Motor start current

Hello. A motors start current is about 6 times it' nominal current. But what if I start the motor at 50% load?

At no load, the initial inrush current will still be there. A lighter load or one with less rotating mass will accelerate faster and thus the inrush current will be present for a shorter period of time.

It is important to note that the old 6/1 inrush current applied to standard efficient NEMA Design B motors only. As efficiencies increased during the 1990's, the inrush currents began drifting up toward 8/1. With today's premium efficient motors, you can expect inrush currents in the range of 14/1. That's why many premium efficient motors are labeled NEMA Design A rather than B. NEMA specs hold inrush to 6-8 times FLA for Design B but there is no limit on Design A.

This is the price that is to be paid for the higher efficiencies. In some cases, a soft starter is the only way to get these motors going.

Sorry Karsten, I didn't notice your location in Denmark. IEC motors are seeing the same thing but I don't know the IEC designations for the different designs or the limits, if any, that the rules place on inrush current.

In some cases, a soft starter is the only way to get these motors going.

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Dick, Doesn't a Variable Frequency Drive provide the soft starting advantage, as well as other functions?

Lancie1 said:

Dick, Doesn't a Variable Frequency Drive provide the soft starting advantage, as well as other functions?

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Yes but if you don't need speed control you will still get all the negatives of a VFD such as induced shaft voltage, EMI /RFI Noise from the drive output cables, Harmonics on the power system.

True their are devices to mitigate the negatives of a VFD but that will cost even more if you don't need speed control then soft start is the best but if you need speed control VFD is the best.

At least that my opinion and always what I have found to be true. Solid state soft start prices are the same if not lower than VFD's now from what I see in my purchasing.

Yes, I agree. My point was that a soft starter may not be the only way to get NEMA Design A motors going. I think Dick was probably speaking generically and including VFDs as a type of motor "soft starter". There are also still a few SCR motor controllers for DC motors.

The inrush current of a motor can always be calculated, and is the same regardless of load. As the motor is not turning initially there is no inductive reactance to 'resist' the applied voltage. Thus Ohms Law is in full effect, the inrush current for a given motor is the voltage divided by the resistance of the windings, and when the motor starts turning you will then start to get inductive reactance to lower the current.

That being said, the less load on a motor at starting, the quicker the motor will speed up (acceleration is dependent on difference between motor torque and load torque)and the inrush current would decay faster.

As for soft starters, many brands have a bypass contactor which connects the motor directly to line voltage when the motor is up to speed, thus averting some of the issues you get with a VFD.

Mark Snodgrass

The inrush current of a motor can always be calculated, and is the same regardless of load.

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Except when started with a soft starter device. Then the inrush can be limited to FLA or less.

Lancie1 said:

Except when started with a soft starter device. Then the inrush can be limited to FLA or less.

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Yes, the soft starter limits the applied voltage, thus Ohms Law still applies. But lets not quibble about this.

In my experience a reduced voltage non-reversing solid state soft starter (RVSS) is about 1/2 to 2/3 the cost of a VFD of comparable quality. I like drives, but there is not much point in using a VFD if a RVSS will do the job.

The main reason for a VFD is the variable speed control. But a VFD or soft starter also reduces the code size of the fuses or circuit breaker, any disconnect switch, and the wire, because the start-up current is much smaller than with an across-the-line starter. I am not quibbling, just pointing out important advantages for the specifying engineers and the people who write the checks. Installers and maintenance men may not be too concerned about those things.

Hmmm! A number of issues here. First, I don't believe that adding a VFD will reduce the code requirements for conductor and fuse sizing. Second, the cost comparison between a RVSS and VFD is related to HP. Under 10hp they are nearly the same price. By 100hp (both of these are assuming 480VAC) the ratio is about 4/1 with the VFD being more expensive, of course. Third, about the best a RVSS can do even on a free shaft zero starting torque load is to cut the inrush by 60%. There will always be some starting inrush. On the other hand, since a VFD starts a motor nearly synchronously, there is no inrush as long as the starting torque does not exceed the nameplate torque. So a VFD does a much better job of reducing inrush while preserving starting torque.

Just for reference, when you reduce the starting voltage on an induction motor, the inrush current comes down about in direct proportion to the voltage reduction. But, and here is the problem, the starting torque comes down as the square of the voltage reduction. So, by the time you reduce your inrush by one-half, you only have one-quarter of the torque left. By one-third, it's down to one-ninth and the motor probably won't start itself, much less any connected load.

First, I don't believe that adding a VFD will reduce the code requirements for conductor and fuse sizing.

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The code is interested in limiting the largest size of the fuse or circuit breaker. It doesn't care if smaller sizes are used. In fact, it would encourage using the smallest fuses and breakers as possible to feed the load. I think designers have gotten in the habit of thinking the "largest allowed" rules for motor overloads and short-circuit protection are also the smallest possible sizes. The smallest size of branch-cicuit device is the size that will handle the load without tripping. Ideally that is the size to use, although with across-the-line motor starting, that size has to be severely increased.

With a motor starter or other line-voltage starting device, the short-circuit and motor overload devices cannot be reduced below the size needed to handle the large start-up current.

But with a VFD, it is not necessary to oversize the fuses or circuit breaker (at least not as much). Once the branch circuit device is reduced (due to the smaller start-up current), the downstream wiring can also be reduced to match the smaller branch circuit device to handle the largest expected load. That is all that is required. I designed VFD circuits that way for years, saving a lot of customer money by using smaller wire, wire sized for the new smaller branch circuit size, wich in turn was slightly larger than the largest VFD overload trip current setting for the motor size. The only concern is there must be labels or drawing notes saying that if the VFD is removed, or a larger motor is installed, then a larger breaker and wire will also be needed.

the larger fuse or breaker is needed for across the line starting. but a larger conductor is not. purely just 125% of the motor fla. and that is just because the inrush is only there for a short bit of time. VFDs allow for the smaller overcurrent protection, but still should have cables rated for more than the motor FLA

At least thats how the code is up here in canada lol

With VFD we can get more torque and faster start if needed?
Also control with field-bus as Profinet is easy, all process data are usable to PLC.
Variable speed is extra usable property.