scr

WARNING- the output terminals and anything associated with SCR drives should be considered LIVE

If you should kill yourself by following the below, don't come back to me and complain that I told you to do it, its under your responsability to follow propper saftey guidelines and practices, if your not qualified to play with the equipment, then get someone who knows what they are doing.

Have you tested your setup with a volt meter across the motor, and a current meter in series with the power feed to the motor?

Yes, the SCR is a electronic switch (Silicone Controlled Rectifier)

I am guessing your SCR is a motor drive?

are you 100% sure that the loading on mudpump #1 is the same as on #2? if there is more load on #1 then the motor will get hot, checking the voltages and currents on each drive with the same load, you should be able to determine whats happening?

Are you knowlegeable in the design and operation (internal design) of SCR drives? if not, stay out of them, as they are generally running LIVE. Been bitten in there a few times over the years.

Testing with a battery and light globe will only show that the SCR is off in the rest state, and full on when the Gate is triggured.

Replacing the load with a Incadecent light globe, you should be able to test the output for full voltage swing from Zero to 100% on the motor drive control knob (with your Volt Meter set to DC-Volts)

If your motors have a wound field (non permanent magnet) you should be able to disconnect the field winding, stall the armature, and test the Armature current that the setting is correct, don't do this test for too long as the Arm will be running FULL LOAD CURRENT, and could burn out.

Are the replacement SCR devices the same specification as those that are being removed?

Are you measuring the G-K resistance in circuit or out of circuit? if in cct, then this could account for errors in your measurements, as there is usually a resistor in the circuit.

sorry sir

when i changed the pump on scr#2 we run it on the same load and was pulling the same amps, same strokes, same volts and the motors temprature goes down quickly.

i know that scr is electronic switch but i am asking because i tested it as switch with bulb and it was ok, now if the scr will be different when it drives heavy loads? thats some thing different. what is the different?

when the ac input sine wave is positive, the scr switched on early to get full volts and delay it to get less volts output! now if the scr on heavy load, will the response to the firing signal change or why we can not take the bulb test?

the replacement scr is not the same specifications, thats why i am still waiting.

i am looking for the answer of mr ''bubbapebi'' saying that some power scrs have internal resistor to eliminate false triggering. and maybe thats why it is showing 14 ohms, but will it be problem if this resistance changed to 12 ohms.

for sure i tested the resistance when the scr is out!

regards

12R against 14R will have a differance with the driver cct of the SCR.

Its difficult to test the loading of a SCR drive with a light bulb, as it will only test the swing of voltage with little or no current swing.

If the load of the motor increases, the voltage F/B cct will compensate by driving the SCR harder (activating earlier in the cycle) to try and keep the voltage stable, this will also increase the current proportionally, when the load disappears, the voltage will rise and the F/B cct will compensate by retarding the triggure pulse on the SCR.

there are 2 types of trig mechanism used on SCR's (that are common)

1/. Hold the gate active from the triggure time to the zero crossing

2/. triggure the gate with a pulse transformer

Lowering the resistance on the gate could put more loading on the driver cct, making it fail in other parts as well.

Whats the manufacturers spec for the SCR drive and the related SCR's used in the drive?

I'm guessing the drive is out of warranty as you have been playing with it.

i am away from the location now but what i can tell u that the scrs used are 2000v, 1400amp as per the manufaturer!!!

rgds

Use a scope and see if you are getting AC ripple on your DC line voltages.

AC will cause a DC motor to heat up really fast. Check your pucks in the SCR for failure, they can be replaced.

Hope this helps.

Paul, Sr. Rig Mechanic. Weatherford Intl.

you'll get AC ripple on the DC load side of the SCR drive, there is no need for inductors to smooth the output of the drive, as the motor doesn't care about "clean" DC driving it.

