PWM a Pump?

[erice1984]

Would it be bad to PWM, basically control pressure on a water pump? given it is supposed to run constantly at 150psi?

or would you control the current into it to control the pressure?

[skimask]

Would it be bad to PWM, basically control pressure on a water pump? given it is supposed to run constantly at 150psi?
or would you control the current into it to control the pressure?
I'm guessing some sort of A/C motor/pump assembly. Sounds to me like throwing PWM at any kind of motor not designed for it would be awful hard on it, especially an A/C motor, 'cause basically, they don't usually really work that way.

If I had to go that route, I'd use some sort of pressure vessel with a regulator on the output, like a well pump for a house might have.
Regulator set at 150psi, run the pump until you get 180psi (or whatever), turn off the pump, wait until the pressure dropped to 151psi (or whatever), turn the pump on, wash, lather, rinse, repeat... The pressure vessel maintains the output.
But...in reality, it really sounds to me like something that would just be a heck of a lot easier and probably a lot cheaper to buy off the shelf at your local hardware store. And 150psi sounds like a LOT of water pressure. My house water pressure only runs 65psi, and I've got the pressure cranked up on it.

[Melanie]

Yes, you can use PWM to control a pump (although indirectly), but it's not as straightforward as you think.

skimask has got close. Use the pump to maintain pressure in a hydrosphere (pressure vessel) at a higher pressure than you need (say 180 or even 200 psi). The pressure vessel feeds a Pressure Reducing Valve (PRV) which is set to your constant pressure. So if you need 150psi, you set the PRV to deliver that, but the high-side (input) to the PRV must always be greater than the set pressure. The pressure vessel is there to take-up the slack (like a Capacitor), so if you need a small amount of liquid (say to fill a kettle) the pressure vessel delivers it rather than kicking-in a pump to deliver half a glass of water. The secret is to install the biggest pressure vessel that space and money allows. The smaller the vessel, the more your pump is going to run. You then also set the largest differential (the difference between the On and Off pressure settings running the pump). So in the above example, pump-ON at 155psi and OFF at 200psi is great.

If you're feeding water to the top of a building several stories high, then 200psi is quite normal. Usually, you need about 15-20psi per floor. If your roof tank is 10 floors up, then yes, 200psi.

As another (expensive) option, you can get PRV's that can be modulated by a PWM signal. That's not really for a constant-pressure application, but we use such devices in the Laboratory to give a broad range of pressures (all adjustable by a PIC) for testing and calibration of pressure sensors and transducers.

[erice1984]

Cool, thanks for the thoughts guys, BTW the motor is DC driven, automobile application. Just trying to explore different ideas for some things.

[skimask]

If you're feeding water to the top of a building several stories high, then 200psi is quite normal. Usually, you need about 15-20psi per floor. If your roof tank is 10 floors up, then yes, 200psi.
Aye...now that you bring that up, good point. Not a lot of tall buildings around where I live, in fact, nothing over 5 floors.

[Melanie]

Curious what your automobile application is?.... the only thing mobile I can think of that requires water at 150psi is a Firetruck...

[erice1984]

water injection, its a technique used to cool the cylinders, rather than fuel dumping (using gasoline to cool the cylinders)