# Motor Control

Resistors

Fortress Systems manufacture resistors for a number of different types of motor control applications including:

- Wound rotor AC motors with slip ring starting resistors
- Star-delta closed transition starting resistors
- Squirrel cage AC motor resistors
- Crane control systems
- DC series wound motor resistors
- Field discharge resistors

Although AC variable speed drives have, in many cases, replaced these types of motors and control systems, they are often still used on grounds of cost, simplicity and reliability.

Fortress can assist you to replicate existing equipment, even where the original design data is not available and the original equipment manufacturer either no longer exists or no longer manufactures resistors.

### Wound Rotor AC Motors with Slip Ring Starting Resistors

The resistor is wired into the motor secondary slip rings, where it dissipates torque energy in the form of heat. It also provides “soft starting” as resistance is removed in steps.

Initial information needed for sizing your resistor:

- Application
- Power
- Secondary volts
- Secondary amps
- Minimum & maximum starting torque
- Start time
- Number of consecutive starts
- Number of equally spaced starts per hour

### Star-Delta Closed Transition Starting Resistors

Star-delta is a reduced inrush starting method in which 3 phase motor stator coils are initially connected in a star configuration and switched to delta as the motor’s speed increases. The resistor is in circuit for a very short time during the switchover from star to delta operation.

Initial information needed for sizing your resistor:

- Application
- Power
- Line voltage
- Full load amps
- Duty

### Squirrel Cage AC Motor Resistors

Commonly known as a “reduced-voltage” or “ballast” resistor, this resistor acts as a voltage divider for soft starting of the motor.

Initial information needed for sizing your resistor:

- Application
- Power
- Line voltage
- Full load amps
- Duty cycle

### DC Series Wound Motor Resistors

The resistor limits torque by limiting current flow to the motor. It is used for starting and stopping the motor.

Initial information needed for sizing your resistor:

- Application
- Power
- Line voltage
- Full load amps
- Starting torque
- Number of speeds
- Duty

For other applications, such as small kW motors, dynamic braking, and field discharge, please give us a call. We offer free engineering assistance to our customers for all motor control applications.