Figure 1. Typical ac induction motor starting characteristic.
With increasing emphasis on power quality, voltage transients within a facility associated with starting large motors are gaining more attention. Add the fact that the new energy-efficient Design E motors have higher starting currents than we are used to, and it becomes increasingly important to have a good handle on the starting methods of large hvacr equipment motors. This month, we'll define the motor starting challenge, and follow up in the next column with a discussion of various methods of meeting it.

Inrush Current or Locked Rotor Current?

The current transient that occurs when a motor is energized is variously called inrush current, starting current, or locked rotor current. In most cases, those using these terms are all referring to the same thing: high current that flows during acceleration of the motor from rest to rated speed. In actuality, this current transient is composed of two separate components that are properly referred to individually as inrush current and locked rotor current.

Inrush current is required to establish a magnetic field in the core steel of electric machines such as motors and transformers each time voltage is applied. The peak value of inrush current, which can be as high as 20 times rated current, will be different each time the equipment is energized due to residual magnetism in the core. While reaching a very high peak value, the inrush component in a typical motor decays within the first few cycles of the line voltage. The peak value and duration are characteristics of the motor and completely independent of the load. This inrush current must be considered when selecting circuit breakers for motor circuits, but is only a minor part of the starting transient; the locked rotor current is usually the obstacle to successfully starting the motor without adverse effects on the power system.

At rated load the rotor of an ac induction motor turns at slightly less than synchronous speed, the difference being termed slip. For example, a nominally 3,600-rpm motor may have a rated speed of 3,540 rpm; a nominally 1,800-rpm motor a rated speed of 1,760 rpm, and so on. Rotor slip is what allows the motor to develop torque, and in fact, if the load increases, tending to increase the slip, the effect is an increase in rotor torque (and current) to match the load requirement. This situation is analogous to matching a pump to a system using curves of head vs. flow. For any given load, the operating speed of the motor will be determined by the point where the motor characteristic speed-torque curve crosses the load characteristic speed-torque curve.

Whenever the motor is operated with higher-than-rated slip, the current will exceed the rated value. Overloads decrease the rotor speed slightly and result in slight increases in current. However, if the rotor speed is reduced below about 85% of rated, the current rises to a high value that is relatively independent of speed. Conversely, when starting, the motor current remains at this high level until the motor and load combination has accelerated to about 85%, at which point it will begin to drop off rapidly to normal running current. Typical variation of motor current and torque with speed is shown in Figure 1.

Voltage Drop is the Culprit

When this transient starting current flows through the power system from the utility source to the motor, a voltage drop equal to the product of the current magnitude and the system impedance is produced. Just as pressure drop associated with a high flow rate reduces the pressure along a pipe, this voltage drop reduces the voltage along the circuit supplying the motor, creating opportunities for problems.

Motors are designed to start with a specified voltage at their terminals and their developed torque is proportional to the square of the voltage. Excessive starting voltage drop between the source and the motor can result in low terminal voltage and inadequate torque to accelerate the load to rated speed within the design time. At best this results in motor trips during starting attempts; at worst it can damage the motor due to excessive heating during acceleration.

Other loads connected to the power system between the source and the motor may also be affected by reduction in voltage during the starting period. Because the sensitivity of many types of equipment to voltage sags depends on the duration, this effect tends to be worse the longer the acceleration time of the motor/load combination.ES