Motor inrush current

By Andy Sagl

Motor inrush current – the large spike of current drawn by a motor as it starts up – can have an adverse effect on supply networks and protection systems, as Andy Sagl, Product Manager at Megger, explains.

When an AC motor is energised using a conventional contactor starter, a large spike of current flows through the motor and the conductors feeding it. This current, which is well in excess of the rated current shown on the motor’s nameplate, is needed to overcome the combined inertia of the stationary motor shaft and the load the motor is driving.

When three-phase power is applied to a motor, the stator windings, which are the stationary windings in the motor frame, are energised. The current in these windings generates a rotating magnetic field which induces current in the rotor winding, which is the winding on the rotating part of the motor. The rotor current also produces a magnetic field and the fields produced by the stator and rotor interact in a way that causes the rotor to rotate.

Figure 1: Motor inrush current at starting

Figure 2: Line voltage (top) and motor current during motor starting

Figure 3: Effect of load on motor inrush current duration

Figure 4: Effect of supply voltage on motor inrush current duration

The rotor speeds up until it reaches a speed close to synchronous speed, which is the speed of the rotating field produced by the stator. The rotor never quite reaches synchronous speed however; because if it did, there would be no induced rotor current and the motor would produce no torque. The difference between the actual speed of the rotor and the synchronous speed is usually expressed in terms of slip, where: slip = (synchronous speed - speed of rotation) ÷ synchronous speed

When the motor is stationary, the slip is 1. When it is running normally, the value of slip depends on the load, but is typically ranges from around 0.05 for small motors to as little as 0.01 for large motors.

At start up, the slip = 1 and this large value of slip is the biggest contributor to the inrush current. As the rotor speeds up, the slip decreases and the inrush current falls to the normal running current of the motor, as shown in Figure 1. The magnitude of the inrush current depends on the type of motor and the starting method. For standard industrial motors started directly on line, inrush currents between eight and ten times the normal running current are typical. For high efficiency motors, the inrush current can be even higher.

Figure 5: Inrush current variation for a motor over a period of two weeks

Figure 6: Current transients (bottom) and voltage dips resulting from changes in load on a motor

The inrush current can cause motor protective devices (overloads and fuses) to operate if these have not been correctly selected, but more typically the voltage dip caused by the large current flow (see Figure 2) causes malfunctions in other devices connected to the same circuit as the motor. The voltage dips can trip controllers and loads off line. Constant power devices will increase their current draw to compensate for the lower voltage, which can lead to the tripping of over-current protection devices. And, in severe cases, the voltage dip may be so great that the motor cannot develop enough torque to start.

Motor load, as well as the characteristics of the supply system, affect the motor inrush current. A heavily loaded motor will draw inrush current for a longer time than a lightly loaded motor, as can be seen in Figure 3. Similarly, if the supply voltage is low, the motor starting time will be extended along with the duration of the inrush current, as is shown in Figure 4. This makes it more likely that protective devices will trip.

When characterising motor inrush current, a waveform capture of at least several seconds is required so that the current can be observed from inrush through to steady state. In addition, a single inrush test may not provide enough information to resolve problems. Motors often start and stop multiple times throughout the day, with varying loads and changing supply voltages affecting the inrush current. Monitoring should, therefore, be carried out over an extended time period. The motor in Figure 5 was monitored for two weeks and it is easy to see that inrush current varied significantly over that time.

In facilities where process changes take place, these changes can alter the load on the motor and this will affect the current it draws. In applications of this type, it is essential to monitor the motor throughout the production cycle as changes in motor load can cause current transients which trip protection devices or create voltage dips that trip other equipment off line (see Figure 6).

To accurately assess the operation of a motor and its impact on other equipment, it is essential to monitor the motor for at least one full production cycle and ideally for several cycles. Voltage, current, active power, reactive power, apparent power and power factor should all be monitored on a production-cycle-by-production-cycle basis, because values aggregated over time are often misleading.

In addition, a current swell trigger should be implemented such that when the current in the circuit being monitored exceeds a preset value, it will trigger waveform capture. The capture should last for at least 10 seconds, looking at all channels simultaneously. This will ensure that all of the data from the initial inrush through to steady state is captured every time the motor is started.

As we have seen, the inrush current associated with motor starting can lead to a range of problems in power systems. However, with a good power quality analyser, tracing the source of the problem is usually a straightforward matter.

The remedy will depend on the application, but in some cases it may, for instance, be possible to replace conventional electromechanical motor starters with soft starters or variable speed drives. These provide controlled acceleration of the motor during starting, and greatly reduce the magnitude of the inrush current. They also reduce mechanical wear and tear on the motor and the load it is driving and, in the case of variable speed drives, they often make it possible for energy efficiency to be increased significantly.