Join Date: Dec 2002
Location: NE Ind
Thanked 0 Times in 0 Posts
Re: SCR\'s as rectifiers? - building a DC welder
The gating circuit looks ok. I do not know much about light dimmer triacs but I assume they adjust the firing angle the full range. Full range of firing angle would be very handy. If you fire at 0 deg as the ac wave is going up you will get full current available. If you fire at 90 deg when the wave is at the peak you will get half the available current. If you fire at 180 deg when the wave is crossing 0 the SCR will reverse bias and you will get 0% of the avail able current. I think you could use that circuit as a good starting point and monkey dink with it and get it to work.
That sounds right about Thyristors and SCRs being the same component. I don't remember if triacs make SCRs or SCRs make triacs, but if you looked it up I'm sure you are correct. After a few beers I will burn this out of my brain yet agian and go back to calling them all SCRs.
I do not want to sound mean, but you are incorrect. I know a smidgen about DC drives. The speed regulation of a DC drive is through adjustment of the armature current. The armature firing angle is adjusted by an error signal. Which is generated by the action of the motor speed feedback on the speed refrence to the drive. Although most digital DC drives have SCRs for the field current. These SCRs are used to control the field current to a steady amount, usually the value that is listed on the motor nameplate. The field firing angle will adjust to keep the field current constant at that setpoint. Most changes in field firing angle are due to changing resistance of the field windings because of heat or action of the armature current on the field windings. The only time the field is lowered to adjust speed is if a drive is being run in field weaking mode. The field is lowered a prescribed % resulting in lower CEMF. This makes the armature current higher for any given armature firing angle. Higher armature current results in higher RPM/torque. Field weaking is mainly used when a motor is being run above base motor nameplate speed or extremly hard starting loads. Either way its hard on motors. I have never commisioned a motor/drive combo to run in this manner but it is done. It is true a DC motor can run away if the field is disconnected or dropped, but most modern drives have field detection circuits that fault the drive out in this event.