# Electromagnet Polarity Switching for Reversed Motion

• mattwkeller
In summary, traditional solenoids with ferrous plungers cannot fire in both directions with polarity switching due to the induced magnetic field in the plunger. However, linear motors with induction or permanent magnets can be fired in both directions, allowing for polarity switching. The function of electric motors also supports this. To reverse the direction of a solenoid or motor, a permanent magnet or a controlled polarity field winding is needed. Non-mechanical linear motors, such as coil guns or induction rails, work similarly through induction. Three-phase solenoids with a coil for each phase can also be bidirectional for a non-magnetic plunger, similar to an induction motor. However, a reversible solenoid with a plain steel plunger may

#### mattwkeller

TL;DR Summary
Why do some electromagnetic designs allow for motion reversal with reversed polarity?
I am trying to understand the differences in design between a traditional solenoid with a ferrous plunger vs linear motors that use either induction or permanent magnets. From my understanding, a traditional solenoid, whether DC or AC, cannot fire in both directions with polarity switching since the magnetic field in the plunger is induced into it.

In the case of linear motors however, it seems that they all can be fired in both directions, AC or DC. Now this would make sense to me if all linear motors were permanent magnet designs, but some linear motors use induction with copper or aluminum bars in the plunger or non coil piece. This seems to agree with the function of electric motors too, both induction and PMAC motors can flip directions.

So my question is, what sets traditional solenoids and linear motors apart to allow for one to reverse the Lorentz force and flip directions when polarity is flipped while the other can't? In a garage project application, I would like the ability of the linear motor to switch directions, but do not want to use a complex plunger with permanent magnets or induction copper/aluminum inlays.

The linear motors I am familiar with are simply permanent magnet fields with conventional armature and brushes that turn a lead screw. The lead screw moves the plunger in and out.
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Solenoids usually are configured to draw the plunger in. Specialized designs may have multiple coils configure to pull from either end.
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Have you googled any of this? There is bound to plenty of info out there along with animation, etc.

hutchphd
To reverse the direction of a solenoid requires a permanent magnet, or a controlled polarity field winding.

To reverse the direction of a motor requires the same, or a multi-phase AC power source that can then control the direction.

hutchphd
Averagesupernova said:
The linear motors I am familiar with are simply permanent magnet fields with conventional armature and brushes that turn a lead screw. The lead screw moves the plunger in and out.
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Solenoids usually are configured to draw the plunger in. Specialized designs may have multiple coils configure to pull from either end.
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Have you googled any of this? There is bound to plenty of info out there along with animation, etc.
I am referring to non mechanical linear motors, similar to coil guns or induction rails on roller coasters. I thought I understood the concept in solenoids until I found out about induction linear motors without permanent magnets, which seem to counter the operation of a one-way solenoid.

Baluncore said:
To reverse the direction of a solenoid requires a permanent magnet, or a controlled polarity field winding.

To reverse the direction of a motor requires the same, or a multi-phase AC power source that can then control the direction.
So could a three phase solenoid with a coil for each phase be bidirectional for a ferrous non-magnetic plunger?

mattwkeller said:
So could a three phase solenoid with a coil for each phase be bidirectional for a ferrous non-magnetic plunger?
That is how an induction motor works.
A current is induced in the "ferrous non-magnetic" armature by the travelling virtual field. The direction is reversed by reversing the phase direction.

Baluncore said:
That is how an induction motor works.
A current is induced in the "ferrous non-magnetic" armature by the travelling virtual field. The direction is reversed by reversing the phase direction.

Understood. The difference in that case is the rotor has copper inlays or aluminum bars arranged on the steel shaft. In this case would the reversible solenoid need those in order to reverse? Would it still reverse but not be as efficient with a plain steel plunger?

mattwkeller said:
Would it still reverse but not be as efficient with a plain steel plunger?
Yes. The moving field induces eddy currents in the surface of the iron, that then drag the steel in the direction of the cyclic field.