100HP motor control options

What would be your go-to design for a 100HP motor?

Typically I deal with 40HP or smaller and almost always use a combination starter - which I usually find to be the most cost efficient. This is the first time I've had a motor this large.

This and one other motor (7 1/2 HP) will be the only two - and they are interlocked - so a PLC isn't necessary and I'll be using pushbuttons to start/stop for the controls.

I'm looking into soft-starters and VFDs, and the VFDs look pretty silly price-wise (and size-wise). I would imagine a circuit breaker, contactor, and overload would be pretty worthless with the inrush current? I'm assuming that is why they don't make many motor protection circuit breakers above about 50A or so.

Thanks in advance!

I've ran both up in high Hp applications. Our 750 Hp compressors use a soft start while our 450 Hp extruders use a vfd for variable speed control.

Do you need to vary speed? How often will the motor be started? What is the applicaation? Is the motor starting against a big load?

You may not be able to easily find the combination motor protector circuit breakers (that do the ocpd and overload in one), but you can find breakers, contactor, and overload for this size all day long.

This 100HP motor will be a fan for a collector, and it should run 24/5 (M-F not on weekends).

If your mechanics can handle the starting torque, and your supply can handle the starting current, DOL 100HP motor circuit breaker / contactor are available from almost any one that deals in those sorts of things and is the cheapest solution for the closed requirement "how to start a 100HP motor".

To deviate from the cheapest solution is either:
* spending money on the starter to save you money elsewhere,
* or spending money on the starter to work within the limits of your supply
* or the limits of your load
* or the requirements of your application

75kW motor running at full load 95% efficent 24h*365d is in the region of 700,000 kWh/year. (paying 8c/kWh is USD55k/year)With that you can see there is money to be made in OPEX by spending money on efficiency. If spending 5k on a VFD for example gives you 1% efficiency saving, return on investment would be one year. More expensive motor (eg ie4) is also good value. Note that at nominal speed and nominal torque, a VFD run motor usually uses more kW than a DOL motor, but below nominal torque there are significant efficiency savings (20% is typical). Then of course if you have the opportunity to decrease the speed, there is also a huge energy saving.

So your motor starting torque is maybe 2.5 to 3 times nominal torque. Can your mechanics handle that? If they can handle that, will they wear out faster? How often are you starting?
So for mechanical reasons you might want a different starter.
Depending who you buy from, star delta starter can be much cheaper or a bit more expensive to buy than a soft starter. You get more possibilities with a soft starter, sure, and you reduce jerk with a soft starter. Star delta starter you are limited to about 75% starting torque though, so you have to know if this is acceptable. Soft starter you can get to about 150% nominal torque when starting, but your starting current requirement can make this impractical. The VFD obviously is unmatched for starting torque and starting current and controllability of the same.

With starting current, you are often charged for the maximum instantaneous current your facility pulls, so using a star delta starter would reduce your starting current by 3 over DOL. A VFD can reduce it to BELOW nominal motor current, a reduction of almost 10 times Vs DOL for some motor windings.

running current of a VFD vs running current when using a DOL / star delta / soft starter is reduced. At nominal load, for a 97% efficient VFD and a .88 pf motor, your running current is reduced by roughly 9%. This can mean the difference between paying to have your supply upgraded, or for a new supply it can drop you down to a lower Ampere level (ie. Save some money.)

VFDs also come in a nice IP66 box similar to the combi starters you are used to. See for example Eaton DA1/DQ1.
A VFD is also easy to integrate into your PLC and wire. It handles every supply and every load situation. No, "oh, we didn't realise that motor needed to reverse." No "oh, we didn't realise we needed to know the amps of that motor." Or worse "you don't have the amps of that motor, you will need to figure out another clever way of solving it without purchasing any more hardware. The hardware budget is closed, and your time doesn't land on the budget of this project ..."

So if I take for example this project you have presented. I know nothing about the load, I know nothing about the supply. I know nothing about the control requirements. It is getting itself a VFD. Sure I would ask if you are running this one machine on a diesel generator with no other loads... When you say yes, really this is a whole excel spreadsheet of knowledge and not suited for this thread.

tl;dr: VFD, always, in every size motor.

Minimum soft-start and, as mentioned, VFD for speed control if desired.

