QSH6018 Manual (V1.08 / 2014-SEP-04)

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5 Considerations for Operation

The following chapters try to help you to correctly set the key operation parameters in order to get a

stable system.

5.1 Choosing the Best Fitting Motor for an Application

For an optimum solution it is important to fit the motor to the application and to choose the best

mode of operation. The key parameters are the desired motor torque and velocity. While the motor

holding torque describes the torque at stand-still, and gives a good indication for comparing different

motors, it is not the key parameter for the best fitting motor. The required torque is a result of static

load on the motor, dynamic loads which occur during acceleration/deceleration and loads due to

friction. In most applications the load at maximum desired motor velocity is most critical, because of

the reduction of motor torque at higher velocity. While the required velocity generally is well known,

the required torque often is only roughly known. Generally, longer motors and motors with a larger

diameter deliver a higher torque. But, using the same driver voltage for the motor, the larger motor

earlier looses torque when increasing motor velocity. This means, that for a high torque at a high

motor velocity, the smaller motor might be the fitting solution. Please refer to the torque vs. velocity

diagram to determine the best fitting motor, which delivers enough torque at the desired velocities.

5.1.1 Determining the Maximum Torque Required by Your Application

Just try a motor with a torque 30-50% above the application’s maximum requirement. Take into

consideration worst case conditions, i.e. minimum driver supply voltage and minimum driver current,

maximum or minimum environment temperature (whichever is worse) and maximum friction of

mechanics. Now, consider that you want to be on the safe side, and add some 10 percent safety

margin to take into account for unknown degradation of mechanics and motor. Therefore try to get a

feeling for the motor reliability at slightly increased load, especially at maximum velocity. That is also

a good test to check the operation at a velocity a little higher than the maximum application velocity.

5.2 Motor Current Setting

Basically, the motor torque is proportional to the motor current, as long as the current stays at a

reasonable level. At the same time, the power consumption of the motor (and driver) is proportional

to the square of the motor current. Optimally, the motor should be chosen to bring the required

performance at the rated motor current. For a short time, the motor current may be raised above this

level in order to get increased torque, but care has to be taken in order not to exceed the maximum

coil temperature of 130°C respectively a continuous motor operation temperature of 90°C.

Percentage

of

rated current

Percentage

of

motor torque

Percentage

of

static

motor power dissipation

Comment

150%

≤150%

225%

Limit operation to a few seconds

125%

125%

156%

Operation possible for a limited time

100%

100%

100%

= 2 * I

Normal operation

85%

85%

72%

Normal operation

75%

75%

56%

Normal operation

50%

50%

25%

Reduced microstep exactness due to

torque reducing in the magnitude of

detent torque

38%

38%

14%

-“-

25%

25%

6%

-“-

0%

see detent

torque

0%

Motor might loose position if the

application’s friction is too low

Table 5.1: Motor current settings