AC Drive Technical Information

3 Phase Squirrel Cage Induction Motor

In order to understand the speed control, theory and methods, It is important that the basic theory of the controlled component i.e. the motor is understood. The Squirrel Cage induction motor is the most commonly used motor in industry. It is low cost and requires low maintenance in comparison with other motor types. It converts electrical energy into mechanical energy and it is used to provide motion. The conversion process is electro-magnetic induction.

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Soft Starter versus VSD Control

Soft Starters:-

Used for fixed speed application. Not suitable for speed/torque control during running. Motor speed is controlled during starting by applying a reduced voltage, which increases until full voltage is reached. The main benefit of this unit is that eliminates the starting inrush current which occurs with D.O.L starting of motors. (Sudden torque).

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VSD (Variable Speed Drive)

Why VSD?

Used for simple process control.

In most process there may be a need for controlling a variable of the process. This variable may need to be kept constant or be changed according to a pre-set pattern. variables such as:-

Speed, Pressure, temperature ,flow, etc. Reduce fan noise. Enable operation above motor base speed.

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Flux Vector Control

Flux vector control is another method of speed control and this feature is also available on some standard inverters which normally operate under V/F control.

There are two methods of flux vector control:-

  • Open Loop (sensor less vector)
  • Closed loop (with encoder feedback).

The torque producing and magnetising current vectors are independently controlled.

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Motor Load Types

In order to make a correct selection of a VSD unit for an application, the load type and the demand of the driven system needs to be established. The load demand is achieved by changing the speed of the motor driving the system. Motor torque / Speed characteristic are very important, because the motor provides the torque that the load requires and not the VSD.

VSD (Inverter) only provides speed change and...

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Motor Size Selection

Due to the effect of PWM on motor insulation and possibility of running at low speeds and the related reduction of cooling, it is necessary to de-rate standard induction motors for VSD use.

Graphs below show de-rating required for class "B" and "F" temperature rise.

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VSD Selection

When selecting VSD’s the following should be considered:-

  • Application. Load types and T-N Curve.
  • Environment. IP00, IP20, Cubicalised IP54.
  • In coming Supply & protection. Circuit breaker Protection. Customer responsibility, unless cubicalised.
  • KW Power & Output Current.
  • Overload and Rapid Acceleration/Torque
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Dynamic Braking with VSD

  • When a stop signal is given to a VSD, the VSD output decelerates to stop and is switched off, but the motor may carry on running due to load inertia. Mechanical energy now is running the motor and motor acts as a generator.
  • Energy flows back from the motor into the DC link capacitors within the drive. This causes the DC link voltage to rise.
  • If the DC link voltage exceeds 750V then...
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In order to meet the European directive 89/336/EEC Electro-Magnetic Compatibility (EMC).

The equipment manufacture, systems integrator and installer shall comply with the requirements of the directive.

Need to consider the mains interference and harmonic distortion. Harmonics are a sinusoidal component of the mains fundamental frequency (50 Hz).

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Other VSD Main Features

  • VSD & Motor Overload.
  • VSD O/L trips Current > 150% of rated Current for 1 Min.
  • Motor Electronic Thermal Overload.
  • Torque Boost
  • Slip Compensation
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