# PM generator direction vs. polarity

• colintonks
In summary, Colin is seeking help with maintaining a constant polarity for his 24v 250w PM DC motor that is being used to generate electricity. Bob suggests using a 4-diode bridge with a 1.2 volt drop to make the voltage output independent of direction. They also discuss the difference between a brushed and brushless DC motor and the potential challenges with generating electricity in both directions. Bob recommends a suitable bridge rectifier for Colin's motor.
colintonks
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

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" !

Thanks
Colin

Last edited by a moderator:
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:

## 1. What is the difference between PM generator direction and polarity?

The direction of a PM generator refers to the physical orientation of the generator, while polarity refers to the direction of the magnetic field within the generator. These two terms are often used interchangeably, but they have distinct meanings.

## 2. Why is it important to know the direction and polarity of a PM generator?

The direction and polarity of a PM generator determine how it will interact with other components in a system. For example, if the polarity is reversed, it could lead to inefficient power generation or damage to other components. Additionally, knowing the direction and polarity is crucial for proper installation and maintenance of the generator.

## 3. How can I determine the direction and polarity of a PM generator?

The direction and polarity of a PM generator can be determined by looking at the markings on the generator or by using a compass. The direction can also be determined by looking at the rotation of the shaft or blades attached to the generator.

## 4. Can the direction and polarity of a PM generator be changed?

Yes, the direction and polarity of a PM generator can be changed by physically rotating the generator or by reversing the electrical connections. However, this should only be done by a trained professional to avoid damaging the generator or other components.

## 5. Are there any safety concerns when working with PM generator direction and polarity?

Yes, it is important to exercise caution and follow safety protocols when working with PM generator direction and polarity. The strong magnetic fields produced by these generators can be hazardous, and proper safety equipment should be used to prevent injury.

• Electrical Engineering
Replies
7
Views
1K
• Electrical Engineering
Replies
14
Views
1K
• Electrical Engineering
Replies
8
Views
1K
• Electrical Engineering
Replies
1
Views
1K
• Electrical Engineering
Replies
13
Views
4K
• Electrical Engineering
Replies
1
Views
1K
• Mechanical Engineering
Replies
34
Views
2K
• Electrical Engineering
Replies
8
Views
3K
• Electrical Engineering
Replies
12
Views
6K
• Electrical Engineering
Replies
4
Views
1K