Electric Motor Selection on Generator-Powered Sites: Starting Current and Solutions

On construction sites where mains electricity has not yet arrived or is unreliable, all machinery — concrete pump, crane, crusher, compressor, water drainage pumps — is fed from a generator. And on almost every site the same scene plays out: a generator whose power is more than sufficient on paper either goes into protection by dropping the frequency when a sizeable motor starts, or collapses the voltage and disables the other equipment on site as well. The source of the problem is not that the motor or the generator is "faulty", but that the two were selected without being considered together. This article explains, in commercial language, the starting-current problem, its relationship with generator sizing and the solutions that work in the field, for companies buying motors for generator-powered sites. As an electric motor factory producing since 1979, HEM Motor supplies motors to site equippers and contractors from its Turkiye stock, together with a starting-solution recommendation.

The Starting-Current Reality: 3–7 Times the Rated Current

When a three-phase asynchronous motor starts from standstill, it draws from the grid roughly 3 to 7 times its rated current; in standard industrial motors this ratio is typically around 6 times. A 30 kW motor with a rated current of about 55 A can exceed 300 A at the moment of start with direct-on-line starting. In a facility fed from the grid, this surge dissolves within the power of the transformer and is most of the time not even noticed. With a generator, the picture is completely different: behind the generator there is not an infinitely powerful grid, but a limited diesel engine and a limited alternator. The sudden current demand causes a voltage drop at the alternator and a sag in speed and frequency on the diesel side. Since the torque the motor produces decreases with the square of the voltage as the voltage drops, the motor's start lengthens even further and the system enters a self-feeding vicious circle: the voltage drops, the torque drops, the start lengthens, and it continues drawing current. The result is either the generator's protection tripping, or a half-completed start that heats the winding.

A typical scenario from the field makes the picture clear: on an infrastructure site, while lighting, the tower crane and small hand tools run without trouble, the generator trips on under-frequency protection every time the 45 kW submersible drainage pump is to be commissioned. The total load is half the generator capacity; the problem is not in the total, but in the surge of hundreds of amperes the pump draws at the moment of direct start. The solution was not to enlarge the generator but to soften the pump's start. The lesson in this example reflects directly on purchasing: the motor and the starting equipment are two halves of the same decision and must be discussed together at the quotation stage.

Relationship with Generator Sizing: The Decisive Factor Is the Largest Motor

When a site generator is selected, the total installed power is summed and a margin is added on top; yet what really strains the generator is not the total load but the starting surge of the largest direct-started motor. In practice a two-way rule works. Looking from the generator side: for a motor to start without trouble on direct start, it is a common field rule for the generator to be roughly three times and above the motor power in kVA; with a softened start, this ratio drops markedly. Looking from the motor side: if your existing generator is fixed, the largest motor power you can start direct is also fixed; a larger motor can be connected to that generator only with a soft-start solution. The practical equivalent of this in purchasing is: when placing the motor order, tell your supplier the kVA value of the generator on site, the other loads that will run at the same time, and which machine the motor will drive. In machines that start under load, such as a crusher, this analysis is even more critical; the details of that application are covered separately in our crusher plant motor selection guide.

Solution 1: Star-Delta Starting — Economical and Common

A star-delta starter first brings the motor up in star connection (with a lower voltage per phase), and after the speed settles switches it to delta. The starting current drops to roughly one third compared with direct-on-line starting; this seriously eases the surge on the generator. Its cost is low, and it is a classic solution the site electrician is familiar with. It has two conditions: the motor must be suitable for delta operation on a 400 V grid, that is, have a 400/690 V nameplate, and the machine must be able to tolerate the torque also dropping to one third in the star step. It runs without trouble in machines that start off-load or under light load (fans, empty belts, unloaded compressors); in machines that have to start under full load, however, the motor cannot accelerate in the star step and a large current surge occurs again at the transition to delta. For this reason, star-delta should be placed correctly as the "cheap but not cure-all" solution. Confirming in the order that the motor's terminal box is six-lead prevents a bridging crisis in the field.

