Description

Automatic Starter for Slipring Motors

The Magnastart product line consists of a range of steel cored Inducers from 37 to 370kW, each individual model spanning a variety of machine sizes. When built into rotor starter equipment, the Inducer acts as the current-limiting rotor impedance during run-up, and is bypassed by a shorting contactor when near full speed is reached.

A number of advantages to the manufacturer and maintainer of starters results from this approach:

Size

Though physically smaller than conventional systems of resistors and contactors, the large thermal capacity of Magnastart permits the starting of difficult and high inertia loads without problems.

Simplicity

Magnastart requires only one shorting contactor, reducing the system complexity, size and cost.

Performance

Magnastart is ideal for inching, reversing, and plug braking without any additional rotor controls.

Maintenance

Magnastart Inducers have no moving parts, and do not use liquids or chemicals. Magnastart will give many years of service without any maintenance or adjustment.

Reliability

The robust construction permits heavy usage under the most arduous conditions. Built-in thermal cut-out provides protection against damage.

Flexibility

Each Inducer model is designed to start a wide range of motors. The front panel allows the user to select the exact starting torque and stator current required, on commissioning if appropriate. Unlike resistors, the total energy capacity of Magnastart remains constant regardless of the impedance tapping selected .

Smooth acceleration

Smooth acceleration to full speed reduces electrical and mechanical stresses in the motor and drive.

How Magnastart Works

The Magnastart Inducer consists of 3 coils wound on specially shaped steel cores. Eddy currents in these cores are reflected in the windings as an impedance which is dependent on rotor
frequency. This is connected across the rotor windings on starting.

When power is applied to the stationary motor, the frequency in the rotor is the same as the stator resulting in the maximum Inducer impedance. As the machine accelerates, the rotor frequency (the slip frequency) and Inducer impedance decrease. This results in a smooth acceleration to nearly full speed at which point the rotor is short circuited by the rotor contactor.

The starting power is dissipated in the heavy mass of the steel cores, which allows the inducer to absorb a considerable amount of energy. Advanced thermal insulation protects the coils which remain relatively cool, and several consecutive starts may be obtained on drives with high inertial loads.

Each of the coils has four connections brought out to studs on the front panel. By using various star and delta combinations up to 17 different starting impedances are available, to provide the exact starting characteristics required on a wide range of motors.

Notes:

1. With 50Hz, 3 phase supply. For other supply arrangements contact ADWEL. It may be possible to parallel models for greater capacity in particular circumstances. Please consult ADWEL with specific details.

2. This is based on the maximum motor size starting against 1.5 x full load torque for 15 secs, mounted in a typical unventilated metallic enclosure. For heavier duties extra or forced ventilation may be required, or a larger Inducer chosen.

3. Maximum consecutive starts are from cold. Note that the motor rating may also limit consecutive starts.

4. Normal starting duties, occasional inching in both directions.

5. Heavy inching, and plug braking.

6. Impedance is approximate per coil at 50Hz, full load, 100% tapping. Typical PF is 0.8. Tappings at 100%, 84% and 56% per coil allow a choice of star and delta impedances from 100% down to 3% in 17 steps.

7. The Thermal Overload Cut-out should be wired to trip the starting system in case of overload (contacts open).

8. On the M30, M45, M90, M125 and M175 the mounting channels can be removed and the Inducer mounted on the M8 studs protruding from the bottom of the rubber mounts. This allows these Inducers to be mounted in a 400mm deep enclosure.

9. Weight is without packing. For normal road shipment, the Inducers are mounted on 50 x 75 x 430mm wooden bearers (to allow fork-lift carriage) and shrink wrapped. For air/sea export, extra packing can be arranged.

The Magnastart System – Application

Magnastart Inducers are intended to be built in to slipring motor starter equipment by the customer to satisfy the technical requirements of the specific motor and load. They may also be retro-fitted into existing rotor starter systems to replace failed starter equipment.

In normal application the Magnastart is connected across the rotor windings, selecting the impedance to suit the starting current/torque requirements. A shorting contactor operated by a timer is connected across the Magnastart, to short out the rotor once near full speed is reached. During commissioning, test starts followed by adjustment of Magnastart tappings allow the best balance between start current and run-up time to be achieved.

Like all purpose-built electrical control systems, the system design requires the consideration of an experienced electrical engineer.
Magnastart design aspects that should be addressed include:

• Stator current overload protection and co-ordination with starting surge 
• Matching rating of Magnastart Inducer to rotor and duty cycle
• Setting Magnastart impedance to meet run-up time, current and torque requirements
• Timing of rotor shorting contactor for full-speed operation
• Controls and protection in case of rotor stall or overload during starting

Further guidance notes on application and selection of tappings are provided with each unit, and are available on request.

ADWEL has designed and manufactured complete Slipring Motor Starters utilising Magnastart Inducers for many Industries, and is able to provide this service to customer specification. Please