circuit uses the combined inductance of a Variac and the motor, along with a large uF value capacitor, to alter the phase angle. As the motor's own inductance value affects things, the only way to get the right value is by trial and error. I found a 124 uF of capacitance with my 2 Hp motor (3000rpm) and a 34uF with the 0.5 Hp motor (1500rpm) was needed.*** Update 16th Oct 2010 ***John
Freau, said in a recent TCML
posting that he thinks that a 120v motor (60Hz USA & others) may require up to 4 times as much capacitance as a 240V 50Hz motor would. So it seems there are many factors at play when deciding on the capacitor size required, the most important one being the distribution system in use where you live (240v 50Hz or 120v 60Hz).The
best way to ascertain the capacitance value is to start with around 20 to 30uF and the Variac set to zero, so it is not introducing any inductance into the circuit at this stage. You then need to slowly increase the Variac and at the same time monitor the voltage across the motor looking for a rise.
You only really need to have up to a maximum of a 10 volt rise at some point after which the voltage should then drop down again as you further increase the Variac. You need to go slowly though as it is all to easy to get a potentially damaging 30 volt rise or greater if your capacitance value is too large to start with. My 0.5Hp motor actually uses a 15v rise without suffering however.
If you find you get no rise at all you will need more capacitance. Too much voltage rise means the capacitance value is too large. With both my motors I found steps of around 5 uF in value can make quite a difference - but other motors may differ.You
often find that even with a capacitance value which is correct (i.e. giving a modest 5 or 6 volt rise) that as you approach maximum inductance on the Variac's scale the motor will loose sync'. But as long as you have around 60 to 70 degrees of electrical phase adjustment for a 100bps or 40 to 50 degrees for a 200bps you will probably find that is sufficient to operate with initially.After
initial setting up you can then move the rotor's position on the shaft so it is very near its optimum position prior to starting. Trying to start the motor with some phase adjustment already "dialled-in" can make it draw a high starting current, and also cause a slow run-up to normal speed.
I then found that when you switch the power off to the SRSG it exhibits a severe self braking effect, where you can actually hear the motor noise being rapidly de-accelerated. This is caused by the phasing circuit and can, I would imagine, damage the motor over time. I found a multiple switch that disconnects both the mains feed to the controller and the feed to the motor at the same time solves this.However
have some bleed resistors across the capacitors though if you use this method. Otherwise the exposed metal pins of the plug will have the capacitor's stored voltage across them. On one occasion I pulled my phase controller's mains plug out and laid it onto the metal work-top and there was an almighty "crack" as 124 uF charged to 250v discharged itself into the bench!
Also don't run the circuit with the motor disconnected either, unless you want some smoke from your Variac. That is the reason for the fuse in series with the capacitor.