Basics of DC Drives

Controlling a DC Motor

A thyristor bridge is a technique commonly used to control the speed of a DC motor by varying the DC voltage. Examples of how a DC rectifier bridge operates are given on the next few pages. Voltage values given in these examples are used for explanation only. The actual values for a given load, speed, and motor vary.

Armature windings are commonly wound for 500 VDC. The control logic in the drive must be adjusted to limit available DC voltage to 0 – 500 VDC. Likewise, the shunt field must be limited to the motor’s nameplate value.

Basic Operation

A DC drive supplies voltage to the motor to operate at a desired speed. The motor draws current from this power source in proportion to the torque (load) applied to the motor shaft.

100% Speed, 0% Load

In this example an unloaded motor connected to a DC drive is being operated at 100% speed. The amount of armature current (Ia) and unloaded motor needs to operate is negligible. For the purpose of explanation a value of 0 amps is used.

Remember that V_a = I_aR_a + CEMF. In this example I_aR_a is 0, therefore CEMF will be approximately 450 VDC.

100% Speed, 100% Load

A fully loaded motor requires 100% of rated armature current at 100% speed. Current flowing through the armature circuit will cause a voltage drop across the armature resistance (Ra). Full voltage (500 VDC) must be applied to a fully loaded motor to operate at 100% speed. To accomplish this, thyristors are gated earlier in the sine wave (36.37°).

The DC drive will supply the voltage required to operate the motor at 100% speed. The motor accelerates until CEMF reaches a value of V_a – I_aR_a.

Remember that Va = I_aR_a + CEMF. In this example armature current (Ia) is 100% and Ra will drop some amount of voltage. If we assume that current and resistance is such that R_a drops 50 VDC, CEMF will be 450 VDC.