To minimize the high-damage problem of switching transients on motor circuits, specify switching system controls that will alleviate the transients themselves.
By R. L. Nailen, P.E.
Electrical Consultant, Hales Corners, Wis.
What is a switching transient, and what can it do to a motor? Taking the second part of the question first: the damage can be severe--shafts twisted, rotors loosened on shafts, coils bent out of shape, or a machine ripped clear off its foundation.
These are the results of high transient (short-time) forces produced by sudden peak currents when a motor circuit is opened then quickly reclosed with the machine running. These switching transients may be repeated frequently in some applications.
As an extreme example, a 300-hp fan motor shaft broke twice before the user realized something was basically wrong. More commonly, however, the damage is gradual. Its true cause goes undetected and the blame for the eventual fatigue failure is placed elsewhere.
Fast switching can have another damaging consequence: transient overvoltage. During starting, a motor gets no more than normal system voltage; often less. Motor standstill impedance is quite low. Current is therefore high, producing high internal forces that the motor is designed to sustain safely.
On the other hand, at or near full speed, the running motor's impedance is much higher. Therefore, normal voltage produces only rated amperes--far below the starting current level. However, if the circuit to that running motor is opened for any reason, then suddenly reclosed, some residual voltage remains within the winding. The magnetic field or flux in the laminations cannot disappear instantly when the winding is de-energized.
Conductors in the coasting rotor cage cut through that field, causing the machine to generate a voltage at its open-circuited terminals that later dies away. A quick reclosure applies the full bus voltage in parallel with that residual amount, producing a total winding voltage that can be destructively high.
Capacitors on the circuit worsen the situation. A power system engineer noted, "In one case, a single fast out-of-phase transfer of a 200-hp vertical induction pump motor sheared four half-inch coupling bolts and noticeably twisted the motor shaft. In another, several years of frequent fast reclosing severely distorted the stator end turns of a 2.3-kv induction air conditioning chiller motor; the machine was on the verge of electrical failure when the damage was revealed by a routine inspection.
"In both of these cases, the problem was exacerbated by the presence of a bank of power-factor-correction capacitors, sized without regard to the possibility of self-excitation during the interruption." More about that problem later.
Which common operating conditions involve fast switching? One is open-transition starting (star-delta, or autotransformer), in which all or part of the motor winding is suddenly disconnected, then reconnected while the motor is either partly or fully up to speed.
In an open transition star-delta starting scheme, switching from "start" to "run" momentarily opens the winding circuit much the same as switching a multispeed motor from one speed to another. One 250/125-hp, 4/8-pole mixer motor failed repeatedly after speed changes of up to 100 times daily. The winding breakdowns were traced to voltage transients caused by the open transition switching. Delaying circuit reclosure long enough for residual voltage to disappear caused a loss of processing time. The mixer might even stop. Restarting could be impossible because of stiffening of the mix during processing.
Jogging is another problem. Used to position machinery for loading/unloading, or for process setup changes, this may involve repeatedly opening and closing the motor circuit below full speed (sometimes called teasing the starter).
A further source of damaging transients is automatic transfer of a motor from one power source to another. This may not occur often, but the transfer is made because continued drive operation after a power failure is important. A transfer that leads to winding damage clearly is self-defeating.
Source transfer on both low- and medium-voltage power systems is increasingly likely in commercial buildings and institutions because of growing concern about power blackouts and the need for uninterrupted data-processing power.
Source transfer allows an operator to feed a motor or group of motors from an alternate supply without shutting down drives. Switching may be automatic or manual. It may occur during a rare emergency or regularly through exercise of control equipment to be sure it is in working order.
In systems of less than 600 volts, the usual switching means is the automatic transfer switch, a pair of multi-pole contactors operated by solenoids or motors. During a transfer, one contactor opens to disconnect the normal power source. Then the second contactor closes, tying the alternate source to the same load. All transfer switches include, to some degree, a way of controlling the dead time during which the load is de-energized. This must either be extremely short, relatively long to allow decay of motor residual voltage, or controlled by circuitry sensing when the residual and oncoming source voltages can safely be paralleled.
Fast transfer switching usually takes four to 10 cycles of dead time, although 10 to 20 cycles is more usual for medium voltage switchgear supplying local distribution systems and for motors above 600 volts. If both sources are live and in phase with each other prior to transfer, an extremely fast transfer can be safe. Residual motor voltage may be high, but it will be so nearly in phase with oncoming source voltage that reclosing damage is not likely. But if the alternate source is a standby generator just being started, that favorable phase relationship cannot be counted upon.
