what does it mean in the ME0913 specs when it refers to 50rpm per volt..... is that some sort of fuzzy idealized number..... i know that i have read that somewhere
From: Eric <ewdysar@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Tuesday, April 24, 2012 3:37 PM
Subject: Re: [Electric Boats] Conversion kit
Hi Michael.
The RPM per volt is more of a DC motor attribute. For a PMAC motor like the ME0913, it doesn't really apply, the AC controller is varying the voltage, amperage and phasing of the current to match up to where you set the throttle. But even on a DC motor, load is matched by current and a given throttle setting will find an equilibrium between free spinning RPM and too many amps. For many electric motors, adjusting the load via gearing so that max throttle generates 75-80% full RPM seems to be a good balance.
But you are correct that runnning a motor at amps that exceed the motor's ability to cool will "burn up" the motor, but it's an issue of how much for how long. Because it takes time for a motor to heat up, most electric motors will handle brief times of excessive current. You can see this in the ratings, using the ME0913 for an example, the motor can take 420A for one minute, but only 125A continuous. It could probably withstand 1000A for a second or two, but I personally wouldn't recommend it.
But on boats, the ramp rate towards a given throttle setting isn't as much of a problem as you stated. Most advanced controllers, like the Gen4, have a ramp rate (acceleration, deceleration) setting, where you can pick how fast the controller responds to a throttle change. These parameters (often in milliseconds) are usually used to protect other driveline components, the motor can typically withstand much higher accel, decel rates than other parts of the system. Boats often have more give (prop slip) than cars do, you can imagine the driveline load from switching to full reverse while cruising down the road with sticky tires. On a boat, spinnning the prop backwards is the normal braking process. But marine electric drives do have the ability to break couplings, drive shafts and mounts if throttle ramp rates are set too agressively, primarily due to driveline inertia.
I know that I've covered this in broad layman's terms, each part of this post could be expanded into a much longer conversation. Hopefully, this is enough for now....
Eric
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> My understanding is that the "throttle" sets the rpm of the motor based on the voltage that is allowed through. If the motor cannot meet that rpm due to excess resistance then it will draw max amps trying to get to the revs as set by the voltage. If the motor is correctly sized then pushing the throttle all the way forward should not hurt the motor but still a gradual application is better to avoid even the slightest amp surge. Not so slow a throttling up as a ICE but you can't just throw the switch to full power and survive if things are incorrectly configured.
>
> I believe that the ME0913 provides about 50rpm per volt. If one throws the whole 48v at it then it wants to spin the motor at 2400 rpm and if it can't reach that 2400 rpm then it will draw more amps until it burns itself out trying to get there.
>
> Someone please correct me if I have this wrong.
>
> I have seen it written in more than one place that one need not pay any attention to the speed of throttle application when using electrics and especially if one is trying to build ones own system this seems extremely bad advice.
>
> (God, my spell checker just caught me typing "expecially".... someone just shoot me.
>
The RPM per volt is more of a DC motor attribute. For a PMAC motor like the ME0913, it doesn't really apply, the AC controller is varying the voltage, amperage and phasing of the current to match up to where you set the throttle. But even on a DC motor, load is matched by current and a given throttle setting will find an equilibrium between free spinning RPM and too many amps. For many electric motors, adjusting the load via gearing so that max throttle generates 75-80% full RPM seems to be a good balance.
But you are correct that runnning a motor at amps that exceed the motor's ability to cool will "burn up" the motor, but it's an issue of how much for how long. Because it takes time for a motor to heat up, most electric motors will handle brief times of excessive current. You can see this in the ratings, using the ME0913 for an example, the motor can take 420A for one minute, but only 125A continuous. It could probably withstand 1000A for a second or two, but I personally wouldn't recommend it.
But on boats, the ramp rate towards a given throttle setting isn't as much of a problem as you stated. Most advanced controllers, like the Gen4, have a ramp rate (acceleration, deceleration) setting, where you can pick how fast the controller responds to a throttle change. These parameters (often in milliseconds) are usually used to protect other driveline components, the motor can typically withstand much higher accel, decel rates than other parts of the system. Boats often have more give (prop slip) than cars do, you can imagine the driveline load from switching to full reverse while cruising down the road with sticky tires. On a boat, spinnning the prop backwards is the normal braking process. But marine electric drives do have the ability to break couplings, drive shafts and mounts if throttle ramp rates are set too agressively, primarily due to driveline inertia.
I know that I've covered this in broad layman's terms, each part of this post could be expanded into a much longer conversation. Hopefully, this is enough for now....
Eric
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> My understanding is that the "throttle" sets the rpm of the motor based on the voltage that is allowed through. If the motor cannot meet that rpm due to excess resistance then it will draw max amps trying to get to the revs as set by the voltage. If the motor is correctly sized then pushing the throttle all the way forward should not hurt the motor but still a gradual application is better to avoid even the slightest amp surge. Not so slow a throttling up as a ICE but you can't just throw the switch to full power and survive if things are incorrectly configured.
>
> I believe that the ME0913 provides about 50rpm per volt. If one throws the whole 48v at it then it wants to spin the motor at 2400 rpm and if it can't reach that 2400 rpm then it will draw more amps until it burns itself out trying to get there.
>
> Someone please correct me if I have this wrong.
>
> I have seen it written in more than one place that one need not pay any attention to the speed of throttle application when using electrics and especially if one is trying to build ones own system this seems extremely bad advice.
>
> (God, my spell checker just caught me typing "expecially".... someone just shoot me.
>
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