jdbrandt said:
You may also want to use an analog output from your blower drive to use blower horsepower as a measure. I don't have the curves for a PD blower, but I'll be[t] the manufacturer can say how many amps equal how much air. Remember, a deadheaded impeller type fan moves no air, and hence, draws very little current. I don't know what the rules are for a PD.
I can help there.
A PD blower (Positive Displacement) is essentially a constant volume flow rate device at a constant rpm. I never use a meter on PD blowers for air flow - I just calculate the flow based on the following formula:
CFM = CFR x (rpm - slip)
CFM = Cubic Feet per Minmute, of course, and is the inlet side volumetric flow rate
CFR = Cubic Feet per Revolution, also known as the displacement of the blower
rpm = actual blower drive shaft speed, which may not equal motor speed
slip = slip rpm, which is the extra turns the blower impellers need to make up for leakage between impellers and housing
So, if you know the ratio of blower speed to motor speed, and you get the slip and CFR from the manufacturer's data you can get the flow rate of the blower. Unlike a centrifugal fan or blower a PD cannot be dead-headed - it will continue to push air. If the discharge is plugged the pressure will rise until something blows. That's why you always need a pressure relief valve on PD blowers!
You are right that the pressure is related to amps, but flow isn't. This again is different from centrifugal fans and blowers, where amps is roughly proportional to flow rate. For a PD blower:
bhp = (FHP x rpm/1000) + rpm x CFR x dP x Fg
bhp = blower shaft horsepower
FHP = friction horsepower = losses for bearings gears etc. and is usually expressed in bhp per 1000 rpm
dP = pressure difference from inlet to outlet
Fg = gas horsepower factor, usually expressed as bhp/psi/CFM
FHP and Fg are also available from the blower manufacturer.
So, for a given blower at a constant rpm the motor current draw will increase roughly proportionally to discharge pressure. At a constant discharge pressure the blower will draw roughly the same amps regardless of speed and flow rate, because torque is related to amps and torque is roughly proportional to pressure.