Induction motors at starting draw larger current than normal operation current. This starting current is represented as kilo volt ampere power. Each motor has its own starting power and is set on the motor nameplate as KVA code.
KVA code of electric motors meaning
Motors KVA code is represented on the name plate on form of letters referring to starting kVA of the motor. Depending on the horsepower (H.P.) of the motor, the KVA code is used to describe the (locked rotor ampere) LRA in KVA. There are several code letters for motors with locked rotor KVA per horsepower that range from lower than 3.14 to higher than 22.4 KVA per horsepower. These codes are defined by a series of letters from A to V.A.
The letters indicate the Kilo Volt Ampere that is drawn by the motor when its rotor is locked reported in kVA / HP. Code letters are ranged from A to V, Code V motors have highest starting kVA value while Code A motors have lowest.
Code letter further from A indicates a higher inrush current per H.P. Compared to those of large motors, the LRA per horsepower requirement for small motors tends to be much more stringent as a correspondence to their higher code letter. The KVA requirements of fractional horsepower motors are even higher than that of fixed motors.
Why Do We Use KVA Code?
By using the KVA code, we can easily calculate the starting current of a motor based on its letter code. There is a mathematical relationship between the code letter value, the horsepower, the motor’s rated voltage, and its operating power factor. You can find the information on the motor nameplate itself.
Whenever a motor needs to be replaced, the KVA code should be noted. It is important to note that if you replace a lower code letter with a higher code letter, other electrical equipment upstream, such as the motor starter, may require a change.
Motor Kva Code Calculation With Example
Using the KVA code table below, the KVA drawn by a motor is given by the formula:
Motor KVA = Motor power (HP) * KVA value from the table.
From the below table, a 10-hp induction motor would draw:
- 10 (hp) * 3.14 (KVA code) = 31.4 kVA, if its code letter is A.
- If the same motor KVA code letter is D it would draw 10 * 4.3 = 43 kVA
The below table gives the code letters and the values of KVA drawn in starting.
Motor KVA Code Table
In order to classify motors based on the ratio of the locked rotor kVA to the horsepower, the National Electrical Manufacturer’s Association (NEMA), which lays out the design standards for motors, has established the NEMA Code letters.
It is possible to determine the starting kVA needed to start a motor at full voltage either from the motor nameplate or by finding out from the manual provided by the manufacturer.
|CODE LETTER’S||KVA/HP 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|
What Does KVA Code G Means?
The kVA code letters in the National Electrical Code (NEC) represent a range of kilovolt-amps per horsepower during locked rotor (starting) of induction motors.
KVA code G means this motor can consume KVA power ranged between 5.6 to 6.29 kVA/hp at its starting. We use this value to calculate the motor KVA at the start as follows.
If the nameplate rating is 300 hp, With a KVA code = G, When the motor first starts (rotor speed is at zero), its apparent power will vary from 5.6 *300 = 1680 kVA to 6.29 * 300 = 1887 kVA.
Does Motor KVA Code Affect Performance Of Motor?
KVA Code didn’t affect the performance of motors; however, using the KVA code, you are able to determine what type of reduced voltage starter is required for the motors.
It is more crucial and common to use a reduced voltage starter for higher horsepower three-phase motors, mostly because three-phase motors consume more amperages than single-phase motors.
In order to accommodate the growing size of submersible installations and the motors used in them, Locked Rotor Amps are becoming increasingly larger as well.
If a 50 horsepower, 460 V motor is installed in a system, the maximum running load will be 77 amps in this case. That same motor, however, has a locked rotor amp of 414. Upon starting the motor directly across the line (direct-on-line), it will attempt to draw 414 amps at the moment of switching on. It is often far more amperage than the electrical service can handle.
In order to solve this problem, reduced voltage starters can be used. Reduced voltage starters allow the motor to ramp up gradually rather than applying full voltage to the motor at once.
The reduced voltage starter can be used to protect a system that is pulling too many amps at start-up. In this scenario, the motor never encounters the 414 amps of locked rotor amps, preventing it from tripping and possible overload.
For more details about motor starting methods read my articles:
What Is The Motor LRA(Locked Rotor Ampere)?
LRA, or Locked Rotor Amps, is the initial current drawn by an electric motor when it is turned on for the first time. Locked rotor amps are those amps that an electric motor draws for about half a second that are about 5X the motor’s normal amp draw.
Generally, DC motors are characterized by the voltage across the motor being proportional to the motor’s RPM. As a result, the current through the motor will be proportional to the torque it produces.
