pond pump comtrol

[jlockham@jlockham.karoo.c]

hi,
I am trying to produce a circuit to control 4 water pumps for my fish tank to create a wave system, for the main control i am using a pic and have programmed a simple time delay the problem is trying to make a soft start system for the pumps i have thought of using a light dimmer circuit but after reading a few forums it seams that they are not suitable for controlling an AC motor, any ideas would be greatfully appriciated.

p,s Anyone know how i change my username put my email in by accident.

 

[Hero999]

That depends on the type of motor you're talking about. Universal motors with carbon brushes are easy but squirrel cage motors can't be used with a lamp dimmet.

The question is what sort of motor does your pump use?

If you don't know then it's easy to find you but you'll need to take the pump apart which mightn't be a good idea as you could ruin the seals.

Perhaps you should ask the manufacturer.

 

[jlockham@jlockham.karoo.c]

i am using a SEIO M820 water pump 240v it has an electromagnet incased in epoxy and has a magnetic drive shaft. apparently these type of pumps cant just be turned on and off as they have a tendency to lock up, I was thinking if i could drop the voltage to say 25% and then gradualy increase this to 100% over 3sec then run at 100% for 5sec then gradally decrease to 25% over 3sec run at 25% for 5sec then start again.

**broken link removed**

 

[Hero999]

That's a synchronous motor so you can't change the speed using a simple light dimmer.

The only way to alter the speed is to change the frequency of the power supply. This would require a complex inverter and won't give you a very wide range of speeds.

Reducing the voltage will only reduce the torque until it stalls, then the current draw would rocket until it overheats.

 

[jlockham@jlockham.karoo.c]

do you mean something like this **broken link removed**
ifso could the 555 be replaced by a pic and then programed to produce the required frequency pulses.

 

[jlockham@jlockham.karoo.c]

do you mean something like this **broken link removed** if so could the 555 be replaced by a pic and programmed to produce the required pulse

sorry youll have to copy and paste in to browser post will not stick when i put in a link

 

[Hero999]

That link didn't work even though I coppied and pasted it into my browser's address bar.

I'm afraid it's a little more complicated than a 555 timer.

A bridge rectifier and filter capacitors directly convert the mains to DC, then MOSFETs or IGBTs are used in an h-bridge configureation to produce a modified sinewave for the motor. The frequency and duty cycle are accurately controlled to alter the speed of the motor.

This still might not work because the motor is designed for 50Hz and the harmonics produced could also cause problems.

 

[jlockham@jlockham.karoo.c]

So would adjusting the input frequency not work? I know you can buy a system from the munfacturers to create a wave system so it must be posible to control the speed. The link was to an AC-DC converter it took the pulse from the 555 timer and turned that into AC ranging from 120v-240v at 50hz-60hz i thought if i could modify it to produce a lower frequency that it might work.

 

[Sceadwian]

About 90% of the time because of pump design, regulating the flow using a restrictor valve on the output is actually a better idea than trying to modulate the motor itself, and much much cheaper. As long as you keep the pump within it's operating paramaters.

 

[jlockham@jlockham.karoo.c]

I cant restrict the outlet, apart from it being unsightly the outlet of the pumps are about 2inches diameter and the cost of an adjustable valve of that diameter (and i need 4 of them!) would be alot more exspensive then the manufaturers controller.

 

[Sceadwian]

You're going to waste 125-200% or more of the overall energy usage of the pumps as heat with a motor controller as oppossed to a flow constrictor. And all that heat is equivilant to wear in an electrical system. Keep in mind that energy costs for a given setup are usually a few HUNDRED times higher than the initial cost of the system over it's life. You're designing your system from the wrong end. Energy cost is almost universallly overlooked, and nowdays especially it's important.

 

[jlockham@jlockham.karoo.c]

thanks for all the help, i think i might have to change the design perhaps 1 large pump and 2 electronic control valves could turn them on and off alternately with a simple flip flop

 

[Hero999]

You're going to waste 125-200% or more of the overall energy usage of the pumps as heat with a motor controller as oppossed to a flow constrictor.

I disagree, variable motor speed drive inverters are quite efficient. But I agree that a flow constrictor would be more suitable here because a speed controller probably won't work.

 

[jlockham@jlockham.karoo.c]

thought id take a look inside one of my old pumps all that was in it was a magnetic coil with the AC power wires connected to it. Now im confused i thought synchronous motors ran on DC?

 

[Hero999]

No synchronous motors are AC, that's where the name stems from the rotor is in sync with the rotating magnetic feild.

 

[jlockham@jlockham.karoo.c]

this is what ive foundView attachment Drawing1 Model (1).pdf
surely the ac current must be changing the polarity of the electro magnet attracting and repelling the magnet drive through each half cycle so in theory if the AC Hz was lowered then the drive would turn slower?

 

[Hero999]

That's exactly what a motor speed control inverter does, it lowers the frequency to reduce the speed.

The problem is your motor isn't designed to be operated from a lower frequency than 50Hz. Because the impedance of an inductor is directly proportional to the frequency more current will flow causing it to overheat. It might be possible to use a modified sine wave to keep the current down but there might be issues with harmonics and the motor's phase shifting mechinism not working at the lower frequency.

Normal AC motors won't work from a single phase mains supply, they require a rotating magnetic field to work, otherwise how do the magnets know which way to turn? To get round this problem all sorts of techniques are used, like a run capacitor or shaded pole, the idea is to provide a phase shift and a rotating field. The problem is often these techniques are highly frequency dependant, for example at a lower frequency a start capacitor will have a much lower phase shift which might not be enough to start the motor

 

[jlockham@jlockham.karoo.c]

So how can the pump which i have drawn create a rotating magnetic field if the plates from the magnet don't continue all the way around the drive.

 

[Hero999]

The electromagnet doesn't have to completely surround the permanent magnet rotor, all that's really required is opposite phases at either side of it.

 

[jlockham@jlockham.karoo.c]

So a synchronous motor creates a magnetic field that rotates around the magnetig drive like pulling a magnet round a compass the needle follows the magnet.
A motor like mine create a magnetic field of changing poles over the magnetic dirve like placing the magnet over the compass and flipping it over the needle will swing round to the opposite end each time you flip the magnet?
if you changed the frequency of the magnetic field then the motor would rotate half a turn and then stop until the magnetic field shifts the other way
sorry for sounding dumb but ac motors arnt my thing give me dc anyday