Chuck Yung
EASA Technical Support Specialist
Rather than moving DC frames, strip heaters, etc. to the test panel for testing, wouldn’t it be convenient to have a portable and variable voltage supply? There are quite a few EASA members who have built a simple voltage supply using readily available components.
Not only does it save timeless time spent using the crane to handle heavy parts – but such a voltage supply can be useful for field service jobs. This article will describe how to build it, provide a parts list, drawing of the complete unit, wiring diagram, and even suggest economical sources for some of the items.
Start with a vari-ac
The heart of the power supply is a vari-ac — a variable AC power supply — similar to that shown in Figure 1. The bigger the vari-ac, the more useful it is, but the trade-off is ease of portability. A 20-amp vari-ac is a practical choice for most uses. If you are on a budget (who isn’t?) you can often find them at places such as eBay for less than $100 U.S. While bargains are great, you won’t regret spending a little more money to purchase a unit with a higher current rating. While a 5-amp unit is adequate for testing most shunt fields, a 20-amp unit is more versatile.
A box to contain the vari-ac and other components is needed to protect the components. Some techs use wood, others use standard metal NEMA enclosures. One significant advantage of the metal enclosure is that it is easier to achieve proper grounding. See Figure 2.
Suitable enclosure
Ebay or surplus dealers often have suitable enclosures for $50 U.S. and up. The box is more than just an enclosure; it must also provide adequate structure to mount the vari-ac, fuse holder, switch and meters.
You will also need a 2-pole double-throw switch, an AC voltmeter, a fuse holder (sized to protect the vari-ac), and a bridge rectifier, 2 alligator clips, a standard plug for the outlets in your region, and enough wire to connect everything.
Use a short length of industrial duty cord, such as S.O. cord, with a standard plug appropriate for your region, connected to the vari-ac input. The ground wire should be secured to the inside of the metal case.
Connect the switch so the vari-ac output may be routed through the rectifier (for DC output) or bypass it (for AC output). The center position of the switch is “off” with “AC” to one side and “DC” to the other position.
Connect the fuseholder between the vari-ac and the switch. The rectifier is connected as shown (Figure 3) on the bridge rectifier (AC in, DC out.) Note: The DC output voltage will be approximately 90% (0.9) of the AC input voltage. Depending on the vari-ac you obtain, and your needs, this can be constructed as a 120vac or 240vac unit.
Output cord
The output cord can also include a ground which can be connected to the equipment being energized or tested. You may find it more convenient to install a separate ground lead instead. Alligator clips for both output leads increase convenience, but size them for higher current than the rating of the vari-ac. Be sure to use the correctly sized insulated boot for the alligator clips – use red for positive and black for negative, to minimize confusion.
Include a carrying handle – a drawer handle from a local hardware store works well for this. You can even add a carrying strap if you like, so the box can be slung on a shoulder. As a safety matter, one addition I suggest is a separate ground lead with alligator clamp so that the operator can securely ground the box (if it is metal) to a known ground before use. Not all of our customers’ outlets are wired correctly, hence the extra safety.
Use this portable power supply to perform AC and DC drop tests of DC field frames and synchronous rotor poles, to test strip heaters, polarity test shunt field coils, check interpole polarity, etc. Since it’s portable, it can be used to perform these and other tests in the field as well as in the service center. Another on-site use is to flash the fields of self-excited generator fields that have no or low residual magnetism.
Related Reference and Training Materials
Print