Constant Voltage Transformer plus Buck Boost?

[tshea]

I have an application where a constant voltage transformer will work much better than the existing (antique) voltage regulator. I can only find 120 volt units.

The customer uses 122v to calibrate an appliance, so I need to give them more than 120v preferably 125v or 130v, but all I find are 120v outputs. On the test bench they have a variable voltage supply where they can fine tune to 122v.

My thought is to buck-boost the 120V to 132V then step it down at the bench to 122.

My first thought is this is OK because the CV xfmr has done all the work. The buck-boost is simply upping the volts to 132.

Am I out in left field on this? Looking for some opinions.

Thanks!

 

[jim dungar]

My thought is to buck-boost the 120V to 132V then step it down at the bench to 122.

Go for it.

 

[GoldDigger]

A variable autotransformer (Variac) can often be configured optionally to deliver overvoltage output (~0 to 132V output with 120V input) instead of just stepping down from the input voltage.
How much power is required for the application and how critical is voltage regulation with changes in output current?

Tapatalk!

 

[tshea]

A variable autotransformer (Variac) can often be configured optionally to deliver overvoltage output (~0 to 132V output with 120V input) instead of just stepping down from the input voltage.
How much power is required for the application and how critical is voltage regulation with changes in output current?

Tapatalk!

Maximum is 2000 VA, on a 20 Amp circuit. Most of the appliances are 65 watt or less. They just get fine tuned at 122 volts.

Present unit is an (antique) Variac. It has seen better days. Voltage regulation bounces between 118-126. Can't seem to hold 124 anymore.
Will look into newer Variac.

 

[GoldDigger]

:thumbsup:
Often the problem with an older unit is mechanical pitting of the commutator surface and/or wear of the carbon based brush.
The brush assembly can be replaced and the commutator can sometimes be resurfaced if the damage is minor.
The brush needs to always contact at least two turns of the winding surface to give stable adjustment, and the design of the brush material is such that the series heat losses in the brush are roughly equal to the losses from the bridged turns.
A new Variac may be the best way to go, but if you only need a narrow range around 122 volts you should use the variac to drive the primary of an isolated buck or boost winding.
If the line voltage is unstable, feeding the whole thing from a CVT makes a lot of sense.
But recognize that some CVTs produce a waveform with high harmonic content.
Finally, if just the range of the Variac close to 122V is bad, a boost transformer will move you into a different part of the mechanical range.


Tapatalk!

 

[gar]

140227-1346 EST

tshea:

Get a Sola constant voltage transformer with a VA rating of possibly double the maximum load you expect. These transformers are quite good relative to regulation against input line voltage variations, but possibly poorer output regulation for varying loads. In the low harmonic output units there is still moderate harmonic distortion.

Connect a small Powerstat (Superior Electric) to the Sola output. Get a small transformer (Stancor possibly) with an output current rating 2 times the maximum load current you expect. Possibly use a 12.6 V filament transformer, but you could possibly use an 8 or 10 V unit. The output is used to provide the boost voltage. This arrangement will probably provide 0.1 V adjustability.

An old 120 VA Sola (55 years old) produced the following results:

Input ... No Load ... 60 W ... 100 W

095 ....... 120.4 .... 122.1 .... 120.3
120 ....... 121.5 .... 120.0 .... 119.4
135 ....... 119.4 .... 119.4 .... 120.0

I think there may be some changing temperature in the transformer. This was not a controlled test after all components had reached a stable temperature. The 122.1 looks out of place, but this is at the low end of the regulating range. Incandescent bulbs were the loads.

.

 

[Open Neutral]

WhatHeSaid re: line vs load regulation.

If you have a Variac after the FR, just strap it to boost.

There are/were motor-driven Variac voltage regulators; they cranked the Variac up/down until happy.

 

[gar]

140227-2050 EST

Open Neutral:

A reason to use a Variac with a stepdown transformer is to obtain finer resolution of voltage adjustment.

My 1 kVA Powerstat can be adjusted to within about 0.7 to 0.3 V of a desired value. A Variac plus 10 to 1 stepdown transformer will improve this setability resolution to about 0.1 V. Also a smaller Variac can be used which is a lower cost and has finer wire and therefore less change in voltage from one Variac position to another.

.

 

[GoldDigger]

A properly designed variac should not discrete steps or positions in its rotation. The change in voltage may not be linear, but any steps would be small and spaced along a curve of continuous changes.
But it would certainly be easier to get the mechanical position you want with the adjustment extending over a larger fraction of the rotation.

Tapatalk!

 

[gar]

140228-1031 EST

GoldDigger:

I checked a real General Radio Variac, 5 A rating, and it was similar to the Powerstat with step changes in the range of 0.3 to 0.7 V and hard to set to a close value.

Using the 1 kVA Powerstat, a 120 to 10 V transformer, the previously mentioned Sola CVT, and a 60 W incandescent load I could adjust the output voltage within 0.1 V at 120 V.

What Variacs have you tested and been able to directly adjust within 0.1 V?

.

 

[GoldDigger]

None recently.
I would not expect to be able to control within .1 volts purely because of mechanical limitations (stick-slip), so the stepdown method is clearly superior.
I have found that the ability to closely adjust the output voltage deteriorates with age and in particular with any overload events that can warp or pit the winding or damage the brush.
Was the GR unit you tested with reasonably new or visually verified to be in good condition?

Tapatalk!

 

[gar]

140228-1153 EST

All of my adjustable autotransformers are either General Radio (1) or Superior Electric (several), on the order of 50 years old, and having little use, and no overloads. I could not presently find one of my small Powerstats that has much finer wire and check it.

.

 

[gar]

140228-1140 EST

Another observation. The Sola CV transformer alone with a 60 W incandescent load has a short time voltage fluctuation of about 0.02 V at 120 V, and over a little longer time about 0.1 V.

.

 

[tshea]

Thanks All!
I presented the information to the customer. They will let me know next week.

I tend to go with a CVT, buckboost, & 100Watt Lamp. The lamp will preload the circuit to slowdown any voltage craziness. It will also be a 130v lamp.

Thanks again!

Tim