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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
Switched reluctance motors operate on basically the same principle as coilguns - electromagnetic coils act on iron, which then moves to reduce reluctance. They can reach 80% efficiency. Reluctance coilguns have gone beyond 10% efficiency. How can the SRM be much more efficient? Is it because it stays in one place an rotates, instead of firing off?
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
It's because the iron in the motor moves at a very high speed. When the motor turns slowly as it first starts up, it's much less than 80% efficient. Think of coilguns (for which each shot starts with iron at rest) as constantly being in that starting up phase.
It has to do with the voltage induced in the drive coils by the magnetized iron, which is proportional to speed. The induced voltage opposes the forward voltage, which reduces current and heat loss (I^2R) in the drive coils.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
A coilgun runs more efficiently at high projectile speeds for the same reason that an electric motor runs more efficiently at high RPMs.
A magnetized object induces voltage in surrounding loops of wire proportional to it's speed. This induced voltage is opposite to the forward voltage in the coils, effectivley reducing the voltage that drives the coils and thus the current. Less current means less heat which means better efficiency. Obviously less current means less force (or torque in a motor) so efficiency and power are opposed to eachother.
With coilguns, this effect is compunded by the fact that later coils are on for shorter periods of time (since the projectile is moving faster), meaning the current doesn't have time to rise up to the max value.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
That means that a circular coilgun (possibly accelerating a steel ball bearing) that used the same coils over and over could probably be fairly efficient! Has anyone ever tried this?
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Why can switched reluctance motors acheive high efficiency when coilguns cannot?
