# DC Motor auto-reverse at low RPM

1. Sep 9, 2014

### HuJunia

Hi All,

Much respect and admiration to the majority of you highly ordered beings. My question is a simple one I hope, how can I get a 24v DC motor to reverse ~half a rotation when forward RPM get low (0-10)?

Thanks

2. Sep 9, 2014

### skeptic2

Why is your motor speed dropping to 0-10 (rpm)? Are you reducing the speed with a speed control or is the load slowing the motor?

Do you want to reverse a half rotation and then stop? To reverse a half rotation you need a way to determine the rotational angle.

Is this motor connected directly to 24 volts or does it have some kind of motor controller driving it?

3. Sep 9, 2014

### HuJunia

It is a mechanically powered motor that's producing electricity. The set up is belt drive to motor to battery.

The half rotation in reverse is required to reinitiate the mechanical process, and ideally that would happen when the motor almost stops producing electricity so as minimize the workload on the power source while still maintaining a constant current. Does that seem reasonable?

Yes, half rotation and then stop, just a little jerk back.

Thanks

4. Sep 9, 2014

### skeptic2

So if the battery were used to reverse the motor, it would have to oppose the mechanical force that is turning the motor?

5. Sep 9, 2014

### HuJunia

Yes. But the amount of force to oppose at that point would be relatively little.

How could I use the battery to do that? Would it damage the motor? How could I ensure that the only time the current back flowed was when I wanted it to in the absence of a diode?

Thanks

6. Sep 9, 2014

### montoyas7940

Could you post a sketch? It is difficult to imagine the process.

7. Sep 9, 2014

So you want to use the battery to reverse the motor when it's speed "while generating" gets to <10 RPM? ( So switch from generating in on direction to a motor in the other direction) -- normally you would almost have to protect from this behaviour -- as the prime mover ( mechanical force, torque) is reduced to where the generator can no longer "push" charge to the battery, then the power from the battery will start flowing back to the motor... usually not the desired behavior - however this is the principal of re-generation in a vehicle.

8. Sep 9, 2014

### HuJunia

I'll have to experiment once I have the pieces in place. Judging by the replies thus far, the current design may already provide the momentary reverse that I'm looking for (might require a tweek or two).

Thanks for your help and I'll follow up in a couple weeks.

9. Sep 10, 2014

### skeptic2

It kind of sounds like you are suggesting that as the voltage of the battery exceeds the voltage generated by the motor, the motor will reverse it's rotation. I'm not sure this is correct. The voltage generated by the motor is the same polarity as the back emf generated by running the motor off the battery, so I would expect the motor to continue turning in the same direction.

I'm thinking the motor should be isolated from the battery with a diode that allows current to flow from the motor to the battery. On the motor side of the diode you could have a relay across the motor that releases at a specific voltage. The use of a lower voltage coil with a zener in series may work. When the RPM is reduced and the voltage drops, the relay would fall to normal and the normally closed contacts would reverse the voltage from the battery to the motor, bypassing the diode. Since you want the motor to reverse only about a half turn, you may want to make the reverse voltage an AC coupled pulse by using a large capacitor or a transformer (or both).

10. Sep 10, 2014

### jim hardy

That sounds contradictory. If current is 'constant' then it hasn't stopped 'producing electricity'.

Automobile voltage regulators from 1930's through about 1965 detected the condition of "almost stops producing electricity" , see leftmost coil in this diagram:
http://www.secondchancegarage.com/public/83.cfm

When generator voltage reaches about 15 volts the voltage coil closes leftmost contact connecting the generator to the battery. Voltage coil is the "Shunt winding" drawn with a dashed line.
A current sensing coil is wound on same pole throuhg which all generator current flows. Current coil is the "Series winding" drawn with with heavy solid line.
So long as current direction is out of generator the current coil flux aids the voltage coil flux so contact stays closed.
When generator "almost stops procucing" , current reverses flowing now from battery into generator motorizing it.
So flux from current coil now opposes instead of aids flux from voltage coil and the contact will open, disconnecting battery so as to not run it down by sending current backward through generator..

Be aware skeptic is right - a DC motor/generator won't reverse its direction when it swaps from generator to motor and curent reverses.

11. Sep 13, 2014

### HuJunia

What about a DC motor specifically designed to be reversible?