We want longevity in our motor applications to avoid expensive maintenance costs, productivity loss, unnecessary wear and tear, service life, part replacements, and much more. A critical part of an operation begins at startup, where various factors combine to dramatically increase the stress of the motor and infrastructure that it is connected to. This stress can be mitigated by utilizing a soft starter.
Common Issues at Startup
An AC induction motor can draw an initial current seven times more than the normal load while at a standstill before dropping down as the motor approaches full speed. The abnormally high currents can potentially damage the motor and electrical system due to heat stress. Oversized cables and components are a requirement for this type of start-up, but they may incur high electricity costs.
Additionally, with full power applied, the motor will accelerate very quickly, producing up to three times the normal load torque. Especially with large, powerful motors, this can cause mechanical shock to the motor, drivetrain, and the driven load, with potential consequences including:
- Worn or failed bearings
- Belt drive slipping
- Gearbox and drivetrain wear and damage
- Water hammer in pumping systems
- Damage to driven machinery
To mitigate these problems, it is recommended to install a soft starter. This will allow the user to gradually increase the voltage supplied to a motor when it starts, preventing the current and torque from building up too quickly. In a typical AC induction motor, the current drawn by the motor is proportional to the square of the supply voltage, and the torque is proportional to the current. This means that ramping up the voltage slowly results in a smooth build-up of the motor's current and torque.
Most commonly, a soft starter consists of two thyristors (or silicon-controlled rectifier as they are commonly known) connected back-to-back for each phase, which work together to control the voltage supplied to the motor. The AC voltage is rapidly changing between positive and negative at the line frequency. The supply voltage can be controlled by switching the power on and off during specific parts of the AC voltage waveform. Thyristors operate like electrical switches, making them ideal for this task.
Soft starters include a logic controller that determines the timing of the power supply to the motor, and this can be an open-loop or closed-loop. Open-loop devices do not have a feedback system; they do not modify their output based on the motor's behaviour. Instead, they ramp up the voltage according to a pre-selected set of values. Closed-loop devices monitor the motor's current build-up with a feedback loop, allowing them to closely manage the motor's start-up for maximum efficiency.
Variable Frequency Drives (VFDs)
A Variable Frequency Drive (VFD) is a device that controls the speed of a motor by modifying the frequency of the AC supply power. While used primarily for precision speed control, a VFD can also sometimes be used as a soft starter and is best suited for applications where the acceleration characteristics must be highly consistent even with variable loads. However, VFDs have some downsides, being relatively expensive and requiring management of potential bearing currents.
Ideal Applications of Soft Starters
When using soft starters, however, the load on the motor impacts how fast it will accelerate. If the main concern is to limit the current surge in the motor rather than achieve a precise acceleration time, a soft starter is often a much more cost-effective choice, especially in large, high-power applications.
The initial voltage supplied by a soft starter has a significant impact on how effective it is. When connected to a high-inertia load, a motor may require a substantial amount of torque to begin rotating. When insufficient power is supplied to create rotation, heat builds in the windings and quickly reaches damaging levels. Therefore, it is important to choose an initial voltage high enough to get the motor rotating straight away – ideally within 1-2 seconds of energization.
The ramp time is how long the soft starter takes to go from starting voltage to full line voltage. A longer ramp time reduces the torque in the motor during start-up, resulting in slower, smoother acceleration, and is common for applications such as water pumps and conveyors where the infrastructure that the motor is connected to is at greater risk from mechanical shock.
For applications where the load on the motor during start-up varies considerably, it is recommended to use a soft starter that monitors the current in the motor to prevent excessive current surges in the system when the load is high.
Applying full power to a motor at start-up often results in mechanical and electrodynamic stress on the motor, electrical system, and connected machinery that reduces service life and increases costs. This problem can be avoided by utilizing a soft starter, maximizing the application's productivity and operational efficiency.