Solution 2: Soft Starter — Stepless Soft Start

A soft starter raises the motor voltage electronically and steplessly, holding the starting current at an adjustable limit (typically in the band of 2.5–4 times the rated current). There is no transition surge as in star-delta; the ramp time and current limit are set according to the machine. For generator-powered sites it is in most cases the most balanced solution: it brings the pressure on the generator kVA down to a predictable level, requires no special winding on the motor side, and runs lossless by dropping out of the circuit (bypass) when the start is complete. Its limitation is this: since the soft starter also reduces the torque, in very hard-starting loads the motor cannot start if the current limit is set too low; a machine-specific balance is established between the current limit and the starting time. This setting must be verified with a clamp meter during commissioning — HEM Motor provides phone-based commissioning support for the soft-starter parameter selection of the motors it sells.

Solution 3: Frequency Drive (VFD) — The Softest Start, Extra Benefits

Because a frequency drive accelerates the motor starting from a low frequency, it can hold the starting current around the rated current, or even below it; this is the most comfortable solution from the generator's point of view. Moreover, while the starting problem is solved, two more benefits come along: speed control (flow in a pump, air control in a fan) and energy saving under load changes. Its cost is the highest investment among the three solutions and the maintenance sensitivity of an electronic device in the site environment; the drive panel must be protected from dust and its ventilation filtered. When using a drive on a generator supply, two technical details must not be skipped: because the harmonics on the input side of the drive can disturb the generator regulator, it is common practice to select the generator somewhat larger relative to the drive load, and the motor must have an insulation structure suited to running with a drive. For motors to be used with a drive, this suitability should be written into the order note; the IE4 series motors dispatched from stock are produced with an insulation package suited to drive-fed operation.

Which Solution for Which Machine? Decision Summary

If we translate the three methods into site practice, the picture is set up as follows. In light-starting machines such as fans, empty-starting belts and compressors with an unloading valve, star-delta gives a sufficient result with the lowest investment; the only conditions are a 400/690 V wound motor and a six-lead terminal box. In drainage pumps, mixers and equipment that may start under load, a soft starter should be the standard choice; the current limit is set according to the generator and the start happens with the same softness every time. In applications where speed control adds extra value to the operation — pumps with varying flow, belts where speed adjustment is wanted — a frequency drive solves both the starting problem and the process need with a single investment. Direct-on-line starting, meanwhile, should be left on a generator-powered site only for small powers (roughly motors below one third of the generator capacity). On sites with more than one large motor, the healthiest approach is a mixed solution: a soft starter or drive for the largest one or two motors, star-delta for medium powers, and direct start for small powers. In this way the investment budget is concentrated on the points where the surge is really a problem, and the generator capacity is used efficiently. When this decision summary is added to your quotation request, the motor winding and the starting equipment are priced in a single package, compatible with one another.

Five Common Mistakes in the Field

A large part of motor-related stoppages on generator-powered sites arise from the same five mistakes. The first is connecting a 230/400 V wound motor to a star-delta panel; the motor sees overvoltage in the delta step and the winding is damaged in a short time. The second is calculating the generator capacity only on the total kW and ignoring the starting surge of the largest motor. The third is panels without automation that allow large motors to start at the same time; making the motors start in sequence with a simple time delay seriously reduces the peak load on the generator. The fourth is long, thin supply cables; on a site, motors may run hundreds of metres away from the generator, and the voltage drop on the cable makes an already marginal start impossible. The fifth is raising the thermal relay setting because it "trips often"; this hides the problem and sacrifices the motor. The correct thing is to solve the starting or load problem at the root of the frequent tripping. None of these mistakes requires an expensive engineering service; they are prevented by asking the right questions at the ordering and commissioning stages. Your supplier not asking these questions is in itself a warning sign: the difference between a company that sells kW from its catalogue and a manufacturer that asks about your generator, your cable and your machine on site emerges at the first start of the first large motor.

