PIC controller with a mapping table in code...) to translate Flow and Return temperatures into PWM output to control the pump.

    Sounds like a Project! :rolleyes:
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Those are either a straight DC or a BLDC motor in the pump. All the data sheet says is "permanent magnet motor". Both the DC and BLDC motors fit that description.

    By adding one of the three way valves to your existing system you could do the same thing. The valve would be used to shunt a portion of the water out of the system back to the main sump. That would allow the volume in the circulation system to be controlled without changing the pump.
     
  9. croydoncorgi

    Thread Starter New Member

    Oct 25, 2011
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    Actually not.

    The pump is inside the boiler, on the Return pipe, so there's no route avalable back to the header.
     
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