LOCKED ROTOR CODE LETTERS
and
REDUCED VOLTAGE STARTING METHODS
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When AC motors are started with full voltage (Across-the-Line Starting), they draw line amperage 300% to 600% greater than their full load running current. The magnitude of the “inrush current” (also called locked rotor amps or LRA) is determined by motor horsepower and design characteristics. To define inrush characteristics and present them in a simplified form, code letters are used. Code letters group motors depending on the range of inrush values and express the inrush in terms of KVA (Kilovolt Amperes). By using the Kilovolt ampere basis, a single letter can be used to define both the low and high voltage inrush values on dual voltage motors.

The code letter designations and their values appear in Table I.

CODE
LETTER
KVA/HP
RANGE
APPROX.
MID-RANGE
VALUE
CODE
LETTER
KVA/HP
RANGE
APPROX.
MID-RANGE
VALUE
A 0.00 - 3.14 1.6 J 7.10 - 7.99 7.5
B 3.15 - 3.54 3.3 K 8.00 - 8.99 8.5
C 3.55 - 3.99 3.8 L 9.00 - 9.99 9.5
D 4.00 - 4.49 4.3 M 10.00 - 11.19 10.6
E 4.50 - 4.99 4.7 N 11.20 - 12.49 11.8
F 5.00 - 5.59 5.3 0 12.50 - 13.99 13.2
G 5.60 - 6.29 5.9 R 14.00 - 15.99 15.0
H 6.30 - 7.09 6.7

To determine starting inrush amperes from the code letter, the code letter value (usually the mid-range value is adequate), horsepower and rated operating voltage are inserted in the appropriate equation. The equation to be used is determined by whether the motor is single or three phase.

INRUSH AMPERES(SINGLE PHASE MOTORS)
= ((CODE LETTER VALUE) X HP X 1000)/(RATED VOLTAGE)

INRUSH AMPERES(THREE PHASE MOTORS)
= ((CODE LETTER VALUE) X HP X 577)/(RATED VOLTAGE)

The following simplified equations for three phase motors will give approximate results for 3 phase motors rated for 200, 230, 460 or 575 volts:

200 volts LRA = Code Letter Value x HP x 2.9
230 volts LRA = Code Letter Value x HP x 2.5
460 volts LRA = Code Letter Value x HP x 1.25
575 volts LRA = Code Letter Value x HP x 1.0

STARTING METHODS

Across the line starting is used on a high percentage, probably over 95% of normal motor applications. Other starting methods (reduced voltage) are used mainly to control inrush current and limit it to values that can be safely handled without excessive voltage dips and the accompanying light flicker. Occasionally, reduced voltage starters are used to reduce starting torque for smoother acceleration of loads. Various methods of reduced voltage starting have been developed. Table 2 shows the common reduced voltage starter types and the results that can be expected in terms of motor voltage, line current, and the output torque of the motor. Caution should be used in applying reduced voltage starters on certain types of loads. For example, a centrifugal pump, which is very easy to start, can be operated with either wye-delta starting or part winding starting. These starting methods produce 33% and 50% of rated motor starting torque respectively and can

easily start centrifugal pumps. They could also be expected to start a compressor so long as it is unloaded. They could have difficulty starting a loaded inclined conveyor or a positive displacement pump because of high starting torques required on these types of loads. The best starting method has to be one that achieves the desired result in inrush reduction and yields adequate starting torque to reliably start the load.

SQUIRREL CAGE INDUCTION MOTORS

* Percent of ?Across The Line Value?.
** Autotransformer magnetizing current not included. Magnetizing current usually less than 25 percent motor full-load current.
Table 2
Starting Method % of Full Voltage Value*
Voltage
at
Motor
Line
Current
Motor
Output
Torque
Full Voltage
100
100 100
Autotransformer
80 % tap 80 64** 64
65 % tap 65 42** 42
50 % tap 50 25** 25
Primary reactor
80 % tap 80 80 64
65 % tap 65 65 42
50 % tap 50 50 25
Primary resistor Typical Rating 80 80 64
Part Winding High Speed Motors (1/2 - 1/2) 100 70 50
Wye Start – Delta Run 100 33 33

In all cases, reduced voltage starters will cost substantially more than full voltage (across the line) starters. Standard motors can be used with autotransformer, primary reactor and primary resistor type starters. In addition, dual voltage motors can usually be utilized with part winding starters but only at the low voltage. Generally speaking, wye delta motors and part winding motors for higher voltage (for example, 460 or 575 volts) must be made to order. This will raise the cost of the motor over a standardly available motor suitable for use on other types of reduced voltage starting.

SOLID STATE STARTERS

So far we have discussed the traditional types of reduced voltage starters. The newer arrival is the solid state soft-start control. With this type of device, the voltage is gradually raised electronically from a low value at which the motor starts to turn the load up to the final across the line operating voltage. This type of starter has the advantage of giving smoothly controlled acceleration and substantially reduced inrush current.

SUMMARY

Overall, it is important to note that one of the primary objectives of reduced voltage starting is to limit the inrush current to a value that the power system or the local utility will accept. There are fringe benefits derived from all types of reduced voltage starting. Reduced torque values that result from the lower applied voltage also reduce wear and tear on couplings, belts, gears and other equipment that is being powered by the motor. Solid state starters offer a smooth transition from standstill to full speed with reduced line current, controlled motor torque and acceleration.




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