Another source of fast switching trouble for motors is a reclosure on the original circuit (no alternate source involved) following a fault trip-out. A multiple shot recloser may reclose a tripped circuit breaker several times before locking out. The purpose is to avoid shutting down an entire circuit on a transient fault that quickly clears itself, as on an outdoor power line during a storm
Let's examine how these short switching times affect phase relationships between motor residual and line voltages. During normal running, a steady-state magnetic field rotates at synchronous rpm within the motor, pulling the rotor after it at a slightly lower speed. Suppose we call the bus voltage at the motor terminals VB. A balance exists between this and the motor's internally generated counter emf voltage VG (see Figure 1).
Suppose we suddenly disconnect the motor from the bus. The internal magnetic field cannot disappear until the energy stored in it is somehow dissipated. Some of the stored energy is used up as iron losses within the laminations. As the squirrel-cage rotor bars cut through the decaying field, voltages are induced in them. The resulting cage current converts more of the stored energy to heat. Torque is produced, siphoning off still more energy to slow the drive down.
The magnetic field also was interacting with the revolving cage to generate voltage and torque before the circuit was opened. But the field then was rotating at synchronous rpm in step with the applied bus voltage. After disconnection, the only rotation present is the rotor rpm itself. Hence VG must assume the lower frequency associated with that somewhat lower rpm--a frequency that continues to drop as the coasting drive slows down.
Opening the circuit removes the load current, causing the stator IZ drop to vanish. VG must now appear directly at the motor terminals. This voltage is called residual because it resides or remains behind for some time following disconnection.
How long it remains will depend on how quickly the stored electromagnetic energy dissipates. Magnetic field strength varies as
-t/ec
or
1/et/c.
The time after circuit opening is "t"; "c" is called the motor's "open circuit time constant." Residual terminal voltage in an open-circuited motor is directly proportional to magnetic field strength (flux density). Therefore, that voltage also will vary as
t/et/c.
Suppose we let t = c. That reduces the formula to simply 1/e, or 0.368. At c seconds after circuit opening, then terminal voltage will have decreased to only 36.8% of its initial value. Choosing other values of t leads to the characteristic voltage decay curve for any motor.
Connected capacitors have an important effect on this time constant. They remain in parallel with the disconnected machine while it coasts. The capacitance helps maintain winding residual magnetism by storing energy during one half of each voltage cycle, then feeding it back to the winding during the other half. Capacitors that are large enough actually will cause residual voltage to rise, rather than begin to decrease, when the circuit is first opened. Losses and friction still use up the available energy eventually, but it might take a much longer time.
For example, the open circuit time constant of a typical 400-hp, 460-v, 6-pole motor is 0.35 second (21 cycles). A terminal capacitance of 110 kvar will give a VG of 460 volts when the circuit is opened. That capacitance, however, also raises the open circuit time constant to 5.8 seconds--more than 15 times longer.
Inasmuch as the residual voltage VG falls into step with rotor rpm, its frequency gradually drops. The bus voltage, however, remains unchanged at its original frequency. When the circuit is reclosed, the two voltages, bus and residual, are likely to be out of phase with one another. Figure 2 illustrates how this situation develops.
Finally, the peak current at reclosure can trip upstream protective devices. The drive is then inoperative even though no fault exists.
Unfortunately, during a gradual deceleration, these dangerous results can occur at not just one but many different reclosure times. Imagine two drivers starting an auto race. Number 2, unable to hold the pace, falls farther and farther behind. The actual distance around the track separating him from Number 1 at first will increase. Then it begins to decrease as Number 1 gains on him. Eventually, the two meet once more, but Number 2 now is a full lap behind. Again they separate, coming together still later with Number 2 two laps behind, and so on. This can be seen from Figure 2. The resultant voltage VR periodically rises to a peak, then drops off again, each successive peak somewhat reduced by the gradual decline in VG.
Thus, while the circuit is open, danger zones periodically occur during which reclosing must be avoided. It cannot be said that "up to this time reclosing is safe," or "it is only safe after that time." Rather, several safe time regions exist. How many and how broad these zones are depends on the rate of drive slowdown. High-drive inertia may result in only a single zone of any interest. With low inertia, deceleration is fast enough to produce conditions as in Figure 3.
Volts = 1 time rated
Amperes = five to seven times rated
Torque = 0.8 to 1.5 times rated.
If the reclosing VR were 1.25 times rated voltage, as an example, we could expect:
Volts = 1.25 times rated
Amperes = about eight times rated
Torque = 1.25 to 2.4 times rated.
