Voltage loss control to coil of motor contactor 3

[moondog]

We are looking into ways to minimize the effects of voltage loss to our plant.We run a continous process,with many motors mainly running pumps. We have boilers & chillers also. We lose power several times a month (power blips) these blips only last about 3 cycles. Sometimes these power blips turn our motors off. We have to rely on the plant operators getting everything turned back on. We are looking at putting something on the coil that will give us a 1/2 second delay before the contactor drops out. I have done this with vacuum before I am not familiar with any other type. Could someone give me the pro's and con's of this? Would a capacitor bank help this problem?

 

[electricpete]

From IEEE Buff book below is excerpted a long discussion on undervoltage protection for motors. One thing I note it appears they are suggesting motors <200hp are not very susceptible to damage from reclose.

9.3 Types of Protection
9.3.1 Undervoltage
9.3.1.1 Purpose, The usual reasons for using undervoltage protection are
as follows:
(1) To prevent possible safety hazard of motor automatic restarting when
voltage returns following an interruption
(2) To avoid excessive inrush to the total motor load on the power system, and
the corresponding voltage drop, following a voltage dip, or when voltage returns
following an interruption
9.3.1.2 Instantaneous or Time Delay. Undervoltage protection will be
either instantaneous (no intentional delay) or of the time-delay type. Time-delay
undervoltage protection should be used with motors important to production
continuity of service, providing it is satisfactory in all respects, to avoid
unnecessary tripping on voltage dips that accompany external short circuits,
Examples follow of nonlatching starters where time-delay undervoltage protec-
tion is not satisfactory and instantaneous undervoltage must be used.
(1) Fused or circuit breaker combination motor starters having ac voltage held
contactors, used on systems of low three-phase fault capacity. With the usual
time-delay undervoltage scheme the contactor could drop out on the low voltage
accompanying the fault before the fuse or circuit breaker opens. The contactor
could then reclose into the fault. This problem does not exist if the fault capacity
is high enough to open the fuse or circuit breaker before the contactor interrupts
the fault current. (See note following the next example.)
(2) Synchronous motors used with starters having ac voltage held contactors.
With the usual time-delay undervoltage scheme the contactor could drop out on
an externally caused system voltage dip and then reclose reapplying the system
voltage to an out-of-phase internal voltage in the motor. The high initial inrush
could damage the motor winding, shaft, or foundation. This problem could also
occur for large-horsepower high-speed squirrel-cage induction motors. It usually
is not a problem with the 200 hp and smaller induction motors with which
voltage held contactor starters are used because the internal voltages of these
motors decay quite rapidly.
NOTE: The foregoing two limitations could be overcome by using a separate ac power source for
control or dc battery control on the contactor to prevent its instantaneous dropout. In other words, the
time-delay undervoltage feature can be applied directly to the main contactor.
(3) Motors used on systems having fast automatic transfer or reclosing where
the motor must be tripped to protect it before the transfer or reclosure takes
place.
(4) When the total motor load having time-delay undervoltage protection will
result in more inrush and voltage drop after an interruption than the system can
satisfactorily cope with. The least important of the motors should have instan-
taneous undervoltage protection. Time-delay undervoltage protection of selec-
tively chosen delays could be used on the motors whose inrush the system can
handle.
9.3.1.3 With Latching Contactor or Circuit Breaker. These motor
switching devices inherently remain closed during periods of low or zero ac
voltage. The following methods are used to trip (open) them:
(1) Energize shunt trip coil from dc battery.
(2) Energize shunt trip coil from a separately generated reliable source of ac.
This ac source must be electrically isolated from the motor ac source in order to be
reliable.
(3) Energize shunt trip coil from a capacitor charged through a rectifier from
the ac system. This is commonly referred to as capacitor trip.
(4) Deenergize a solenoid and allow a spring to be released to trip the contactor
or circuit breaker. This is commonly referred to as a dc trip scheme.
Items (1)-(3) are usually used in conjunction with voltage-sensing relays (see
9.3.1.6).
Item (4) could have the solenoid operating directly on the ac system voltage.
Alternatively, the solenoid could operate on dc from a battery, in which case a
relay would sense loss of ac voltage and deenergize the solenoid. The solenoid
could be either instantaneous or time delayed using a dashpot arrangement.
9.3.1.4 With AC Voltage Held Main Contactor. Since the main contactor
(which switches the motor) will drop out on loss of alternating current, it provides
an instantaneous undervoltage function. There are two common approaches to
achieve time-delay undervoltage protection:
(1) Permit the main contactor to drop out instantaneously but provide a timing
scheme (which will time when ac voltage is low or zero) to reclose the main
contactor providing normal ac voltage returns within the preset timing interval.
Some of the timing schemes in use are as follows:
(a) Capacitor charged through a rectifier from the ac system, The charge
keeps an instantaneous dropout auxiliary relay energized for an adjustable
interval, which is commonly 2 or 4 S.
(b) Standard timer that times when deenergized (pneumatic or induction
disk, etc).
(2) Note that two-wire control is sometimes used with an ac voltage held main
contactor. This control utilizes a maintained closed start button, or operates from
an external contact responsive to some condition such as process pressure,
temperature, level, etc. The main contactor drops out with loss of ac but recloses
when ac voltage returns. This arrangement does not provide undervoltage
protection, and should not be used if automatic restarting could endanger
personnel or equipment.
9.3.1.5 With DC Voltage Held Main Contactor. With this arrangement
the contactor remains closed during low or zero ac voltage. Time-delay under-
voltage protection is achieved using voltage-sensing relays (see 9.3.1.6).
9.3.1.6 Voltage-Sensing Relays. The most commonly used type is the
single-phase induction disk undervoltage time-delay relay. Since a blown control
fuse will cause tripping, it is sometimes desirable to use two or three of these
relays connected to different phases and wire them so that all must operate before
tripping will occur.
Three-phase undervoltage relays are available. Many operate in response to
the area of the voltage triangle formed by the three-phase voltages.
In applications requiring a fixed time delay of a few cycles, an instantaneous
undervoltage relay is applied in conjunction with a suitable timer (see 9.3.19).
When applying undervoltage protection with time delay, the time-delay
setting should be chosen so that time-delay undervoltage tripping does not occur
before all external fault-detecting relays have an opportunity to clear all faults
from the system. This recognizes that the most frequent causes of low voltage are
system faults, and when these are cleared most induction motors can continue
normal operation. In the case of induction disk undervoltage relays it is
recommended that their trip time versus system short-circuit current be plotted
to ensure that they do not trip before the system overcurrent relays. This should
be done for the most critical coordination condition, which exists when the system
short-circuit capacity is minimum.
Typical time delay at zero voltage is 2 to 5 S.
For motors extremely important to continuity of service, such as some auxilia-
ries in electric generating plants, the undervoltage relays are used to alarm only.

 

[jbartos]

Suggestion: The previous posting just confirms the potential advantage of Energy Storage Modules (EMSs). They are heavily researched and designed for such applications as ships, where the overall space is limited and hardware weight is carefully scrutinized. The flywheel energy storage is one of the most effective for very short energy needs.
The Buff Book reference is not focusing on the global solution. It is concentrating on protective solutions that are also good; however, on the large scale, they can become less reliable and costly.

 

[moondog]

Thanks Pete,
This information is very useful. I spoke with the utility company they said they made some changes (would not give details) and the power blips should decrease in number. We have not had one (power blip) in the last two weeks. In combination with the utility company and this information I belive we can greatly reduce the number of upsets.
Thanks again for everybodys time on this issue.