Motor-Starting

Motor-Starting Sags

Motors have the undesirable effect of drawing several times their full

load current while starting. This large current will, by flowing through

system impedances, cause a voltage sag which may dim lights, cause

contactors to drop out, and disrupt sensitive equipment. The situation

is made worse by an extremely poor starting displacement factor—usually

in the range of 15 to 30 percent.

The time required for the motor to accelerate to rated speed increases

with the magnitude of the sag, and an excessive sag may prevent the

motor from starting successfully. Motor starting sags can persist for

many seconds, as illustrated in Fig. 3.31.

Motor-starting methods

Energizing the motor in a single step (full-voltage starting) provides low

cost and allows the most rapid acceleration. It is the preferred method

unless the resulting voltage sag or mechanical stress is excessive.

Autotransformer starters have two autotransformers connected in

open delta. Taps provide a motor voltage of 80, 65, or 50 percent of system

voltage during start-up. Line current and starting torque vary

with the square of the voltage applied to the motor, so the 50 percent

tap will deliver only 25 percent of the full-voltage starting current and

torque. The lowest tap which will supply the required starting torque

is selected.

Chapter Three

Resistance and reactance starters initially insert an impedance in

series with the motor. After a time delay, this impedance is shorted out.

Starting resistors may be shorted out over several steps; starting reactors

are shorted out in a single step. Line current and starting torque

vary directly with the voltage applied to the motor, so for a given starting

voltage, these starters draw more current from the line than with

autotransformer starters, but provide higher starting torque. Reactors

are typically provided with 50, 45, and 37.5 percent taps.

Part-winding starters are attractive for use with dual-rated motors

(220/440 V or 230/460 V). The stator of a dual-rated motor consists of

two windings connected in parallel at the lower voltage rating, or in

series at the higher voltage rating. When operated with a part-winding

starter at the lower voltage rating, only one winding is energized initially,

limiting starting current and starting torque to 50 percent of the

values seen when both windings are energized simultaneously.

Delta-wye starters connect the stator in wye for starting and then,

after a time delay, reconnect the windings in delta. The wye connection

reduces the starting voltage to 57 percent of the system line-line voltage;

starting current and starting torque are reduced to 33 percent of their

values for full-voltage start.

Estimating the sag severity during

full-voltage starting

As shown in Fig. 3.31, starting an induction motor results in a steep dip

in voltage, followed by a gradual recovery. If full-voltage starting is

used, the sag voltage, in per unit of nominal system voltage, is

where V(pu) = actual system voltage, in per unit of nominal

kVALR = motor locked rotor kVA

kVASC =system short-circuit kVA at motor

Figure 3.32 illustrates the results of this computation for sag to 90 percent

of nominal voltage, using typical system impedances and motor

characteristics.

If the result is above the minimum allowable steady-state voltage for

the affected equipment, then the full-voltage starting is acceptable. If

not, then the sag magnitude versus duration characteristic must be

compared to the voltage tolerance envelope of the affected equipment.

The required calculations are fairly complicated and best left to a

motor-starting or general transient analysis computer program. The

following data will be required for the simulation:

v  Parameter values for the standard induction motor equivalent circuit:

R1, X1, R2, X2, and XM.

■ Number of motor poles and rated rpm (or slip).

■ WK2 (inertia constant) values for the motor and the motor load.

■ Torque versus speed characteristic for the motor load.