How a Slip Ring Motor Starts

The stator of a high voltage slip ring motor is connected to the three-phase power supply. The rotor contains a three-phase winding whose terminals are connected to slip rings mounted on the shaft.

During startup:

1. External resistance is connected to the rotor circuit.

2. The motor develops the required starting torque.

3. Starting current is controlled according to the system design.

4. Rotor resistance is reduced in stages as speed increases.

5. The rotor circuit is short-circuited when the motor reaches operating speed.

6. The motor continues to run as an induction motor.

The starter may use a liquid resistance system, grid resistance bank or another application-specific rotor-control method.

Typical Applications

Ball Mills and Grinding Mills

Grinding mills combine high rotational inertia with demanding breakaway torque. The motor, starter, coupling and mill load must be evaluated as a complete drive system.

Important project information includes mill type, loaded starting condition, total rotating inertia, required acceleration time and number of starts per hour.

Cement Mills and Raw Mills

Slip ring motors are commonly considered for cement-production equipment where reliable starting is required under dusty conditions.

Cooling, enclosure protection, bearing seals and maintenance access should be selected according to the actual installation environment.

Crushers and Shredders

Crushers may experience high breakaway torque, variable load and occasional material blockage. The motor design should consider both normal startup and abnormal operating conditions.

Conveyors

Long or fully loaded conveyors require controlled acceleration to reduce belt stress and mechanical shock.

The motor and starter should be selected using conveyor length, slope, belt load, inertia and acceleration requirements.

Large Fans and Blowers

Large fans may have high rotating inertia even when the steady-state torque demand is moderate.

A high voltage slip ring motor can provide controlled fan acceleration without imposing excessive current demand on the electrical supply.

Compressors

Compressor starting requirements depend on compressor type, inlet condition, pressure, unloading system and operating cycle.

A complete torque-speed curve should be supplied when evaluating a slip ring motor for a compressor.

Hoists, Winches and Material-Handling Equipment

Selected hoisting and material-handling systems require high torque, frequent starts or controlled acceleration.

Duty cycle, braking, reversing and safety requirements must be reviewed before motor selection.

High Voltage Slip Ring Motor vs. Squirrel-Cage Motor

A squirrel-cage motor may be the preferred option when starting requirements are moderate and minimum maintenance is the primary goal. A slip ring motor becomes especially valuable when the load requires high torque, low starting current or controlled acceleration.

Starting System Options

The rotor starter is a critical part of the complete slip ring motor system.

Liquid Resistance Starter

A liquid resistance starter changes rotor resistance through an electrolyte system. It is commonly considered for large motors requiring smooth acceleration over an extended starting period.

Project evaluation should include:

• Rotor voltage

• Rotor current

• Required starting torque

• Load inertia

• Acceleration time

• Starts per hour

• Ambient conditions

• Starter installation location

Grid Resistance Starter

A grid resistance starter uses metallic resistor elements and staged switching to reduce rotor resistance as the motor accelerates.

It may be selected when defined resistance steps and a robust industrial structure are required.

Brush-Lifting and Short-Circuiting Device

Selected motors can be evaluated with a brush-lifting mechanism. After startup, the rotor circuit is short-circuited and the brushes may be lifted from the slip rings.

This option can reduce continuous brush contact, but it increases the mechanical complexity of the slip-ring system and must be matched to the maintenance strategy.

Cooling Options

The cooling system should be selected according to motor power, ambient temperature, contamination level, available cooling water and installation space.

Possible configurations include:

• Open-circuit ventilation

• Totally enclosed fan cooling

• Tube cooling

• Air-to-air heat exchanger

• Air-to-water heat exchanger

• Independent forced ventilation

Open cooling may be suitable for clean, well-ventilated installations. Enclosed air-to-air systems can isolate the internal air circuit from dusty ambient air. Air-to-water cooling is often considered when compact size or low heat release into the motor room is required.

Only list the IC cooling codes that are available within the actual product range.

Protection and Environmental Options

High voltage slip ring motors can be configured for different industrial environments, including:

• Cement and mining dust

• High ambient temperatures

• Outdoor installations

• Humid environments

• Corrosive atmospheres

• High-altitude sites

• Low-temperature installations

• Hazardous areas

• Restricted ventilation

• High vibration

Possible options include increased enclosure protection, space heaters, winding-temperature sensors, bearing-temperature sensors, corrosion-resistant paint, special seals and hazardous-area construction.

Bearings and Mechanical Construction

Bearing selection depends on rotor weight, speed, radial load, axial load, coupling arrangement and maintenance requirements.

Available designs may include:

• Rolling-element bearings

• Sleeve bearings

• Insulated bearings

• Forced-lubricated bearings

• Water-cooled bearings

• Bearing vibration monitoring

• RTD temperature monitoring

The customer should provide coupling weight, external forces and driven-equipment shaft information during technical evaluation.

Replacement High Voltage Slip Ring Motors

Older mills, fans and compressors may use a motor that is no longer available from the original supplier.

A replacement motor can be designed to match an existing installation, including:

• Foundation dimensions

• Shaft height

• Shaft extension

• Coupling position

• Rotation direction

• Terminal-box location

• Motor footprint

• Cooling interfaces

• Rotor terminal data

• Existing starter characteristics

• Electrical performance

• Overall space restrictions

Provide the original motor nameplate, dimension drawing, performance data and photographs. When possible, information about the existing starter and driven equipment should also be supplied.

Custom High Voltage Wound Rotor Motors

Custom engineering may include:

• Special voltage and frequency

• Custom power and speed

• Modified starting torque

• Special rotor voltage and current

• Custom shaft and flange

• Existing-foundation adaptation

• Horizontal or vertical mounting

• Alternative cooling systems

• Increased IP protection

• Sleeve or rolling-element bearings

• Vibration-monitoring provisions

• Temperature sensors

• Special terminal-box arrangements

• Brush-lifting devices

• Hazardous-area options

• Special paint systems

Custom design must be confirmed through an approved technical datasheet and dimensional drawing before manufacturing.

Testing and Quality Control

Recommended test and inspection items include:

• Visual and dimensional inspection

• Winding resistance

• Insulation resistance

• Polarization index

• High-voltage withstand test

• No-load current and losses

• Locked-rotor or starting-performance evaluation

• Vibration measurement

• Bearing-temperature measurement

• Noise measurement

• Direction-of-rotation check

• Rotor-voltage verification

• Slip-ring and brush inspection

• Overspeed test when required

• Load or temperature-rise test when specified

Factory acceptance testing requirements should be agreed before order confirmation.