Induction motors at starting draw higher current than normal operating 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.
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KVA code of electric motors meaning
The Motors KVA code is represented on the nameplate as a letter referring to the motor starting kVA.
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.
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 the highest starting kVA value while Code A motors have the lowest. The higher the KVA code the higher the motor starting current.
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.
Knowing the starting current of the motor, which is represented as KVA, is important to ensure that the network is capable of starting the motor with an acceptable voltage drop, and to ensure the overcurrent protection won’t trip during the starting.
When replacing a motor, the KVA code should be noted. It’s 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.
Motors with higher KVA codes may require a starting method, like a soft starter, to decrease starting current. A higher starting current causes a higher voltage drop and affects other loads and can cause them to stop.
Once we have a large induction motor with a high starting current installed in a water plant at my work. The starting of the motor caused the other motor to stop due to an under-voltage fault, however, the motor has a soft starter. To overcome this issue we replaced the transformer with a larger one.
I’ve written a detailed article about, What are different types of motor starting methods? you can read it for more information.
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 (for A 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.
|KVA/HP WITH LOCKED ROTOR
|APPROXIMATE MID-RANGE VALUE
|0 – 3.14
|3.15 – 3.55
|3.55 – 3.99
|4.0 – 4.49
|4.5 – 4.99
|5.0 – 5.59
|5.6 – 6.29
|6.3 – 7.09
|7.1 – 7.99
|8.0 – 8.99
|9.0 – 9.99
|10.0 – 11.19
|11.2 – 12.49
|12.5 – 13.99
|14.0 – 15.99
|16.0 – 17.99
|18.0 – 19.99
|20.0 – 22.39
|22.4 – and up
What Does KVA Code G Means?
KVA code G means this motor can consume KVA power ranging 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 the Performance Of Motor?
KVA Code didn’t affect the performance of motors, it’s just an indicator of how much current the motor draw at starting. Using the KVA code enables you to determine what type of reduced voltage starter is required for the motors.
It’s more crucial and common to use a motor starting method for higher horsepower three-phase motors, mostly because three-phase motors have higher KVA.
No matter what the KVA code of a motor is, the performance of the motor remains the same.
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 from the stationary state with rated voltage applied to the motor.
Locked rotor amps are those amps that an electric motor draws for about half a second which is about 5X the motor’s normal amp draw.
Induction motors, 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.
How to convert LRA to hP?
Yes, is it possible to convert locked rotor amper (LRA) to HP using the 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 the formula:
HP=22.5 x 440 / 746Watts
HP=9900 / 746 Watts
What Is the Difference Between FLA and LRA?
Locked Rotor Amps-LRA: this 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, the maximum current the motor will draw under full load and normal operating conditions.
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.
FLA is set on the nameplate as a current value, while LRA is set on the motor nameplate as a letter code, as I explained above.
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, starting method, and overcurrent protection device.
We ensure that the circuit breaker, fuse, or overcurrent protection device 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.
LRA should never be compared to an overcurrent device’s continuous current 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 applied 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.
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.
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