During our Column-by-Column series on single-phase motors, we talked about Locked Rotor Amps and briefly touched on KVA Code, saving it for a three-phase discussion. Since we discussed the three-phase motors specifications listed on pages 22 – 28 of the Franklin Electric AIM Manual in our last post, now is the perfect opportunity to revisit KVA and where that code letter comes from.
KVA Code is most commonly used to specify a reduced voltage starter. Reduced voltage starters are more common and important for higher horsepower, three-phase motors since they pull more amps than single-phase motors. As submersible installations and the motors in them get larger, Locked Rotor Amps get much larger as well.
For example, a system with a 6-inch, 50 hp, 460 V motor has a maximum running load of 77 amps. However, that same motor has locked rotor amps of 414. As we discussed in the post Locked Rotor Amps and KVA Code, if the motor is started directly across the line (called DOL for direct-on-line), it will try to pull 414 amps at the moment of start-up. In many cases, this is far more amperage than the electrical service can provide.
That’s where reduced voltage starters come in. Reduced voltage starters allow the system to ramp up instead of applying full voltage to the motor all at once. The reduced voltage starter aids a system pulling too many amps at start up; in this case, the motor will never see the locked rotor amps of 414, saving the system from tripping and possible overload.
The KVA Code is used to specify which reduced voltage starter is needed for a specific motor. This code letter defines a group of motors based on a combination of their voltage, locked rotor amps, and horsepower.
So how do we get a specific KVA Code letter? This range of numbers is found using the following equation:
K = Kilo (1000)
V = Voltage
A = Amperage (Locked Rotor AMPs)
(Volts x Amps) / 1000 = KVA single-phase
(Volts x Amps x 1.73) / 1000 = KVA three-phase
KVA / HP = Rating (Code Letter)
Each KVA Code letter corresponds to a universal KVA/HP range, as defined by NEMA.
Going back to our 6-inch, 50 hp, 460 V motor:
(460 x 414 x 1.73) / 1000 = 329
329 / 50 hp = 6.6
By referencing the KVA Code chart we see the corresponding letter is H, and we have reached the correct KVA rating. Luckily, the math is already done for us and all KVA Codes are located in the AIM Manual, as well as on the motor nameplate.
That wraps up our discussion on Locked Rotor Amps, KVA Code, reduced voltage starters and the columns of three-phase motor specifications, starting on page 22. Come back next week when we will review “what you really need to know” from this series.