At start, the reactance of the rotor is high because the slip of the motor is equal to unity. The value of Rr/s increase as the slip gets decrease. When the induction motor is started the rotor reactance is more than the rotor resistance and because of the large Xr/Rr ratio the motor takes large inductive current.
What causes high starting current in motor?
Probable causes of high current with load include mechanical overload, excessively high magnetic flux densities and, less frequently, an open rotor. An error in winding data that results in lower-than-design-level magnetic flux also can cause high current with load.
How can the starting current of a motor be reduced?
Starting current of three phase induction motor can be reduced to safe value by including:
- resistor in rotor circuit at instant of starting.
- capacitor in stator circuit at instant of starting.
- capacitor in rotor circuit at instant of starting.
- inductor in rotor circuit at instant of starting.
Why do motors experience large inrush current when first started?
The difference in the magnetic fields between the stator winding (stationary copper winding group within the motor) and the rotor winding (rotating shaft winding) is the biggest contributor to the initial inrush current experienced at startup.
Why is inrush current so high?
High inrush current is the result of the maximum instantaneous input current drawn by an electrical device during the initial power up. As technology continues to advance, most systems today run efficiently and maintain a low impedance which in turn contributes to high inrush current.
How can I reduce my inrush current?
Inrush current can be reduced by increasing the voltage rise time on the load capacitance and slowing down the rate at which the capacitors charge.
How does VFD reduce starting current?
VFDs deliver much higher frequency source power during startup to reduce current due to the increased inductive impedance. This can be advantageous if control is desired after full speed is reached. This control allows for continuing control as the load on the motor is changing.
How do you calculate motor starting current?
An induction motor, starting “across the line”, will typically have an instantaneous draw 6x the kw it is rated to run at. e.g. a 10 hp motor is rated at 7.46 kw, so on starting it may draw 75 kw. On 480 volts 3 phase, the amperage will be I = W / (E x 1.73) = 75,000 / (480 x 1.73) = 90.4 amps.
What is the difference between inrush current and starting current?
In rush is a short transient not dependant on the mass or load on a motor. The starting current is however dependant on what the motor has to drive and will be influenced by the torque needed to get the load spinning. “Starting Current” is that current which is required by any motor to first start it.
Is inrush current the same as starting current?
Typically, during the initial half cycle, the inrush current is often higher than 20 times the normal full load current. After the first half-cycle the motor begins to rotate and the starting current subsides to 4 to 8 times the normal current for several seconds.
Is Locked rotor the starting current?
The term Locked rotor current usually used in blocked rotor condition. It is the current drawn by the motor when the load applied is heavier such that the motor stops rotation. The starting current is the initial current flow into the motor which is usually very high and controlled by using starters.
What are starting amps?
Cranking amps (current), also known as ‘CA’, starting amps or starting current, is the amount of power (electrical current) a battery can discharge and sustain for at least 30 seconds during normal climate conditions of 32 degrees Fahrenheit or 0 degrees Celcius.
What is maximum inrush current?
Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on. Alternating-current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform.