# Generating Electricity with a PM DC Motor **URL:** \ **Category:** Factual Questions\ **Created:** [May 20, 2011, 9:08pm UTC](https://boards.straightdope.com/t/generating-electricity-with-a-pm-dc-motor/582620 "2011-05-20T21:08:05Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![davidaugustmoon](https://avatars.discourse-cdn.com/v4/letter/d/7ab992/32.png) [@davidaugustmoon](https://boards.straightdope.com/u/davidaugustmoon)\ **Post date:** [May 20, 2011, 9:08pm UTC](https://boards.straightdope.com/t/generating-electricity-with-a-pm-dc-motor/582620/1 "2011-05-20T21:08:05Z")
Does it matter which direction you spin a PM DC motor to generate electricity? If I spin it clockwise my voltmeter reads a positive voltage, but if I spin it counter clockwise it reads a negative voltage. Does this negative voltage mean that I would be drawing current from my battery bank, rather than charging it? Thanks for any help everyone! ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [May 20, 2011, 10:16pm UTC](https://boards.straightdope.com/t/generating-electricity-with-a-pm-dc-motor/582620/2 "2011-05-20T22:16:47Z")
The negative voltage just means that the motor is still generating electricity. It’s just going in the opposite direction. If you are charging a battery, then you want to reverse the leads on your battery to reflect the opposite voltage. Otherwise the battery and the motor will be fighting each other and you will be discharging your battery. If you reverse the leads on your battery when you spin the motor in the opposite direction the motor will still charge the battery. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [May 20, 2011, 10:36pm UTC](https://boards.straightdope.com/t/generating-electricity-with-a-pm-dc-motor/582620/3 "2011-05-20T22:36:14Z")
mount the motor (now generator) in a way that works well mechanically and spins the way you can make it spin. then connect the wires as engineer\_comp\_geek said to have it charge the battery. the mechanics are the hardest to work out then connect the wires whatever way needed. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 20, 2011, 10:42pm UTC](https://boards.straightdope.com/t/generating-electricity-with-a-pm-dc-motor/582620/4 "2011-05-20T22:42:20Z")
And if you don’t know which way the generator is going to turn but want to charge the batteries regardless, then you can rig up a rectifier from four diodes so that the current is always going the same way at the battery, regardless of what the generator is doing. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [May 21, 2011, 3:43am UTC](https://boards.straightdope.com/t/generating-electricity-with-a-pm-dc-motor/582620/5 "2011-05-21T03:43:00Z")
> [@Chronos](#): > > And **if** you don’t know which way the generator is going to turn but want to charge the batteries regardless, then you can rig up a rectifier from four diodes so that the current is always going the same way at the battery, regardless of what the generator is doing. True, but the operative word there is “if.” Diodes can ruin your efficiency. So if you know your polarity, then it’s better not to use diodes. ---
**Author:** ![wbeaty](https://avatars.discourse-cdn.com/v4/letter/w/439d5e/32.png) [@wbeaty](https://boards.straightdope.com/u/wbeaty)\ **Post date:** [May 22, 2011, 3:32am UTC](https://boards.straightdope.com/t/generating-electricity-with-a-pm-dc-motor/582620/6 "2011-05-22T03:32:40Z")
[quote=“davidaugustmoon, post:1, topic:582620”] Does it matter which direction you spin a PM DC motor to generate electricity? [quote] Not for heaters and incandescent lights. But for most everything else, yes, polarity is important. > [@davidaugustmoon](#): > > If I spin it clockwise my voltmeter reads a positive voltage, but if I spin it counter clockwise it reads a negative voltage. Does this negative voltage mean that I would be drawing current from my battery bank, rather than charging it? Yes and no. Trouble is, if you’re spinning the generator and charging the batteries, if you spin it a little slow, then the current direction reverses and the batteries get drained. You don’t have to stop or reverse the generator, just spin it slow. Why? Because if the generator voltage is slightly higher than the batt volts, you’re charging them …but if the generator voltage is slightly lower than the batt volts, then current is reversed and the generator becomes a motor. Or think of it this way: hook a DC motor-generator to batteries. It starts spinning. If the bearings are very low-friction, then the motor is basically free-running, and almost no charge is flowing in the wires. Now manually spin the motor faster than it’s free-running RPM. You’re doing work. The current gets large, and the batteries are being charged. OK now instead hold your thumb against the motor shaft. It spins slower, the current is large and backwards, and the batteries are being drained. Note: all motors are generators, just as all drive pulleys can become driven pulleys. Even 3-phase AC motors are generators (but they need a resonating capacitor to replace the function of the missing AC power grid, see [http://www.qsl.net/ns8o/Induction\_Generator.html](http://www.qsl.net/ns8o/Induction_Generator.html)