What is the difference between soft starter and VFD soft starter?
Today we briefly explain a topic that may sometimes confuse: What is the difference between soft starter and VFD soft starter in motor starting applications?
Direct on-line motor start
Across the line or direct on-line (DOL) motor start is the simplest possible way to start an electric motor. The advantage is the absence of additional hardware and extra cost. On the other hand, there are couple of drawbacks associated with DOL start (high starting current, grid voltage sag, thermal stress for the motor, oscillating motor torque etc). See [1] for more details.
Alternative starting methods
Due to above disadvantages, DOL start brings several concerns and may even be prohibited, especially for larger motors. Several other motor starting methods exist [2]. They differ in their complexity, performance, investment cost and, of course, the quality of the motor start.
Soft starting solutions
One category is so called “soft starting”. However, there are two types of soft starting equipment with very different performance. Key is whether the device adjusts only the output voltage or both voltage and frequency. In this post we aim to explain the difference between soft starter and VFD soft starter.
Soft starter
Conventional soft starter is a power electronic based equipment. It can adjust the output voltage in order to reduce the motor starting current. The output frequency is the same as the input frequency – the soft starter is not a frequency converter. Major limitation comes from the starting capability itself: Reduction of output voltage means significant reduction of motor starting torque (torque proportional to the square of the stator voltage). The soft starter reduces the motor starting current compared to a DOL start. Nevertheless, the starting current still remains 2-4 times the nominal motor current. The reduction of starting torque may be a show stopper for loads that require high starting torque at low speed.
VFD soft starter
In contrast, VFD soft starter is much more sophisticated device. It is basically a full-blown variable frequency drive (VFD). Sometimes, the dimensioning of the VFD soft starter can be optimized considering the specific duty cycle. On the other hand, power semiconductors have very short thermal time constant. Therefore, the power hardware of a VFD soft starter is usually very similar (if not identical) to a VFD rated for continuous operation. Other system components, such as transformer(s), can be somewhat optimized using the duty cycle (there the thermal time constant is much longer).
VFD soft starter is the most expensive motor starting method (CapEx point of view) with highest quality of the motor start [3].
Comparison
Let’s bring it to the point and compare the classical soft starter with VFD soft starter. As a reference, we add also the DOL motor start for a better illustration. The performance comparison is shown in Table 1.
Table 1: Comparison of DOL motor start, classical soft start and VFD soft starting
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Summary
DOL motor start is the simplest starting method but also the method with most severe drawbacks. Thus, multiple alternative starting methods exist. When it comes to “soft starting” there are two options: a conventional soft starter and a VFD soft starter.
Conventional soft starter can only adjust the output voltage (range 0…input voltage). The output frequency remains the same as the input frequency (e.g. 50 Hz or 60 Hz). The motor starting current is reduced compared to DOL start but remains 2-3 times higher than the rated current. Moreover, conventional soft starter always reduces the available motor starting torque which can be a major issue.
In contrast, VFD soft starter is a device working the same way as a normal VFD, i.e. adjusting output voltage and output frequency. The starting current remains significantly below the nominal current while the motor can provide full torque from zero speed (if required). Thus, it is suitable also for heavy duty starts.
References
[1] Motor starting methods: Direct on-line start, https://mb-drive-services.com/direct-on-line-start/
[2] Motor starting methods: Overview, https://mb-drive-services.com/motor-starting-methods/
[3] VFD as a soft starter, https://mb-drive-services.com/vfd-as-soft-starter/
1 Comment
Motor start with VFD - MB Drive Services · February 25, 2024 at 9:25 am
[…] (variable voltage – variable frequency) is sometimes used as a synonym for VFD starter. See [3] for more details on soft starter and VFD soft […]
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