Motor Currents

Good day lightening arrester,

herewith:

D.O.L. starting: max starting current: 6 x rated current Maximum starting time: 5 sec.

Star Delta starting: Maximum starting current: 2 x rated current Maximum starting time: 15 sec. motor overload relay in phase current: set to 0.58 x rated current.

will send you a page on Locked Rotor Current.

hope this helps,

regards,

Leon

Lightning arrester,

The full load current (nominal current, rated current) is the current drawn by the motor to develop the rated output power (mechanical power at the shaft, [kW, HP]).

It is designated as In (nominal current), FLC (full load current), FLA (full load amperes), etc

The locked rotor current (short circuit current) is the maximum current that is drawn by the motor with a locked (blocked) rotor. If you keep the rotor blocked for a longer period of time, it heats up and the current decreases in time.

It is designated as Isc (Short-circuit current), LRC (locked rotor current), LRA (locked rotor amperes), etc

For general use motors, the locked rotor current is (6 - 7) * In

The starting current is the current drawn by the motor in the starting process. Depending on how it starts (no-load or full-load) this current is usually lower that the short circuit current, since when applying a voltage the rotor starts turning and the drawn current decreases.

It may be designated as Ip. It is usually 2 * In (for general use motors).

By general use motors are meant 3-phase, squirrel cage induction motors, direct start (across the line, no Y/delta, no low voltage, transformer, resistance in series, frequency converter, etc).

For these general use motors regulations (standards) set a maximum value of locked rotor and starting currents, either as a ratio between them and the full load current, or directly as the respective current(s).

These ratios (currents) are given to allow the selection of the overcurrent protection devices to be provided on the power supply side, to avoid nuisance tripping during the staring process but at the same time to trip in case of a fault (short circuit (blocked rotor) or abnormal operation (overload).

Lightning.arrester,

These terms can be confusing. When you first apply voltage to the motor, its shaft is stationary, so the current flowing at that instant is the locked-rotor current for that particular motor. As the shaft begins to rotate, however, a counter-emf begins to be produced. As the rotational speed increases, this counter-emf also increases until you get to a steady-state of the motor running at its (designed) full speed. This counter-emf has the effect of reducing the apparent voltage to the windings, so the current flowing through them is less than the locked rotor current. If the motor is driving a load equal to its nameplate power, this running current is the full-load current.

During the time the motor shaft is gaining speed, the motor is drawing what I would call the "starting current". This current begins at the locked rotor current and decreases rapidly until the end of the starting period, when the starting current has become equal to the full-load current. Depending on the inertia of the load the motor is starting, the starting time can be as short as a fraction of a second, but occasionally can be as long as a minute or even longer.

To add to the confusion between these terms, different designs of motors result in different relationships between these various currents. The most common motor design has a locked-rotor current approximately 6x the full-load current. In premium energy-efficient motors, the locked-rotor current is usually higher, in the range of 8x full-load. This seemingly modest increase in current has required many exceptions or modifications to the "customary" selections of equipment for motor disconnects and motor starters. There are other, less common, motor designs than the two mentioned above (such as high-slip, high-torque, etc).

Most electric utilities have rules for how you can start motors above certain specified sizes. This is because of the high current draw during the starting period. All of these methods have the effect of reducing the voltage applied to the motor during part (or all) of the starting period. Some of them require the motor to be wound in certain ways (part-winding and wye-delta). Another requires slip-rings and windings on the rotor (secondary resistor). Others add resistance in series (primary resistor and reactor). Then you have electronic devices (soft start and VFD). If the motor's starting current is a fairly large percent of the entire load for a building or plant, it is possible to cause voltage-sensitive equipment to fail. If it is a large percent of the capacity of a utility's distribution system, you can even brown-out the entire area! There have been many threads on the CR4 site about problems with starting motors on generator-supplied systems.

In addition to all this, you have to worry about overheating the motor during its starting period and making sure that the motor has enough torque during this period to accelerate its load. If the starting period is significantly long (I've seen large fans take 20-30 seconds to accelerate to full-speed, and a log chipper take a few minutes), you have to pay very careful attention to overload protection, starting method, etc.

regards--JMM