Closed-loop control of multi-motor drive

In a multi-motor drive, one machine has several motors, each driving a separate part (section) of it, e.g. paper machines, textile machines, rolling mills, cranes (hoist, trolley, travel motors). The sections must run at precise speed ratios (or tension), so closed-loop feedback control is used.

Block diagram

                 Master speed reference w*
                         |
         +---------------+----------------+
         |               |                |
      [ratio k1]      [ratio k2]       [ratio kn]
         |               |                |
  w1* -->(+)-->[SC]-->(+)-->[CC]-->[Conv]-->M1--> section 1
          ^-  speed      ^-  current            |
          |   ctrl       |   ctrl               |
          |              +-- current sensor <---+
          +------------- tacho/encoder <--------+
         ...
  (same loop for sections 2 ... n)
         |
   [Tension/load sensor between sections]
         +---> trims speed refs of next section

Working

1. Master reference: an operator sets the line speed $\omega^*$. Each section's speed reference is obtained by multiplying by a ratio $k_i$ (draw ratio), so all sections change speed together.
2. Outer speed loop: the speed sensor (tachogenerator or encoder) measures the actual speed $\omega_i$. The error $\omega_i^* - \omega_i$ goes to the speed controller (PI), whose output is the current (torque) reference.
3. Inner current loop: the current controller compares the reference with the measured armature (or stator) current and sets the converter firing angle/duty cycle (for DC drives) or the inverter PWM (for AC drives). It limits current during starting and overload and gives fast torque response.
4. Power converter (controlled rectifier, chopper or inverter) supplies the motor.
5. Tension/position (dancer) feedback between sections adds a trim signal so the material is neither slack nor over-stretched.
6. Load sharing: when several motors drive the same shaft, their current loops are coordinated so each carries its share.

Advantages

• Accurate speed ratios and tension; good product quality.
• Fast response to disturbances (current loop corrects supply and load changes).
• Protection against overcurrent; smooth starting of the whole line.
• Easy changing of line speed with one master control; suitable for PLC/computer control.

Applications

Paper and textile mills, steel rolling mills, printing presses, cranes, conveyor lines.