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?

 

[HawkBrazil]

Hello Friend,

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.

 

[HawkBrazil]

Hello Friend,

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.

 

[electricpete]

Tough question.

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.

 

[moondog]

I appreciate the replies on this topic. This is a big discussion at our plant. The people at the top of the food chain are pressing hard to solve this problem. I want to find all the information, & receive all the input I can to move this project forward or close it.
Thanks

 

[rbulsara]

While holding on to contactors during a dip may or may not be the greatest idea. But here is an relatively economical and correct way of doing it:

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.

 

[edison123]

I agree with electricpete. Removing and restoring supply to motors within a very short period (like auto reclosures)can damage motors and sometimes driven loads too seriously.

 

[d23]

moondog:

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!

 

[jbartos]

Suggestion: The short blips 3-10 cycles might be solved by energy storage modules based on flywheel, fuel cells, batteries and inverters (i.e. UPS), etc. This solution will also be beneficial to electronic devices, electric clocks, etc. When it comes to motors, 3-10 cycles are detrimental to motors that propel loads with small inertia. Motors that have large inertia will not be very sensitive to 10 cycles, i.e. 166 milliseconds since the electromechanical transfer switches, (this is where those blips may be coming from), may have 200 millisecond blips (the shortest ones are about 50 milliseconds). Static transfer switches can transfer in about 4 milliseconds.

 

[markac]

See

http://www.pqsi.com

for coil lock devices for momentary ride through. As noted by electricpete, be aware of re-energizing motors after such "blips".

 

[busbar]

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.

 

[Marke]

I totally agree with electricpete on this one, the re energisation after a few cycles will cause massive current and torque transients which will cause fuse and breaker trips and will likely cause major mechanical damage as well. It may be a problem bringing up all the circuits again, but it will be more major if you hold the contactor circuits in with a UPS and try to ride through the auto reclose. You will get away with it a few times, but not every time and when it hits, the damage can be severe. - broken shafts, couplings, chains, rotor bars etc.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[PUMPDESIGNER]

We have always considered power outages serious stuff.
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

 

[ELECTRICALTESTING]

The problem under discussion can be solved by providing "re-acceleration" to the drives. The drives i.e. the motors can be divided into a) contactor operated b) breaker operated c) VFD operated

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

 

[moondog]

Sorry it took me so long to reply. Could not get online monday. I really appreciate all the replies, I am going to talk to upper management today. I feel the same way as most of you. I do not want to hold the contactors in because of some of the same concerns. I hope all of the input that has been given will help swing them my way. Thanks very much I will let everyone know how it turns out.

 

[DanDel]

moondog, I would get in touch with the utility providing your power and indicate to them your concern about the financial losses for these power outages. Recorded information from a power monitoring instrument will provide some proof. This is an unacceptable condition, and if it is their fault, they should attempt to help rectify it.

 

[moondog]

DanDel, We had a power momitor hooked up for 60 days. The power company installed it on the load side of their transformer. This is where we got the information about the 3 cycle loss. I suppose the utility company cannot prevent this from happening because they met with upper managment and this is why we are talking about this. I met with upper managment and they are open to hear my issues with this. I did not see anything about the cap. bank. I know the cap. bank will help on demand with power usage. Does any one think the cap. bank discharging would help us ride out the 3 cycles?

 

[DanDel]

moondog, any power disruption that causes even a partial plant outage 'several times a month' (your words) and is measured on the utility side of your system is unacceptable. Ask the utility what is causing these outages from their side.

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?

 

[moondog]

DanDel, after receiving your post today I made some phone calls to the utility company. They are sending someone out to talk with me. I appreciate your and all the other's time on this. Thanks
We will see how it goes.

 

[jbartos]

Suggestion: As power electronics becomes more affordable, static transfer switches transferring buses, generators, etc, will reduce blips caused by the electromechanical transfer switches or transferring schemes. The difference in the transfer time is about 50 to 200ms for electromechanical switching versus about 4ms or so for the static switching.
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.