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soft starter and vfd

Created by: Chetan2490 at: 5/26/2014 12:12 PM (20 Replies)

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5/26/2014 12:12 PM
Chetan2490 Advanced Member Joined: 5/12/2014 Last visit: 9/9/2020 Posts: 149 Rating: (16)	Dear experts, How soft starter is differ from VFD? Explain in Brief.

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5/26/2014 2:30 PM
jeet2907 Posts: 235 Rating: (52)	Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Dear Chetan2490 The difference is the Soft Starert is primarily a mechanical soft start. You reduce the voltage and limit the current to ease the mechanical stresses associated with motor starting. You do have the potential to reduce the current associated with starting, but this benefit is highly dependent on many variable and can change dependent on the particular voltage condition at the time of starting. Medium Voltage Motors do not have the generous amounts of starting torque many people take for granted with 460v motors. Most Medium Voltage Motors (2300v and up) have between 70 -85% starting torque (this various from supplier to supplier and is also a function of the rotor construction - die cast vs copper bar). When you use a reduce voltage starter, think of it as a reduced torque starter. They do not call them reduced current starters for a reason. Your starting torque will drop by the square of the voltage so a 80% RVS will result in 64% of the torque available (.80 x .80 = .64) Because you have a medium voltage motor that may only have 75% of its full load torque during acceleration,you now only have 64% of the available 75% during start. 64% of 75% is 48%. If your speed torque demand exceeds the available 48% pull up torque, your motor will stall. On a centrifugal load, this is usually not a problem as long as you can tolerate 350% inrush, but there are some serious asterisks with that statement. Are you trying to load the pump during starting? You have the issue of line notching that occurs when the SCR's fire which causes additional voltage demand (the scr's firing is basically a short between phases), the size of your transformer relative to the linear load - voltage drop during starting and flicker restriction requirements by the utility. Constant torque loads are even more problematic if you have to start the equipment loaded or if you begin loading the equipment before the motor is up to full speed. The soft start works mind you, it just takes some due diligence. The VFD is a completely different technology. PWM drives can actually vary the frequency and the voltage with a fixed v/hz ratio. By reducing the frequency, but maintaining the ratio to voltage, you can smoothly accelerate the motor and generate full starting torque in the process while NEVER EXCEEDING THE FULL LOAD AMPS OF THE MOTOR. The gives you unlimiteed starts with your motor because the motor never sees more than it's Full Load Amps it would see running at full load under normal conditions. Think of the VFD like a dimmer switch on your ceiling fan. You can ease the switch up and slowly make the motor go faster and faster (by the way...that's a totally different technology (using DC power and I just made the comparison to provide a loose visual aid). Once the motor is up to speed: The Reduced Voltage Starter will have a bypass that closes after the motor is up to full speed and now your motor is running at full speed, producing full torque and the SCR's are no longer in cirucit. The VFD can still trim load based on the speed reference from the user. (many ways to do this) and provides additional benefites such as process control, huge huge huge electrical savings (see the afinity law) and excellent motor protection. Much more to go into, but I hope this helped.
jeet2907 Posts: 235 Rating: (52)
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5/26/2014 4:37 PM	Rate (1)
metwally mustafa Silver Member Joined: 10/10/2010 Last visit: 7/5/2025 Posts: 309 Rating: (15)	VFD or variable frequency drive is a speed controller used the thoery of operation of voltage and frequency control to produce a variable voltage vriable frequency to control the speed the soft starter form its mane is a starting device it's just working during the starting period of the motor and after the motor reaches its full load speed the bypass contactor is working after soft starter outage the soft starter has a SCR or thyristor devices controlling the firing angle of the thyristor to control the voltage magnitude and the frequency is fixed

