Full wave 3 phase rectifier. - Page 1
Forum section: electronics / projects

author: geggi1
author_rank: Frequent Contributor
timestamp: January 13, 2021, 09:05:23 pm
content: Hi all!I got a project at work where we need a DC current of about 500A but the voltage is low about 50V.The plan is to use a 3 phase transformer and rectify the output AC. The transformer will be controlled by a VFD to keep things simple. The VFD will probably be controlled from a PC to also be able to monitor the power. Output power does not have to be very accurate measured as long as its less than a certain level because the DC will be used for heating purposes.From college I remember that there where certain factors between the DC current and AC current and also between DC voltage and AC voltage, but college where close to 30 years ago and the factors forgotten the only thing I remember is that the equations where starting done form RMS values.Can anybody help me getting these factors or show me where to find them?

author: Benta
author_rank: Super Contributor
timestamp: January 13, 2021, 09:27:34 pm
content: Three-phase rectification is not difficult, this page might help you further:https://www.electronics-tutorials.ws/power/three-phase-rectification.htmlI'm more worried about your use of a VFD for this. Mains transformers don't behave very well at high frequencies, and the VFD will operate with a PWM carrier frequency of at least 20 kHz.The iron losses in the transformer will be immense. Have you considered this?Also, 25 kW is quite a lot of power.

author: trobbins
author_rank: Frequent Contributor
timestamp: January 13, 2021, 09:46:52 pm
content: Yeh, the typical approach is a phase controlled SCR bridge, but the actual setup depends on how much control range is needed (ie. 0-100% or say 70-100%). Full control is normally done with a 5-limb transformer and double-star rectifier, whereas battery charger style range is just a simpler 3-limb transformer and full bridge rectifier. The 500A output requirement will typically mean large air-cooled heatsinks with stud or puck or flat-base module style semiconductors. Input PF won't be good. Current monitoring can be done with primary side phase CT's summed in a loading resistor.

author: geggi1
author_rank: Frequent Contributor
timestamp: January 13, 2021, 09:57:35 pm
content: The setup is only going to be used for a test for a few hours.Because the DC is going to be used for heating so power quality is not important.Our company have experience running transformers form a VFD so that will not be a problem.The transformer is probably going to be something purpose build or a modification of an existing transformer with tapping on the secondary side.

author: drvtech
author_rank: Regular Contributor
timestamp: January 13, 2021, 10:15:33 pm
content: Assuming the heater assembly has at least a little thermal mass then you would probably be better off using a burst-fire technique to control the power. That way you're only dealing with low frequencies, your transformer will be perfectly happy and, as the switching will be occurring at the zero crossing, you won't have significant EMC problems.

author: Circlotron
author_rank: Super Contributor
timestamp: January 13, 2021, 10:22:26 pm
content: If you are using the low voltage output for heating, why do you need DC? What are you using for heating elements? Won’t they tolerate AC?

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 13, 2021, 10:44:54 pm
content: The DC current will be approximately 1.732 times higher than the three phase AC current.The DC output voltage will be approximately 1.414 times higher than the RMS AC input voltage with a full wave rectifier.Running a transformer with a vfd is quite doable in V/Hz mode, the carrier frequency of the vfd should be raised to as high as it will go, the higher the better as this moves heat generation from the transformer to the drive which is easier to cool, up to a point. Oversizing the vfd and transformer by 25-30% would be ideal if it will run for any length of time.You might need to use closed loop control of the power, depending upon how stable you need the output to be and the variation in output load with respect to heating.

author: jmelson
author_rank: Super Contributor
timestamp: January 14, 2021, 12:39:27 am
content: Quote from: geggi1 on January 13, 2021, 09:05:23 pmVFDs do not produce varying voltages. They rectify the mains and then chop that at some KHz rate. So, for a 240 V VFD, you will get 340 V square waves ofvarying duty cycles. They depend on the huge inductance of induction motors to produce roughly sine wave currents in the motor windings. Transformers have relatively low mutual inductance, so the peak voltage in will be the peak voltage out, divided by the winding ratio.What you want to do is in THEORY possible by adding large inductors on the input of the transformer, but that may cause problems with regulation.There might be a better way to do what you want, but you haven't given us a lot of information.Jon

author: trobbins
author_rank: Frequent Contributor
timestamp: January 14, 2021, 12:50:04 am
content: Another option would be to beg/borrow/steal a 3ph variac on the input (that's just a 50A phase current rating if you were on 415V 3ph). Then just need to cobble together some diodes for a bridge rectifier, and increase your AC feed CB rating to manage the variac in-rush

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 14, 2021, 01:47:49 am
content: Just use a few welding power supplies in parallel?