Induction motors have this specification. At the maximum slip angle (s = 1) caused by the actual 3-phase stator coils, the magnetic field surrounding the rotating rotor is at the maximum slip it can sustain (s = 1).
Thus, the voltage induced in the rotor coils (or bars) is also maximized, leading to much greater rotor currents. All the energy transferred across the air gap to the rotor is dissipated as heat since there is no mechanical power output (the shaft is locked). As a result, the induction motor acts as a (poor) transformer.
It is possible to determine the equivalent circuit components of an induction motor by measuring the locked rotor current (in the stator or rotor). As a result of the high current, this is a stress point on the rotor coils/bars. A motor will start with one slip and hence generate high currents in the rotor at startup.
Brushes and slip rings can be used to bring out the coils of wound rotor induction motors (as opposed to squirrel cage motors).
With a resistor in series with each coil, they can manage rotor currents at startup (and increase starting torque). Eventually, the resistors can short as the motor approaches the full-load speed
How to convert LRA to hp?
Yes, is it possible to convert LRA to HP using given formula: HP=LRA (Locked Rotor Ampere’s) x Volts / 746Watts
Let’s Calculate the HP for Three Phase Motor when LRA is Known:
LRA = 22.5
Voltage (Three Phase) = 440V
Put the given values in formula:
HP=22.5 x 440 / 746Watts
HP=9900 / 746 Watts
What Is the Difference Between FLA and LRA?
Locked Rotor Amps-LRA: is the maximum current you can expect from a motor when applying full voltage to it during start-up conditions.
Full Load Amps-FLA: Under any operating condition, a motor must draw the maximum current it can consume, defined as FLA. Its the maximum current the motor will draw under full load and normal operation condition .
FLA and LRA differ in the sense that while for FLA, the rating of the motor will be the source of the evaluation, for FLA, you will be evaluating the rated power delivered to the load. Therefore, when you look at FLA, at the time of operation or under recommended conditions, the HP would be the actual shaft hp.
Do LRA Of Motor Matter?
The LRA of the motor matters because knowing the locked rotor amps (LRA) and acceleration time of a motor aids in the selection of upstream breakers.
We ensure that the circuit breaker or fuse can supply the locked rotor amps for the duration required to bring the motor load up to speed and that it will trip if the locked rotor amps exceed the permissible stall period.
The lock rotor current is the amount of current drawn through the motor against the rated voltage when the rotor is maintained at a fixed position, or, more precisely, it isn’t moving or turning.
It should never be compared to an overcurrent device’s continuous current rating. It is comparable to the OCPD immediate rating. Locked Rotor Current is a decent estimate of the short circuit current added by a motor to a power system short circuit.
Is LRA the Same As Starting Amps?
Starting current is the current drawn by the motor when it starts from rest. The locked rotor current is the current drawn by the motor as it accelerates from rest while rated voltage is connected to the stator.
As a result, the starting and locked currents are the same in some cases. However, it is not necessary the starting current and locked current are the same. Let’s look at the relationship between locked current and beginning current.
A motor starter is used to start the motor. The most common starters are direct online starters and star-delta starter. The rated voltage is provided to the stator when the motor is accelerated from rest with a DOL starter. Yes, in this scenario, the starting and locked current are the same since the following requirements are met.
- As the motor is idle, it is at rest.
- The stator is subjected to the rated voltage.
Now, consider the Star delta starting. The motor is activated by a star when the motor is at rest. After increasing to a certain speed, the motor’s winding will be automatically connected using the delta configuration, allowing it to speed up the motor to its maximum speed.
As you know, the star configuration voltage is 58.7 percent of the line voltage. Thus, the current at the start is less than those motors that are running in a star configuration.
Could we say that the start current of the star motor is equal to the locked rotor’s current? Absolutely not. We aren’t applying the full rated voltage to the stator in this case. This means that the current at the start is lower than the locked rotor’s current.
Induction motor starting current
- As mentioned above induction motors draw starting current higher than normal operation current. This starting current is as high as 5-7 times normal current. So starting methods are used such as soft starter and star delta.
- This high starting current could cause voltage drop during starting period. This drop affects the operation of other motors which are working on the same bus. Starting methods of induction motors reduce this current and ensure smooth starting.
- Large motors starting can cause windings to heat up. So the manufacturer sets limited number of starting trials per hour. The more you start the motor the lower its lifetime will be. Motors protection against over-current also should take the starting period into account to avoid protection trip during starting.