Reducing Volts and Amps

[Anonymous]

Posted by Keith Long on 11/08/2018 11:10:10:

Ron you're thinking about this from the wrong end. Your controller will control the VOLTAGE to your motors which will then draw whatever CURRENT they need at that voltage setting. If the motors are running light – ie not driving anything and you set the voltage to 24 volts then they will draw little current. If however you were to lock the shafts and then apply the 24 VOLTS they will try and draw a lot more CURRENT.

+1

Any half decent controller should incorporate a current limit at it's maximum rating to protect itself in the event of an external fault. But that's not the same as trying to control the current.

Andrew

[RobinParticipant @robin]

Posted by Les Jones 1 on 11/08/2018 11:44:54:

Robin, Why do you say CHEAP controllers use PWM. A linear controller for this sort of power would generate a lot of heat and waste energy. The inductance of the motor windings helps PWM to work by trying to maintain current through the winding during the off state of the PWM cycle.

When you go beyond cheap it is like pwm but the ON phase is determined by the current, the OFF phase is determined by cleverness. You want to measure the generated Volts to get the speed but the moment you cut the supply, inductance starts flowing big current through the coils via the protection diode which confuses everything. The controller has to make a guess and that is the expensive bit.

[Neil WyattModerator @neilwyatt]

You can get modules with voltage and current limit setting potentiometers and an LED display of volts and amps. Typically with 12 or 15A maximum current.

Replace the voltage pot with a remote 'speed' pot. Set the voltage quite low.

Set the 'amps' pot to minimum then short the terminals (use a darn fat wire and a high current ammeter) and adjust to give the required current limit.

You can then use the voltage pot to control speed and the current limit will keep everything safe.

Neil

[Ron LadenParticipant @ronladen17547]

I doubt it helps much but a picture below of the 60 amp controller I am using on the 0-4-0 loco. Dont ask me anything technical about it as I cant tell you. All I know is it rated at 60 amps (40 amps continuous) 12 to 36 volts, it is wired with a forward/reverse switch and a rotary speed control.

The picture of the 100 amp version looks just the same but I will try and track down some technical info on it.

[SillyOldDufferModerator @sillyoldduffer]

Wild guesses but I suggest:

• The lack of coils imply it's a PWM controller. (This is good)
• The two relays provide forward / reverse switching by swapping the polarity of the output terminals. Don't know if the electronics protect against the operator switching from full speed ahead to full speed reverse. Does the switch have a central neutral position?
• There's what looks like a preset potentiometer under the upside down word 'POWER'. I think this is most likely to set the current overload trip value. I wouldn't mess with it.

If the controller were mine I would connect a car headlamp across the output as a test load and, with the pot OFF/minimum, apply power to the input. Then vary the pot to see if the lamp brightness varies. If all is well substitute motors and confirm they work too.

Fitted to the loco the driver wouldn't have much clue from the controller if he was overdoing it or not. So I'd fit an ammeter to the input and display the current in the cab. This one might be suitable, particularly as it also displays Watts.

Watts are good because – within reason – the motors don't care about particular volts and amps values. What matters is how hot they get, which is indicated by the watts consumed. Say you have 12V, 15A motors that's 180W, and as there are 4 of them, a total of 720W. Provided the driver keeps below 720W and doesn't overheat the motors all should be well. (This is how people get away with 24V on a 12V motor. ) The actual value depends on how hot the motors get in practice. As this depends on the cooling arrangements and track/load profile, I'd recommend some test runs to see how warm the motors get. If they get hot, reduce the drivers watt limit.

Overheating may not happen at all in practice – infrequent lightly loaded trains – and it could be you flatten the batteries before the motors get too hot however hard you drive them.

To avoid fires and prevent overloading the controller, fit a 60A automotive fuse to the input.

If the controller 'just works' there's no need to fret about the volt-amp relationships. Fingers crossed – it might be easy.

Dave

[Ron LadenParticipant @ronladen17547]

Thanks Dave,

That all makes perfect sense so I will go with the full 24 volt and as you say the amperage can be controlled via the throttle and been able to read the amps and watts level via a cab ammeter.

