Soft starter for an electric motor: the principle of operation of an asynchronous electric motor

An asynchronous electric motor has the ability to start independently due to the interaction between the rotating magnetic field flux and the rotor winding flux, causing a high current in it. As a result, the stator draws a large current, which, by the time the motor reaches full speed, becomes more than the rated current, which can lead to overheating of the motor and its damage. To prevent this, an electric motor soft starter (SCD) is required.

The principle of operation of the starter

It consists in the fact that the device regulates the voltage applied to the motor during start-up, controlling the characteristics of the current. For induction motors, the starting torque is approximately proportional to the square of the starting current. It is proportional to the applied voltage. The torque can also be considered approximately proportional to the applied voltage, thus adjusting the voltage during starting, the current consumed by the machine and its torque are controlled by the device and can be reduced.

By using six SCRs in a configuration as shown, the soft starter can regulate the voltage supplied to the motor at start-up from 0 volts to nominal linear voltage. Soft start of the electric motor can be carried out in three ways:

1. Direct start using full load voltage.
2. Applying gradually downgraded.
3. Application of starting a partial winding using an autotransformer starter.

SCP can be of two types:

1. Open control: the starting voltage is applied with a time delay regardless of the motor current or speed. For each phase, the two SCRs are initially delayed 180 degrees for the respective half-wave cycles (for which each SCR is performed). This delay gradually decreases over time until the applied voltage reaches the nominal value. It is also known as a temporary stress system. This method does not actually control the acceleration of the engine.
2. Closed loop monitoring: Monitors any characteristics of the motor output such as current or speed. The starting voltage is varied accordingly to obtain the required response. Thus, the task of the soft starter is to control the conduction angle of the SCR and control the supply voltage.

Benefits of soft start

Solid state soft starters use semiconductor devices to temporarily derate parameters at the motor terminals. This allows the motor current to be monitored to reduce the torque of the motor limit. The control is based on the voltage control of the motor terminals on two or three phases.

There are several reasons why this method is preferred over others:

1. Increased efficiency: The efficiency of a soft-state soft starter system is mainly due to the low voltage state.
2. Controlled launch: The starting parameters can be controlled by easily changing them, which ensures that it starts without any jerks.
3. Controlled acceleration: The acceleration of the engine is controlled smoothly.
4. Low cost and size: This is done using solid state switches.

Solid State Components

Power switches such as SCRs that are phase controlled for each part of the cycle. For a three-phase motor, two SCRs are connected to each phase. The motor soft start relays must be rated at least three times the line voltage.

Working example of a system for a three-phase asynchronous motor. The system consists of 6 SCRs, a control logic circuit in the form of two comparators - LM324 and LM339 for obtaining the level and voltage of the ramp and an opto-isolator to control the application of the gate voltage to the SCR on each phase.

Thus, by controlling the duration between pulses or their delay, the controlled SCR angle is monitored and the power supply during the motor start phase is regulated. The whole process is actually an open loop control system that controls the timing of the gate trigger pulses for each SCR.

SCR basics

SCR (Silicon Controlled Rectifier) ​​is a high power DC controlled power regulator. SCR induction motor soft starters are PNPN four-layer silicon semiconductor devices. It has three external terminals and uses alternate symbols in Figure 2 (a) and has a transistor equivalent circuit in Figure 2 (b).

The main use of the SCR is as a switch with an anode positive with respect to the cathode, controlled when the machine is started.

The main characteristics of the SCR can be understood using these diagrams. The motor soft starter can be turned on and made to act as a silicon forward bias rectifier by briefly applying a gate current across S2 to it. The SCR quickly (within a few microseconds) automatically latches into the on state and remains on even when the shutter motor is removed.

This action is shown in Figure 2 (b) the initial gate current turns on Q1 and the collector current of Q1 turns on Q2, the collector current of Q2 then holds Q1 even when the gate drive is removed. The saturation potential is 1 V or so and is created between the anode and cathode.

Only a short shutter pulse is required to turn on the SCR. Once the SCR is fixed, it can be turned off again by briefly reducing its anode current below a certain value, typically a few milliamps, in AC applications, shutdown occurs automatically at the zero-cross point at each half-cycle.

Significant gain is available between the gate and anode of the SCR, and low gate currents (usually a few mA or less) can control high values ​​of anode current (up to tens of amplifiers). Most SCRs have anode ratings of hundreds of volts. The characteristics of the SCR gate are similar to those of the transistor junction - the emitter of the transistor (see. Rice. 2 (b)).

An internal capacitance (a few pF) exists between the anode and the gate of the SCR, and the surging voltage appearing at the anode can cause enough gate breakthrough to turn on the SCR. This “speed effect” can be caused by power line transients, etc. The problems with the speed effect can be overcome by running a CR smoothing network between the anode and cathode to limit the rate of rise to a safe value.

Variable speed operation

AC mains voltage (Fig. 5) is rectified using a passive diode bridge. This means that the diodes are triggered when the line voltage is greater than the voltage across the capacitor section. The resulting waveform has two pulses during each half cycle, one for each diode conduction window.

The waveform shows some continuous current as conduction moves from one diode to the next. This is typical when it is used in the DC link of a drive and some load is present. Inverters use wide pulse modulation to generate output signals. The triangle signal is generated at the carrier frequency with which the IGBT inverter will switch.

This waveform is compared to a sinusoidal waveform at the fundamental frequency that must be driven to the motor. The result is the U waveform shown in the figure.

The inverter output can be any frequency below or above the line frequency up to the limits of the inverter and / or the mechanical limits of the motor. Note that the drive always runs within the motor slip rating.

Start control process

SCR timing is the key to controlling the voltage output for a soft starter. During start-up, the soft starter logic determines when to turn on the SCR. It does not turn on the SCR at the point where the voltage goes from negative to positive, but waits for a while after that. This is a well-known process called "gradual recovery" of the SCR. The SCR switch point is set or programmed so that the start torque, start current, or current limit is strictly controlled.

The SCR step recovery results in a non-sinusoidal undervoltage at the motor terminals as shown in the figures. Since the motor is inductive and the current lags behind the voltage, the SCR remains on and conducts until the current reaches zero. This happens after the voltage has turned negative. Individual SCR voltage output.

Compared to the total voltage waveform, it can be seen that the peak voltage is the same as the total waveform voltage. However, the current does not increase to the same level as when full voltage is applied due to the inductive nature of the motors. When this voltage is applied to the motor, the output current looks like the figure.

Since the voltage frequency is the same as the linear frequency, the current frequency is also the same. SCRs gradually transition to admittance, the current gaps are filled in until the waveform looks the same as the motor.

Motor characteristics using a soft starter

Such a soft start of an asynchronous electric motor, in contrast to an AC drive, has the characteristics of the current in the network and the motor current are always the same. During start-up, the change in current depends directly on the magnitude of the applied voltage. Motor torque varies as the square of the applied voltage or current.

The most important factor in evaluating is engine torque. Standard motors produce approximately 180% of full load torque when starting. Therefore, a 25% derating will be equal to full load torque. If the motor draws 600% of the full load current when starting, then the current in this circuit will reduce the starting current from 600% to 450% of the load.

Starter wiring diagrams

There are two options with which the starter starts the electric motor: the standard circuit and inside the delta.

Standard scheme. The starter is connected in series with the line voltage supplied to the motor.

Inside the triangle, there is another circuit according to which the starter is connected, called the internal delta circuit. In this diagram, two cables that connect to one of the motors are connected directly to the I / P power supply, and the other cable will be connected through the starter. One feature of this circuit is that the starter can be used for large motors such as 100 kW motors, since the phase currents are divided into 2 parts.