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Forum Index : Electronics : Single phase Induction motor speed control
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Haxby Guru  Joined: 07/07/2008 Location: AustraliaPosts: 423 |
More pool questions! I have a single phase induction motor running the pool and it is too big for the pool I have. I need to decrease the speed. I read the article in Silicon chip on control of these motors, and it looks like both the voltage and the frequency has to be lowered to maintain temperature stability. I'll get this kit unless there are other ways to do it these days???? |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
There are a number of similar VFDs (VSDs) on the likes of AliExpress or BangGood for a lower price, even allowing for their inflated power ratings. If you need 1.5kW choose a 2.2kW unit. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Its all about controlling the motor current. As the frequency reduces the inductive reactance decreases, so to prevent the motor winding current from becoming excessive, the voltage needs to be reduced in proportion to the reduction in frequency. How well this works depends on the nature of the load. Centrifugal fans and pumps can control quite well, because the shaft power required increases cubed law with rpm. So as speed is reduced, the load on the motor falls away very dramatically, and the motor speed can be readily adjustable over a usefully wide range. Edited 2021-12-09 08:28 by Warpspeed Cheers, Tony. |
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poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1418 |
I have a variable speed pool pump (onga 3 speed) The 3 speeds available are useful (low, medium and full speed) High speeds for cleaning, medium for normal filtering and running through the solar heater and low for the most effective fine particle filtration. Power consumption of a pump increases by the cube of flow rate have a look here I fully support you in driving the pump at appropriate speeds for the application. It will save you a lot of energy. VFD frequently are configured to reduce the voltage as the speed slows. This is needed because the motor winding will saturate more easily as the motor drive frequency reduces. Edited 2021-12-09 08:30 by poida wronger than a phone book full of wrong phone numbers |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
We posted at the same time, but yes it will work. Cheers, Tony. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 458 |
There may be an easier way to do it. If you have a speed controller varying the frequency and voltage the unit needs to keep the voltage and frequency up on start mode. If the motor happens to have a centrifical starting switch then it will not like starting on low frequency and voltage. If the motor is purely a capacitor start capacitor run unit it may not matter as much. Have you thought about just bypassing some of the water from the output back into the input. That should take some of the load off the motor and reduce the flow into the pool as well as reducing the power drawn by the motor. I know it is a low tech approach but the speed controller you pointed to sounds pretty expensive. Pete |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 423 |
Cheers all. Alas I think it's best if I change the pump over to one with 3 speeds as per Poidas setup. I found a new old-stock Zodiac pump with this feature for $150. You can actually set the RPM in 50 rpm steps from 1000 to 2400. I just hope it remembers in non volatile memory what it was set to, and resumes at this speed after a power cut. If it doesn't do that, I'll have to re-visit the whole thing as I want it on a manual timer that turns on during daylight hours to capitalise on the solar production. |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
"Have you thought about just bypassing some of the water from the output back into the input." This will increase the flow through the pump a little and, as noted above increase the power use. Restricting the flow with a throttling valve reduces power use a little but is less efficient than a VFD. Before VFDs became affordable the efficient solution was a Pre Rotation Vane device. It is attached to the inlet of a centrifugal pump and forces the water to rotate in the same direction as the impeller as it enters the pump. The impeller can add less extra velocity to the water and the motor uses less power. The vane angle can be adjusted to get the desired flow or pressure. They seem to have disappeared completely. VFDs rule. |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 423 |
Ok I hooked up the variable speed zodiac pump to the pool. I tested it, and all was good. Curiosity got the better of me and I took apart the controller to see what's inside. It appears it is a 3 phase pump. Ok that makes sense. The controller is just a VFD. Power usage: it's a 1000w pump, but at the lowest setting, it's drawing 350w. I'd like a setting even lower than this, but that's good enough. Certainly better than drawing 1kw all day! Now the important test of seeing if it remembers the last speed setting after a power outage..... Yep. All good. It does. Now here is the interesting thing. I pressed the soft "off" button around 6pm last night.... To my surprise, it was on when I woke up today. No idea why! The switches are tactile, and there is no way anyone else would have pressed the buttons. |
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| poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1418 |
Haxby: I finally got around to it and measured the 3 motor speed settings. Max: 1083W at 0.69 power factor medium: 664W at 0.68 pf low: 138W at 0.67 pf given that power is the cube of flow rate low = 1 unit flow rate meduim = 1.69 x low flow rate (cube root of (664/138) ) and high is 2.0x low flow rate (cube root of (1083/138) ) Funnily enough the motor seems to sing at an octave higher than low when running on high speed. wronger than a phone book full of wrong phone numbers |
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wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 1156 |
Interesting, so 2 motor/pumps at low speed (276W) will pass the same volume as one motor at high speed 1,083W. Is that the right deduction - Id rather drive 2 at 276W if I needed the flow rate? Seems like increased flow rate is a very diminishing return for the amount of energy input to a 1 pump setup. What happens if you gang two pumps on the one motor shaft at the low speed setting, will it pass ~ twice the volume for a modest increase in power ie 276W or less? - This is not my field of expertise...... BTW Merry Xmas to all on the Forum which has been very quiet of late...... Edited 2021-12-25 18:27 by wiseguy If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 423 |
Wiseguy I think 2 pumps at low speed will use about half the energy but only if you have 2 sets of pipes going in/out of the pool. Or much bigger diameter pipes to keep friction low. It all starts to get complicated. But a low flow rate for my modestly sized pool is perfectly fine. It's been a few days now, and I'm very happy with the variable speed pump and heat pump water heater. Since I have a 10kw solar system, the energy for the pool is completely free. I turn off the system at sunset, and the pool drops from the 31degree setpoint to around 25 degrees in the next morning. Only to be back up to 30 odd degrees by midday. The next phase of my experiments will involve a new pool that will be insulated all round. A bit like a swim spa. I'm hoping to get close to year-round free energy warm swims. |
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