Speed Regulation, Percent Slip and Start Current.docx

7/27/2019 Speed Regulation, Percent Slip and Start Current.docx

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Speed Regulation, Percent Slip and Start Current

Posted byZone 4infoAugust 18 - 314 views - Filed inPower Systems

The squirrel-cage induction motor has very good speed regulation characteristics (the ratio of

difference in speed from no load to full load). Speed performance is measured in terms of

percent slip. The synchronous speed of the rotating field of the stator is used as a reference point.Recall that the synchronous speed depends on the number of stator poles and the operating

frequency. Because these two quantities remain constant, the synchronous speed also remains

constant. If the speed of the rotor at full load is deducted from the synchronous speed of the

stator field, the difference is the number of revolutions per minute that

the rotor slips behind the rotating field of the stator. The value is expressed as percent slip

according to the following formula:

Example 2:

If the three-phase AC induction motor used in Example 1 has a synchronous speed of 1200 RPM

and a full-load speed of 1140 RPM, find the percent of slip.

Synchronous speed (Example 1) = 1200 RPM Full-load rotor speed = 1140 RPM
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For a squirrel-cage induction motor, as the value of percent slip decreases toward 0%, the speed

performance of the motor is improved. The average range of percent slip for squirrel-cageinduction motors is 2% to 6%. Figure

shows a speed curve and a percent slip for a squirrel-cage induction motor operating between noload and full load. The rotor speed at no load slips behind the synchronous speed of the rotating

stator field just enough to create the torque required to overcome friction and windage losses at

no load. As a mechanical load is applied to the motor shaft, the rotor tends to slow down. This

means that the stator field (turning at a fixed speed) cuts the rotor bars a greater number of timesin a given period. The induced voltages in the rotor bars increase, resulting in more current in the

rotor bars and a stronger rotor field. A greater magnetic reaction between the stator and rotor

fields exists, which causes a stronger twisting effect, or torque. This also increases stator currenttaken from the line. The motor is able to handle the increased mechanical load with very little

decrease in the speed of the rotor. Typical slip-torque curves for a squirrel-cage induction motor

are shown in Figure .


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The torque output of the motor in pound-feet (lb-ft) increases linearly with an increase in the

value of percent slip as the mechanical load is increased to the point of full load. Beyond full

load, the torque curve bends and finally reaches a maximum point called the breakdown torque.

If the motor is loaded beyond this point, there will be a corresponding decrease in torque untilthe point is reached where the motor stalls. However, all induction motors must have some slip

in order to function. Starting torque is not shown, but is approximately 300% of running torque

for a generic AC motor.

Starting Current

When a three-phase AC induction motor is connected across the full line voltage, the startingsurge of current momentarily reaches as high as 400% to 1000% or more of the rated full-load

current. At the moment the motor starts, the rotor is at a standstill. At this instant,

therefore, the stator field cuts the rotor bars at a faster rate than when the rotor is turning. Thismeans that there will be relatively high induced voltages in the rotor, which cause heavy rotor

current. The resulting input current to the stator windings is high at the instant of starting.

Because of this high starting current, starting protection rated as high as 300% of the rated full-load current for nontime-delay fuses is provided for squirrel-cage induction motor installations.

Most squirrel-cage induction motors are started at full voltage. If there are any questions

concerning starting large motors at full voltage, the electric utility company should be consulted.In the event that the feeders and protective devices of the electric utility are unable to handle thelarge starting currents, reduced voltage starting circuits must be used with the motor.

Reference:

- See more at: http://www.zone4info.com/articles/558/speed-regulation-percent-slip-and-start-

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Speed Regulation, Percent Slip and Start Current.docx