International English

Selecting a Drive

3.4.2Other Loads.

Many other loads have non-linear or varying torque relationships. The torque requirement of the load should be understood before the inverter and motor is selected.

By comparing the load/speed requirement with the motor capability, the correct motor can be selected. Remember a different pole pair arrangement may give a better match to the load needs.

Starting torque may need special consideration. If a high starting torque is required this must be considered during rating.

Short term (e.g. starting)

operation possible

Torque

150%

100%

 

Continuous operation possible

 

Load

 

 

Characteristic

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.5

1.0

1.2

1.5

 

 

 

 

 

 

 

 

 

Speed ( X 50/60)

Figure 3-7 Matching the load to the Motor/Inverter Capabilities

3.5Acceleration and Braking requirements

If the load has high inertia and there is a requirement for fast acceleration or braking, the load due to the inertia must be considered.

During acceleration, additional torque will be needed. The total torque needed will be the sum of the steady state torque and this additional torque. Details of these calculations are described in Appendix B.

During braking, the inertial energy of the load must be dissipated. If a mechanical brake is used this is no problem, providing the inverter is disabled during brake operation. If the motor is decelerated by reducing the inverter output frequency, the energy from the load will be returned to the inverter. Other options such as DC braking and Compound braking will minimize regeneration to the inverter, but in this case the energy will be dissipated in the motor windings. Braking methods and options are described in detail in section 8.2.

MICROMASTER Applications Handbook

15