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Timothy Forde
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PWM

Ok just trying to clarify a few things
if the ibc is puting out say 12v of 24v (50% on 50% off)
The volage and so speed is halfed but is the torqe/ammps pulled half as well?
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Post Fri Jan 20, 2006 11:08 pm 
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Philip
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Torque should be halved and your stall current should also be halved if you give the motor half the average voltage in a permanent magnet motor. You can measure your stall current at 1.2 v and again at 12 v and the second current measured should be 10 times the first, ignoring the effects of heat on the coils.

This is a useful method of determining stall currents without risking damaging a motor. It is also handy if your multimeter only goes to 10 amps and you don't want to set up a shunt resistor.
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Post Sat Jan 21, 2006 4:31 am 
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Timothy Forde
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Thanks was wondering if the fact it still pulses 24v dose anything
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Post Sat Jan 21, 2006 11:48 am 
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Spockie-Tech
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The answer to your question is complex..

The IBC (and all PWM Controllers) outputs pulses at the full battery voltage at about a 3Khz rate.

The "Duty cycle" (on vs off time) is varied according to the throttle.. This means the "on" pulses will be from ~3 microseconds in length up to ~300 microseconds long. At a properly calibrated full-throttle, full DC power will be switched through with no PWM pulses.

What this means to your motors is difficult to say exactly. It depends on the *impedance* (frequency sensitive resistance) and *inductance* (magnetic energy storage) of your motors circuit and windings as to how much the full-voltage pulses will be smoothed out into an equivalent-DC voltage.

The *current* will also be pulsing at full power regardless, which is why PWM controllers give more part-throttle torque than variable-voltage controllers do.

Current drawn through any high resistance items (like brush contacts) generates heat, the PWM just means it is generated in short bursts of full power, rather than a constant equivalent lower value. The effects of this are again variable depending on the thermal mass vs. the dissipative capability of your bits that are getting hot.

You normal brushed motor Windings for example have a high thermal mass (meaning they take a while to heat up since they are heavy chunks of metal), but have a relatively low ability to dissipate heat. Since they are only in direct contact with anything else through the motor shaft, they can only conduct heat away through that path. Most of their cooling comes from the airflow through the motor. In our application, since we are only running the motors for a few minutes, we can risk blocking this source of cooling and rely on their thermal mass to protect them from getting too hot too quickly.

The brushes and commutator however have a better heat-conduction path to get rid of stored heat into other things around them (brush plate, rotor), but have a relatively low thermal mass of their own. So if you exceed their ability to dissipate their heat, their low mass means they will get very hot *fast*

To sum up all that waffle - 50% width PWM bursts of high-power (full voltage) will probably cause items that are the weakest link in the power chain to get hot faster than 50% of the voltage would.

So, dialling back your speed controller to 50% will reduce the average continuous power used (and hence extend your battery life), but will not save your brushes from short term destruction.

Anyone see a problem with that logic ? Since I already know what happened to Tims motors, I might be reasoning backwards to a known result and have made an error in there somewhere. Wink
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Post Sun Jan 22, 2006 1:00 pm 
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kkeerroo
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This is my thinking.
The heat generated in the brushes is proportional to the electrical power dissipated by the entire motor. If the motor is driven at 50% duty cycle then the total power disipated by the motor will be 1/4 of that disipated at 100% (Power = 2 * Voltage / total Impedence). The problem is the transient voltages generated by the switching on and off of the coils in the motor. If you can filter these out with noise suppresion capacitors accross the motor terminals then I don't see a problem.
After all, quite a few American robots use limiting on thier drives to save motor wear and tear (eg Totaly Offensive).
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Post Sun Jan 22, 2006 3:34 pm 
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Spockie-Tech
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quote:
Originally posted by kkeerroo:
The heat generated in the brushes is proportional to the electrical power dissipated by the entire motor.


Is this the case when PWM'ing a higher than rated voltage through the brushes though ?

In a steady-state, I imagine it should average out, but I'm thinking that short bursts of high voltage would increase the arcing and heat on the brushes at a rate higher than that indicated by the motors averaged power dissipation.

> After all, quite a few American robots use limiting
> on thier drives to save motor wear and tear (eg Totaly Offensive).

