PM generator direction vs. polarity

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Discussion Overview

The discussion revolves around the challenges of using a 24v 250w PM DC motor as a generator, particularly focusing on maintaining a constant polarity regardless of the motor's rotational direction. The conversation includes technical considerations related to the motor type, rectification methods, and the implications of using a brushless versus brushed motor.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Colin seeks a solution to maintain constant polarity from a PM DC motor that can rotate in either direction while generating electricity.
  • Bob suggests using a 4-diode bridge rectifier to achieve independent voltage output from the motor's direction, noting the importance of grounding.
  • Colin clarifies that the motors are brushless, questioning the relevance of this detail.
  • Bob responds that the brushless nature of the motor is significant due to the presence of Hall Effect sensors and H-bridge circuitry, which may affect voltage requirements and control.
  • Colin mentions prior experience using the motors for generating electricity, indicating a level of familiarity despite the current challenge.
  • Bob confirms that a brushed motor can generate DC power in both directions and shares his experience with a similar motor.
  • Colin inquires about the suitability of a specific bridge rectifier, indicating a need for confirmation on component compatibility.
  • Bob affirms that the suggested bridge rectifier specifications are adequate for the application.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using brushless versus brushed motors, with some technical details remaining unresolved. There is no consensus on the best approach to maintain polarity in the context of the motor type.

Contextual Notes

Participants discuss the technical requirements of the motor control circuitry, including voltage limits and the potential need for PWM speed control, which may not be fully addressed in the conversation.

Who May Find This Useful

Individuals interested in motor applications, particularly in generating electricity with DC motors, and those exploring rectification methods for variable direction outputs may find this discussion relevant.

colintonks
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Hi,

I have a 24v 250w PM DC motor that is being used generate electricity.

Problem is that the way it is installed, it can be turned in either direction which of course gives me a polarity problem.

I'm sure there is a simple solution, but could someone tell how I maintain a constant polarity regardless of the motor's direction.

Furthermore, the generator will be charging a battery so will need a diode of about 25amps.

Thanks
Colin
 
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You will need a 4-diode bridge, with about a 1.2 volt drop, to make the voltage output completely independent of direction. Also, neither side of the motor (e.g., motor case) output can be a good ground. The ground will be one side of the bridge. (I presume you are talking about a brushed dc motor with a PM stator).
Bob S
 
Bob S said:
You will need a 4-diode bridge, with about a 1.2 volt drop, to make the voltage output completely independent of direction. Also, neither side of the motor (e.g., motor case) output can be a good ground. The ground will be one side of the bridge. (I presume you are talking about a brushed dc motor with a PM stator).
Bob S

Thanks Bob,

I just had a look at a http://upload.wikimedia.org/wikipedia/commons/1/13/4_diodes_bridge_rectifier.jpg" for a bridge rectifier and it does make perfect sense!

Thanks-you.

ps the motors are brushless but I suppose that makes little difference?
 
Last edited by a moderator:
colintonks said:
ps the motors are brushless but I suppose that makes little difference?
It make a lot of difference. A BLDC (brushless dc) motor your size probably has a Hall Effect sensor (to sense rotor position) in it, along with an H-bridge polarity switch, to control the stator windings. The motor control circuitry requires a voltage within narrow(?) limits to operate. Your motor may also have a speed control based on PWM (pulse width modulation). Is your motor bi directional? Have you used this motor already for generating electricity?
Bob S
 
Bob S said:
It make a lot of difference. A BLDC (brushless dc) motor your size probably has a Hall Effect sensor (to sense rotor position) in it, along with an H-bridge polarity switch, to control the stator windings. The motor control circuitry requires a voltage within narrow(?) limits to operate. Your motor may also have a speed control based on PWM (pulse width modulation). Is your motor bi directional? Have you used this motor already for generating electricity?
Bob S

Ah I see, the motor we are using is this http://www.monsterscooterparts.com/24v30mowisp.html" .

To answer your question regarding generating electricity! Yes, we believe it or not we used them to do http://www.bbc.co.uk/programmes/b00p8469" !

Look forward to your reply.

Thanks
Colin
 
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Nice motor. I have one. it has brushes. It will generate dc power in both directions. It may have 4 (four) brushes (I forget). Bob S
 
Bob S said:
Nice motor. I have one. it has brushes. It will generate dc power in both directions. It may have 4 (four) brushes (I forget). Bob S

Yeah they are an excellent motor, well built and made from decent metal!

Bob I was wondering, do you think http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2278621" would suit?

Really appreciate your help. You'd think after powering an entire house with these we'd know what we are talking about, its just that we've never had to use them in a b-direction manner.

Thanks
Colin
 
Last edited by a moderator:

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