Three Phase Induction Motors

Since its invention one hundred years ago, the standard 3-phase induction motor has become one of the most familiar items of industrial equipment ever known. Due to its simplicity of construction, low cost, reliability and relatively high efficiency, it is likely to remain the prime source of mechanical energy for the foreseeable future.

The main problems are the motor's inability to match motor torque to load torque both during the high starting current and normal running.

During starting the motor usually produces 150 - 200% torque accelerating the load to full speed in a fraction of a second, which can cause damage to the drive train. At the same time the motor can commonly draw 8-10 times nominal current (In) causing supply stability problems

When the motor is operating at light load for extended periods, the motor¹s efficiency falls due to the over-fluxing of the windings for the particular torque required to drive the load. At a constant terminal voltage this flux, often referred to as magnetising current, is fixed and accounts for around 30-50% of the motors total losses.

Soft Start & Soft Stop

Motorboss has a selection of ramp times to provide controlled acceleration and deceleration of motors and can effectively control the current without the levels of harmonic distortion normally associated with inverters. There are numerous problems associated with starting and stopping AC Induction motors - all of which can have serious implications if not rectified.

Motorboss has an unrivalled ability to change the shape of the applied voltage waveform, thus reducing the impact of the standing losses existing in all AC Induction motors. In common with conventional soft start devices Motorboss uses thyristor¹s to accurately control the voltage applied at the motor terminals. A characteristic of the thyristor is to switch rapidly from "OFF" to "ON" when pulsed, and to remain "ON" until the current through the device falls to zero at the end of each half-cycle in the AC supply, this is called self commutation.

The Control Technology

By controlling the switch-on point relative to the voltage zero crossing in each half cycle of the supply, it is possible to regulate the current flowing through the thyristor. The closer the turn-on point is to the end of the cycle the smaller the value of current that will be allowed to flow. Conversely, the closer the turn-on point is to the beginning of the cycle the higher the value of current will be. Using this principle, and by connecting two thyristor¹s in anti-parallel to each of the phase connections to a motor. Motorboss can continuously adjust the voltage to the motor terminals by precisely controlling the thyristor¹s turn-on points. This provides just sufficient voltage for the motor to accelerate the load.

So, for instance, by starting with a large delay to the turn on point in each half cycle, and progressively reducing it over a selected time period, the voltage applied to the motor starts from a relatively low value and increases to full voltage. Due to the motor torque being proportional to the square of the applied voltage, the starting torque increases in a stepless manner ensuring a soft start for both the motor and the driven load.

Optimisation

Continuous dynamic control is applied to the motor by a microcontroller embedded with unique software. Thyristor phase control is used to vary the motor terminal voltage in response to changes in motor loading. By reducing the terminal voltage of a motor under light or variable loads, it is often possible to reduce the magnetic core saturation, and copper winding losses, thereby increasing overall motor efficiency - saving on electricity costs.

Any variation of load, up or down, is automatically compensated for by changing the output from Motorboss to satisfy the change of circumstance. Because Motorboss is an intelligent device which serves the specific needs of each motor, you must install one Motorboss on each motor. Parallel connections are ineffective.

Suitable Applications

Motorboss can be installed on any Caged Rotor (squirrel cage) AC Induction motor. These represent the vast majority of motors in use in commerce and industry. Motorboss can be installed direct on line or in line with an existing star-delta starter. The suitability, and relevant benefits, are determined by the application and duty cycle of the motor.

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