It would limit maximum output torque and should save stress on the rest of the drivetrain from higher-torque reversals, so its probably worth doing in that respect.

I'm not yet sure that a 12v *motor* running at 50% PWM drive from 24v is going to suffer no more stress as than at 12V motor @ 100% drive though.
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Post Sun Jan 22, 2006 5:48 pm 
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Timothy Forde
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thanks and no it was not for the drills lol
Was more wondering if it could get any more torqe that way did think so but worth arsking
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Post Sun Jan 22, 2006 11:36 pm 
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Philip
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If you are prepared to loose 50% of your speed, you can double your torque through gearing.

If this is for a drive motor, you might be able to use a smaller diameter wheel to get more force but less speed.
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Post Mon Jan 23, 2006 5:27 am 
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Timothy Forde
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Trust me the wheel is as small as it will go Very Happy
Thanks for the help maybe I will just need gearing
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Post Mon Jan 23, 2006 2:49 pm 
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Valen
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@ Brett i think you are missing the inductance

my understanding
an inductor acts to resist change in current flow.
the coil/winding in the motor is for all intents and purposes an inductor.

V = IR is the "steady state" for current flow, IE the current through the motor at stall, where the motor is trying to be.

when you PWM the input voltage to the motor the *voltage* across the motor will go to full battery voltage and then down to a voltage sufficent to maintain the freewheeling current through the resistance of the coil (probbly 2 V or so gut feeling) at a rate of 3khz but the current through the coil and from the battery will be smoothed out by the inductance of the coil.
assuming that the inductance of the coil is signifigant.

heat disipation in the brushes will be I^2 R (and since they have a signifigant R this isnt such a good thing) there will be current through the brush 100% of the time (it may stop when the motor changes winding) and will have a triangle wave ontop of the average level, the triangle wave would be the influence of the PWM.

so the brush heat should be reduced by the PWM
damage to the brushes due to arcing and the like will probbly also be helped as (i think) most of the arcing happens when the brush leaves the contact and the inductance of the coil sends the voltage shooting up to try to maintain current flow.
therefore with a lower current flow that arc at least shoul be reduced.

the "make" arc should be pretty minimal i'd think, a 24V potential is going to make for a stuffall arc length, and 12 isnt going to be much less.
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Post Tue Jan 24, 2006 9:56 am 
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Nexus
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I wont pretend to understand everything that has been said but I have a question that will make it clearer for me.

I have for whatever reason been led to believe that if you run your 12volt motors at 24volts and limit the radio to 50 percent , that in the event of a stall at full throttle you will get the stall current of 24 volts.

Is this true?

Have also got the impression that current is dependant on the torque applied to the motor so does limiting the motors from your radio reduce the current when stalled.
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Post Tue Jan 24, 2006 11:26 am 
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DumHed
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a stalled motor at 50% duty cycle on 24v will get the same peak current (and torque) as a full 24v supply, but the average power is still less.

Due to inductive losses it will dissipate more power than it would if it was stalled at 12v, but nowhere near as much as it would at 24.
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Post Tue Jan 24, 2006 11:32 am 
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Valen
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With a stalled motor you have basically just made a buck converter (driving a short circuit)
you have

VBat+ --- Mosfet --- Brush --- Coil --- Brush --- Mosfet --- VBat-
and backwards across the coil is a diode

there is a resistance/Load in the coil/brush etc

starting at 0 voltages everywhere
Power is applied
The Coil is acting to resist the change in current so initally it appears as an open circuit.
over time however the current will start to flow, now if the motor stays stalled and the current is allowed to flow then the current will reach a maximum determined by V=IR, so something daft like 100A

Now assuming we are PWMing at 50% duty cycle. The current will be rising, but then the FET's will turn off. At this point the coil will again act to resist the change in current by increasing the voltage across itself to the point where it excedes the diode drop and then the current will flow through the diode and back into the start of the coil.
Due to the resistance of the coil and the diode the current will start falling again but (hopefully) before this has gotten signifigant (say 10% or so) the Mosfets will turn on again and the current will start rising.
In theory what this means is you should be opperating with about 50% of the current through the coil as you would have with no PWM and it oscilating (triangle wave) by +- 10%.
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Post Tue Jan 24, 2006 2:37 pm 
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