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Electric Performance Dimensioning
Performance Dimensioning 95
Torque constant
The torque constant specifies the ratio between the motor current and the motor
torque (Nm/A). The torque constant is the result of the required maximum speed, dy-
namics, the efficiency and the quality of the magnetic material.
Inductive coil resistance
The inductive coil resistance (ωL) results from the number of windings of the coil. At
standstill, it is zero. The resistance increases with the frequency.
Ohmic coil resistance
The ohmic coil resistance R results from the length and size of wire. At standstill, it
is the only value which determines the coil resistance.
Electric time constant
The electric time constant results from the ohmic and the inductive resistance (y =
L/R)
Nut setting motors
Nut setting motors are usually high-dynamic motors with a high maximum speed,
high maximum torque, a low inertia of masses and a low nominal torque. As a result,
the voltage constant and the inductance are low, the wires of the coil are thin and the
rotor diameter is small. Due to the low inductance, nut setting motors are operated
with a high pulse-width modulation frequency (PWM frequency 16 KHz) in order to
keep the current ripple low.
11.2 Power Consumption of a Drive
If a constant torque is taken from the drive, the power consumption will depend on
the current speed.
Examples:
From this results a motor current of
The motor requires a voltage of U = 1 Ω × 30 A = 30 V
0 RPM, standstill
From this results a power of P = 30 V × 30 A = 0.9 kW.
At an intermediate circuit voltage of 300 V an input current results from the supply
voltage of I = P / 300 V = 3 A.
Thus, considerably less current flows in the power supply unit than in the motor. This
calculation is very important especially for nut setting applications, since the high
torques and thus currents are only required for low speeds.
Preset torque: 30 Nm
Intermediate circuit voltage: 300 V
Voltage constant:
50 mV min
-1
(50 V / 1,000 RPM)
Coil resistance: 1 Ω
Torque constant: 1 Nm / A
I
30Nm
1Nm / A
--------------------------30A==
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