# Will a motor run at the same speed whether its on a VFD or a starter?



## John Valdes (May 17, 2007)

If the base speed of a motor is 1800 RPM it will run at 1800 RPM give or take provided its being applied its name plate voltage, and Hz.
Now this is adjustable so you must program the speed or speeds you want to use.


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## Breakfasteatre (Sep 8, 2009)

John Valdes said:


> If the base speed of a motor is 1800 RPM it will run at 1800 RPM give or take provided its being applied its name plate voltage, and Hz.
> Now this is adjustable so you must program the speed or speeds you want to use.


Ok good, that was my understanding. If the voltage and Hz were the same, the speed would be the same


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## SWDweller (Dec 9, 2020)

Yes that is correct. volts and Hz the same on both methods.

Note anyone installing a VFD with out the need for variable loads or speeds could be wasting money, A soft start has the same front end as a VFD.

NOT all motors accept electronic starting well. Extending the start time can do bad things for the heat in the motor. 
Min speeds like 30% start increasing the heat in the motor. Make sure the motor is ready from the MFG for a VFD or soft start.


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## JRaef (Mar 23, 2009)

SWDweller said:


> .., A soft start has the same front end as a VFD.


Not really, but I know what you were getting at. A VFD converts AC to DC, then back to AC. A soft starter just controls the RMS voltage that is allowed to get to the motor, which lowers the torque and reduces the current that the motor pulls on starting. 

SOMETIMES… a VFD is used as a glorified soft starter because a VFD is the only way to accelerate a motor with the least amount of current. You can program a VFD to never exceed the motor FLA as it accelerates, so long as you don’t need to worry about the time it takes. You cannot do that even with a soft starter. Also, a VFD can accept single phase power in and run a 3 phase motor. Neither of those situations however would be something that would allow the VFD to be replaced by an Across-the-Line starter though.


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## paulengr (Oct 8, 2017)

Breakfasteatre said:


> I did some googling and couldn't find any definitive info, but comparing apples to apples, the motor running at 60hz through the VFD, I imagine the motor would be running at practically the same speed as it would be going through a starter.
> 
> Customer recently changed their specs so that the minimum size VFD is 3hp. the cost difference between a 1hp and 3hp vfd is$1500 so hilariously, the customers own engineer wants to run a starter to save money.
> 
> bonus, name the multinational customer!


Depends on how you run it. In sensorless flux vector mode it will run at synchronous speed! If you mess with the voltage curves in V/Hz mode it can run slower yet.

As long as it is adequate and not too large, VFD size does not matter. If you get it too oversized you will run into issues with the impedance mismatch in some VFDs. And buying a “3 HP” normal duty VFD intended for say a centrifugal pump but using it on a 3 HP conveyor will fail.

The VFD you use should be the size and type for the job.


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## mburtis (Sep 1, 2018)

paulengr said:


> Depends on how you run it. In sensorless flux vector mode it will run at synchronous speed! If you mess with the voltage curves in V/Hz mode it can run slower yet.
> 
> As long as it is adequate and not too large, VFD size does not matter. If you get it too oversized you will run into issues with the impedance mismatch in some VFDs. And buying a “3 HP” normal duty VFD intended for say a centrifugal pump but using it on a 3 HP conveyor will fail.
> 
> The VFD you use should be the size and type for the job.


Do they make application specific drives? I always figured that for most normal applications that would be a matter of the drive programing, acceleration times, torque at start, etc .


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## backstay (Feb 3, 2011)

It kind of depends. Some only give you a few hundred parameters. They would not be adaptable to all situations. I’ve seen some with thousands of parameters.


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## gpop (May 14, 2018)

mburtis said:


> Do they make application specific drives? I always figured that for most normal applications that would be a matter of the drive programing, acceleration times, torque at start, etc .



If you accidentally purchase a air-con vfd you will find its very much a application drive.


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## paulengr (Oct 8, 2017)

mburtis said:


> Do they make application specific drives? I always figured that for most normal applications that would be a matter of the drive programing, acceleration times, torque at start, etc .


