Motor diagrams, operating ranges The catalog contains a diagram of every maxon DC and EC motor type that shows the operating ranges of the different winding types using a typical motor. Permanent operating range The two criteria “maximum continuous torque” and “maximum permis- sible speed” limit the continuous operating range. Operating points within this range are not critical thermally and do not generally cause increased wear of the commutation system. Short-term operating range The motor may only be loaded with the maximum continuous current for thermal reasons. However, temporary higher currents (torques) are allowed. As long as the winding temperature is below the critical value, the winding will not be damaged. Phases with increased currents are time limited. A measure of how long the temporary overload can last is provided by the thermal time constant of the winding (line 19 of the motor data). The magnitude of the times with overload ranges from several seconds for the smallest motors (6 mm to 13 mm diameter) up to roughly one minute for the largest (60 mm to 90 mm diameter). The calculation of the exact overload duration depends highly on the motor current and Due to the winding resistance, the motor current causes the winding to heat up. To prevent the motor from overheating, this heat needs to be dissipated to the environment via the stator. The maximum winding temperature (line 22 of the motor data) must not be exceeded even for a short time. For graphite brush motors and EC motors with their usually higher current load, it is 125°C (in some cases up to 155°C). Precious metal commutated motors only allow for low current loads, so that the rotor temperature must not exceed 85°C. Precautions taken during installation, such as good air circulation or cooling plates, may signifi- cantly lower the temperature. Permissible continuous current, permissible continuous torque The electrical heat losses define the max. permissible continuous current at which the maximum winding temperature is reached under standard conditions (25°C ambient temperature, no heat dissipation via the flange, air circulating freely). Larger motor currents result in too high winding temperatures. the winding temperature at the beginning. Max. permissible winding temperature The nominal current is selected to correspond with this maximum permissible continuous current. It is highly dependent on the winding. Windings with thin wire have lower nominal currents than windings with thick wire. In the case of windings with very low resistance, the current capacity of the brush system can further restrict the permissible continuous current. The graphite brush motors significantly increase the friction losses at high speeds. In EC motors, the eddy current loss in the magnetic return increases when the speed increases and gener- ates additional heat. The maximum permissible continuous current decreases at higher speeds. The nominal current assigned to the rated torque is practically constant within the winding type of a motor type and is one of the characteristics of the motor type. The maximum permissible speed for DC motors is primarily limited by the commutation system. The commutator and brushes wear more rapidly at very high speeds. The reasons are: − Increased mechanical wear because of the large traveled path of the commutator − Increased electro-erosion because of brush vibration and spark formation. A further reason for limiting the speed is the rotor’s residual mechanical imbalance which shortens the service life of the bearings. Higher speeds than the limit speed n max (line 23) are possible, however, they are “paid for” by a reduced service life expectancy. The maximum permissible speed for the EC motor is calculated based on service life consid- erations of the ball bearings (at least 20 000 hours) at the maximum residual imbalance and bearing load.
Speed [rpm]
25000
20000 15000 10000 5000
30
10 20
40
Current [A] Torque [mNm]
0.4
0.8
1.2
Operating range diagram
Intermittent operation Switch-on duration and current
I ON / I N
T
5
4
Time
3
2
1
0
10 20 30 40 50 60 70 80 90 t ON%
ON OFF
Motor in operation Motor stationary Max. peak current
I ON
Max. permissible continuous current (line 6) ON time [s] , should not exeed τ w (line 19) Duty cycle as percentage of cycle time. The motor may be overloaded by the relationship I ON / I N at X % of the total cycle time. I on = I N T t ON Cycle time t ON + t OFF [s]
I N
t ON
T
t ON%
80 maxon
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