ABB
Abb AC Servo motors
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77
5
The next step is to determine requirements for a suitable control or drive (amplifier).
The control must be able to supply sufficient
acceleration current (for the application’s acceleration requirements), as well as continuous current (or “RMS” current for the application’s
duty cycle requirements).
Acceleration current which must be supplied to the motor is calculated from:
current (amps), acceleration
=
acceleration torque (lb-in)
motor torque constant (lb-in/amp)
I
acc
=
T
acc
K
tHOT
Where K
tHOT
= K
tCOLD
x 0.9 (Typical derate for Neodymium magnets).
=
13.75 (lb-in)
= 3.18 amps
Metric = 1.55 Nm
= 3.18 amp
4.8 (lb-in/amp) x 0.9
0.54 Nm/a x 0.9
RMS current over the duty cycle, which the control must be capable of supplying to the motor, is calculated from:
current, RMS
=
RMS torque (lb-in)
(amps)
motor torque constant (lb-in/amp)
I
RMS
=
T
RMS
K
t
=
7.73 (lb-in)
= 1.78 amps
Metric = 0.86 Nm = 1.78 amp
4.8 (lb-in/amp) x 0.9
0.54 Nm/a x 0.9
Thus the servo control which would be selected must have the capability of supplying currents of 3.18 amps for acceleration and 1.78
amps continuously (RMS over the duty cycle).
Temperature approximation
The temperature of the internal motor winding, or how hot a motor gets, depends upon the power dissipated inside the motor, and the
motor’s ability to eliminate itself of that heat. A measure of the motor’s capability to eliminate heat is expressed as the thermal resistance.
The first step in determining the motor’s winding temperature is to calculate power dissipation (watts dissipated). Using the previous
determination of the applications current over the duty cycle, or I
RMS
, of 1.78 amps, and the motor’s resistance:
P
DISS
= I
2
x R
HOT
Where R
HOT
= R
COLD
x 1.5
P
DISS
= (1.78)
2
x 4.5 x 1.5 = 21.3 watts
Then multiply times the motor’s thermal resistance (deg C/watt) to obtain the winding temperature rise:
Temperature Rise = P
DISS
x R
th
= 21.3 x 1.56 = 33.3 deg C rise
Thus total temperature rise in a 25 deg C ambient would be:
Total temperature = ambient temp + temp rise = 25 + 33.3 = 58.3 deg C
Total temperature in a 40 deg C ambient would be:
Total temperature = 40 + 33.3= 73.3 deg C
Since the motor is designed to handle a total temperature of 155 deg C, both would be within the capability of the motor design. This
easy calculation works well for speeds below 4000 rpm. At high speeds other dissipation issues such as friction and damping must be
considered. If the 155 deg C temperature is exceeded, a larger motor should be investigated for the application.
Control section
Abb AC Servo motors