Site Conditions: Dust, Voltage Fluctuation and the Correct Motor Specification

Starting is not the only thing that strains a motor on a generator-powered site; the operating conditions are also harsher than in a grid-connected factory. First, dust: in an excavation and concrete environment, IP55 protection class should be accepted as the minimum requirement, and the regular cleaning of the motor fan cowl should be written into the maintenance plan; cooling fins clogged with dust steal even a correctly selected motor's thermal life. On high-altitude dam and road sites, the drop in air density weakens motor cooling; since the nameplate values are given for standard conditions, reporting the altitude in the order ensures the motor is recommended with the correct thermal reserve. Second, voltage and frequency fluctuation: a generator on a site that takes on and sheds load cannot produce voltage as steady as the grid; therefore the temperature reserve provided by Class F insulation and a wide voltage tolerance become important in the motors. Third, mechanical shock and transport: site motors are frequently dismantled and moved; standard industrial motors with a cast-iron frame and a robust terminal box withstand this life better than light aluminium series. The equivalent of this profile is the general-purpose industrial motors family available in stock in every power and speed: from 0.55 kW to 355 kW, with 2-4-6-8 pole options, it meets all the pump, belt, crane and compressor needs of the site in a single frame standard. The motor replacement of the compressor on site deserves a separate heading; the details of shaft-flange matching are explained in our compressor motor replacement guide.

Purchasing Scenario: Provide These Five Pieces of Information When Ordering

There are five pieces of information that speed up the motor quotation process for a generator-powered site: the kVA value and brand of the generator; the machine the motor will drive and the starting form (off-load or under load); the planned starting type (direct, star-delta, soft starter, drive); the other large loads that will run at the same time; the environmental conditions and the mounting form. With these five pieces of information our technical team draws up the motor, the winding voltage and the starting recommendation together; if necessary, it talks at the same table as your generator supplier. The advantage of being a manufacturer that reflects on the site is seen in the lead time: standard motors are dispatched from Turkiye stock, special needs such as 400/690 V wound or two-speed are solved on the production line with a short lead time, and every delivery is completed with phone-based engineering support during commissioning. When the site finishes, the motors are moved to the next project; a correctly selected motor fleet is the fixed-asset value of the company.

On sites running with a rental generator there is one more variable: as the project progresses, the generator can change, the capacity can shrink or it can be split into two separate generators. If your motor fleet is built from 400/690 V wound motors suited to starting with a soft starter or drive, these changes do not produce a problem; the starting setting is updated according to the new generator and the work continues. A fleet built for direct start, on the other hand, comes under risk again at every generator change. For this reason, gathering the motor fleet into specific frame and winding standards — just like a formwork and scaffolding standard — is a long-term efficiency decision for site companies.

Frequently Asked Questions

Up to what power can I start a motor direct with my existing generator?

The rough field rule is that, in direct start, the generator kVA value should be at least three times the motor power (kW); however, the result depends on the alternator type of the generator, the other loads and the machine's starting character. If you send us the generator nameplate and the machine details, we will size the motor and, if necessary, the soft-start solution together.

Should I prefer star-delta or a soft starter?

In machines that start off-load or under light load, star-delta is economical and sufficient; in applications that start under load, are sensitive to the transition surge or have a generator at its limit, a soft starter gives a safer result. If speed control is also wanted, going straight to a frequency drive combines two investments into a single line.

Does voltage fluctuation on the generator affect the motor warranty?

Our standard motors run within warranty within the voltage and frequency tolerances in the norms. In generator supply, what matters is preventing continuous and excessive deviations; if you report the supply conditions at the quotation stage, we recommend the motor according to these reserves and evaluate the current-voltage measurements together at commissioning.

Get a Quote

For your generator-powered site, send your motor list together with the generator kVA information; let us price it the same day together with a starting recommendation, the stock status and the lead time. Pump, belt, crane, compressor and crusher motors are dispatched from Turkiye stock; commissioning support is part of the supply. You can reach us on +90 (532) 345 49 86 or send your request through our contact us page.