# Switching positive and negative voltage

I struggle to understand how can you switch positive or negative supply from a battery.

Firstly, what is a positive voltage and negative voltage?

I read many posts stating it all depends on your point of reference. To my understanding, current flows from negative to positive. Positive and negative current is just a difference in current flow.

The reason I am asking is I am in a process of studying Inverter Field control for Hybrid vehicles.

The booklet states:

By switching the positive or negative post of the battery to the three MG (Motor Generator) connections a current starts flowing through the MG's coils. The current through the MG coils creates a revolving or still magnetic field, making the rotor magnets in the MG's follow this field.

Please notice the current flow highlighted orange. I don't understand how can you switch positive or negative voltage and how does that affect anything. I also don't understand how can you have a positive voltage flowing in and the other end has negative voltage flowing out.

Moreover, how can 'give' or rectify negative voltage to the negative terminal a positive voltage to the positive terminal depicted below. What does that even mean?

I would appreciate an answer from an engineer explained in non-engineer terms. I know there are a lot of smart folks out there, but please keep it simple.

Thanks.

• You might need to clarify that. The top diagram appears wrong to me. The U and V phase negative transistors are on so why is there no current flowing in the W wire? The bottom diagram arrows match the switch status. Apr 28, 2018 at 21:58
• As you see the first diagram is a bit cut off at the bottom, you can see only half of the 150V and W, let's assume there is an arrow it just been cut out. Apr 28, 2018 at 22:19
• Do you understand that a battery has a positive side and a negative side? Apr 28, 2018 at 23:06
• @Transistor - for some reason my mind was in 'delta' mode ... Apr 29, 2018 at 14:11

simulate this circuit – Schematic created using CircuitLab

Figure 1. A single-coil motor in a 'H-bridge' configuration.

Really easy:

• In Figure 1 left current flows through SW1, the motor and SW4. The left motor terminal is positive and the right is negative.
• If we switch the switches carfully to avoid 'shoot-through' (e.g., if SW1 and 2 were on simultaneously enough current might flow to destroy the circuit) we can reverse the current flow through the motor. In Figure 1 right current flows through SW7, the motor and SW6. Note that the motor polarity has been reversed.

The result is that either motor terminal can be connected to battery positive or negative. For a simple DC motor this allows reversal of rotation.

The three-phase situation is more complex but still understandable:

• If one winding is connected to positive then at least one of the other two has to provide the return path to the supply negative.
• To approximate the sinusoidal waveform pulse-width modulation (PWM) is used with progressively wider pulses averaging to the rising voltage on the sine.
• Funny. The motor "polarity" is shown differently in two cases of H-bridge state. Is there a special meaning? :-) Apr 28, 2018 at 21:59
• Hmmm. I'm trying to show that the motor terminal voltages have swapped. I'll add some terminal designations to clarify. Any better? Apr 28, 2018 at 22:01
• Okay, I see what you meant. Maybe the (+) and (-) would be better placed near the blue arrow, but I just picking on you... Apr 28, 2018 at 22:07