SCR motor control

[Norad]

Hello-
I'm using an SCR motor controller for a brushed DC motor 0-90VDC.
The input power voltage to the SCR is 240VAC and it is rated for both 50/60Hz. The control voltage is 0-10VDC from a PLC.
I've tested it at 60Hz, and it seems to work fine.
At 50Hz, a branch breaker to the SCR blows.
After blowing the breaker several times, the SCR faults_forever and needs to be replaced.
This has happened twice.

I am not sure that the frequency is the problem, or if there is some other issue. Can the frequency make a difference with SCR controls?

Another thought is that the SCR controller has acceleration/decceleration potentiometers on it. I may have these set incorrectly, allowing the control voltage to get to the motor before the motor is ready to accept. I had been using these potentiometers for ramping rate.
I have multiple motor speeds programmed in, that jump up and down rpms quickly.
I can't tell if the motor blows the breaker during start or during ramp.

Any thoughts at all are welcome!
Norad

 

[Skogsgurra]

Yes, frequency is very important to SCR controllers. Especially older ones, and especially^2 if they are analogue.

The reason is that the firing angle, which determines how much of the mains voltage is "let through" to the motor, is based on the standard length of a mains period. Standard length is 20 ms in 50 Hz grids and 16.666 ms in 60 Hz grids.

If the drive is rated for 50/60 Hz, there is probably a jumper or a setting that you shall change. If it is a modern digital drive, this adjustment is usually done automatically. There might be other problems also. No diagnostics in the drive?

"Jumping" up and down quickly is not good for a drive. No ramp available? No current limits? Is it a four quadrant type of drive? Then, the beta limit (firing angle when braking) may be set wrongly.

RTFM carefully if nothing else helps.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Norad]

Thanks.
Unfortunately there is no M to FR.
It is circa early 1990s so probably analogue. I don't actually know how it works.
The only ramping available are potentiometers on the board.
I can write ramping into software.

There is also a pot for IR Comp. Any idea what that is?
Any thoughts on how much to limit the ramp?
Norad

 

[Skogsgurra]

The IR is for voltage drop compensation if you run without a tachometer. IR stands for I*R, which is the voltage drop caused by armature current in the armature and cable resistance. Turn it down first. Then, if you think that your speed is too load dependent, turn it up and check if speed is more load independent. Too much IR will make the drive unstable (overcompensation).

What drive is it? I may have an FM in the attic.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Norad]

I attached the FM that came with the drive. I'm using a 1/2hp motor with it.
Should I be able to adjust it for 50Hz or do I need to purchase one made specifically for 50?
The seller of the drive told me it was rated 50/60 but they just told me they cannot test it at 50 because they don't have access to it. Grrrr.
Norad

 

[Skogsgurra]

That must be the K-TROL from ELECTROL in Pennsylvania.

I happen to have a manual that is very similar to the one you scanned pages from. It says explicitly that you can connect to 50 OR 60 Hz. There seems to be no adjustments for 50 or 60 Hz. Just connect and run. Well, there are a few adjustments to done. But none for frequency.

There are both ramp and current limit adjustments. So, if properly adjusted and with right motor selection, it should just work.

You can find the complete manual at

http://www.electrolco.com/files/Open Manual.PDF Below/K-Trol_787B.pdf

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Norad]

Gunner, that was ridiculously helpful, thank you!
Norad

 

[Norad]

Hello again-
After speaking with Electrol, they've told me that they've had sometime problems in the past with 50Hz.
I wasn't able to gain an understanding about what they actually meant.
Can anyone recommend an SCR drive that will work at 50Hz ?

Also- is the modern motor control VFD? Or is everyone still using SCR?
Thanks!
N

 

[itsmoked]

VFD are not for running 90VDC motors. You'll still need a 90VDC drive.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Technically, VFD would be for an AC motor. For DC motors, you would call it a VSD.

Either that, or explain to me about the Frequency of DC!




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

Norad,

You asked about SCR drives for 50Hz: the Control Techniques 'Mentor II' series was one of the best DC drives of its era, and I think is still in production. It is certainly worth a look: it probably looks a little dated in appearance by modern standards, unless it's had a facelift, but underneath it was a very well engineered drive.


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If we learn from our mistakes I'm getting a great education!

 

[Norad]

Thanks- The Mentor II looks like it had a facelift!
Do I need anything else in between my control input and the Mentor II?
N

 

[ScottyUK]

From memory no, but check in the manual. You'll need to register - which is why I didn't check for you, I get enough sales guys ringing me - but here's the link:

http://www.controltechniques.com/CTcom/products/dc_drives/mentor_ll/downloads.aspx

The one below is new to me - this is what I meant by a 'facelift'. It appears to be based on the old Mentor power circuit with the brain of a Unidrive. As I say it is the first one I've seen so I can't comment objectively, but the Unidrive is a decent product and so was the Mentor II.

http://www.controltechniques.com/CTcom/products/dc drives/mentor mp/overview.aspx

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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

I can objectively comment.. "That thing looks freakish!"

Keith Cress
kcress -

http://www.flaminsystems.com

 

[safiabd73]

the frequncy is very importent to control a scr
the firing angle debended on the frequncy

 

[Norad]

What is the difference between tachometer input and 0-10V (or 05V) input. Can I use the tach input in the same way?
Thank you for replies-
N

 

[Protectionfailer]

There is a company called Electronic & Power Manufacturing in South Africa that still manufactures DC drives , battery formation rectifiers, soft starters, etc. Thyristor controlled and build to customer requirements. Could be worth contacting them.

http://www.epm.co.za