Easy to set up and use as a roast profile controller. KRM's variacs are less expensive than other manufacturers'. Extremely reliable. Built like a tank.
Negative Product Points
Also heavy as a tank. Dimmers may be a less expensive alternative. Use may void warranties and be at your own risk.
WHAT'S A VARIAC?
It's an accessory for home coffee roasters; a variable transformer that can change the voltage supplied to the roaster from 0V to 135V. With the variac, one can change how quickly or slowly one roasts.
The KRMs are a cylinder about the same size as a hearthware roaster base, with a box attached on the side. The cylinder case has slots revealing the heavy copper winding of the transformer; the box has a standard 3 prong electric outlet for variable voltage, a fuse and an analog voltmeter showing the output level. The voltage control knob is at the top of the cylinder. It weighs in at a hefty 17lb. The case is heavy iron, painted with an industrial beige rustproofing paint. It isn't, and doesn't look like a consumer item, since it's primarily used to control the stamping depth of embossing machines in print shops. It's plain, solid, indestructable looking, and oddly confidence inspiring. I think a company could make a small fortune marketing regular consumer items built in this plain massive way, rather than the usual plastic designer glitz.
WHY USE A VARIAC?
A roast profile is a graph of bean temperatures over time. Home roasters follow a single inflexible profile. For instance, my unmodified Freshroast+ reaches 415F, a city roast, in about 4 1/2 minutes, and 445F, a dark full city roast, in about 5 1/2. However, it's well known that a slower roast to a given temperature will produce greater body and sweetness, but more subdued origin and roast flavors, when compared to a faster one to the same temperature. This is a taste dimension that's separate from stopping the roast at lower or higher temperatures (lighter or darker roasts) which produce more origin or roast flavors respectively.
Obviously, being able to control the profile as a whole, rather than just the finishing temperature, will allow for better and more customized roasts. But since purpose built home roasters and poppers supply constant heat, one is stuck with the fixed profile of the roaster one buys; fast for the FR, slow for the Z&D or Alp. I don't think this is acceptable; all commercial roasters have heating controls that allow roast profiling. Think of it this way: would you buy an oven which can only be set to one temperature? A variac allows one to change the roaster's temperature, and gives home roasters the same profiling abilities as their commercial counterparts.
Dimmers are a lower cost alternative to variacs. However, inexpensive ones tend to cut the highest voltage one can get to about 105V; while good ones designed for flourescent lights tend to be nearly as expensive as a low cost variac. The variac has the additional advantage of allowing an increase in voltage. This can be used to increase the capacity of air roasters by starting at 125 - 135 volts in the first minutes of the roast, and supplying enough airpressure to move the heavier load of beans.
TIPS ON USING A VARIAC
It takes practice to become proficient. It took me longer to learn how to use a variac than to home roast. I hope these tips shorten the process for others.
When an airroaster is plugged into the variac, changing the voltage changes both fan speed and heat. Since the beans are heavy at the start of the roast, one has to supply high heat to get enough airflow to move them. This means that the timing early in the roast cannot be controlled. I believe the roasting speed below about 325F (just prior to the first crack) doesn't make much difference to the taste, but having control here is convenient if one wants to dry out beans to improve roast evenness. If you do wish to control roasting speed at this early stage, you'll need to go into the roaster and install a separate supply line to the fan.
The way to fully profile the roast is to use a roast thermometer and stopwatch, and to adjust the voltage to get a steady rise to the finishing temperature you want in the time you want, starting just prior to the first crack.
I use a profile, favored by many experts, that goes quickly through the first crack, and slows down thereafter. For this, one has to bump the voltage up high (120V-130V) to get the first crack racing along, then reduce it steadily (to 105V-110V) during the first crack to keep the roast from running away, then nudge it up very gradually to get a steady rise to the finish. Other, equally formidable, experts swear by a straight line profile, which requires a gradual raising of the voltage (with a bit of fiddling at spots) from about 340F to the end; this is a more difficult profile to pull off.
It pays to have the temperature target for each minute of the roast written down, so one doesn't have to do too much mental arithmetic. This allows one to make small timely adjustments, rather than laggard large ones; and improves roast quality.
