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Selecting a Drive

Torque

Possible limited

 

operation due to

 

motor cooling

150%

Short term overload capability

 

 

(60 seconds)

100%

 

Continuous operating area

0

0.5

1.0

1.2

1.5

Speed ( X 50/60)

Figure 3-4 Operating Capabilities of Motor/Inverter Combinations

High speed operation of standard motors is usually limited to twice the normal operating speed (i.e. up to 6000 or 7200 rpm) of a two-pole motor because of bearing limitations. However, because the flux level will reduce above base speed (because the output voltage is limited to approximately the input voltage) the maximum torque will also fall in inverse proportion to the speed above base speed.

However, if a motor is connected as a low voltage motor (delta) and operated on a higher voltage inverter, full torque may be obtained up to 1.7 times base frequency if the inverter is correctly set up. The correct voltage/frequency curve may be defined by setting the appropriate motor voltage (e.g. 400 V) and frequency

(87 Hz).

3.4Load Considerations

The inverter and motor requirements are determined by the speed range and torque requirements of the load. The relationship between Speed and Torque is different for different loads. Many loads can be considered to be Constant Torque loads. That is, the torque remains the same over the operating speed range. Typical constant torque loads are conveyors, compressors and positive displacement pumps. See Figure 3-5

Torque

Extruder, Mixer

Conveyor, Compressor

Pump, Fan

Speed

MICROMASTER Applications Handbook

13