There are definitely application specific drives. For instance if you use a diode front end you cannot do regenerative braking. A diode front end only allows power flow from line to load. “Pump/fan” drives also usually do not have a brake chopper transistor so the only deceleration they can do is coasting. Try to stop too fast and they trip. Adding a brake chopper and an active front end makes it more if a servo drive suitable for say machine tools. Also in some applications you may want a drive with 12, 18, 24, or 36 pulses because the output and input has a lot less harmonics. This is more efficient than say using a sinus wave filter if you are pushing line length between the drive and motor ir you want a cleaner output for use as a variable frequency power supply. Also many drives are available with different amounts of IO or option cards for applications that need say more than 1 or 2 analog outputs. On the software side some have extra application specific settings. For instance crane drives contain special safety features for hoisting and anti-sway algorithms for trolley functions.

You can split drive sizing pretty much into 3 applications. First are centrifugal pumps and fans. As long as inertia isn’t a big deal you can use smaller VFDs. The overload rating will only be 110% of output for 30 seconds. Often called “standard” or “normal” duty.. Almost everything else is “heavy duty”. Same drive but slightly oversized, 135% overload for 30 seconds. Then for crushers, rock quarry screens, and Banberry mixers with very high shock loads you size to 200-250% of load rating for the “extreme” loads.


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## paulengr (Oct 8, 2017)

Also forgot to mention brushless permanent magnet DC motors which can be driven by an AC drive, and reluctance motors. Then we get over into the DC side which includes synchronous motors, DC shunt motors, and a whole gaggle of other variations. These don’t require an inverter or a DC link. A diode bridge might be used for say a fixed field output but mostly these are SCR bridges. The drive “programming” spans a huge range and DC drives can be quite a bit more “stupid” than AC. For instance servo controllers for robotics really just require a large DC power source. A +/-10 VDC signal controls a much larger DC output voltage or current. There is no “brain” needed. Some DC drives are effectively on board servo drives. For instance the legendary Siemens Simoreg 6RA series DC drives can be used as a traditional DC drive for older variable speed machinery or you can program it to do full servo control with as complicated of a control loop as you want.

Another last option has to do with safety features. Drives can be equipped with various braking functions. A feature sometimes seen on cranes is a brake test mode where it fluxes up the drive until the motor starts to move with the brakes applied to test how much torque is required. “Safe torque off” gives you both the require stop command and a special extra inout that cuts off the power source to the gate circuits to implement two independent shutoff functions. These are not necessarily different drives, just added features.

So one issue with drives and there is a good but annoying reason for this is that you are stuck with a 100,000 hour (about 10 year) life. This number drives the whole industry. With very rare exceptions. The limitation is not so much a technical one and I’ve done a 250,000 hour drive but a technological obsolescence issue. Chip manufacturers estimate product life cycles of 10 years before they obsolete a product.

The lifespan issue means you need to PLAN for your drives to all be replaced roughly every 10 years. Unlike most electrical equipment which has an estimated 25+ year life that often lasts much longer, drives have short lives I do a ton of retrofits. The thing with retrofits is that for 90% of applications, a drive is a drive. Since most of the market is very simple pump and centrifugal fan applications usually with very simple signals (run command, one or two speed references, zero or one speed feedback, one or two relay feedbacks). Drives are interchangeable commodities. Many guys don’t even understand or try to set up vector mode despite the advantages and a lot of them never even auto tune the drive. BUT then there are the other 10% of applications. If you do retrofits you need to know how to spot these.

On a recent job we had a customer with a VFD that failed. The VFD itself was nothing special but it is fairly small as VFDs go. It was a pump drive running a water pump. The customer wanted to switch brands. The brand of choice is one where the VFDs are fairly large to start with and none would fit in the cabinet. To make things worse what was actually delivered from the drive manufacturer was effectively a crane drive. It did not even have a display. Displays are back ordered months out That manufacturer is having serious issues with the chip shortage and this clearly specialty drive was probably something they had sitting on the shelf. It was 3 times longer than the enclosure for the pump drive.