Generally the SCR circuit is a 1/2 Bridge SCR and 1/2 Diode, where you have 2 diodes from -ve to the 2xAC lines and the SCR's from the 2xAC lines to +ve and a "Freewheeling diode" across the -ve -> +ve the motor load is connected directly across the +/- on the output, unless you have low inductance motors, then you'll possibly have a series inductor in the circuit (a few Russian built motor drives I have worked on will have this)

Greetings from Alaska,

If your drive is a copy of a Ross Hill, it should have a seperate field power supply. Do some comparison between the two drives. Check voltage on the fields when under same load on each of the drives.

If a motor gets hot with one drive, the #1 drive but not the other, the #2 drive, it is more than likely the drive or field power supply.

If you have access to a scope meter look at the feed back voltage on both drives at the same load settings. Don't go right to the load leads with the scope, that's a good way to smoke a meter and bet a good bite as well.

You might find this article interesting.

http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

If you test an SCR with an ohmmeter, it will indicate conduction if the positive (red) lead is connected to the gate and the negative (black) lead is connected to the cathode. Since you are measuring a PN junction, reversing the leads will show no conduction, unless the SCR is defective.

Some high-power SCR's have an internal resistor connected between the cathode and the gate to eliminate false triggering. In that case, the resistance will be the same with either orientation of the ohmmeter leads.

Electric motors heat up as a result of I²R effects. Since the winding resistance is essentially constant, an increased load will cause the windings to generate more heat because this load tends to slow down the rotational speed of the motor, reducing the back EMF.

The current will also increase if the SCR is leaking current of the wrong polarity. A DC ammeter may not indicate this. An oscilloscope would show it clearly.

You may not have an oscilloscope available. However, replacing the suspect SCR with a known good component will probably restore your system to normal operation even without further analysis.

That is what you get for buying a cheap knock-off piece of crap from China. How long will it take for you and everyone else out there to wake up.

I doubt the gentleman asking for help with this issue is the one responsible for specing out this drive and the purchase of it.

He is having a problem and has asked for HELP not condemnation.

hi sir

i am pretty sure that you are not chinese?

we are talking about technical problem now, business problems are not my business!!.

kindly if you can supply any useful informations? your help will be highly appreciated.

we say if you do not know just do not say? no body will complain!!!!!

rgds

Will the offending SCR cause elevated temps on the DW motor/motors when assigned to that load.

If it is limited to the MP #1 motors, you may need to look at the contactors and leads to and from.

hi

yes, it heat up the drawwork motor aswell if it is assigned to it?

So from that we can conclude that it is not a faulty motor, not a faulty cantactor or power leads from SCR house.

How about the field power supply? is the voltage, current the same between the offending SCR and another SCR?

Try to get you hands on a scope meter and have a look at the output.

Log.

hi

yes the volts and amps are the same.

i am still waiting for the scope meter to come and i will do these checks as soon as the meter arrive.

I have seen a field power supply with a DC output that had a fair amount of AC ripple in it that caused the same sort of problems you are having.

On th efield power supply, try to measure both DC voltage and the AC voltage. If it has ripple the Fluke meter will show this.

I am assuming the rig has 600VAC primary power to the SCR @ 60Hz. Is that correct?

Just food for thought.

hi

now there is some body else in the rig, i will ask him to do these checks.

the rig is 600v 50hz

I use high voltage probes on my scope and verify that all six pulses are in place and are evenly spaced. At 60 Hz there are 360 pulses per second. They should be very close to 1/360 seconds each, and symmetrical.

I curve trace the SCR's and take them up to the rated PIV of the device. Then verify the gate firing. With older devices I often find the reverse blocking voltage breaks down at less than line voltage and so will conduct at 100 or 200 volts. If the SCR is de-capped I often find the vacuum was broken and there is corrosion inside the device.

Hand held multimeters cannot determine the overall condition of an SCR, just dead shorts.

If all the devices curve trace OK, then the drive circuits for the gates need to be verified. If the SCR is not triggered properly it will not turn on.

Check the field amps to make sure it is running at the proper field strength. A week field will cause you to draw more armature amps at a reduced voltage for the same speed. Too strong a field and you will cook the field windings.

The beauty of a DC drive is armature volts x amps is close to output power and it is linear. Amps^2 will drive heat in the motor. Field strength is adjustable and will set the CEMF.