What is the NEMA KVA letter code for?
The NEMA KVA Code found on a Three Phase AC Motor helps users select a suitable starter.
The current drawn by an induction motor is a function of its rotor speed. The voltage applied to the motor creates a rotating magnetic field. The input current reduces to a minimum when the rotor is rotating at the same speed as this magnetic field, as it does at no load, and reaches a maximum when the rotor stops.
The running rotor produces a back EMF that opposes the applied voltage, thereby reducing the input current. If the rotor is stationary, there is no back EMF, and the input current is the highest. The same maximum current is also drawn if the rotor were to be locked in place and not allowed to turn. In this situation, the motor draws the Locked Rotor Amperage (LRA).
This is also the situation when starting a motor, as its rotor is then at a position of rest, with the motor drawing a starting current equal to its LRA. However, as the rotor is not locked, it starts to turn, building up back EMF, and reducing the current drawn. Depending on the torque, the motor has to produce to turn the load, its rotor speed remains below the speed of the rotating magnetic field, thereby determining the load current drawn by the motor.
Sample Motor Specification Data with Listed KVA Code H
Although over 95% of normal motor applications start the motor directly across the power line, the starting current drawn may be as high as 300-600% greater than its full load running current. In many cases, specifically for higher horsepower, 3-phase motors, this may lead to far greater starting currents than the electrical service can provide. That is why reduced voltage starters are used.
Reduced Voltage Starters
Reduced voltage starters do not apply full voltage to the motor when it is starting, rather allowing it to ramp up with a reduced voltage instead. This prevents the motor from pulling its full LRA, saving the system from possible overload and tripping when starting. As there is a huge variety of motors available, it becomes rather difficult to determine the type of starter a specific application requires.
The National Electrical Manufacturers Association (NEMA) has defined KVA codes that specify the type of reduced voltage starter a specific motor should use. Coded on the motor nameplate, the KVA code letter defines a group of motors based on a combination of their supply voltage, LRA, and horsepower.
NEMA KVA Code
A NEMA KVA code defines the LRA in KVA on a per-horsepower (HP) basis. The code letters cover motors with a locked rotor KVA per horsepower ranging from lower than 3.14 to higher than 22.4. A series of letters from A to V define the codes (as in the standard MG 1-10.37.2). In general, farther the code letter is from A, higher is the inrush current per HP. It is common for small motors to have a higher NEMA code letter with a related higher LRA per horsepower requirement as compared to those of large motors. Fractional horsepower motors have even higher locked rotor KVA requirements.
Significance of the KVA code
The KVA code makes it easy to determine the starting current of a motor from its code letter. The LRA is dependent on a mathematical relationship between the code letter value, the HP, the rated voltage of the motor, and its operating power factor. All these are available on the motor nameplate.
The KVA code is important when replacing a motor. If replaced with one of a higher code letter, other related upstream electrical equipment, such as the motor starter, may require a change as well.
Motor Starting Methods
Starting a centrifugal pump is easy and reduced voltage starters such as wye-delta or part winding starters may be adequate. These starting methods typically produce 33% and 50% of the rated motor starting torque respectively. Such starters would also be adequate for starting unloaded compressors.
However, starting a positive displacement pump or a loaded inclined conveyor poses additional difficulties, as they need higher starting torques demanded by these types of loads. Therefore, the best starting method would be one that helps achieve the desired result in starting current reduction while yielding an adequate starting torque to start the load reliably.
with locked rotor
|Approximate Mid-Range Value|
|A||0 – 3.14||1.6|
|B||3.15 – 3.55||3.3|
|C||3.55 – 3.99||3.8|
|D||4.0 – 4.49||4.3|
|E||4.5 – 4.99||4.7|
|F||5.0 – 5.59||5.3|
|G||5.6 – 6.29||5.9|
|H||6.3 – 7.09||6.7|
|J||7.1 – 7.99||7.5|
|K||8.0 – 8.99||8.5|
|L||9.0 – 9.99||9.5|
|M||10.0 – 11.19||10.6|
|N||11.2 – 12.49||11.8|
|P||12.5 – 13.99||13.2|
|R||14.0 – 15.99||15.0|
|S||16.0 – 17.99|
|T||18.0 – 19.99|
|U||20.0 – 22.39|
|V||22.4 – and up|