Motor protection delivers reliable, efficient operation

How to make the right choice between a soft starter and a variable frequency drive

04/15/2013


How to make the right choice between a soft starter and a variable frequency drive. Courtesy: EatonMotors often require large amounts of energy when quickly accelerating to full speed. Soft starters and variable frequency drives can both be used to reduce inrush currents and limit torque—protecting your valuable equipment and extending the life of your motor by reducing motor heating caused by frequent starts and stops. Choosing between a soft starter and a variable frequency drive often depends on the application, system requirements, and cost (both for initial start-up and over the lifecycle of the system). 

Extending motor life

A soft starter is a solid-state device that protects ac electric motors from damage caused by sudden influxes of power by limiting the large initial inrush of current associated with motor start-up. They provide a gentle ramp-up to full speed and are used only at start-up (and stop, if equipped). Ramping up the initial voltage to the motor produces this gradual start. Soft starters are also known as reduced voltage soft starters (RVSS). 

Applications

Soft starters are used in applications where:

  • Speed and torque control are required only during start-up (and stop if equipped with soft stop)
  • Reducing large start-up inrush currents associated with a large motor is required
  • The mechanical system requires a gentle start to relieve torque spikes and tension associated with normal start-up (for example, conveyors, belt-driven systems, gears, and so on)
  • Pumps are used to eliminate pressure surges caused in piping systems when fluid changes direction rapidly  

How does a soft starter work? Electrical soft starters temporarily reduce voltage or current input by reducing torque. Some soft starters may use solid-state devices to help control the flow of the current. They can control one to three phases, with three-phase control usually producing better results. 

Figure 1. Soft Starter Schematic. Courtesy: Eaton

Figure 1. Soft Starter Schematic 

Most soft starters use a series of thyristors or silicon controlled rectifiers (SCRs) to reduce the voltage. In the normal Off state, the SCRs restrict current, but in the normal On state, the SCRs allow current. The SCRs are engaged during ramp-up, and bypass contactors are pulled in after maximum speed is achieved. This helps to significantly reduce motor heating.

Soft starters are often the more economical choice for applications that require speed and torque control only during motor start-up. Additionally, they are often the ideal solution for applications where space is a concern, as they usually take up less space than variable frequency drives.

Driving energy efficiency

A variable frequency drive is a motor control device that protects and controls the speed of an ac induction motor. A VFD can control the speed of the motor during the start and stop cycle, as well as throughout the run cycle. 

Applications

VFDs are used in applications where:

  • Complete speed control is required
  • Energy savings is a goal
  • Custom control is needed  

VFDs convert input power to adjustable frequency and voltage source for controlling speed of ac induction motors. The frequency of the power applied to an ac motor determines the motor speed, based on the following equation: 

Speed (rpm) = 120 x Frequency (Hz) x number of motor poles 

For example, a four-pole motor is operating at 60 Hz. These values can be inserted into the formula to calculate the speed:

120 x 60 x 4 = 1800 rpm 

Figure 2. The function of a VFD. Courtesy: Eaton

Figure 2. The function of a VFD

  • ac supply: Comes from the facility power network (typically 480V, 60 Hz ac)
  • Rectifier: Converts network ac power to dc power
  • Filter and dc bus: Work together to smooth the rectified dc power and to provide clean, low ripple dc power to the inverter
  • Inverter: Uses dc power from the dc bus and filter to invert an output that resembles sine wave ac power using a pulse width modulation (PWM) technique.
  • Pulse width modulation: Switches the inverter semiconductors in varying widths and times that, when averaged, create a sine waveform 

Figure 3. Pus Width Modulated Waveform. Courtesy: Eaton

Figure 3. Pus Width Modulated Waveform 


<< First < Previous Page 1 Page 2 Next > Last >>

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Doubling down on digital manufacturing; Data driving predictive maintenance; Electric motors and generators; Rewarding operational improvement
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Research team developing Tesla coil designs; Implementing wireless process sensing
Commissioning electrical systems; Designing emergency and standby generator systems; Paralleling switchgear generator systems
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me