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5/26/2014 4:48 PM
BhaPraJ Silver Member Joined: 1/28/2012 Last visit: 7/23/2024 Posts: 519 Rating: (83)	[quote user="Chetan2490"]Dear experts, How soft starter is differ from VFD? Explain in Brief. Go through following link for application based article. www.controlglobal.com/articles/2010/vfd1005/
	Regards, BhaPraJ
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5/26/2014 5:27 PM
@nkur Posts: 202 Rating: (16)	metwally mustafa VFD or variable frequency drive is a speed controller used the thoery of operation of voltage and frequency control to produce a variable voltage vriable frequency to control the speed the soft starter form its mane is a starting device it's just working during the starting period of the motor and after the motor reaches its full load speed the bypass contactor is working after soft starter outage the soft starter has a SCR or thyristor devices controlling the firing angle of the thyristor to control the voltage magnitude and the frequency is fixed Hi there im am not getting your point related to the Bypass contactor can you please elaborate..
@nkur Posts: 202 Rating: (16)
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5/26/2014 5:29 PM
@nkur Posts: 202 Rating: (16)	Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Hi all I would like to add a question to it in which cases i need a softstarter???
@nkur Posts: 202 Rating: (16)
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5/26/2014 6:34 PM	Rate (0)
Ayman Elotaify Platinum Member Joined: 5/28/2008 Last visit: 9/3/2025 Posts: 4522 Rating: (843)	Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Hello, Detaled answer is here: "Theory of operation of inverter from scartch" The theory of the softstarter Hope this helps.
	Ayman Elotaify
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5/26/2014 6:39 PM
Ayman Elotaify Platinum Member Joined: 5/28/2008 Last visit: 9/3/2025 Posts: 4522 Rating: (843)	@nkur Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Hi all I would like to add a question to it in which cases i need a softstarter??? Hello, Drive is a device to control speed of a motor. While Softstarter is a starting device. We use it to ensure that the motor start working smoothly and then motor work at fixed speed as if it is connected as a DOL. Softstarter is a replacement of Star-Delta starter. In many cases, it's better to use softstarter to ensure smooth start. We can see it in pumps for instance to avoid water hummering. Hope this helps.
	Ayman Elotaify
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5/27/2014 6:31 AM
jeet2907 Posts: 235 Rating: (52)	Ayman Elotaify @nkur Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Hi all I would like to add a question to it in which cases i need a softstarter??? Hello, Drive is a device to control speed of a motor. While Softstarter is a starting device. We use it to ensure that the motor start working smoothly and then motor work at fixed speed as if it is connected as a DOL. Softstarter is a replacement of Star-Delta starter. In many cases, it's better to use softstarter to ensure smooth start. We can see it in pumps for instance to avoid water hummering. Hope this helps. Dear Ayman Elotaify As you said that, Drive is a device to control speed of a motor. So want to know that DRIVE is used just for the speed control only??.....what is the main advantage of DRIVE over SOFT STARTER??
jeet2907 Posts: 235 Rating: (52)
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5/27/2014 7:46 AM
Chetan2490 Advanced Member Joined: 5/12/2014 Last visit: 9/9/2020 Posts: 149 Rating: (16)	jeet2907 Ayman Elotaify @nkur Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Hi all I would like to add a question to it in which cases i need a softstarter??? Hello, Drive is a device to control speed of a motor. While Softstarter is a starting device. We use it to ensure that the motor start working smoothly and then motor work at fixed speed as if it is connected as a DOL. Softstarter is a replacement of Star-Delta starter. In many cases, it's better to use softstarter to ensure smooth start. We can see it in pumps for instance to avoid water hummering. Hope this helps. Dear Ayman Elotaify As you said that, Drive is a device to control speed of a motor. So want to know that DRIVE is used just for the speed control only??.....what is the main advantage of DRIVE over SOFT STARTER?? Dear jeet , Speed control is the main advantage of VFD as wellVFD usualy includes additional features such as multiple speed settings and S curve deceleration. Consider the needs of your application and the benifits of additional flexiblity versus the difference in cost. For smaller motors, a VFD will often provide a lot of additional flexiblity and future proofing for a small additional cost. But, this is not always the case so it really depends on the details of you application and budget. The VFD is as the acronym suggest a variable frequency drive which allows the motor to be operated over a range of speed in a controlled manner, and since you can control the speed of the motor it could have the effect of a soft starter

