Stepped Compressor Speed with VFD?

[bad_idea]

I have a Quincy 325, powered by a 5HP 3PH motor, run off of a Huanyang 7.5kw VFD. I would like to install two pressure switches to have two speeds for the compressor. For 80% of my uses (home garage), the lowest speed the pump can turn is sufficient.

My thought is to have one switch cut in at 125 PSI and cut out at 175 PSI running the motor at ~30HZ, and the second switch cut in at 100 PSI running the motor at 60HZ.

I am very green to the controls side of a VFD. Anyone have some first hand experience, and can guide me in this? Thanks all.

 

[dutchgray]

It's most likely a bad idea as you won't get full torque at 30Hz from the motor and it will likely stall, or if you turn the power up enough where it will run at 30Hz the motor will most likely overheat.
Unless you have a VFD rated motor with an independently powered cooling fan.

If you were going to do it I would consider a pressure transducer of some sort wired into the VFD and then set the thing up so it could increase the motor speed as pressure increased, rather than having a set step.

 

[Firebrick43]

It's most likely a bad idea as you won't get full torque at 30Hz from the motor and it will likely stall, or if you turn the power up enough where it will run at 30Hz the motor will most likely overheat.
Unless you have a VFD rated motor with an independently powered cooling fan.

If you were going to do it I would consider a pressure transducer of some sort wired into the VFD and then set the thing up so it could increase the motor speed as pressure increased, rather than having a set step.

With a vfd that has vectorless torqu you get full torque at anything below 60hz. It only starts decreasing torque above 60 hz.

You are possibly right about cooling, it depends on design and insulation class of the motor (found on the name plate)

Ideally you would adjust your sheave size on the motor(decrease) or increase size on the pump so the motor at the new lower pump speed the motor is turning at 60 hz and turning at say 75hz to drive the pump at its original rpm. A 1725 rpm motor can be driven by a vfd safely to 120hz.

Issue with that is it will probably need to be the pump pulley get larger as the motor sheave is typically already to small for belt lack of belt wrap and making it smaller only worsens the situation

Most VFDs don’t have logic built in to use a pressure transducer output for variable speed. They would have to be processed by a mini PLC or something similar and output a 0-10v signal for the VFD.

A stepped speed change can however be easily done by the vfd using pressure switches.

 

[cvairwerks]

Bad idea, as turning it too slow will burn up the crank. The 325 is a pressure lubricated pump and requires a minimum rpm for the pump to work and lube everything. Run it a a single speed, but if you want to set the cut out lower, that works better if you don't need the full 175 psig.

 

[dutchgray]

With a vfd that has vectorless torqu you get full torque at anything below 60hz. It only starts decreasing torque above 60 hz.

You are possibly right about cooling, it depends on design and insulation class of the motor (found on the name plate)

Ideally you would adjust your sheave size on the motor(decrease) or increase size on the pump so the motor at the new lower pump speed the motor is turning at 60 hz and turning at say 75hz to drive the pump at its original rpm. A 1725 rpm motor can be driven by a vfd safely to 120hz.

Issue with that is it will probably need to be the pump pulley get larger as the motor sheave is typically already to small for belt lack of belt wrap and making it smaller only worsens the situation

Most VFDs don’t have logic built in to use a pressure transducer output for variable speed. They would have to be processed by a mini PLC or something similar and output a 0-10v signal for the VFD.

A stepped speed change can however be easily done by the vfd using pressure switches.

Every VFD I have played with has the standard 4-20 mA inputs that industrial controls use, as well as 0-10V inputs, those would not be hard to couple to a pressure transducer (with an interface unit to convert to 4-20 mA which is common)

I still maintain its a bad idea to run the motor slower than intended, non VFD rated motors do not like running at significantly lower frequencys under high loads in my (fairly limited) experience.

 

[micromind]

This would be easy if the VFD has 'preset speed' ability. Some do, some don't.

You'd need to program a digital input to say, preset speed 1 then program that speed to what you want. Program another digital input to another preset speed and program that speed to what you want.

Some VFDs can be programmed so that one preset speed will overrun the other but most of them will go into fault if more than one preset speed is active.

At slow speeds, there's a chance the VFD will fault out on 'high DC bus voltage'. This is because after the compression stroke, the remaining air in the cylinder will drive the motor forward to more than its present speed, it'll turn into a generator and the only place the excess power can go is the filter capacitors. If the voltage gets too high, the VFD will fault in order to keep the capacitors from exploding.

Also, as noted motor cooling will likely be an issue as will compressor lubrication.

 

[bad_idea]

Thanks for all of the input guys. I am confident the pump would be fine, it is rated for 400-900 RPMs. The pulley on the motor is for the max of 900 RPMs. My thought was to run it at the lower end of it's range to save on noise levels. BUT, not willing to deal with burning the motor up.

