Toroidal transformer power limits?

[TheTick]

I'm working on a mechanical lift using DC motor powered linear actuators. The control box for the motors has a toroidal transformer which has 365W max available power according to the manufacturer.

I need to know more about the performance characteristics of such a transformer, especially how much power is available vs. demand. I assume it's not as simple as reaching the limit and then that's all there is.

 

[davidbeach]

It is a thermal issue. The transformer itself can sustain overloads for some period of time without immediate damage, the higher the overload the shorter the time. Rated life of the insulation system is based on never exceeding a certain design temperature which will be the equilibrium temperature when carrying rated load in the design ambient temperature. Any time operating above that insulation design temperature takes life out of the insulation.

The DC side may or may not much more limiting than the transformer itself. In the limiting case there may be a crowbar circuit that shuts down the device at 366W. Even without a crowbar circuit, the electronics are much more sensitive to overload conditions and much more willing to let go of their magic smoke.

 

[TheTick]

Basically what we are seeing is when two actuators run, there simply isn't enough power. They run at spec speed when run individually or on separate motor controllers, but not when both are run on same MC at high power.

Mfr says toroidal xfmr has 365W limit. We are not exceeding per-channel current, but apparently somehow we can not pull enough power through the motor controller.

 

[davidbeach]

Too much current draw can result in reduced voltage, which for a constant power means more current is needed, resulting in more voltage drop, resulting in more current draw......

Sounds like you need two controllers.

 

[TheTick]

What we really need is two controllers for the cost of one!

Thanks for your time, David.

 

[waross]

How about a larger transformer?
Are you getting excess voltage drops in your supply conductors under load?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Possibly inadequate reservoir capacitors after the rectfier too.

Any chance you have access to a multimeter, TheTick? Some judicious measurements, carefully taken, would probably resolve this quite quickly.


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

 

[TheTick]

Unfortunately I don't have a lot of control (zero, actually) over our choices of actuators & motor controllers.

The frustrating thing is that we believed the manufacturer specs, only to find that the really stretched the truth of their system's capabilities. I think the manufacturer gambled that no one would come up with a design that would spend so much time at the top end of their envelope.

 

[ScottyUK]

Sounds like you might have a case against the manufacturer, but you first need to prove it's not something your supply is causing before you blame their equipment. You need data!

Anyone electrical work at your place? Get them to read this thread, or sign 'em up as a member if they qualify.


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

 

[desertfox]

Hi Tick

Can you give us some details about the lift mechanic's of your system?


regards

desertfox

 

[Skogsgurra]

"ScottyUK (Electrical) 18 Oct 08 4:13
Possibly inadequate reservoir capacitors after the rectfier too"

Definitely. I was asked to help a company producing lift tables for operating rooms. Should be able to lift up to 150 kg persons.

Met specs with light load, but slowed down a lot when lifting me (not quite 100 kg, yet). It was all because the DC capacitors were too small. Adding 2200 uF was sufficient in our case.

If, however, the thermal rating is inadequate - then nothing but a blower or larger transformer works.

Gunnar Englund

http://www.gke.org

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

 

[TheTick]

The lift mechanism is a hospital bed. It is a 4-bar parallelogram with an actuator providing the final constraint across one of the corners, like an extending gusset.

Many beds have similar setups. Typically, the resulting load on the actuator decreases during lift from bottom to top, usually following a cosine curve.

My genius boss (yes, really) found a way to adjust the actuator position throughout the lift so that the actuator is able to exert nearly full force throughout the lift, maximizing lift speed. This is taxing the control box beyond its actual limits. The manufacturer admits to publishing their specs with the assumption that none of their customers would ever have a mechanism which applies a load at the top of their specs for an entire stroke of the actuator.

Meanwhile, we are trying to determine the true operating envelope of our controller/actuator system. Somehow, the control box is choking on the amount of current it needs to deliver. I am trying to gain a deeper understanding of the limiting factors.

 

[itsmoked]

Best would be a scope to look between the supply and the motors. Barring that look at the voltage at the motor terminals. Disconnect one of the motor's leads and run the system. Look at the voltage. Then do it with the lead connected but with no load.. Then loaded.


If there is a huge drop off on voltage you will be able to add a capacitor as Skogs suggests and see if this raises the voltage appreciably. Caps are really expensive these days so the maker could certainly be skimping.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TheTick]

Had a meeting with vendor today. I gave them a detailed test plan of what I want to know (forces, speeds, volts, amps at various configurations and loads). They are going to oblige. I think they are curious where I am going with this.

The vendor already has a test apparatus. I think they realize that if we build our own, we will test whatever we want, including competition's stuff, without being constrained by confidentiality. At this point I can move past trying to predict the behavior of the components and concentrate on optimizing w.r.t. known system parameters.

The project is getting exciting, as I have gained some insight that will give us an advantage over our competitors, who are pursuing increased performance but looking at misleading parameters.

Thank you everyone for your input.