Electrical Apparatus magazine, April 2004 The Trouble with Plugging

"Plugging" is the deceleration of a polyphase motor by reversing applied voltage phase rotation at full speed. Although this draws high line current, even exceeding that of a normal start, plugging is the simplest and quickest braking method (sometimes within less than one revolution).

Except for small motors, however, manufacturers recommend against plug-stopping for two reasons. First, it generates heat losses in both stator and rotor windings that far exceed those during normal acceleration. Second, contactor opening and out-of-phase reclosing causes severe torque and current transients. The shaft torque may be several times the normal breakdown value. Stator coils can undergo damaging stress.

Bringing the inertia of any machine to full speed generates major heat loss in the driving motor. That heat is a direct function of the square of the initial slip (1.0 for a motor starting from rest) minus the square of the final slip (usually a small fraction). For a phase reversal with the rotor still turning at full forward speed, initial slip becomes 2.0, and the final slip at standstill is 1.0. The difference in their squares is 4.0 minus 1.0, or 3.0. Hence, motor heating loss for a plug stop is three times that for an acceleration.

That ratio is not exact, for two reasons. First, torque required by the load retards motor acceleration, adding heat loss, whereas during plugging the load torque aids during deceleration. Second, during deceleration the system inertia tends to oppose the change in speed. Still, designers normally consider one plug stop as the thermal equivalent of three starts. Besides the thermal effect, plugging adds torsion bal shock to couplings and other mechanical system components, and transient torques may excite damaging mechanical resonance.

Finally, plugging cannot provide holding torque to keep the motor at standstill. If power is not removed slightly before zero rpm is reached, the motor will begin to re-accelerate in the reverse direction. Special speed-sensing controls may be needed to prevent that.

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Featured Technical Article
April 2004

On this page is a summary of the Electrical Apparatus April 2004 featured technical article, by Richard L. Nailen, P.E. , "The Trouble with Plugging" Probleme mit Gegenstrombremsungen ... Le problème du freinage par inversion de phases ... El problema del frenado por inversión de fases....

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