5
Project Planning for Gear Units Multi-stage gearmotors 5.3Multi-stage gearmotors
General information You can achieve particularly low output speeds by using multi-stage gear units or multi-stage gearmotors. This means an additional second gear unit, usually a
helical pinion unit,is installed in front of the gear unit or between gear unit and motor.The resulting total reduction ratio may make gear unit protection necessary. Limiting the motor power You have to reduce the maximum output motor power according to the maximum per-mitted output torque on the gear unit (M
a max ). For this purpose you first have to deter-mine the maximum permitted motor torque (M
N zul ).You can calculate the maximum permitted motor torque as follows:Use this maximum permitted motor torque M
M
N zul =
M i
ges η
gesa max 59717ADE N zul and the load diagram of the motor todetermine the associated value for the motor current.Take suitable measures to prevent the continuous current consumption of the motorfrom exceeding the previously determined value for the motor torque M
N zul . A suitablemeasure is, for example, to set the trip current of the protective circuit breaker to thismaximum current value. A
motor cutout relay offers the option to compensate for abrief overload, for example during the startup phase of the motor. A suitable measurefor
professional inverter drives is to limit the output current of the inverter according to the determinedmotor current. Checking brake torques If you use a multi-stage brakemotor, you have to limit the braking torque (M
B ) accordingto the maximum permitted motor torque M
N zul . The maximum permitted braking torqueis 200 % M
N zul .M
B max ≤ 200 % M
N zul If you have questions on the starting frequency of multi-stage brakemotors, please con-sult SEW-EURODRIVE. Avoiding blockage Blockage on the output side of the multi-stage gear unit or multi-stage gearmotor is notpermitted. The reason is that indeterminable torques and uncontrolled overhung and ax-ial loads may occur. The gear units may suffer irreparable damage as a result. Consult SEW-EURODRIVE if blockages of the multi-stage gear unit or multi-stage gear-motor cannot be avoided due to the application.
42
Catalog – DR Gearmotors 01/2008