Motor controls
Home » Blog » Motor controls » Voltage transient/inrush current in induction motors

# Voltage transient/inrush current in induction motors

The voltage transient which occurs whenever there is a sudden change in current in an inductive device. Inductors resist a sudden current change.

V=L di/dt

In electric motors this occurs at start up when the contactors close and shut down when the contactors open. Soft starters reduce the start up transient, but not the shutdown transient.

This also occurs with variable frequency drives which switch the current rapidly and repeatedly.

Voltage transients of 2 to 5 times line voltage are common. This is a primary reason for failure of weakened motor insulation systems. Test standards require high voltage Hipot and Impulse testing of insulation systems in order to ensure that a motor can withstand these transients.
Inrush is something we have always had to deal with, especially with motors that are direct on the line start. The inrush can be as high as seven times the nameplate current. The damage created can be minimal if the motor is started up in the morning and them runs all day.

A motor that runs on a variable frequency drive can be subjected to the same level of voltage spike all day long. The transients are so bad that the conductor we used in electric motors was no longer good enough. The covering has to be modified to be spike resistant. We now use "PULSE SHIELD INVERTER DUTY CONDUCTOR"
There is one situation that creates a huge inductive spike. Take a motor, lets say it is driving a fan, and it is coasting to a stop. The operator decides to push the start button while it is still coasting. It is a misconception that because the motor is already in motion that you will reduce the starting inrush. You will cause more damage to the insulation system by doing this than you could ever imagine.

The inrush current at start-up for a motor is not an inductive spike. In fact, the small inductance in a motor winding is a slight impedance to the inrush (hence the term), though very slight unless it is a high inductance winding.
An inductive spike is the spike that occurs when voltage is quickly switched between windings. The inductance will not allow current to change instantaneously and must go somewhere.
Changing voltages when the motor is moving because the inductance is an energy storage device. If you reverse voltage on a winding in a permanent magnet motor while the motor is active, the voltage on the winding is momentarily doubled, in theory, but the released energy in the winding can cause huge spikes when the back EMF is no longer opposed by the applied voltage, etc.

Calculate (1 + 3) =

You may also like:

Well first let get one thing straight for transformers: the higher the line frequency, the lower the core (iron) losses! The core power loss are proportional to kf*B^2 approximately for any machine, dynamic or ...
When a choosing a power cable for a motor, we prefer using one larger diameter cable than two smaller diameter cables in parallel, although it would cost less to do so. Why?
The rotating electrical machines have basically three noise sources: The ventilation system The rolling bearings Electromagnetic excitation Bearings in perfect conditions ...
Making a connection of 3 phase motor the nameplate shows different voltages for delta it is 380-400 volt and 660-690 volt for star, what option should be selected? the supply Line to Line voltage is ...
This is a finite element analysis tool for various applications. In power we get the voltage (stress) distribution in equipment like cables, bends in cables etc including stator winding of generators.
Gozuk Blog: all about electric motor control & drives industries development in energy saving applications.

Featured

Like pumps, fans consume significant electrical energy while serving several applications. In many plants, the VFDs (variable ... A frequency inverter controls AC motor speed. The frequency inverter converts the fixed supply frequency (60 Hz) to a ... Motor starter (also known as soft starter, motor soft starter) is a electronic device integrates soft start, soft stop, ... Soft starter allows the output voltage decreases gradually to achieve soft stop, in order to protect the equipment. Such as the ... Soft Starter reduces electric motor starting current to 2-4 times during motor start up, reduces the impact to power grid during ...

In Discussion