Voltage loss control to coil of motor contactor
Voltage loss control to coil of motor contactor
(OP)
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?





RE: Voltage loss control to coil of motor contactor
We have a different ways to fix this problem but I recomend, by a better way, one associate between one "time relay with a delay in a de-energization. This equipment, very cheap, can be sold in a different scales of times and may provide other regulations if a half second shows opperate not appropriated.
This "relay" is mounted in a rail or by a screws and have reduced dimensions, but ONLY operate at low voltage ( I didn't saw anything greather than 380 Vac - single phase ). By a suggestion you can monitore only one or the three fases of your plant, using a control in a serial connection ( if one goes out - everybody goes out ) or a parallel ( only if everybody goes out, the process goes out ), as your exclusive choice.
I hope have helped you, givind my apologizes for my poor english.
Nice to type.
RE: Voltage loss control to coil of motor contactor
We have a different ways to fix this problem but I recomend, by a better way, one associate between one "time relay with a delay in a de-energization. This equipment, very cheap, can be sold in a different scales of times and may provide other regulations if a half second shows opperate not appropriated.
This "relay" is mounted in a rail or by a screws and have reduced dimensions, but ONLY operate at low voltage ( I didn't saw anything greather than 380 Vac - single phase ). By a suggestion you can monitore only one or the three fases of your plant, using a control in a serial connection ( if one goes out - everybody goes out ) or a parallel ( only if everybody goes out, the process goes out ), as your exclusive choice.
I hope have helped you, givind my apologizes for my poor english.
Nice to type.
RE: Voltage loss control to coil of motor contactor
One thing that comes to mind is that the contactor's dropping out may serve a useful purpose in protecting the motor. Removing and restoring power after an interval 3-10 cycles can result in out-of-phase reenergization before the motor voltage has decayed... large transient torques which can do big damage.
I don't think that contactors are specifically selected to protect against that scenario... but it is an added benefit which may be important to you considering that your power environment includes those spikes.
I'm sure there are a lot more pertinent factors that the other folks can add.
RE: Voltage loss control to coil of motor contactor
Thanks
RE: Voltage loss control to coil of motor contactor
Install a UPS (rectifier/invertor and charger) system with batteries sized only for the control power requriements. You need to feed all control power from this separate UPS system or systems.
The size of UPS will be small as you are not backing up the motor power but only controls. If the problem is as big a concern as you are suggesting, the management should not have problem spending some money.
Try on one or two MCCs first and see how it responds.
RE: Voltage loss control to coil of motor contactor
RE: Voltage loss control to coil of motor contactor
Electricpete is absolutely correct about torque loading and big problems. The first time you break a shaft or destroy a pump due to some type of “ride-through” control system you will have second thoughts. The blinks you describe in a lot of cases are due to power system auto-reclosers. You may want to check with the power company and see if that is your problem and what help they can offer. Shorter times on the recloser is not the solution.
For your more critical loads like boiler feed pumps you may want to consider a VFD. Most VFD's I've seen have enough stored energy in the bus to ride through a 3 cycle blip. They also offer an auto-restart that will catch a motor on the fly and prevent the related torque problems.
Pete:
Good reply!
RE: Voltage loss control to coil of motor contactor
RE: Voltage loss control to coil of motor contactor
RE: Voltage loss control to coil of motor contactor
There are electrically held versus mechanically held contactors in some sizes. But, there is also a very serious tradeoff between equipment and personnel safety with the two.
RE: Voltage loss control to coil of motor contactor
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Voltage loss control to coil of motor contactor
For 15 years, every PLC program we have is designed to hold off for 5 minutes after an outage to allow the line to stabilize.
Hard to analyze results, very hard. But our feeling is that we have saved ourselves a lot of trouble.
I realize that perhaps your plant cannot wait 5 minutes to restart. But perhaps if management knew more they would be willing to invest sufficient money to solve problem.
How about a scope data logger to see how the power looks just before and after these blips? If the voltage is sagging just before, or spiking just after, could be useful information in your solution.
PUMPDESIGNER
RE: Voltage loss control to coil of motor contactor
For a) contactor operated motors, there are several ways of achieving like 1. by providing control supply of contactors from UPS or DC supply. 2. by providing a timer contact parallel to the start push button contact (if this is provided without auo-manual switch then the stop push button need to be lockable type).
3. by providing mechanically latched contactors
4. by providing auto-start from Process conditions like say discharge of pump, flow, level, etc.
For b) breaker operated motors, trip through under voltage protection can be time delayed by 1 or 2 secs as per the requirement.
For c) some VFDs are provided with "ride through feature" which can be used , but at the same time for contactors and control supply , if used in vfd scheme, treatment as mentioned in a) above need to be done. Again for vfds one has to be careful about the setting of overcurrent protection i.e. the drive must not trip while re-accelerating.
For all the above cases, the motor need to be suitably designed for the worst case of imposition of out of step voltage at 180 deg
RE: Voltage loss control to coil of motor contactor
RE: Voltage loss control to coil of motor contactor
RE: Voltage loss control to coil of motor contactor
RE: Voltage loss control to coil of motor contactor
I've seen lightning hits or animal interference cause intermittent outages from surge supressors and reclosers, which are unavoidable. I've also seen switching transients from cap banks on the line, voltage sags and swells from incorrectly operating utility voltage regulators, and many other assorted problems caused by the utility. But I've never seen an outage problem like yours where the customer is required to absorb all costs of this utility problem.
There are thousands of continuous and non-continuous-process plants in the world (literally hundreds of which are customers of mine) with motor starters just like yours which don't have outages like you do. Why should you just accept this as normal?
RE: Voltage loss control to coil of motor contactor
We will see how it goes.
RE: Voltage loss control to coil of motor contactor
If there are many loads affected by the short blips, then the global solution in terms of energy storage modules may become competitive with various local "approximate" and "risky" fixes that may or may not work very well.
50ms to 200ms is harmful to many motor loads except those that have a high inertia, e.g. large axial fans, rolling mills, etc. Fully loaded low inertia motor-load sets decelerate very quickly, and the motor internal emf decays fast. It also becomes out of the power supply synchronism. This causes noticeable mismatch in the motor and power supply voltage, which is detrimental to the motor-load set.
If some VFDs have ride-through feature, than the short blips will be safely overcome.
RE: Voltage loss control to coil of motor contactor
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.
RE: Voltage loss control to coil of motor contactor
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.
RE: Voltage loss control to coil of motor contactor
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.