Synchronous motors in ELSM^® control mode

Since no encoder system is used for motor control in ELSM^® control mode, control mode-specific restrictions arise compared to operation in CFC control mode with a motor encoder.

Up to the transition speed, the control mode in open-loop operation only works in open-loop speed control. Above the transition speed, the control mode operates in closed-loop operation and is comparable to the CFC control mode with a resolver. The speed setting range is 1:50. Since no encoder system is used in ELSM^® control mode, the absolute position of the motor shaft is initially unknown. This is why it is determined by test pulses every time the device is enabled. This results in a start delay of 25 ms.

When starting up from idle state, the device first starts up in open-loop operation. Regardless of the load for stabilizing the drive, the start-up in open-loop operation takes place with a current injection of 150% of the nominal current of the motor in operation. In idle state with zero speed, a current of 120% of the nominal current of the motor in operation is injected.

If the current cannot be provided by the inverter, the current is limited to the thermal current limit of the inverter. This results in restrictions in relation to the usable torque in open-loop operation.

As soon as the transition speed is reached, the operation switches to closed-loop mode. The transition speed is approximately 2% of the nominal speed of the motor in operation. In controlled operation, the absolute position of the motor shaft is then derived from various measured variables. In closed-loop operation, the usable dynamic torque depends on both the motor as well as on the motor/inverter combination.

If the speed falls below the transition speed again, the inverter switches from closed-loop to open-loop operation. This means that open-loop operation is always active at the start of acceleration, but also at the end of a deceleration, as soon as the transition speed is undershot. Open-loop operation may only be passed through. Permanent open-loop operation below the transition speed is not permitted.

Switching to a rotating motor is possible. A restart is also possible if the drive has not yet reached speed 0.

Depending on whether the control mode is in open-loop or closed-loop operation, various limit values must be checked for drive project planning in order to be able to make a statement about whether the motor can provide the required torques.

We recommend using the SEW-Workbench project planning software for project planning.

The permanent magnets increase the voltage induced in the winding of the motor as the speed increases. This limits the amount of the current that can be injected, as only the difference between the voltage in the DC link of the inverter and the induced voltage is available as the voltage. When the voltage limit is reached, the torque drops at higher speeds. To enable operation to the right of the voltage limit characteristic curve, a field-weakening current must be injected. The field weakening function (in preparation) is active by default. Note that the required current for the same torque is significantly higher in field weakening.