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5/27/2014 8:22 AM	Rate (0)
Ayman Elotaify Platinum Member Joined: 5/28/2008 Last visit: 9/3/2025 Posts: 4522 Rating: (843)	jeet2907 Ayman Elotaify @nkur Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Hi all I would like to add a question to it in which cases i need a softstarter??? Hello, Drive is a device to control speed of a motor. While Softstarter is a starting device. We use it to ensure that the motor start working smoothly and then motor work at fixed speed as if it is connected as a DOL. Softstarter is a replacement of Star-Delta starter. In many cases, it's better to use softstarter to ensure smooth start. We can see it in pumps for instance to avoid water hummering. Hope this helps. Dear Ayman Elotaify As you said that, Drive is a device to control speed of a motor. So want to know that DRIVE is used just for the speed control only??.....what is the main advantage of DRIVE over SOFT STARTER?? Hello, They are totally different. Softstarter is a starting device and you can not control speed from it Drive is speed control device. Hope this helps.
	Ayman Elotaify
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5/27/2014 1:48 PM
kriipc Silver Member Joined: 4/11/2011 Last visit: 7/11/2025 Posts: 132 Rating: (7)	Ayman Elotaify Drive is speed control device. Hope this helps. Also it may work as torque control device.
	I'm just a human. Mistakes are allowed. Sorry for my english. Only 3 months of training and manuals :)
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5/27/2014 4:49 PM
metwally mustafa Silver Member Joined: 10/10/2010 Last visit: 7/5/2025 Posts: 309 Rating: (15)	VFD has diffirent types of control modes to control torque and speed -V/F control (linear- parabolic- user define) -vector control with sensor(feedback encoder) -vector control sensorless all control modes are working in open loop ( V/F) or in closed loop ( closed loop vector )
	Last edited by: metwally mustafa at: 5/27/2014 4:50 PM
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5/28/2014 7:04 AM
Chetan2490 Advanced Member Joined: 5/12/2014 Last visit: 9/9/2020 Posts: 149 Rating: (16)	metwally mustafa VFD has diffirent types of control modes to control torque and speed -V/F control (linear- parabolic- user define) -vector control with sensor(feedback encoder) -vector control sensorless all control modes are working in open loop ( V/F) or in closed loop ( closed loop vector ) Dear , Can you explain in detail about vector control??

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5/28/2014 7:19 AM	Rate (0)
umencho Diamond Member Joined: 3/21/2006 Last visit: 3/30/2025 Posts: 10239 Rating: (1183)	Knowledge BanksDrives

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5/28/2014 7:39 AM
Ayman Elotaify Platinum Member Joined: 5/28/2008 Last visit: 9/3/2025 Posts: 4522 Rating: (843)	Chetan2490 Dear , Can you explain in detail about vector control?? Please read this Tip: (Theory of inverter from scratch) new episode !
	Ayman Elotaify
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5/28/2014 8:25 AM
ashishmistry390 Posts: 21 Rating: (0)	Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Dear Chetan2490 The primary purpose of a variable frequency drive (VFD) is to control the speed of an AC motor. It works by electronically changing the frequency of the power supplied to the motor. The applied voltage is also changed so that the ratio of voltage to frequency (V/Hz) is approximately constant. Since the operating speed of an AC motor is mostly determined by the frequency of the applied power, adjusting the frequency is the best means of adjusting the speed. A VFD also provides "soft" starting. When the motor is started, a very low frequency is applied. and the motor starts and runs without drawing a high current. If the voltage is properly controlled, the motor can produce up to 150% of rated torque while drawing proportional current. To accelerate the motor, the frequency is gradually increased so that the required torque is provided while accelerating with limited current. A soft starter electronically reduces the voltage applied to the motor during starting. That limits the starting current, but also limits the starting torque. Since AC motors can produce considerably more torque than their rated continuous running torque, starting with limited torque is usually acceptable. Once the motor reaches full speed, the voltage is increased to the normal running voltage for efficient operation. Regards, Ashish Mistry
ashishmistry390 Posts: 21 Rating: (0)
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5/28/2014 11:00 AM
N. Vijay Silver Member Joined: 3/17/2013 Last visit: 4/28/2025 Posts: 481 Rating: (43)	HI You should use softstarter in applications where normal running load on the motor is variable like pulpers, compressors etc where there is no need to apply full voltage across the motor windings all the time while it is running. If mechanical load is reduced on motor voltage across windings also reduced, so that it will reduce the stress on motor winding. As mentioned by experts, it uses antiparallel SCR bridge to have controlled voltage. VFD regulatesboth Voltage and frequency there by speed is controlled Regards, Vijay

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5/28/2014 12:12 PM
jeet2907 Posts: 235 Rating: (52)	Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Dear, Here is the main advantage of VFD over SS pls follow link Benefits
jeet2907 Posts: 235 Rating: (52)
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5/28/2014 3:23 PM	Rate (0)
MHasan Posts: 63 Rating: (4)	Chetan2490 Dear experts, How soft starter is differ from VFD? Explain in Brief. Its very simple: VFD is usedfor motor: control speed/ switching in fwd-rev direction/ power saving in some application/ motor protection from overload, overheat etc. VFD also have acceleration/ de-acelleration (ramp-up/ ramp-down) options for protection of motor and mechanical connected structure. Whereas softstarters are mostly used for accelleation (ramp-up) functions.
MHasan Posts: 63 Rating: (4)
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