The best way to reverse a DC motor is to use a Four Quadrant, static reversing DC Thyristor Converter controlled by a digital input. A Two Quadrant drive can reverse also, if it includes reversing of the field polarity. A Single Quadrant drive with external contacts can only be reversed at zero speed and zero current.
Four Quadrant static reversing is achieved by the DC Thristor Converter incorporating two full sized fully controlled Thyristor bridges for the Armature. The arrangement is controlled and interlocked such that the polarity required to control the load in both directions of torque, as well as both directions of rotation is applied to the motor with only a few milli seconds interval at the point of change over. The direction of rotation would be controlled by externally reversing the polarity of the speed setpoint signal, or more usually by switching between two analogue inputs (one scaled for reverse), or selecting a reversing block in the drive firmware via a digital input. Reversing the direction of torque would be automatically controlled either by the load overhauling the motor, or by a reduction (or reversal) of the speed setpoint signal.
Two Quadrant reversing is achieved in some DC Thristor Converters by incorporating components to change over the polarity of the field. This arrangement usually provides control of the direction of motor torque, such that regenerative braking can be applied during an emergency stop. The direction of rotation is controlled by digital signals at start up. Reversing the Current is a motor field winding is typically slow by comparison and the time taken for the torque to change over will be governed by the time constant of the field and any settings within the drive product.
Single Quadrant reversing can be achieved by the use of external contacts, or a reversing control card (as our 26037). If the DC Thyristor converter is of the 'half controlled' design (that is incorporating two Thyristors and Two Diodes), care must be taken to prevent the drive reversing sequence being completed before the motor has reached zero speed. This is because the diodes in the pack would be presented to the armature Voltage in the wrong polarity and the motor with be short circuited. When correctly interlocked the change over at zero current, means standard latching AC contactors can be employed.
The reversing card (our 26037) contains all the sequencing and contacts required to reverse a small DC Thyristor drive and motor. The same reversing card can be appled to large drives when suitable slave contactors are included in the scheme.
For a system based on external components only, a relay and contactor sequence would be:-
- Stop button releases the ‘Run Relay’ and removes the run input (drive coasts).
- 'Run Relay' drops out the ‘Forward’ or ‘Reverse’ contactor (current free by this time).
- Timer on the 'Run Relay' prevents the 'Start' buttons from doing anything until timed out.
- 'Start Forward' button (or 'Start Reverse' button) closes relevant DC contactor - either contactor on closing feeds a signal back to via a second contact on the relevent pushbutton to close the ‘Run Relay’ and start the drive.
Note: The timer must be set to exceed the time taken to coast to a stop from maximum speed. DC Contactors are standard AC3 rated contactors, as long as the sequence is arranged for current free opening and closing. Alternatively, the timer could be replaced by a Voltage sensing relay looking for zero motor speed (or Zero Armature Voltage).