How to interpret DC motor performance curves?
DC motor selection must be done by understanding the duty required by the application and comparing this to the dc motor performance of each motor-pump characteristic.
There are two main performance graphs, Motor-pump performance and Motor Ratings.
Motor-pump performance
This graph shows two curves for each pump size(eg 3.3,4.2,5.0,5.8 and 6.1cc/rev). These are typical pump sizes for mini hydraulic power packs.
The curve going down to the right represents the flow rate and this can be read from the right hand axis.
The curve going up to the right represents the current drawn by the dc motor (12v 1.8kW in this case), and this can be read from the left hand axis.
For a given pressure figures for current draw and flow can be found for each pump size.
For example, for 60 bar a 4.2cc pump will deliver 12LPM and draw 155Amps.
Motor-Ratings
This graph shows two figures the S2 and S3 figures. These are specific to each dc motor characteristics and can be read for any current draw (Amp)
The definition of the duty type S2
Short time duty – Operation at constant load for a given time(Minutes). This is a time less than that required to reach thermal equilibrium, followed by a rest and de-energized period of sufficient duration to re-establish initial ambient conditions.
The definition of the duty type S3
Intermittent periodic duty – A sequence of identical duty cycles, each including a period of operation at constant load and a rest and de-energized period. The duration of a cycle is generally 10 minutes. The value S3 is expressed as a percentage For example S3= 25% means that the load on the motor can be constant for 25% of cycle time (2.5 minutes); then you need a rest position for the remaining 75 % of cycle time (7.5 minutes) before starting over a new cycle.
As an example if this motor is draws a current of 125 Amps then it can operate for 6 minutes (S2), before the motor must be rested to ambient conditions. If the motor is to run for repeated intermittent periods the S3 rating is 10% of a 10 minutes cycle. So if the motor is run for 1 minute then it must be rested for 9 minutes.