It doesn't sound like a variable speed application unless a production rate could affect the airflow required. And unless its controlled automatically, the operator will likely have it cranked to full speed 24/5

I would look at a soft-starter with a bypass contactor - heat generation in your cabinet is significantly reduced with one fitted. I always prefer starters with external BPCs even though they take up more space. Allows the contactor to be serviced or replaced without needing to take the soft starter apart.

I also always fit a line contactor. If there is a significant fault it gives the SS a better chance of survival.

We use Schneider ATS48's for critical or larger applications like this. ATS22 for the smaller ones, they have the internal BPCs which i don't like but they're cheap enough you just replace the whole starter if there's a problem.

We also use the AB SMC-3 for smaller pump applications, not sure if they go up to 75kW but they are a reliable and simple starter provided you fit the line side protection module.

Having read now your application requirements, VFD is a clear winner.

You can get the cheaper model VFD that doesn't have the same maximal torque allowance (your fan never need go over 101% nominal torque)

You can show the customer the graph of how much electricity (actually show the dollars) can be saved when the air is easier to push than the maximal design of the system. Your VFD guy should be able to put something together, but also it is good to learn yourself. Tell him the mechanical reliability savings, starting your fan in a controlled ramp.

Also for you, 30% markup works out better on a VFD than a DOL starter. If the math works out, even offer to give him the drive for free, and you will take the "energy saved" figure * 6c/kWh after year one instead... Make sure your facial expression conveys which is the better deal for whom.

unless its controlled automatically, the operator will likely have it cranked to full speed 24/5

I would look at a soft-starter

Click to expand...

I think even at full speed, the system won't draw near enough to nominal torque for the DOL-run to give more efficient operating than a VFD that is designed for reducing volt/Hz ratio automatically when it's not needed (most all nowadays. Definitely if it has the words fan or pump).

The most important thing to deciding what starter to install is the type of load that the motor drives, if it is a load with a lot of inertia, for example a large centrifugal fan, you need a soft starter that can do long-lasting starts up to 30 seconds (I recommend Siemens Sirius starter).

Instead if it moves constant torque load, for example a conveyor at full load , you will also need a starter but for a shorter start time, a few seconds.

On the other hand, if the motor starts without load, and the supply is strong probably a contactor with a protection relay will be enough.

If there is no advantage to varying the speed, a VFD will not only be a waste of cap cost, it will cost you more to operate; they are at best, 97% efficient. So if you leave it running the same speed all of the time, it wastes 3% of the energy needlessly. On the other hand if there is something about the process that would allow you to vary the flow based on conditions AND the machine is a centrifugal fan, a VFD will pay for itself in energy savings, usually in a year at most. It just involves careful examination of how it is going to be used.


many utilities will have a cut-off point in motor HP that will require you to use Reduced Voltage Starting methods, that is often around 50HP but varies from one utility to the next across the country. Start there, you may not have a choice to use Across-the-Line anyway.


If you do have to use RV starting, then a Soft Starter is (in my opinion) the only way to go for a 100HP motor. Get a good one that uses SCRs in all 3 phases. There are some cheap ones that use SCRs in only two of the phases to cut costs; they are not only garbage, but can harm your motor.

I second what Jraef said. I'd add that you can sometimes buy a VFD for about the same cost as a RVSS and have potential energy savings available. Odds are pretty good that the max fan capacity exeeds the dust collector's typical daily needs. The fan is probably sized for 100 °F ambient or thereabouts. That means that at cooler ambient temps the airflow will exceed process needs, allowing slower speed and significant power savings.

If you go with RVSS get one with a bypass contactor that pulls in once the fan is at full speed. That will extend the life of your RVSS.

I get a twitch at using a full voltage starter on 100 hp. You will see some lights dim and hear some ugly belt squeal.

Our equipment uses 75hp-300hp cooling blowers.
I always use a soft starter, and only if the customer wants to do controlled cooling rates, do I use a VFD....and a line reactor.

If a minimum or constant air pressure is needed even if the flow is variable then reducing the speed of the centrifugal fan is not an option since the pressure drops very fast with the loss of speed.

Thanks for all of the comments.

Just received a call from the customer this morning and a soft-starter is required, no VFD or DOL starter.