I'll run the unit at 60hz and look into getting a smaller pulley for the motor. Will be contingent on if I find myself using the higher CFM output of the unit.

 

[F-22]

Pulley is a better choice, gear it up and rather use the VFD to spin it a bit faster if you maybe ever need that. Cooling is a very common issue for motors at low speeds, but it won't mind spinning a bit faster (as long as it is reasonable).

 

[u3b3rg33k]

Every VFD I have played with has the standard 4-20 mA inputs that industrial controls use, as well as 0-10V inputs, those would not be hard to couple to a pressure transducer (with an interface unit to convert to 4-20 mA which is common)

I still maintain its a bad idea to run the motor slower than intended, non VFD rated motors do not like running at significantly lower frequencys under high loads in my (fairly limited) experience.

I disagree. a compressor is a constant torque load, so driving it within rated slip would be fine. trying to get more torque out of it than it can do would present issues, but a properly commissioned VFD will shut down before it cooks the motor. what have you seen fail?

This would be easy if the VFD has 'preset speed' ability. Some do, some don't.

You'd need to program a digital input to say, preset speed 1 then program that speed to what you want. Program another digital input to another preset speed and program that speed to what you want.

Some VFDs can be programmed so that one preset speed will overrun the other but most of them will go into fault if more than one preset speed is active.

At slow speeds, there's a chance the VFD will fault out on 'high DC bus voltage'. This is because after the compression stroke, the remaining air in the cylinder will drive the motor forward to more than its present speed, it'll turn into a generator and the only place the excess power can go is the filter capacitors. If the voltage gets too high, the VFD will fault in order to keep the capacitors from exploding.

Also, as noted motor cooling will likely be an issue as will compressor lubrication.

I don't see this as a problem at all.
1: airflow is directly proportional to fan speed. 1/2 RPM, 1/2 power, 1/2 airflow. perfect scaling.
2: compressor lubrication. set your min/max speeds within the rating of the compressor. problems staying solved.

Thanks for all of the input guys. I am confident the pump would be fine, it is rated for 400-900 RPMs. The pulley on the motor is for the max of 900 RPMs. My thought was to run it at the lower end of it's range to save on noise levels. BUT, not willing to deal with burning the motor up.

I'll run the unit at 60hz and look into getting a smaller pulley for the motor. Will be contingent on if I find myself using the higher CFM output of the unit.

I've got a compressor at work that's VFD driven. literally a standard WEG on it (not even W22), and it's got 600 hours running in the 30-50Hz range. zero issues.

 

[bad_idea]

I recently got the compressor up and running. I have it in a separate room out in the detached garage/shop. I insulated the wall it is against with rock wool. Have not insulated or finished the other walls or ceiling yet. With the door to that room closed, it is very tolerable in the rest of the garage. I think I will just leave it running at 900RPMs (the current pulley setup and the pump's max rpm). Thank you for your input though.

 

[Sleeper]

I disagree. a compressor is a constant torque load, so driving it within rated slip would be fine. trying to get more torque out of it than it can do would present issues, but a properly commissioned VFD will shut down before it cooks the motor. what have you seen fail?

I have seen the reference to a compressor being a constant torque load a few times on here and I’ve also seen claims that the required torque and amp draw increases as the compressor reaches higher pressure. I’m trying to understand if there is some way that these are both correct or is one of these claims just wrong?

 

[Firebrick43]

I have seen the reference to a compressor being a constant torque load a few times on here and I’ve also seen claims that the required torque and amp draw increases as the compressor reaches higher pressure. I’m trying to understand if there is some way that these are both correct or is one of these claims just wrong?

Constant torque doesn’t necessarily have any reference to the load getting higher as pressures go up.

It’s more of a term for positive displacement pumps that torque load at a given pressure is unaffected by the speed that the pump turns, only the HP required is affected by the speed.

Pumps that are not positive displacement like scroll or centrifugal pumps are not constant torque and several factors, speed being just one of those factors, can affect the torque required.

The torque required is rarely linear either where positive displacement pumps can be very linear until you start getting at the extremes of the pumps performance parameters.

 

[u3b3rg33k]

I have seen the reference to a compressor being a constant torque load a few times on here and I’ve also seen claims that the required torque and amp draw increases as the compressor reaches higher pressure. I’m trying to understand if there is some way that these are both correct or is one of these claims just wrong?

Firebrick43 has it right.

basically you need rated torque to turn-over a piston compressor. this is generally a non-issue if you have some kind of working unloader on the system, and your tank check valve doesn't leak. if either of those isn't working properly, you can have issues starting, VFD or not. pushing against full tank pressure can be a tall order!