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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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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. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Sun Jan 22, 2006 1:00 pm |
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