What does zero voltage or negative current mean in H-bridges?
The classic H-Bridge.



There are five LEGAL states in a H-bridge there are then two ILLEGAL &amp; four that do nothing.

The LEGAL states are:


All switches OFF - any current would freewheel by the supply &amp; decay
1 &amp; 2 ON - positive current can build up in the load
3 &amp; 4 ON - negative current can build up in the load
1 &amp; 3 ON - zero volt loop that minimises load current decay OR shorts winding.
2 &amp; 4 ON - zero volt loop that minimises load current decay OR shorts windings.




Your question has identified five conditions 


positive voltage, positive current


This would align to the state2 - charging as positive voltage is applied to the coil and there is positive current flow 


zero voltage, positive current


This would align with State4 +ve or with State5 +ve as positive current carries on flowing via a diode and a switch. As there is no forcing voltage, this is referred to as a "zero voltage" state. The current will decay quite slowly


Negative voltage ,positive current


This would align with State1 +ve freewheeling. The inductive load keeps the current flowing in a positive manner but the voltage applied is negative. This will force the current to decay quite quickly 


positive voltage negative current


This would align with State1 -ve freewheeling. The inductive load keeps the current flowing in a negative manner but the voltage applied is positive. This will force the current to decay quite quickly 


zero voltage, negative current 


This would align with State4 -ve or with State5 -ve as negative current carries on flowing via a diode and a switch. As there is no forcing voltage, this is referred to as a "zero voltage" state. The current will decay quite slowly