Those little compact or micro drives by the way are usually MOSFET drives. An IGBT is effectively a bipolar transistor driven by a MOSFET. This takes advantage of the low power of the FET family and the high current of the bipolar family. MOSFETs are cheaper and have less issues but are limited to under 10 HP. They are also cheaper. This is why the “micro drives” are so much less expensive compared to the same size standard drives. So this also points to a difference in drive design and it’s a big one.

Also in the area of small drives there is a single phase drive on the market that runs PSC motors. Most drives can run single phase anyway if you size them for it but there are micro drives that get rid of the extra unused diodes. There are also voltage doubler drives that take 120 V input and operate 230 V three phase motors.

And then there is the entire world of medium voltage drives. They look and act somewhat like low voltage drives but that’s where the difference ends.

And before I end you’ve got soft starts that aren’t really drives. But they can use an ancient variable speed technology called a cycloconverter to run variable speed.

And for a short period of time one manufacturer sold a VFD called a matrix drive. 9 transistors and NO DC link. It was overpriced and many features didn’t pan out and required a boost transformer so it never sold very well.

Today the latest thing making the rounds (and used a lot in medium voltage) is talking about neutral point clamped drives and cascaded H bridges. This is far removed from a 6 pulse drive but the neutral point clamped drives have advantages in hybrid silicon/silicon carbide hybrid drives. This will reduce or eliminate problems with bearing fluting and possibly boost efficiency and shrink the drive about 10%. But this one is still mostly R&D.


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## paulengr (Oct 8, 2017)

gpop said:


> If you accidentally purchase a air-con vfd you will find its very much a application drive.


Most HVAC drives are very application specific. In fact ABB has two sales channels. Pretty much anyone can buy an ACS or a Simoreg. But those same ones can’t buy an ACH. The concern is that an ACH and an ACS are interchangeable for many applications. But they don’t want you to know you can buy a cheaper drive for a simple fan application just because you are an industrial customer. You need to pay higher margins. As an industrial vendor I’d have to buy one from an HVAC distributor then mark it up higher than the ACS to sell it to you. The only technical difference is the ACH650 has fewer settings than the ACS650 as an example.

It is also helpful to know who the actual manufacturers are vs private branding. In normal years this can save money but right now if I know a manufacture is private labeling drives for say 4 companies and I need a new one I can just call all 5 companies and see who might have one in stock. And knowing that drives are in limited “frame sizes” I might be able to go up a size or two or even jump from say a pump drive to a more general purpose drive for very little cost and find one in stock. An AB customer recently got hit with over 60 weeks delivery. AB claimed supply chain.


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## paulengr (Oct 8, 2017)

Breakfasteatre said:


> Ok good, that was my understanding. If the voltage and Hz were the same, the speed would be the same


Running is the same. Acceleration is not. And there is also the dark side of VFDs (bearing fluting, reflected waves). A contactor can easily handle up to 10 times FLA starting current. The motor design, especially the rotor, can get up to almost 300% of name plate torque for some motors like crusher duty. In addition reversing contactors can plug reverse a motor. And they can run two speed motors. The same motor on a VFD cannot do any of that. Even in crusher duty (high starting torque) if you greatly oversize the VFD ultimately the motor is always riding in the speed range between synchronous and the breakdown point. We just “move” breakdown by changing synchronous speed. But almost no VFD can withstand the current at breakdown, let alone stall or even worse plugging. Fortunately few applications have a torque curve so screwy that they need that much break away torque. In this case turning down max amps/torque prevents starting.


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## Breakfasteatre (Sep 8, 2009)

Paul, you are a wealth of information!

I work for an an industrial solutions company, and we mostly deal with food and beverage. Lots of custom conveyors with light loads.

This would be a small conveyor, fractional horsepower motor. We usually install VFDs to allow fine tuning of speeds for product transfer and such and fractional horsepower motors @ 600v is 95% of our use case.

This customer mentioned in the OP is one of our main customers and has also purchased another one of our main customers so these specs will encompass a lot of our work. 

They also spec a separate HIM for each VFD on the panel face so that parameters can be changed without opening the panel :\


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