I have a system on which two 480 volt 1.5HP hydraulic pumps cycle on and off, causing very large counter emf spikes on the 480 volt power. These spikes are coupled through a step down transformer that powers a 120 volt panel. The spikes then damage sensitive equipment on the 120 volt system. I'm thinking of installing an isolation transformer at the 480 volt pumps, in hopes this will isolate the spikes from the system. I have confirmed with a scope meter that this is the source of the problem. Any suggestions??
480V pumps causing spikes, need suppression
- Thread starter Tfret
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weressl
Esteemed Member
Tfret said:
If you say that the spikes got through the step-down transformer then why do you suspect the same would not happen with the isolation transformer? Sounds like a soft start application.
gar
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- Ann Arbor, Michigan
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Tfret:
What is your definition of "counter emf"?
And definition of "isolation transformer" as compared to "step-down transformer"?
How much power does the 120 circuit require? And the "sensitive equipment" alone?
What is damaged in the "sensitive equipment"?
How large and what is the duration of the transients at the 120 V circuit level?
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Tfret:
What is your definition of "counter emf"?
And definition of "isolation transformer" as compared to "step-down transformer"?
How much power does the 120 circuit require? And the "sensitive equipment" alone?
What is damaged in the "sensitive equipment"?
How large and what is the duration of the transients at the 120 V circuit level?
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brian john
Senior Member
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- Kilmarnock, Va
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- Retired after 52 years in the trade.
How large and what is the duration of the transients at the 480 V and 120 V circuit level?
The spikes after coupling through the step down xfrmr are around 200 volts. Thats what is visible on the digital scope trace. In reality they are probably much higher, the display simply cant capture and show it. Circuit boards are being destroyed on electronic test equipment in a lab that the 120vac serves.
gar
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- Ann Arbor, Michigan
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Tfret:
Consider appropriate LC filters with MOV or other transient limiters on the primary and secondary sides of the step-down transformer. Then maybe followed by a Sola ferroresonant voltage regulating transformer.
An isolation transformer would usually mean a transformer with no common low resistance connection between primary and secondary, and a Faraday shield between primary and secondary. The Faraday shield only minimizes capacitive coupling from primary to secondary and will have no effect on magnetically coupled noise.
The total load will determine if a Sola is a viable option. It could be less expensive to use solid-state switches on the motors to turn off at current zero crossings.
I doubt that circuit boards are being destroyed, rather some specific components on these boards are what is destroyed. Important here is to know if these are power supply components, internal circuitry with little connection to the outside, or mainly interface components such as line drivers to external wiring (RS232 and RS422 type devices).
You need to judge whether damage is coming in thru the power supplies, or thru signal circuits.
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Tfret:
Consider appropriate LC filters with MOV or other transient limiters on the primary and secondary sides of the step-down transformer. Then maybe followed by a Sola ferroresonant voltage regulating transformer.
An isolation transformer would usually mean a transformer with no common low resistance connection between primary and secondary, and a Faraday shield between primary and secondary. The Faraday shield only minimizes capacitive coupling from primary to secondary and will have no effect on magnetically coupled noise.
The total load will determine if a Sola is a viable option. It could be less expensive to use solid-state switches on the motors to turn off at current zero crossings.
I doubt that circuit boards are being destroyed, rather some specific components on these boards are what is destroyed. Important here is to know if these are power supply components, internal circuitry with little connection to the outside, or mainly interface components such as line drivers to external wiring (RS232 and RS422 type devices).
You need to judge whether damage is coming in thru the power supplies, or thru signal circuits.
.
If I understand...
If I understand...
If ref's are correct.... 1.5HP motor FLA 3A LRC 20A ( +/- but close )
The voltage spike ( assuming 3ph 480 and 120 ) is 200vac peak, this should be a 800 vac peak on the 480 side. I haven't seen spikes like these from such small motors unless the service, ( separately derived ??? ), transformer is overloaded. If the main feed is close to max the LRC surge current will cause the voltage peak drop may then surge back sometimes over potential. I am curious was this also seen on the OP's scope???
I agree with the Sola type ISO XMFR to protect sensitive loads, I would also agree it must be installed on the load requiring protection.
If I understand...
If ref's are correct.... 1.5HP motor FLA 3A LRC 20A ( +/- but close )
The voltage spike ( assuming 3ph 480 and 120 ) is 200vac peak, this should be a 800 vac peak on the 480 side. I haven't seen spikes like these from such small motors unless the service, ( separately derived ??? ), transformer is overloaded. If the main feed is close to max the LRC surge current will cause the voltage peak drop may then surge back sometimes over potential. I am curious was this also seen on the OP's scope???
I agree with the Sola type ISO XMFR to protect sensitive loads, I would also agree it must be installed on the load requiring protection.
Last edited:
gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
080826-1828 EST
One can generate rather large voltages without physically large inductors. When a current is flowing in an inductor and you open the circuit to the inductor the inductor will produce whatever voltage is necessary to maintain the current. The voltage is only limited by what breaks down. This can be the gap between the switch contacts or the insulation of the inductor or something else.
Years ago I experimented with an 8 ft Slimline on the shop bench. Cord to the lamp was bout 10 ft long. Inserting and removing the plug to the light and monitoring the peak transient voltage at the outlet I typically saw 2000 to 4000 V transients. A #2 motor starter coil will produce similar transients. The energy dissipated in the spark will be dependent on the energy stored in the inductor and the values of other components making up the total circuit. The magnitude of the transient voltage will depend upon the various parameters. An MOV can greatly minimze the peak voltage, devices like Transorbs can be even more effective.
