RE: chipdir=L 3-Phase motor

[Jeffrey Meade]

Original question:   Hi all   I have a 3-phase motor witch I'd would like to run with only 1 phase, so I need a capacitor to "generate" the missing phase.   How can I determine the value of the capacitor?   The motor has following type label:   220/380V,  1,3/0,75A 0,35PS, 0,25kW 50Hz cos phi 0,70 1400 U/min   Any ideas?   tia Edi Im Hof       It might be less expensive to have your utility install real three phase service, than to produce your own pseudo third phase.     Operating a three phase motor on single phase requires using an idler motor to achieve the best possible third pseudo leg.  Said idler motor will not operate any machinery, but should have a flywheel. The idler motor should be of the old cast iron style design, with emphasis on heavy duty massive armature.  Start the idler motor with at least a 40 mfd AC running capacitor. AC capacitors come in two grades, starting duty and running duty. Using a running duty capacitor to start means extremely reliable operation. You can use a hand operated normally open pushbutton to energize the starting capacitor until the idler motor rotates at full speed. Leaving the capacitor switched on will heat the capacitor and it will last about six months. Once the idler motor is rotating at full speed, it is okay to switch on the work motor that you want to operate. If said idler motor has a massive flywheel, there will be no appreciable reduction in idler motor speed when any load is applied. Your work motor can achieve about 70.7% rated HP. You can switch on additional work motors for additional equipment and they will all contribute to the third pseudo leg. I wired a machine shop with one 5 HP three phase idler motor, and five 3 HP work motors, and one 5 HP work motor. They all can be used at the same time, after the 5 HP three phase idler motor is started. Their 5 HP lathe can bog down if it is being used hard and if all the other motors are being worked hard. The limiting factor is the incoming current limit of the single phase utility power.     Another machine shop that I service has an air compressor motor that incorporates a pseudo third phase capacitor filled box. They do not use an idler motor. I replace their dozen running grade capacitors about every six months.     I too am curious to know the capacitor selection formula.  You asked about a formula to select the running capacitor, and I can only suggest that an ammeter will tell you when the current is correct.  That is, you add capacitors to achieve the rated current for the pseudo leg.  Be careful that you do not exceed the VAC rating of the AC capacitor. You can series several capacitors to increase the withstand VAC, but this reduces the capacitance. You can parallel several capacitors to increase the capacity, if their VAC rating is substantially greater than the operating VAC.     I have built and sold three timer operated self-starting three phase idler motor derived pseudo third phase industrial panels. The electricians who installed these panels, and wired several 3 phase industrial machines, also installed one 5 HP idler motor to produce the third pseudo leg. Each of my panels incorporates a three phase loss detector and low duty cycle industrial contractors. If the three phase power is switched on at the beginning of the business day and used all day, a design that calls for low duty cycle industrial contactors will save a lot of money. If the 3 phase power is to be switched on and off throughout the business day, then use Nema rated high duty cycle industrial contactors, which are reliable and which are expensive.   Remember to have your utility quote the cost to install real three phase service, before you devise a costly means to produce the third pseudo leg.    Jeffrey Meade 713 Seventh Street NE North Canton, Ohio 44720-2016 USA [EMAIL PROTECTED] Tel: 330.497.0736