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Mac's Service Shop: Voltage-Regulating Transformers
July 1965 Electronics World

July 1965 Electronics World

July 1965 Electronics World Cover - RF Cafe  Table of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

Despite having heard of and used the "voltage-regulating transformer" (aka "constant voltage transformer," CVT), I never knew exactly how they work. This installment of Mac's Service Shop in the July 1965 issue of Electronics World magazine happens to address the device, so now I at least have some idea what's going on to magically hold the secondary voltage at an amazingly constant value regardless (within reason) of the primary voltage. It is a purely passive device that uses ferroresonance for feedback. The price to be paid for such a convenience is explained to Barney by Mac after Barney gets befuddling readings on his voltmeters. Mac invokes the name of the inventor, Joseph Sola, who manufactured CVT's in the Sola Electrics Company premises. According to the SolaHD (merger of Sola and Hevi-Duty) About Us webpage, "Sola Electric opened in 1930 as an emerging laboratory in two modest size rooms in a Chicago bank building. It was a small beginning, but Joseph G. Sola had big ideas. Beginning in the 1930's, he practically invented the field of transformer magnetics and went on to revolutionize the fledgling electrical industry - winning 55 U.S. patents along the way."

Mac's Service Shop: Voltage-Regulating Transformers

Mac's Service Shop: Voltage-Regulating Transformers, July 1965 Electronics World - RF CafeThese special transformers, as opposed to conventional transformers, operate on the ferroresonant principle.

By John Frye

Barney came bustling into the service department with a heavy object cradled in his arms. "There's something I've wanted a long time," he announced, depositing a husky transformer from which dangled a metal-encased capacitor on the service bench.

"What do you need with a voltage-regulating transformer?" Mac, his employer, asked.

"My single-sideband ham receiver drifts a few cycles on 15 and 20 meters when the line voltage changes. Plate voltage of the receiver v.f.o. is regulated with a VR tube, and since the frequency change is a slow affair I'm sure it's the result of oscillator filament voltage change. Even a few cycles of drift is annoying on sideband; but after I plug my receiver into this little gem - which a friend just gave me - the line voltage can horse up and down all it wants without making me retune. I'm told this transformer, in some mysterious way, will hold output voltage within 1% while the line voltage is changing as much as 15%. Just for kicks, I thought I'd check it out on our variable-voltage transformer."

Grinning expectantly, Mac watched Barney connect a 75-watt lamp to the output of the transformer and attach leads from the v.o.m. to the lamp terminals. Confidently the youth plugged the line cord of the voltage-regulating transformer into the output receptacle of the service bench variable-voltage transformer, but he hastily jerked out the plug when he noticed the v.o.m. reading. Gingerly he replaced the plug and rotated the voltage-selecting knob of the bench transformer. Next he substituted test leads from the v.t.v.m. for those of the v.o.m. Finally he connected leads from both meters to the a.c. line and compared readings.

"Man, I don't get that!" he exclaimed. "This transformer is supposed to put out 118 volts, but the v.o.m, reads 133 volts, no matter if the input voltage is anywhere from 105 to 130 volts. The v.t.v.m., on the other hand, reads a little under 100 volts for the same range of input voltage. Yet both meters read exactly 118 volts when connected to the line. This July heat must be getting to me."

"Maybe not," Mac said soothingly. "Think a little. The a.c. meter of the v.o.m. is a rectifying type that deflects according to the average value of a sine-wave voltage but has a scale calibrated in r.m.s. units, The v.t.v.m. is deflected in accordance with peak-to-peak values, but the scale indicates r.m.s, voltage of a sine wave."

"A non-sinusoidal waveshape!" Barney interrupted, switching on the scope. Sure enough, cycles of the voltage-regulating transformer output looked like slightly domed square waves.

Conventional transformer Magnetization curve - RF Cafe

Fig. 1 - (A) Conventional transformer. (B) Magnetization curve. (C) Transformer with magnetic shunt and resonating capacitor. (D) Voltage-regulating transformer.

"Your transformer is of the type called normal harmonic," Mac commented. "Output voltage of such a transformer contains from around 14% to more than 20% harmonic content depending on the manufacturer, loading, etc. From the looks of that wave, I'd guess this one is close to 20%. Voltage from these transformers is set with a dynamometer type of voltmeter, and readings from any other type of voltmeter will not be the same. Remember, multiplying peak voltage bv 0.707 to get r.m.s. voltage and by 0.637 to get average voltage only holds true for a sine wave. Actually, in a square wave the peak, r.m.s., and average values are all the same. Let's see what will happen if you feed your receiver from this transformer."

Mac connected a full-wave bridge instrument-rectifier across the 10-volt secondary of a bell transformer and placed a 40-μf. filter capacitor across the rectified output. A 14-volt lamp was connected across the secondary.

"Voltage across the filter capacitor will represent d.c. voltages in your receiver," he said. "The lamp bulb will represent filaments, and the reading of this lightmeter beside the bulb will indicate any change in filament temperature. When the primary is connected straight to the 118-volt line, I see we have 13 volts d.c. across the filter capacitor, and both the v.o.m. and the v.t.v.m. read 10.5 volts a.c. across the secondary.

"Now we switch the primary to the output of the voltage-regulating transformer. The lamp gets a little brighter - I'd guess about 5% according to the lightmeter - but our d.c. voltage has dropped to 11 volts! Secondary voltage has gone up to 11.6 volts measured with the v.o.m., and down to 8.8 volts measured with the v.t.v.m. Besides demonstrating the futility of trying to measure non-sinusoidal voltages with conventional meters, this experiment leads us to expect d.c. voltages will be down by about 15% in your receiver while the filaments burn a little brighter.

