# TORQUE AND PWM



## John J (Dec 29, 2007)

How great is the torque on a engine when runing PWM at reduced speeds as compaired to Liniear control? 

Is the torque as great as runing full power with PWM?

Is it some what less.?

Is it a whole lot less?


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## Del Tapparo (Jan 4, 2008)

Undoubtedly some "brain" will come along and correct me (feel free by the way), but I would think that the torque produced by the motor is strictly a function of what the motor does given a certain voltage. I wouldn't think there would be any difference between PWM and DC. 

The last three lines of your questions don't relate to the first. Running at full power with PWM is certainly not the same as running at reduced speeds with PWM or DC. One is full speed and the other is reduced speed! 

(And by the way ... anymore I really hesitate to attempt to answer questions on this forum for fear of being shot down by someone that claims to know more than me. Now, there are certainly lots of folks out there that do know more than I do about many things. I am also pretty sure there are many out there that are too intimidated to respond to most posts because of this. This is probably the subject of another thread, which I am sure one of the"superposters" will quickly tell me that I'm out of line for saying this, even though we have moderators that are supposed to take care of that job.)


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## TonyWalsham (Jan 2, 2008)

I am not technically qualified to answer that question. So I won't try. 

My practical experience is the same as Del describes. There appears to be no difference between pwm and linear. 
Apart from the voltage as Del describes, torque is also dependent on the gearing of the loco. 

What I can say about pwm compared to linear is that in general more heat is generated at the motor with pwm than with Linear where the heat is developed at the power transistor resulting in much larger heat sinks being required to keep the power transistor cool. 

How much heat is developed at the motor depends on the frequency of the pwm. Low frequency (100 Hz) usually means more heat (and motor buzzing) than a high frequency (over 5 KHz). 
Some motors handle pwm better than others. Motors that have the case vented to the atmosphere can dissipate the heat better than those that do not have venting in the motor blocks.


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## Greg Elmassian (Jan 3, 2008)

I don't know the answer for sure, but I am sure you can google it. 

The torque of the motor is a combination of the current and voltage it is getting. PWM gives you full voltage pulses. The fact that the motor heats tells you that the motor is in a sense "resisting" being driven this way. (it's the rapid switching on and off of the voltage, no inductive load likes this). 

I would imagine just by elementary reasoning smooth DC would probably give you more consistent torque. I doubt if there is any practical difference in torque, or people would have noticed on this before, and also the "pure DC" camp would be usiing it as a selling point. 

PWM has the advantage of working on dirty track somewhat better, the full voltage THEORETICALLY should cut though oxides better. Is that full voltage making a difference? Again I wonder. 

The real reason you are seeing PWM control is because it is much cheaper, smaller, and less heat with a motor controller made this way, rather than what would basically be a 5 amp linear amplifier. 

Greg


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## TonyWalsham (Jan 2, 2008)

Greg. 
As far as I am aware there is no "pure DC" camp. Certainly not for on board use. The heat sinks take up too much space. 

Linear, as in "pure DC", is the only way you should run the QSI sound, for example, on track power. PWM will definitely "confuse" it.


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## East Broad Top (Dec 29, 2007)

Dad's home-brewed R/C throttles provided a linear signal to the motor. Each loco or power car had a power transistor mounted to the underside where it could cool, often attached to a small bit of aluminum (1/2 x 1/8" x 3" or so--small enough to fit between the frame and not be seen). We've since switched over to commercial control systems that use PWM. I've not noticed any _observable_ differences in performance. Maybe there is at some level, but in terms response to the throttle and moving trains up the mountain, it seems to be a wash. About the only "difference" I noticed was that the PWM-controlled motors started a bit slower than they would under linear power, but that, too, is often as much a function of the motor and gears as anything else. 

Later, 

K


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## Greg Elmassian (Jan 3, 2008)

Actually, breaking the "stiction" of a motor and slow speed running is where PWM has an advantage, a clear advantage. 

This is because you are hitting the motor hard with full voltage, so the additional horsepower/torque helps break things free, since static friction is greater than kinetic friction (where you have to bump up the power to get it going, but once going, you can throttle down to maintain a speed slower than you can start at). 

Thus the "pulse power" revolution in loco power in the 60's. 

Greg


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## TonyWalsham (Jan 2, 2008)

The only real problem with Linear on board is they usually have a greater voltage drop from IN to OUT than PWM ESC's do. 
Leaving aside the effects of any type of friction in the mechanism, the "stiction", as Greg describes it, is real with some motors and PWM overcomes it better than Linear does. The "stiction" is less with motors that have a higher number of poles (7 pole Buhler LGB motors for example) and is virtually non existent with (so called) Coreless instrument motors such as those made by Faulhaber (sp) and Cannon. 
LGB motor blocks are so well made that most of them can be pushed along against the twin start worm drive. The Bachmann K-27 can do that too. 
Only some other (very) expensive metal bodied locos can do that, but they have (so called) free wheeling drives. 

That the latest PWM ESC's are silent, is only because the frequency of the PWM is above the threshold of human hearing.


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## Greg Elmassian (Jan 3, 2008)

The "stiction" is also in the gear train, and any moving part in the entire system from the motor rotor to the wheels, including the mass of these components 

Actually, I believe linear can be done with low voltage drop, using FET's in the final stages as opposed to normal transistors. Virtually all PWM systems use FETs (because of their low on resistance). 

As an aside, many DCC decoders let you change the PWM frequency, and several have re-instated the low frequency PWM, apparently some motors work better with it. 

Interesting stuff that has developed over the years. 

Greg


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## John J (Dec 29, 2007)

Doesn't a enging run smoother at slow speed with PWM? 

It looks like it to me.


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## Greg Elmassian (Jan 3, 2008)

PWM has better low speed control. Smoother? well I think so, but some people call it a series of short small jerks. 

As above, you can overcome friction/stiction at slow speeds by hitting the motor with a full voltage pulse. 

Again, this was the "Revolution" in power in the 60's .... remember all the different kinds of "pulse power"? 

Greg


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