Controlling current vs PWM?

Thread Starter

Loz2212

Joined Oct 9, 2018
28
Hi everyone,

I work for a power transmission company that has become more focused on electronically controlled components over the years. The one in question is a torque limiter which has an electromagnetic coil in - the more you increase/decrease the current, the torque is altered proportionally.

A customer has asked me how they would vary this... we have controllers but not for our 24v range (no idea why?). The controller for our 96v range says it adjusts the current.

What would be a suitable option for them to control it? I thought about PWM but I don't know how that would work with coils that are safety critical. Is there a way to vary current from a 24v supply? Maybe something a PLC could offer?

Hope someone can help. Thank you.
 

Thread Starter

Loz2212

Joined Oct 9, 2018
28
Unfortunately not - most, if not all of our 24v components don't have any controllers or accessories. I can only guess it's an industry standard voltage and the customer is left to work that out for themselves.
 

wayneh

Joined Sep 9, 2010
16,399
All PWM provides current control, in the sense that the voltage does not change, only its average over time.

Why not just buy a 24V PWM-based motor controller?
 

Thread Starter

Loz2212

Joined Oct 9, 2018
28
Thanks Wayne, that may be what I should suggest.

The controller for the 96v version has an analogue 1-10vdc input which changes the magnetism in the coil (0v is 25% - 10v is 100%). I'm assuming this must be PWM as surely a change in current only will have the ohms law police knocking at my door.

I'm going to knock up a PWM controller or buy an eBay special and do some tests myself.

Thank you.
 

Thread Starter

Loz2212

Joined Oct 9, 2018
28
If a failure occurs the output would need to go low so the drive would disconnect. A certain part of the clutch moves when it reaches the torque and emits a signal from an integral proximity switch.
 

crutschow

Joined Mar 14, 2008
25,267
PWM should work well for the control, but will need a failsafe circuit to compare the output to the input and shut the output down if they disagree.
 

Thread Starter

Loz2212

Joined Oct 9, 2018
28
PWM should work well for the control, but will need a failsafe circuit to compare the output to the input and shut the output down if they disagree.
Thank you crutschow - I've had some really good info off you in the past which has helped a lot! Appreciate your time.

Do you mean PWM output to coil input? Some sort of feedback circuit? What would this do?

Thank you
 

crutschow

Joined Mar 14, 2008
25,267
The PWM signal is an on-off digital type signal to the coil so you compare the input command PWM signal to the output and shut it down if they differ.
This can be done with a comparator or logic-gate circuit.
That could trigger an SCR crowbar circuit to clamp the output to zero.
 
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MrAl

Joined Jun 17, 2014
7,758
Hi everyone,

I work for a power transmission company that has become more focused on electronically controlled components over the years. The one in question is a torque limiter which has an electromagnetic coil in - the more you increase/decrease the current, the torque is altered proportionally.

A customer has asked me how they would vary this... we have controllers but not for our 24v range (no idea why?). The controller for our 96v range says it adjusts the current.

What would be a suitable option for them to control it? I thought about PWM but I don't know how that would work with coils that are safety critical. Is there a way to vary current from a 24v supply? Maybe something a PLC could offer?

Hope someone can help. Thank you.
Hi,

PWM is a very general description of a means to regulate current or voltage or both. PWM control basically happens with a PWM power supply that can be adjusted for proper current and/or voltage or a chopper that chops up the main power source.

An example would be a buck circuit. This takes a higher voltage and converts it into a lower voltage at decent efficiency but can sometimes work nearly to the full voltage of the input. Thus with 24v input you can get nearly 24v output or less as needed.

There are voltage mode and current mode variations. The voltage mode mainly regulates the voltage, the current mode regulates the current. The difference is the feedback method mostly but may include some other differences.

There's also the EMI issue. WIth any PWM there is going to be some EMI that you would not get with a pure linear control. A linear control can be very inefficient though.

What kind of safety issues are you talking about here? Depending on what safety standards that have to be met you may not even be able to build such a thing or you may have to get approval. Hospital stuff is most critical, consumer stuff is a little critical, industry stuff is more relaxed.
 
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