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Alternators Are Synchronous Motors
Powering the rotor through the slip rings makes the alternator a "synchronous motor" and not an "induction motor".
I think the thought process went like this...
For an automobile there was a desire to improve on the brushed generators they were using and so they went with an alternator that uses the three phase design. This allowed them to remove the permanent magnets and lower the costs. The problem with a pure induction alternator is that it does not produce much power at low rpm and since there is a strong need for autos to be able to recharge while at idle they needed a way to get high charge levels at low loads. It's the low load situation that was the problem. The solution is to "cheat" a little with a slip ring and charge up the rotor so that you get more power even at lower rpm.
Now switch back to the ebikes... what is our thought process?
For a "racing ebike" like I'm trying to make there is no "idle" situation to deal with. You are either at 100% throttle and load or you are off the throttle and braking for a turn. (the rare exception being sweeper turns that use less than full throttle which might be 5% of the time on the road) So for the "racing ebike" we want a 100% induction design because we know that we will be at full throttle and full load most of the time.
Getting back to the auto alternator...
The auto alternator uses slip rings to charge up the rotor and that lowers the efficiency slightly... but the REAL REASON that alternators have low efficiency is that they run at less than full load all the time.
What is needed is to take the auto alternator apart and remove the slip rings and brushes, then short circuit the rotor like in an induction motor so that magnet fields generated on one part of the rotor create counter fields in the other parts. An induction motor's rotor can be nothing but a chunk of iron if you want.
The final product will be an induction motor with fairly low torque at really low rpms, (assuming that your controller limits the currents strictly) but very quickly the torque will rise. Efficiency is always the highest when the load is the largest. The final fully loaded efficiency should be above 80% because many induction motors can get up into the 90% and above when loaded.
It's all about loading... full load good... low load bad....
The "bottom line" is that auto alternators are synchronous and not induction motors so you cannot just plug and play with them. In order to make them of use you need to modify them. That being said, they are still a good foundation to start with and have the potential to deliver the right kind of power.
One of these days we will be able to buy stuff for our ebikes that really deliver what we want out of the box, but it just seems like we are stuck in a loop trying to get what we want. (or figure it out)
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Induction Motors
Class B (IEC Class N) motors are the default motor to use in most applications. With a starting torque of LRT = 150% to 170% of FLT, it can start most loads, without excessive starting current (LRT). Efficiency and power factor are high. It typically drives pumps, fans, and machine tools.
Class A starting torque is the same as class B. Drop out torque and starting current (LRT)are higher. This motor handles transient overloads as encountered in injection molding machines.
Class C (IEC Class H) has higher starting torque than class A and B at LRT = 200% of FLT. This motor is applied to hard-starting loads which need to be driven at constant speed like conveyors, crushers, and reciprocating pumps and compressors.
Class D motors have the highest starting torque (LRT) coupled with low starting current due to high slip ( 5% to 13% at FLT). The high slip results in lower speed. Speed regulation is poor. However, the motor excels at driving highly variable speed loads like those requiring an energy storage flywheel. Applications include punch presses, shears, and elevators.
Powering the rotor through the slip rings makes the alternator a "synchronous motor" and not an "induction motor".
I think the thought process went like this...
For an automobile there was a desire to improve on the brushed generators they were using and so they went with an alternator that uses the three phase design. This allowed them to remove the permanent magnets and lower the costs. The problem with a pure induction alternator is that it does not produce much power at low rpm and since there is a strong need for autos to be able to recharge while at idle they needed a way to get high charge levels at low loads. It's the low load situation that was the problem. The solution is to "cheat" a little with a slip ring and charge up the rotor so that you get more power even at lower rpm.
Now switch back to the ebikes... what is our thought process?
For a "racing ebike" like I'm trying to make there is no "idle" situation to deal with. You are either at 100% throttle and load or you are off the throttle and braking for a turn. (the rare exception being sweeper turns that use less than full throttle which might be 5% of the time on the road) So for the "racing ebike" we want a 100% induction design because we know that we will be at full throttle and full load most of the time.
Getting back to the auto alternator...
The auto alternator uses slip rings to charge up the rotor and that lowers the efficiency slightly... but the REAL REASON that alternators have low efficiency is that they run at less than full load all the time.
What is needed is to take the auto alternator apart and remove the slip rings and brushes, then short circuit the rotor like in an induction motor so that magnet fields generated on one part of the rotor create counter fields in the other parts. An induction motor's rotor can be nothing but a chunk of iron if you want.
The final product will be an induction motor with fairly low torque at really low rpms, (assuming that your controller limits the currents strictly) but very quickly the torque will rise. Efficiency is always the highest when the load is the largest. The final fully loaded efficiency should be above 80% because many induction motors can get up into the 90% and above when loaded.
It's all about loading... full load good... low load bad....
The "bottom line" is that auto alternators are synchronous and not induction motors so you cannot just plug and play with them. In order to make them of use you need to modify them. That being said, they are still a good foundation to start with and have the potential to deliver the right kind of power.
One of these days we will be able to buy stuff for our ebikes that really deliver what we want out of the box, but it just seems like we are stuck in a loop trying to get what we want. (or figure it out)
--------------------------------
Induction Motors
Class B (IEC Class N) motors are the default motor to use in most applications. With a starting torque of LRT = 150% to 170% of FLT, it can start most loads, without excessive starting current (LRT). Efficiency and power factor are high. It typically drives pumps, fans, and machine tools.
Class A starting torque is the same as class B. Drop out torque and starting current (LRT)are higher. This motor handles transient overloads as encountered in injection molding machines.
Class C (IEC Class H) has higher starting torque than class A and B at LRT = 200% of FLT. This motor is applied to hard-starting loads which need to be driven at constant speed like conveyors, crushers, and reciprocating pumps and compressors.
Class D motors have the highest starting torque (LRT) coupled with low starting current due to high slip ( 5% to 13% at FLT). The high slip results in lower speed. Speed regulation is poor. However, the motor excels at driving highly variable speed loads like those requiring an energy storage flywheel. Applications include punch presses, shears, and elevators.
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