LTi Synchronous Motor LST/LSH Replacement Selection Guide

The encoder interface is the biggest compatibility challenge when replacing old motors. If the original system uses a Resolver, the new motor should also choose a Resolver, or confirm that the driver supports other encoder types and reconfigure it. If the original system uses an absolute value encoder, the same protocol (SSI or Endat) and resolution must be selected, otherwise the driver cannot read the position correctly. The plug type and pin definition of the encoder cable must also match, and it is recommended to use LT original cables or confirm the pin compatibility of third-party cables.

Maintain and upkeep the brake system

The LSH and LST series can be optionally equipped with a 24 VDC holding brake, model code B. This brake is a backless permanent magnet single-sided brake that uses the principle of locked rotor current, which requires power to release. The brake is directly installed behind the flange (A side) to provide optimal holding torque. Brakes usually switch when stationary, and if used as emergency stop brakes, attention must be paid to the maximum allowable frictional energy WR.

Taking LST-127 as an example, the brake release current is 1.0 A at 20 ° C, the holding torque is 18.0 Nm, the maximum friction energy is 1.29 × 10 ⁶ Ws, the moment of inertia is 0.000166 kgm ², and the mass is 0.9 kg. The release current of LST-158 is 1.1 A, the holding torque is 36.0 Nm, and the friction energy is 2.90 × 10 ⁶ Ws. The release current of LST-190 is 1.1 A, the holding torque is 36 Nm, and the friction energy is 2.9 × 10 ⁶ Ws. The release current of LST-220 is 2.1 A, the holding torque is 145 Nm, and the friction energy is 13 × 10 ⁶ Ws.

The response time of the brake includes the suction time t1 and the release time t2. Taking LST-127 as an example, t1 is 10 ms and t2 is 50 ms. LST-158 has t1 of 22 ms and t2 of 90 ms. The DC side switch (switching between rectifier and coil) can achieve very short overtravel, and for situations that require precise braking, especially for lifting mechanisms, a DC side switch must be used. During brake maintenance, the air gap and friction plate wear should be checked regularly. If the brake does not release, first check if the 24V power supply is normal and if the release current has reached the rated value. If the brake noise is high or the braking torque decreases, it may be necessary to replace the friction pads or the entire brake.

Mechanical installation and axial/radial forces

The installation forms of LT synchronous motors include B5 (flange installation, entering from the housing side), V1 (flange installation, bottom shaft end facing downwards), and V3 (flange installation, top shaft end facing upwards). B5 is the free shaft end, V1 is the bottom free shaft end, and V3 is the top free shaft end. When installed vertically (V1), the allowable axial force FAm applies. When installing (V3) vertically upwards, the allowable axial force needs to be subtracted from the rotor weight FG.

The allowable radial force FRm and axial force FAm depend on the machine frame size, speed, and application point. Taking LST-127 as an example, at 3000 rpm, the radial force FRm is 590 N and the axial force FAm is 110 N. LST-158 has a radial force of 620 N and an axial force of 120 N at 3000 rpm. LST-190 has a radial force of 1170 N and an axial force of 225 N at 3000 rpm. LST-220 has a radial force of 1490 N and an axial force of 280 N at 3000 rpm. These values are based on a lifespan of 20000 hours and are applied at the midpoint of the shaft end. If the point of force application is not at the midpoint, it can be converted according to the leverage ratio. Attention: Radial and axial forces cannot be applied to the motor shaft simultaneously, only one of them can be applied.

During installation, ensure that the motor flange is tightly attached to the installation surface, and tighten the bolts with torque. For shafts with keyway (option P), a suitable coupling or pulley must be used to avoid excessive impact on the keyway. If there are feather keys on the motor shaft end, they must be fixed before debugging to prevent them from flying out during high-speed rotation. Motor cables must use shielded cables, with both ends of the shielding layer grounded, and the cable length not exceeding the maximum allowable value of the driver.

Practical steps for replacing discontinued motors

When the original LT synchronous motor is shut down or damaged and needs to be replaced, it is recommended to follow the following steps:

Record the original motor nameplate data: model, serial number, rated torque, rated speed, rated current, DC bus voltage, encoder type, brake voltage and torque, flange size, shaft diameter, shaft length, installation form.

Search for new models with the same machine base number and length in the LT selection sample. If the original motor is an old model, it may have been upgraded, but the installation dimensions and electrical parameters should be matched as much as possible.

Check the encoder interface: If the original system uses a Resolver, the new motor should also choose a Resolver; If the original system uses an absolute value encoder, confirm that the protocol and resolution are consistent.