Here is a recent patent reference, but I did not see the curves or other drawings referenced.
http://www.freepatentsonline.com/7339776.html
You can do a Google search on transorb but include transient voltage limiter in the string.
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One can generate rather large voltages without physically large inductors. When a current is flowing in an inductor and you open the circuit to the inductor the inductor will produce whatever voltage is necessary to maintain the current. The voltage is only limited by what breaks down. This can be the gap between the switch contacts or the insulation of the inductor or something else.
Years ago I experimented with an 8 ft Slimline on the shop bench. Cord to the lamp was bout 10 ft long. Inserting and removing the plug to the light and monitoring the peak transient voltage at the outlet I typically saw 2000 to 4000 V transients. A #2 motor starter coil will produce similar transients. The energy dissipated in the spark will be dependent on the energy stored in the inductor and the values of other components making up the total circuit. The magnitude of the transient voltage will depend upon the various parameters. An MOV can greatly minimze the peak voltage, devices like Transorbs can be even more effective.
Here is a recent patent reference, but I did not see the curves or other drawings referenced.
http://www.freepatentsonline.com/7339776.html
You can do a Google search on transorb but include transient voltage limiter in the string.
.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
gar: The voltage surge should dissipate the further away you placed the scope away from the load break?
I would check the contacts in the starters.
What about TVSS? at the transformer primary and at the loads?
I would check the contacts in the starters.
What about TVSS? at the transformer primary and at the loads?
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
080826-2051 EST
Brian:
Yes the further way from the contact you would expect lower voltage because the cabling looks like a low pass filter. However, there may be some resonance effects that might actually increase the voltage.
It is difficult to guess at a point of entry without knowing what is failing, and what existing protection is in the "sensitive equipment.
Tfret:
Can you provide photos of the spikes you see on the 120 V side. The peak of a 120 V sine wave is about 170 V. So the worst case with an added 200 V spike is 370 V. But you said it might be higher.
If you get a computer transient limiting power strip and connect one of your instruments to this, then what peak transient do you see on the output of the strip going into one of your instruments. A short impulse into a low pass filter produces a lower amplitude output pulse, but stretched in length. In other words your input pulse contains a certain amount of energy, and if we assume the filter is relatively low loss, then the output contains about the same energy as the input and therefore a lower output voltage means a longer duration.
If you go inside your instrument and look at the +5 and other DC voltages how large is the transient at this point? With and without a transient filter preceeding the instrument?
Do you have instrumentation errors from the 1 turn loop that inherently exists at the probe input if not elsewhere?
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Brian:
Yes the further way from the contact you would expect lower voltage because the cabling looks like a low pass filter. However, there may be some resonance effects that might actually increase the voltage.
It is difficult to guess at a point of entry without knowing what is failing, and what existing protection is in the "sensitive equipment.
Tfret:
Can you provide photos of the spikes you see on the 120 V side. The peak of a 120 V sine wave is about 170 V. So the worst case with an added 200 V spike is 370 V. But you said it might be higher.
If you get a computer transient limiting power strip and connect one of your instruments to this, then what peak transient do you see on the output of the strip going into one of your instruments. A short impulse into a low pass filter produces a lower amplitude output pulse, but stretched in length. In other words your input pulse contains a certain amount of energy, and if we assume the filter is relatively low loss, then the output contains about the same energy as the input and therefore a lower output voltage means a longer duration.
If you go inside your instrument and look at the +5 and other DC voltages how large is the transient at this point? With and without a transient filter preceeding the instrument?
Do you have instrumentation errors from the 1 turn loop that inherently exists at the probe input if not elsewhere?
.
We have tried both MOV power strips as well as online power conditioning ups systems on the lab equipment. They do work for a while at suppressing the spikes, but as you know, MOV's are consumable, so once they are gone the equipment begins failing again. Right now I'm leaning toward using soft starts on the pump motors. Most of these soft starters go into a "bypass" mode once the motor starts after some time delay. At that point they operate just like a contactor. I don't know if they are available to actually provide a reduced voltage ramp down in order to turn off. I believe most simply open, which will continue to generate spikes. So I need to research what is available. Thank you all for your valued comments and suggestions. It has been very helpful.
beanland
Senior Member
- Location
- Vancouver, WA
Transient Filters
Transient Filters
You need transient filters (LC filters) to eliminate/reduce the spikes. They are low-pass devices that will shunt and block the spike. A transformer will serve no purpose because it is not low-pass.
Check Allied Electronics or any similar vendor for EMI filters. Allied lists 747 options.
Transient Filters
You need transient filters (LC filters) to eliminate/reduce the spikes. They are low-pass devices that will shunt and block the spike. A transformer will serve no purpose because it is not low-pass.
Check Allied Electronics or any similar vendor for EMI filters. Allied lists 747 options.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080827-1351 EST
Tfret:
Motor starting is not like when the spikes occur, although if a lot of contact arcing occurs at start, then this may be a problem. More likely motor turn off and the contact arcing at this time is the source.
There is much information that you have not yet provided.
A motor generator could be a good solution.
.
Tfret:
Motor starting is not like when the spikes occur, although if a lot of contact arcing occurs at start, then this may be a problem. More likely motor turn off and the contact arcing at this time is the source.
There is much information that you have not yet provided.
A motor generator could be a good solution.
.
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