Servo Motors

Servo motors are special category of motors, designed for applications involving position control, velocity control and torque control. These motors are special in the following ways:

1. Lower mechanical time constant.
2. Lower electrical time constant.
3. Permanent magnet of high flux density to generate the field.
4. Fail-safe electro-mechanical brakes.

For applications where the load is to be rapidly accelerated or decelerated frequently, the electrical and mechanical time constants of the motor plays an important role. The mechanical time constants in these motors are reduced by reducing the rotor inertia. Hence the rotor of these motors have an elongated structure. For DC brush type motors, the permanent magnets are mounted on the stator, while the armature conductors are on the rotor. The rotating conductors make contact with the stationary electrical source via a brush-commutator assembly. A DC tacho is mounted on the motor shaft, for indicating the shaft speed in-terms of a voltage. These motors also come with fail-safe electro-mechanical brakes. In the event of failure of the utility mains, the antennas are stopped by these brakes.

Table 20.3: Servo motor specifications

Type	DC brush type, permanent magnet field.
Horse Power rating	6 HP.
Rated motor voltage	150 V (DC).
Rated motor current	80 Amp (Continuous).
Rated motor speed	2250 rpm.
Continuous stall torque	47 N-m.
Peak Torque	111 N-m.
Torque Sensitivity	0.56 N-m / Amp.
Back E.M.F. Constant	59 V / krpm.
Armature resistance	0.045 Ohm.
Armature inductance	0.33 mH.
Tacho sensitivity	17 V / krpm.