"Normal-harmonic-type voltage-regulating transformers are not recommended for use where a high harmonic content may lead to difficulty or where the output is to be rectified. Such is not the case when the output is used for heating or to operate relays and solenoids. In your case, why not use a separate filament transformer working off this gift horse of yours for heating critical filaments in your receiver? You could arrange it so these filaments burn all the time and keep heat-sensitive circuitry warmed up. That will make your receiver even more stable than it is right now."

"You know, you're not so dumb for an older man," Barney said. "I'll do it. Now can you tell me how voltage-regulating transformers work?"

"I'll try, but it's not easy," Mac warned. "Not much has been published on the subject in popular magazines. Most of what I know I've learned from literature furnished by the Sola Electric Company, a pioneer in the field and a major manufacturer of these transformers.

"Let's start by reviewing conventional transformer action," Mac said while he drew some sketches on the blackboard at the end of the bench. "Fig. 1A here is such a transformer; Fig. 1B is the magnetization curve of the core material. A voltage Ep1 across the primary causes a current Ip1 to flow through the primary, producing magnetization flux Φ1. This flux links the secondary and, in accordance with Faraday's Law, produces a voltage Es across the secondary proportional to the primary/secondary turns ratio. Secondary current flowing through a load produces a secondary flux that tends to cancel primary flux so that primary current must increase to maintain the original flux level."

"The conventional transformer is a linear device operating on the linear portion of the magnetization curve. Increasing primary voltage to Ep2 produces current Ip2, causing a flux increase to Φ2resulting in an increase in secondary voltage directly proportional to the increase in primary voltage. But if the transformer is operated to the right of the knee of the magnetization curve, it ceases to be a linear device. Now a similar change in primary voltage causing an Ip3 to Ip4 increase in primary current results in a much smaller increase in flux and, consequently, in secondary voltage. Such a saturated transformer provides a degree of voltage regulation, but it is not practical for any considerable amount of power because primary current approaches short-circuit value as the core saturates. What we need is a transformer in which secondary flux could be saturated without materially affecting primary flux."

"This is accomplished in Fig. 1C by what is called a magnetic shunt or bypass built into the core. Here, as you can see, only part of the primary flux links the secondary, and vice versa. Next, to increase secondary flux saturation, we connect a large value of capacitance across the secondary and establish a condition called ferroresonance. A ferroresonant circuit cannot be exactly the same thing as an ordinary resonant circuit because the inductance in the ferroresonant circuit is non-linear; yet the two circuits have many common properties.

"For example, the resonant tank circuit of your amateur transmitter develops very high voltages across it because of heavy circulating tank currents. So long as the excitation voltage is maintained above a certain minimum value, changes in the level of that exciting voltage have little effect on voltage across the tank circuit.

"High voltage across the ferroresonant circuit is also chiefly due to heavy circulating currents through the capacitor. This voltage is much higher than would be expected from the primary / secondary turns ratio. You will find around 600 volts across that capacitor. And voltage across the ferroresonant secondary is little affected by changes in the primary voltage. Isolation of primary and secondary magnetic fluxes allows the heavy saturating secondary current to be produced without heavy primary current.

"But there is always some increase in secondary voltage with an increase in primary voltage, and this leads to the circuitry of Fig. 1D. Here we see a portion of the voltage developed across the resonant circuit fed to the load in series with a bucking winding wound over the primary. An increase in primary voltage leading to a slight increase in voltage across the tapped portion of the resonant winding also produces a slight increase in voltage across the bucking coil. Since the bucking-coil voltage opposes the voltage of the resonant winding, the two voltage increases cancel each other."

"Those are the highlights of the regulating transformer story, but there's much, much more. For example, if one of these transformers is overloaded, the magnetic field collapses and output voltage falls to zero without primary current increasing enough to do any damage. When the overload is removed, normal operation is automatically restored. And there is another type of regulating transformer, costing slightly more, called the sinusoidal that uses additional 'neutralizer' windings to reduce the harmonic content of the output to less than 3%. It can be used for those applications where the normal-harmonic type is not recommended since it has the same excellent voltage-regulating characteristics. Other voltage-regulating transformers are de-signed to furnish various filament voltages, or combined plate and filament voltages, and for 400-cycle use."

"Is Sola the only manufacturer of these transformers?"

"No. Other companies that manufacture or have manufactured voltage-regulating transformers include Stancor, Triad, Acme Transformer, General Electric, and Raytheon."

"Well, thanks for all the information - I think," Barney said. "I feel a little like the fellow who asked for a slice of bread and got a bakery. But you just let me mull over what you've told me for an hour or so, and I'll bet I can come up with some questions."

"No bet!" Mac answered quickly.



Posted October 21, 2022

Mac's Radio Service Shop Episodes on RF Cafe

This series of instructive stories was the brainchild of none other than John T. Frye, creator of the Carl and Jerry series that ran in Popular Electronics for many years. "Mac's Radio Service Shop" began life in April 1948 in Radio News magazine (which later became Radio & Television News, then Electronics World), and changed its name to simply "Mac's Service Shop" until the final episode was published in a 1977 Popular Electronics magazine. "Mac" is electronics repair shop owner Mac McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.

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