We use transformers for all types of jobs. There are step-up transformers, step-down transformers, potential transformers, current transformers, auto transformers, buck-boost transformers. Each is designed for a specific use.
Basic design involves a magnetic core with primary windings and secondary windings. The number of primary windings to secondary windings (winding ratio) determines the voltage ratio. A 2-to-1 ratio would give us 120V and 240V, 240V and 480V, etc. Transformers have inherent losses built into them. Designers/engineers try to come up with new ways to reduce these losses. One type of loss is eddy current. Ferromagnetic cores are good conductors and constitute a single short-circuited turn throughout its entire length. Eddy currents flow along the core in a plane normal to the flux (magnetic field developed from the applied voltage and current) and create resistive heating. Heat equals energy loss. Eddy current losses are a complex function of the square of supply frequency and inverse square of metal thickness. Most designers now utilize thinner, insulated metal plates laminated together to form the core in order to reduce the effects of eddy currents. By using this design, however, there is another phenomena created. Magnetostriction is the effect of magnetic flux in a ferromagnetic core that causes the expansion and contraction with each magnetic cycle. This is what causes transformers to hum.
Autotransformers utilize a single winding with two end terminals and one or more terminals at intermediate tap points. Primary voltage is applied across the two end terminations and the secondary is usually connected across one end terminal and an intermediate tap. These transformers are cheaper to manufacture but are not as safe as separate primary and secondary windings. Another style utilizes exposed windings in the intermediate area and uses a brush to make the secondary connection. The brush can be moved up or down the exposed section to increase or decrease voltage output. A rheostat is an example of an autotransformer.
Leakage transformers have a significantly higher leakage inductance. This loose coupling between primary and secondary provides an inherent current limiting effect. This makes it possible to create a short on the secondary side and not cause damage to the transformer. Doorbell transformers are an example of this design.
Current transformers and potential transformers, also known as instrument transformers, are used for measuring current and voltage in electrical power systems, and for power system protection and control. These transformers are used where it is impractical or unsafe to use conventional meters to measure voltage or current due to the high values present. Current transformers measure current in a circuit without being electrically connected to that circuit. It consists of a core, usually circular or rectangular, with a single set of windings and two end terminations. The circuit to be measured is routed through the center of the core, and as current flows through the circuit it induces a current in the CT which sends this to either a meter or control device. A potential transformer is designed to be connected in parallel with the circuit to be monitored to provide a consistent value (proportional to circuit values) that can be measured or controlled. These transformers impart a very small load to the circuit, but provide a means to measure the voltage safely and accurately.
Friday, January 21, 2011
Have you ever been drilling with a holesaw and just when the pilot bit breaks through the teeth on the holesaw grab and snap your pilot bit? Well here is a simple trick to help eliminate that. Place a 1/4" X 1 1/2" fender washer over the pilot bit for holesaws up to 1 1/2" (1 1/4" conduit size). When the pilot hole is drilled, the teeth on the holesaw grab the washer, spinning it instead of grabbing. For larger holesaws you can stack 1/4" X 1 1/4" fender washers until there are 2 washers protruding past the teeth on the holesaw.
Submit your own tips and tricks.
Submit your own tips and tricks.
Posted by Norman W at 9:37 AM
Tuesday, January 18, 2011
The new 2011 NEC is out. Hopefully everyone ran out and purchased one for themselves. This year they came with a free .PDF version that can be downloaded at their website, www.2011NECOFFER.ORG. As everyone knows, the code is released every 3 years and it always has tons of changes. I thought I would discuss some of these. I will do more each week so as not to make this one long boring read.
I want to start off with Section 110.11. The new requirement is to protect equipment identified for indoor use, dry locations, or damp locations from damage DURING construction. So don't be hanging panel boxes before the building is dried in unless you have some way to protect it.
110.24 (A) Field Marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum fault current. The field marking shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved.
220.5 (B) Fractions of an Ampere. Calculations shall be permitted to be rounded to the nearest whole ampere, with decimal fractions smaller than 0.5 dropped.
Doesn't seem like that big a deal, until you get an inspector that argues you have exceeded the amperage limits. Suppose you calculate 16.4 amps for a 20A circuit, now you can drop the .4 and use the 20A circuit.
250.24(C)(3) Delta-Connected Service. The grounded conductor of a 3-phase 3-wire delta service shall have an ampacity not less than that of the ungrounded conductors.
You may not derate in this situation.
250.30 (A)(2) Supply-Side Bonding Jumper. If the source of a separately derived system and the first disconnecting means are located in separate enclosures, a supply-side bonding jumper shall be installed with the circuit conductors from the source enclosure to the first disconnecting means. A supply-side bonding jumper shall not be required to be larger than the derived ungrounded conductors. The supply-side bonding jumper shall be permitted to be of nonflexible metal raceway type or of the wire or bus type as follows:
So if you install a transformer in a building and the disconnecting means is not located in the transformer enclosure then you must install a supply-side bonding jumper along with your ungrounded and grounded (neutral) conductor. Do not forget the rest of the requirements in Article 250 regarding separately derived systems.
More to come.....
Posted by Norman W at 2:13 PM
Sunday, January 16, 2011
I'd like to pass on an experience we had recently at one of our new projects. We had just finished installing the parking lot pole lights and were power checking them. As soon as the breaker was turned on it tripped out. The electrician assigned to the task of troubleshooting decided to start at the panel checking for shorts. He then moved to the lighting contactor, then the electronic time clock before he moved to the first pole. He spent 4 hours taking off covers, checking, replacing covers and moving on before he finally found the problem in the first light pole. A good place to start would have been at the first pole. He would have isolated the interior section of wiring from the exterior section. It would have cut the circuit almost in half. Even if the problem had been in one of the other poles he wouldn't have wasted all that time removing and replacing covers.
Troubleshooting should be looked at in a logical manner. What is supposed to be happening? What type of problems could cause this not to happen?
Having the proper testing equipment will also save you time and more importantly keep you safer. I was reading on another forum about an old school electrician that uses a homemade buzzer with a 9 volt battery for a continuity tester. What happens if he were to place his tester on a live circuit by mistake? Chances are it would blowup in his hands. Todays meters are built to stringent standards to comply with not only the technical specifications but the safety specifications too. Having the right type of meter will also make your job of troubleshooting easier.
Here is another example. We received a trouble call from a new client. He had been using one of our competitors for quite some time, but they were swamped and not able to take care of this call. This customer had some nice accent lighting in their display room. It uses a transformer converting the 120V to 24V. It is connected to 2 wires that are run exposed 6" below the ceiling on insulators spaced 36" apart. The light fixtures lay on the two wires and set screws clamp the fixtures to the wires. The problem they were experiencing was the transformer kept going bad. The previous contractor had replaced it twice at a cost of $300 each. We counted the number of fixtures, calculated total watts, checked the transformer output. Everything was correct. We used an ammeter to check circuit amps. It was higher than the calculated load. We removed all the fixtures (there were 8). We used a true RMS DMM (digital multimeter) to check continuity of the two 24V wires. They tested fine. We used the same meter to test each fixture individually. They tested fine also. I decided to bring in the megohmeter and retest the fixtures. We found a high impedance short in one of the fixtures. We ordered a new fixture and the circuit has been trouble free since. Would we have figured this out without the megohmeter? Probably not. Would we have figured this out if we hadn't known to check for a high impedance short? Definitely not. Schooling, equipment, trade experience, interviewing are all necessary parts to becoming a successful troubleshooter.
Posted by Norman W at 5:54 PM