A lower effort alternative: one can roast at high voltage (115V to 125V) until the first crack gets rolling, then make a single adjustment, usually to a lower voltage, for the remainder of the roast. A thermometer is not required for this approach; but one has to keep records, so one knows what roast time comes from each finishing voltage.
For my taste, and friends who've tried my coffee, the best roasts go from the start of the first crack to the end in about 3 to 6 minutes, shorter for brewed coffee, longer for espresso. From the published profiles of home and commercial roasters, this timing seems to hold for nearly all of them (drum roasters warm up very slowly to about 325F - 350F, but finish about as quickly as air roasters). However, as I mentioned, professional roasters and coffee experts disagree on just about every other aspect of roast profiling, and the whole point of the exercise is to try it for yourself; so don't take my word on it!
The KRM Model 590 is rated at 5 amps. But I and many others have used it without problems with 10 to 15 amp home roasters, myself for about a year, doing up to 12 roasts in a row. However, it is an overcurrent use, voiding the variac warranty. So, it's at your own risk. A full assessment of the pros and cons can be found at Coffee Bean Corral. The by-the-book alternative is the KRM model 2090, rated at 20 amps, which costs about $120. Variacs from most other manufacturers are considerably more, since they are precision devices designed for use with Hi-Fi equipment, whereas the KRMs are primarily used to control the stamping force of embossing machines. Their primary use as a motor control may also explain why they work at overcurrents; motors have a powerful surge current when started, and controls for them are overbuilt to handle it.
Using a variac in excess of 120 volts also voids the roaster warranty, and is, again, at your own risk. I have encountered no problems at all with my FR using the variac with spells of 125-130V. Rosto + Variac users report the same.
Full profiling only works for roasters with no electronic controls, such as poppers, the FR or Rosto. However, a variac is a superb filter of voltage transients; so many owners of Hearthwares and Alps use them to protect their electronics. Also, the simpler method of picking a fixed voltage once the beans are light enough will work to extend overall roast times. Shortening roast times on electronically controlled roasters is not possible, however, since the heating coils are thermostatically controlled and will simply cut out more frequently if the coil runs hotter (the thermostats on the FR and Rosto are safety devices and don't cut out under normal roasting conditions).
http://coffeebeancorral.com/ is a great site, and Russ, the owner, provides terrific service. They have a very good selection of green coffees, especially Hawaiians; and sell roasters, spare parts for them, and other coffee equipment as well.
Three Month Followup
Since writing the review, I've purchased a second, smaller variac and control fan and heat separately. The best roast quality is hardly improved for full city or darker roasts; but it does help for city roasts that are stopped prior to the second crack, where the longer drum roast style temperature ramp up preserves the origin flavors while reducing sourness.
I primarily did it to compare the more straight line drum profiles to the more fast rise/slow finish air profiles. If one proceeds carefully, there is little difference between the two for roasts into the 2nd. However, I also think drum style profiles are more forgiving, since the "sweetspot" between baked and sour is wider.
For the Rosto and Freshroast, which use 24 volt fan motors with a drop down resistor, a second variac on the fan also allows a substantial increase in the roast load. For my FR, I've gone from a maximum of 85 grams (3 ounces) to a maximum of 128 grams (4 1/2 ounces)
One Year Followup
I'm still using the unit for every roast on my 10 amp FR and also on some 15 amp P1s. The unit has never heated up, although I did blow a 15 amp fuse on one roast.
NOTE ON USING VARICAS OVERAMPED: variacs designed for motor control have overrated wire sizes to withstand starting surges and can run for short periods (under 1/2 hour) at 3 to 4 times their rated full load, provided they are given time to recover. Since I use my roaster to cool the beans, the periodic over-amped use with the heater running is followed by a rest as the beans are cooled. According to the engineering specifications, this turns out to be a proper use, not a misuse, of the unit. However, people running their roaster continuously, and using another device to cool the beans, would be better advised to buy a 20 amp unit.
I've rarely had as much "bang for the buck" from a product as I've gotten from this variac.