General - Motors are magnetic devices. They rely on the interaction between magnetic fields to create rotation. Before this rotation takes place and the magnetic fields are in their steady-state condition, a large value of current will normally flow into the motor. This is called "inrush".
If we don't properly account for inrush in our designs, there could be two different types of problems. First, if the inrush current is too high, we may see excessive voltage drop in the system. This dip in voltage could affect the performance of other loads and lead to unintended system failures. Alternatively, if there is too much impedance and the motor receives a substantially reduced starting voltage, the motor may take too long to start. Or worse, the motor may fail to overcome its own inertia and will stall out.
Motor Starting Voltage Drop - In order for a motor to be able to turn over during starting, there needs to be sufficient voltage at the motor terminals, even when the locked-rotor-current (inrush current) is initially pulled in.
Imagine a system with a very high impedance . If the motor is switched into service and attempts to draw the inrush current to magnetize, there will be a problem. The voltage will be so low that the motor isn't getting the power it needs.
How low is too low? Motor manufacturers will provide engineers with the minimum starting voltage. 80% of nominal voltage is a typical minimum value. Likewise, the inrush current (locked-rotor current) can be taken from a motor datasheet to perform this voltage drop calculation. Locked rotor current is typically around 6.5x the full load current value.
Motor Starting Inrush Reduction Methods - What happens if inrush causes the motor voltage to dip below its minimum rating? Do we have to redesign the entire system?
No! There are ways that we can reduce inrush that don't require substantial redesign. Here's a quick sampling:
Upsize motor feeder/branch circuit conductors - The voltage drop flowing through these conductors is directly related to their impedance. Larger conductors can significantly reduce this impedance, yielding improved voltage drop. The tradeoff of this is cost and installation effort.
Reduce the impedance of upstream transformers - Ideal transformers don't affect the amount of real and reactive power flowing through them, but real transformers do. Practical transformers have a certain amount of impedance, a predominantly inductive reactance that causes voltage drops across transformers. Larger transformers can usually be specified with lower impedances (to a certain extent). The tradeoff of this is an increased fault current coming from the transformer.
Use a more sophisticated starting method - The discussion of inrush effects up to this point has been based on the use of "full voltage" starting. Full voltage starting consists of closing a contactor, switch, or breaker to energize a motor with the full line-to-line voltage of the source. There are alternative ways to energize a motor, including soft starters and wye-delta starters. Soft starters use electronic components to start a motor with a smooth transition from zero voltage to rated voltage, controlling the current that enters and preventing excessive voltage drop. Soft starters are the most expensive motor starting device and can produce harmonics due to their nonlinear behavior. Wye-delta starters configure a motor to start based on line-neutral voltage across the motor windings until a certain state is reached. Then, the winding is switched to delta, to provide full, rated voltage. A Wye-delta starter is like a soft starter with only 2 discrete steps. They are less expensive than a wye-delta, but less sophisticated.
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