Magnetic Motors

mspicka

Member
Location
Colorado
Occupation
Owner
Attraction or repulsion... Why would you use one over the other?

I am reading about power sources and it came up the there are two types of magnetic power sources. I am struggling to find the reason one would be used over the other. And in reading, attraction has a 5-10% advantage.
 

__dan

Senior Member
In standard three phase motors, the (sum of) the stator magnetic field rotates while the rotor poles are N - S magnets that follow the rotating field, in lockstep for synchonous type, and with slip for induction type. I believe that is arranged attractive. Servo motors and ECM type simulate synchonous type but with PM permanent magnets at the rotor and more field poles (typically).

Why induction type this way, my guess would be jungle rules, most impressive brute force delivery at the lowest cost over the long term, other factors weighing less.

Now with EV;s, there's billions of dollars being spent annually just in engineering the next breakthrough battery, and probably some similar quantity spent trying to engineer the next breakthrough DC powered (probably ECM or Servo type field control). The motors are extremely high tech with PM magnets arranged Hallbach effect for magnetic field bending and other tricks.

The why would be for leading performance battery life in a higher torque less weight package, with marketing bragging rights 0 - 60 and 1/4 mile times. Tesla's early market target was for early adopters who would pay the price to beta test whatever they put on the market. They're selling high priced performance and are probably trying everything to lead and stay there. The million mile car is coming later, right now they're selling ludicrous 10 second performance time.

How they do that you would have to keep reading for.
 

mspicka

Member
Location
Colorado
Occupation
Owner
In standard three phase motors, the (sum of) the stator magnetic field rotates while the rotor poles are N - S magnets that follow the rotating field, in lockstep for synchonous type, and with slip for induction type. I believe that is arranged attractive. Servo motors and ECM type simulate synchonous type but with PM permanent magnets at the rotor and more field poles (typically).

Why induction type this way, my guess would be jungle rules, most impressive brute force delivery at the lowest cost over the long term, other factors weighing less.

Now with EV;s, there's billions of dollars being spent annually just in engineering the next breakthrough battery, and probably some similar quantity spent trying to engineer the next breakthrough DC powered (probably ECM or Servo type field control). The motors are extremely high tech with PM magnets arranged Hallbach effect for magnetic field bending and other tricks.

The why would be for leading performance battery life in a higher torque less weight package, with marketing bragging rights 0 - 60 and 1/4 mile times. Tesla's early market target was for early adopters who would pay the price to beta test whatever they put on the market. They're selling high priced performance and are probably trying everything to lead and stay there. The million mile car is coming later, right now they're selling ludicrous 10 second performance time.

How they do that you would have to keep reading for.
I enjoyed this answer but I was looking for a more simplistic answer. I am making a power point presentation for beginners and was trying to answer it a bit easier.
 

paulengr

Senior Member
It’s detailed but some things are blatantly wrong.

In an induction motor it always slightly lags the rotating magnetic field. The way that you create a rotor current is by cutting the magnetic lines if force. If the rotor speed matched the synchronous speed then rotor current and thus torque equals zeto, not enough to even sustain itself so this is a theoretical condition only.

A similar condition exists in permanent magnet motors. Not sure what you mean by attraction/repulsion but this is one area where we can adjust this somewhat. In a stepper motor we generally apply current to one set of poles and the permanent magnet rotor advances to that step. We do it again and it advances again. We continue the process to cause more or less continuous rotation but there is another option. We can also pulse width modulate two adjacent steps so that we can finely control the rotor position. This is called microstepping. Similar things can also be achieved in BLDC style motors which are more efficient. In fact I should stop here and mention that because they don’t have a rotor current BLDC (brushless DC) motors can achieve high torque and efficiency although there are scaling limits so they are limited in size.

With both of them and actually on any motor if you reversed the rotation of the applied field then the motor becomes a “brake” which is really a generator. For a period of time power flows backward. Continue past idle and it motors again but in the opposite direction. In fact if an external source continues to rotate the shaft it’s a true generator. People rarely think of it that way but generators and motors are the same thing. The only difference is the power flow direction.

Synchronous motors are the only ones I know of where the terms attraction/repulsion might apply. The rotor voltage is DC. The stator is AC. If you adjust the rotor voltage it causes the phase angle to change from nearly 90 degrees leading to 90 degrees lagging. The speed is synchronous. As it goes through leading angles instead of acting inductive and consuming Vars it will act capacitive and produce them! At 0 vars the power factor is zero and the current is at a minimum, making transformer I-squared losses a minimum. As you move into leading power factor the resulting vars cancel those created by other inductive loads, benefitting the entire system.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Attraction or repulsion... Why would you use one over the other?

I am reading about power sources and it came up the there are two types of magnetic power sources. I am struggling to find the reason one would be used over the other. And in reading, attraction has a 5-10% advantage.

Could you explain what you mean by 'magnetic power sources'?

I think understanding your question will be necessary before getting into different motor types.

Jon
 

Carultch

Senior Member
Location
Massachusetts
I am reading about power sources and it came up the there are two types of magnetic power sources.

You are probably thinking of magnetic forces that are either attractive or repulsive, rather than magnetic power sources. The power for a motor doesn't come from the permanent magnet. If you take energy out of a permanent magnet, the magnetization will deplete and it will turn back in to ordinary iron. The energy in the permanent magnet ideally remains constant throughout the motor's service life.

The power in a motor comes from the electrical energy source as it energizes the windings into becoming an electromagnet. That electromagnet will then attract and/or repel the permanent magnets at different positions throughout the rotation. The permanent magnet (assuming it is the stator) is there to set up the field that the electromagnet rotor aims to align with, as it rotates.
 
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