Rollback - DC Injection
DC Injection is commonly used in open loop geared applications to prevent rollback. DC Injection is performed by injecting current into the motor as the brake releases (at start) or applies (at stop) to hold the sheave in position and prevent rollback.
DC Injection is only used in geared motors without an encoder (Open Loop), due to there being no feedback loop.
The sections below detail the DC Injection setup and working principle.
Note
DC Injection is only available in Geared Open Loop Applications (P4-01 = 0 or 1 & P6-05 = 0).
DC Injection at start is enabled by default for 0.5s with 50% of motor rated current injected. This can be adjusted with the following steps:
To adjust the time that DC Injection is active for use P9-11.
Note
This should be enough time so that the sheave does not move.
To adjust the amount of current injected into the motor use P9-12. Typical values should be enough to hold the sheave at all loads. 100% = P4-06 value.
The images below shows what happens if DC Injection is not active during start and the effects of rollback.
DC Injection at 75% for 2 seconds | DC Injection Disabled |
|---|---|
 As you can see the current is increased while DC Injection is active and there is no movement of the motor sheave (shown in green). |  Unlike when DC Injection is active the motor sheave in this example moves backwards (rollback) as the load takes effect once the motor brake is released, due to DC Injection not being active. As you can see the travel and current profile is much worse than when using DC Injection. |
Note
While DC Injection is active the speed error is disabled so if the load is to heavy you may overhaul the motor.
DC Injection at stop works to hold the motor sheave in place before the brake applies so that the motor does not rollback.
DC Injection at stop is enable by default in geared open loop applications.
To modify DC Injection at stop follow the steps below.
To adjust the DC Injection time at stop change P9-13
To adjust the amount of current change P9-14. A value of 100% in P9-14 = P4-06.
DC Injection at stop has the additional feature where you can choose a speed at which DC Injection is applied
By default DC Injection is active at 0 speed.
When setting this parameter the best value is the lowest speed to stop the motor fully. Too high a speed will cause a sudden and harsh stop and the elevator may overshoot.
Parameter Number | Parameter Name | Units | Default | Available Settings | Access | Change During Run | Scaling | Size (Bytes) | CAN Register | CAN Sub Index | Modbus Register |
|---|---|---|---|---|---|---|---|---|---|---|---|
P9-11 | DC Injection Time At Start | sec | 0.5s | 0.0 ... 10.0s | RW | Y | 1 = 0.1 s | 1 | 238Fh | 0 | 911 |
P9-12 | DC Injection Current At Start | % | 50% | 0.0 ... 200.0% | RW | Y | 1 = 0.1 % | 2 | 2390h | 0 | 912 |
P9-13 | Position Lock / DC Injection Time At Stop | sec | 0.5s | 0.0 ... 10.0s | RW | Y | 1 = 0.1 s | 1 | 2391h | 0 | 913 |
P9-14 | DC Injection Current At Stop | % | 50.0% | 0.0 ... 200.0% | RW | Y | 1 = 0.1 % | 2 | 2392h | 0 | 914 |
P9-15 | Position Lock / DC Injection Speed At Stop | Hz / rpm / ft/s / m/s | 0.0Hz | 0.0 ... P8-01 | RW | Y | 1 = 0.01Hz | 2 | 2393h | 0 | 915 |
The drive injects DC current into the motor for the time set in this parameter at the start of the travel sequence at 0.
Function Disabled if value entered = 0.0.
Best value is the shortest time but with the least amount of rollback. Also note that while DC Injection is active the speed error detection is turned off.
Sets the level of current injected into the drive for the time set in P9-11.
Value is set as a % of P4-06 motor rated current.
Sets the time Position Lock or DC Injection will be active for at stop.
Sets the level of current injected into the drive for the time set in P9-13.
Best value is the lowest value but with the least amount of rollback, the higher the value the more heat generated in the motor and nuisance over current trips.
Function Disabled if value entered = 0.0.
Sets the speed at which either Position Lock or DC Injection activates, depending on the motor and encoder configuration.
Best to aim for the lowest suitable value, as too high a value can cause the motor to stop to early resulting in the lift car stopping before reaching the floor.
DC Injection function is disabled if value entered = 0.0.