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The induction motor draws a high starting current as compared to the running condition. The starting current of the induction motor is about 5 or 6 times the motor full load current. An induction motor of rating 11 KW,22 Amps,440 volts takes a high starting current of about 132 Amps.
What is the starting current of a motor?
The current at motor starting can be anywhere from 5-8 times the motor’s full load. Any electrical system is designed to carry and sustain a steady state during the running period. However, motor starting will cause electrical cables to carry a higher current compared to the steady-state.
What is the inrush current for a motor?
Inrush current, also referred to as “locked rotor current,” is the excessive current flow experienced within a motor and its conductors during the first few moments following the energizing (switching on) of the motor.
Why is DOL starter used up 5 hp?
Direct-on-Line (DOL) Starter This heavy current reduces as the motor accelerates to its rated speed. For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points and hence increases the life of starter.
Why is inrush current bad?
The ratio of inrush current to normal full-load current can range from 5 to 100 times greater. This current surge can cause component damage and/or failure within the equipment itself, blown fuses, tripped circuit breakers, and may severely limit the number of devices connected to a common power source.
What is the principle of induction motor?
The motor which works on the principle of electromagnetic induction is known as the induction motor. Electromagnetic induction is the phenomenon in which the electromotive force induces across the electrical conductor when it is placed in a rotating magnetic field.
What is back emf in dc motor?
When the armature of the DC motor rotates under the influence of driving torque, the armature of the conductors moves through a magnetic field inducing an emf in them. The induced emf is in the opposite direction to the applied voltage and is known as the back emf.
Why starters are used in induction motor?
Let us study in detail why starter is required for an induction motor. At starting time, the speed of the motor is zero and slip is at its maximum i.e. unity. So magnitude of the rotor induced emf is very large at start.
Do DC motors have inrush?
DC motors do not have current inrush. And the DC inrush current should be the DC resistance divided into the applied voltage. Once the motor is spinning it generates a back emf which reduces the current draw.
Why no load test is performed in induction motor?
As the name suggest no load test is performed when rotor rotates with synchronous speed and there is no load torque. This test gives the information regarding no-load losses such as core loss, friction loss and windage loss. Rotor copper loss at no load is very less that its value is negligible.
What are the causes of overheating of a DC motor?
The most common causes of overheating include:
- An unsuitable motor: Motors come in a range of sizes.
- The wrong voltage supply: Too many volts or too few volts can be damaging to a motor.
- A poor surrounding environment: A motor needs room to breathe so it can perform at its best.
How does an induction motor start?
A three-phase Induction Motor is Self Starting. When the supply is connected to the stator of a three-phase induction motor, a rotating magnetic field is produced, and the rotor begins rotating and the induction motor starts.
What is the difference between inrush current and starting current?
The inrush current is the current transient that flows in the first cycle or so that establishes the magnetic field in the iron. The inrush current can be significantly higher than the locked rotor current with 2 – 2.5 times the start current being quoted by some.
How can we reduce the starting current of an induction motor?
Some of the techniques used to reduce the starting current of a squirrel induction motor:
- Soft starting (voltage control)
- Variable Frequency Drives (Voltage and frequency control)
- Star/Delta Starting.
- Stator impedance and/or resistance starting.
- Autotransformer Starting.
What is the no load current of induction motor?
The I2R losses on the primary side are neglected as they vary with the square of the current, as we know that the no-load current is 20-30% of the full load current, As the motor is running at no load, the total input power is equal to the constant iron loss, friction, and windage losses of the motor.
Why starting current is high in motor?
Why High Starting Currents: Resistance and leakage reactance circuit consist of resistance and leakage reactance of stator and rotor of induction motor connected in series. A load resistance (variable) is connected in series to the fixed rotor and stator impedance. During starting of the motor, slip will be one.
Why is inrush current high?
Inrush current is the instantaneous high input current drawn by a power supply or electrical equipment at turn-on. This arises due to the high initial currents required to charge the capacitors and inductors or transformers. The inrush current is also known as the switch–on surge, or the input surge current.
What is load current in motor?
The no-load current is the current required just to turn the motor shaft with nothing connected. The maximum load current is the maximum continuous current that can be used to continuously drive a load, If the current is higher than that, the motor will run too hot and the insulation will fail eventually.
What are the types of induction motor?
Types of Induction Motors
- Split Phase Induction Motor.
- Capacitor Start Induction Motor.
- Capacitor Start and Capacitor Run Induction Motor.
- Shaded Pole Induction Motor.
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.
Which motor Cannot be started on no load?
A series motor should never be started at no load. With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by overspeed.
Why is DC motor current so high?
Answer: The starting current is high as there is no back EMF or counter EMF present in the armature circuit because atstarting counter EMF is zero. It’s armature has very less resistance due to this it need more current at starting time. Hence DC starters are used to limit the starting current ofmotor.