Engineers claim, however, that actual reclosure torque can be eight to 10 times rated, because transient motor behavior doesn't match the starting situation. Even during a normal start, a short-time transient torque appears, typically three to five times the locked-rotor value.
Some authorities calculate reclosure torque peaks of 10 to 20 times rated. A formula in an Institute of Electronic and Electrical Engineers paper gives a maximum ratio of 12.3, when VB and VG at reclosure are 120 deg apart in phase (not 180 deg, as one might suppose).
Such extreme values probably will not appear at the shaft. They are theoretical "air-gap torques," existing electromagnetically within the machine air gap. Elasticity of the rotor-shaft structure uses up some transient torque. More is absorbed in changing the drive speed, especially if inertia is low (a high inertia, strongly resisting speed change can cause the shaft to snap). Furthermore, the high reclosing current transient tends to depress bus voltage to diminish the effect.
Conversely, some real life conditions may worsen the torque transient. One is mechanical resonance. Such a coincidence between line frequency (or a multiple thereof) and the rotating systemÕs natural torsional vibration frequency will amplify the reclosing shock.
Another complication is that starters or breakers seldom close in all three phases at once. Even if they did, the relative phase position of the three line voltage vectors would not be the same at every closure. This can cause wide variation in the voltage (and torque) transients.
The degree of slowdown involved in fast switching is not sufficient for concern about restarting. After four cycles dead time, the larger motor of Figure 3 will have decelerated from 1,785 to 1,645 rpm, at which a circuit reclosure imposes no restarting burden on either motor or power system. The higher inertia drive will slow only from 1,780 to 1,580 rpm during an open-circuit interval of 1/4 second.
However, one way for the user to avoid transient damage is to make sure the circuit stays open long enough for VG to dip below any potentially damaging reclosure level. And that may risk a restart.
Industry generally accepts 133% of rated voltage as a maximum safe limit for VR (some engineers prefer 125%). Meeting that limit requires a delay in circuit reclosure of at least one time constant. Designers may stipulate one and a half time constants, or even two. The issue has been under NEMA review for some time.
In a high inertia fan drive, motor speed may drop only slightly even after several minutes of coasting. After one or more time constants have elapsed, reclosing will not produce high sustained inrush current, nor will the motor undergo the thermal stress of bringing the load back up to full speed from some low rpm.
But suppose the motor is driving a low-inertia pump. That pump will continue demanding high torque, available only from the stored rotational energy in the drive plus that in the decaying magnetic field. As pump output absorbs that energy, the drive decelerates quickly. Reduced rpm means lower pump torque demand too, but the net result still will be virtual motor standstill within 1-1/2 to 3 seconds.
A reclosure then will force the motor to entirely restart its load. Several difficulties may result. Suppose, for example, that reduced voltage starting was required originally because of a weak power system unable to sustain full voltage acceleration. A power interruption at full speed may not drop the starter back to the first step. Hence, the reclosure is at almost full voltage, and the system voltage drop may then be too great for reacceleration to succeed.
A similar problem is that the momentary outage may drop many motors off the line rather than just one. System voltage may be inadequate to restart all of them at once.
Delay in reclosing also may disrupt equipment operation. Contactor dropout, loss of fluid pressure, or unexpected machinery stoppage may be costly. These might not occur with a brief circuit interruption.
A different reclosing or transfer control method can help. For example, relays are readily available to monitor the phase position of VG relative to VB. Reclosure is blocked unless that angle falls below a preset maximum. Such a synchronous transfer or reclosing scheme avoids the need to wait one or more time constants. For the smaller motor of Figure 3, for instance, we can see that a reclosure in five or six cycles would be safe. If the controls cannot respond that quickly, another safe period exists between 12 and 15 cycles. Without a phase angle relay, or phase band monitor (see Figure 4), motor safety dictates waiting five to 10 times that long.
Undervoltage relays at the motor will sense VG, allowing reclosure once it is low enough. However, such relays tend to be sensitive to frequency and thus will be affected by the drive slowdown rate. The desirable trip setting also may cause nuisance operation during a normal start.
Why not overdesign the motor itself to safely withstand an out-of-phase reclosure or transfer? That would be extremely costly, at best. Insulation, coil bracing, shafting, keys, frame--all must be beefed up. Just because a shaft may break, for instance, does not mean a stronger shaft will solve the problem. Instead, some other motor component then becomes the weakest, subject to reclosure damage.
To minimize fast switching transient problems on motor circuits, do not expect to find a motor that will withstand the worst conditions. Instead, look for switching system controls that will minimize the transients themselves.
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