by Tony » Tue Dec 26, 2006 4:48 pm
Once an automatic test pressure control system becomes this sophisticated, there are some pretty powerful advantages of using a large single three phase blower run from a commercial variable frequency drive.
* The pressure/flow characteristic of a vacuum cleaner motor changes very steeply, particularly the two stage higher pressure vacuum motors that most people use. As you make a series of increasing flow measurements, the power to the motors needs to be constantly adjusted. The pressure/flow characteristic of a large diameter single stage centrifugal blower will be almost flat. These things develop almost constant pressure over a very wide flow range, so any automatic test pressure control system has much less to do.
* Ac/dc brush type motors are very sensitive to the applied voltage, in fact that is how the speed is varied. But they are also very susceptible to sudden mains supply voltage variations, and the speed will also change substantially with mechanical load. Motor Rpm will change with airflow, and so will pressure. Completely block the flow to any vacuum cleaner, and hear the motor speed up. Motor performance also changes with motor winding temperature, which will vary all over the place, because it is cooled by the airflow, which is changing.... A three phase induction motor driven from a variable frequency drive is by design completely immune to mains voltage variation. But more importantly the SPEED is controlled by the supply FREQUENCY, the motor voltage within reason is far less critical. A three phase motor hardy changes in running speed from zero load to flat out maximum horsepower, it sure will draw a lot more current, but the running speed remains virtually constant. So a three phase motor and VFD system will have a rock solid stable operating Rpm, whereas a vacuum motor will be wandering up and down in Rpm all over the place with changes in internal temperature, flow, and mains voltage.
* Closed loop test pressure control is especially easy with a VFD, because the better ones already have this feature inbuilt. It only requires a manifold absolute pressure sensor from an EFI vehicle, (or even better, a commercial digital manometer), and a control knob to set the test pressure. With anything else, you get to develop your own closed loop feedback system.
For a basic flow bench, vacuum motors are very simple and cost effective, with some sort of manual flow control.
Once you start thinking in terms of high resolution data acquisition, and automatic control of test pressure, and other automated measurement techniques, the old vacuum cleaner motors are less than an ideal source of bench airflow. They are just not going to be stable enough to get the sort of repeatable numbers that the rest of the system may be capable of.
While a high quality variable frequency drive will not come cheap, the three phase motors cost almost nothing. The VFD will have its own internal closed loop speed control system, and quite likely a serial computer communications interface, where you can program the required test pressure into it. It all comes in one attractive box ready to go, and the final results will be far better.
The vast reduction in noise level of a large diameter three phase blower is an added bonus, even flat out it will be as quiet as a window airconditioner, but at more normal operating flows and pressures it will be almost silent. No possibility of burning out the three phase motor or wearing out the bearings either. Vacuum cleaner motors are built to have a rather limited life. This becomes especially true at high test pressures, where the motors will often be running at full flat out power, but with reduced (or even zero) internal cooling airflow. A three phase motor will always have its own independent enclosed fan and cooling system, so it can run flat out continuously with the bench flow completely blocked.
Also known as the infamous "Warpspeed" on some other Forums.