When the loco is ready I can run some track tests and get the best balance between performance and amps/watts whilst checking I,m not cooking the motors. Once I find the maximum throttle position it wouldnt be difficult to fit a mechanical stop to prevent this been increased any further.

The controller in the picture does have a central neutral position on the forward/reverse switch, the 100 amp version controller is the same. I cant find any useful data on the 100 amp version but its spec gives 12 to 50 volts, 100 amps max and a continuous rating of 60 amps. The 60 amps continuous is convenient as I was looking at 15 amps for each of the 4 motors.

Thanks again and thanks to all the guys for their input.

Ron

Edited By Ron Laden on 11/08/2018 20:13:52

[not done it yetParticipant @notdoneityet]

Not been watching this thread, but do check the time allowed for maximum current (as opposed to ‘continuous&rsquo. Any controllers which do not state a time allowed at maximum current may well only withstand a very short term spike, without the circuitry letting out the magic smoke. That is likely the difference between cheap and better quality, but more expensive, controllers.

[Ron LadenParticipant @ronladen17547]

Thanks NDIY, I will be giving the controller fuse protection and allthough I have no spec details I did read that the controller does have some inbuilt protection.

Ron

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ron Laden on 11/08/2018 11:47:49:

Sorry Les, but what do you mean by a PWM controller..?

.

Forgive me for jumping in, but I don't think Les has seen your question, Ron

PWM is Pulse Width Modulation

The DC voltage is chopped-up into a series of rectangular pulses, and the average [DC-ish] voltage is then dependent upon the 'mark/space' ratio [or 'duty cycle'] … we can therefore adjust motor speed.

The BIG advantage of doing it this way is that although the motor 'generally' sees the average voltage, it also benefits from the full voltage in terms of torque pulses. … For this to work well, we need to have the chopping frequency in the right zone for the motor, but it can be very effective.

MichaelG.

.

Variations on the theme of PWM are available, but that's the basic idea.

Edited By Michael Gilligan on 12/08/2018 12:13:29

[Anonymous]

Switching frequencies are normally in the kHz to tens of kHz. The important factor is that the frequency is well above the mechanical response time of the motor so that each switch period is quasi-stationary. If the motor noticably changes speed every switching cycle that wouldn't generally be desirable.

The really big advantage of PWM is that the switching device is either on or off. So the power dissipation is conduction losses plus some switching losses, which are generally much lower than the losses in a linear circuit for reducing voltage. Basically the same idea as a buck converter.

Andrew

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Andrew Johnston on 12/08/2018 12:29:21:

Switching frequencies are normally in the kHz to tens of kHz. …

.

Accepted, Andrew … hence my postscript

I just thought it worth mentioning the 'low switching frequency' which was used to great effect when PWM controllers first made an entry into the world of model locomotives. … Not my thing, but noted then and utilised thereafter.

MichaelG.

[Ron LadenParticipant @ronladen17547]

Thanks Michael and Andrew,

I now know what PWM is, not that I understand it despite your explanations. I think it a case of coming from a mechanical background and thinking electronics a mix of magic and wizardry…lol.

Appreciate you both explaining it though.

cheers

Ron

 

Edited By Ron Laden on 12/08/2018 14:02:47

[Anonymous]

Doooh, posted in the wrong thread.

Andrew

Edited By Andrew Johnston on 12/08/2018 15:05:07

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 12/08/2018 13:01:26:

I just thought it worth mentioning the 'low switching frequency' which was used to great effect when PWM controllers first made an entry into the world of model locomotives. … Not my thing, but noted then and utilised thereafter.

.

This isn't the actual article that I saw many years ago, but it's similar: **LINK**

https://www.hmrg.co.uk/techtops/pulsecontrol.htm

[note the frequency being used]

MichaelG.

.

Edit: I've just found this, which may be of interest:

http://www.sumidacrossing.org/LayoutControl/DCC/DCCMotorControl/PWM/

Edited By Michael Gilligan on 12/08/2018 18:08:18

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