Running a VSD compressor on a diesel generator

What changes when a VSD compressor runs on a DG set

A variable speed drive (VSD) compressor draws current in short pulses rather than as a smooth sine wave, and a diesel generator is far less able to absorb those pulses than the electricity grid. On the grid, the supply transformer and the network behind it are strong, so the distortion the drive causes in the voltage stays small. On a DG set, the alternator’s internal reactance is much higher, so the same current pulses distort the voltage much more. Distorted voltage can upset the generator’s voltage regulator, overheat the alternator and make the drive itself, or other equipment on the same set, trip.

The good news is that a VSD compressor is in other ways the easiest compressor a DG set can carry. It starts without the heavy inrush of a fixed-speed motor, it ramps its load up gently instead of applying it in one step, and its power factor stays high at every speed. This page explains how to get those benefits without the harmonic problems: how to size the generator, how to choose the alternator and drive, how to handle changeover from mains to DG, and how to avoid running the engine too lightly loaded.

It is written for plant engineers, maintenance heads and project teams in factories where power cuts are frequent, on construction and infrastructure sites with no grid connection, and for anyone deciding between a fixed-speed and a VSD compressor where a DG set will carry it for part of the time.

Why a drive draws harmonic current

Harmonic current is current at whole multiples of the supply frequency (250 Hz, 350 Hz and so on for a 50 Hz supply) that a non-linear load draws in addition to the normal 50 Hz current. Most compressor VSDs use a six-pulse diode rectifier: it charges the drive’s DC link capacitors only near the peaks of the supply voltage, so the input current flows in two short pulses per half-cycle. The result is mainly 5th, 7th, 11th and 13th harmonic current.

Current distortion becomes voltage distortion in the source impedance. A generator’s subtransient reactance is several times higher than the effective impedance of a typical grid supply, which is why a drive that behaves perfectly on mains can cause visible voltage distortion on a DG set of the same kVA. The usual measure is total harmonic distortion of voltage (THDv); many sensitive loads, and many alternator voltage regulators, start to misbehave once THDv rises well above about 5 to 8%.

What reduces the harmonic current

• A DC choke or AC line reactor in the drive smooths the current pulses. Most industrial compressor drives include one; check the data sheet.
• A low-harmonic drive with an active front end or a built-in harmonic filter draws close to sinusoidal current and is the strongest single measure when a DG set will carry the compressor for long periods.
• A passive or active harmonic filter fitted at the panel can be added to an existing installation.
• A larger generator lowers the source impedance seen by the drive. It works, but it costs more and makes light-load running worse, as explained below.

Starting, step loads and power factor: where a VSD helps the DG set

A VSD compressor is easier for a DG set to start than a fixed-speed compressor of the same size because the drive limits the starting current to about the rated current. A direct-on-line start typically draws 6 to 7 times full-load current, and a star-delta start about a third of that, roughly 2 to 2.3 times full-load current, plus a brief current spike at the change from star to delta. Each of these is a large, sudden kVA demand that pulls the generator voltage and frequency down.

The VSD also applies its load as a ramp. A fixed-speed compressor on load/unload control switches between unloaded and full load every time it loads, which the generator sees as a repeated step change of roughly two thirds or more of the compressor’s power. The VSD changes speed smoothly, so the engine governor and the voltage regulator have time to follow.

Power factor is the third difference. The drive’s displacement power factor stays at about 0.95 or above across the speed range, so it draws little reactive current, while a fixed-speed motor running unloaded has a low power factor. The drive’s harmonic current still adds to the current the alternator must carry, so its true power factor is lower than 0.95, and that is why the set is sized with a margin.

The screening ratios in the table are a starting point used for early budgeting. The generator maker should confirm the final rating with the drive’s harmonic data, the alternator’s reactance and any other loads on the same set.

Method: sizing the DG set for a VSD compressor

Size the DG set in four steps: find the drive’s kVA, apply the harmonic screening ratio, add the other loads, then check the light-load case.

1. Drive kVA = package input power at full speed (kW) ÷ input power factor. Use the package input power from the data sheet, not the motor’s nameplate kW.
2. Screening generator rating = drive kVA × ratio: about 2 to 2.5 for a six-pulse drive with a DC choke or line reactor, about 1.25 to 1.5 for a low-harmonic drive. A drive without a choke needs more.
3. Add the other loads on the same set, including their own starting demand. A DG set carrying lights, pumps and a VSD compressor together needs all three in the calculation.
4. Check light load: compressor kW at the lowest expected demand ÷ (generator kVA × 0.8). If this stays below about 30% for long periods, plan for it (see below).

Worked example 1: a 64 kW VSD compressor alone on a DG set

A plant has a VSD screw compressor with a package input power of 64 kW at full speed and an input power factor of 0.95. Its lowest regular demand puts the compressor at its minimum speed, drawing 19 kW. The DG set will carry only the compressor.

1. Drive kVA = 64 ÷ 0.95 = 67.4 kVA.
2. Standard six-pulse drive: 67.4 × 2 = 134.8 kVA and 67.4 × 2.5 = 168.5 kVA, so the screening rating is about 135 to 170 kVA.
3. Low-harmonic drive: 67.4 × 1.25 = 84.3 kVA and 67.4 × 1.5 = 101.1 kVA, so about 85 to 100 kVA.
4. Light load on a 160 kVA set: 160 × 0.8 = 128 kW; 19 ÷ 128 = 14.8% of the set’s kW rating.
5. Light load on a 100 kVA set: 100 × 0.8 = 80 kW; 19 ÷ 80 = 23.8%.

What it means: the standard drive needs a set of around 160 kVA, a common rating within that range, which then spends its quiet hours at under 15% load. The low-harmonic drive lets the same compressor run on a set of around 100 kVA that is better loaded and costs less to buy. Even this set falls below about 30% at minimum speed, so plan shared loads or periodic running at higher load. Part or all of the extra price of a low-harmonic drive can be offset by the smaller DG set; compare the two quotations.

Worked example 2: comparing the starting current of a 55 kW motor

The motor of a compressor like the one in example 1 is typically rated at 55 kW. A 55 kW compressor motor on a 415 V supply has a full-load current of about 92.5 A, taking a motor efficiency of 94% and a power factor of 0.88: 55,000 ÷ (1.732 × 415 × 0.94 × 0.88) = 92.5 A.

1. Direct-on-line start at 6 to 7 times full-load current: 92.5 × 6 = 555 A to 92.5 × 7 = 647.5 A.
2. Star-delta start at one third of the direct-on-line current: 555 ÷ 3 = 185 A to 647.5 ÷ 3 = 215.8 A, plus the transition spike.
3. VSD start: limited to about the rated current, 92.5 A or less.

What it means: the star-delta start asks the DG set for about twice the compressor’s running current every time the compressor starts. On a set sized close to the running load this causes a deep voltage dip that can trip contactors and other equipment. The VSD’s soft start removes that problem, which is why a VSD compressor often fits on a DG set that could not reliably start a fixed-speed machine of the same size.

Indian site conditions that change the answer

Frequent power cuts mean the changeover from mains to DG, and back, happens many times a week in many Telangana and Andhra Pradesh plants. Each changeover is a voltage interruption that a drive sees as an undervoltage fault. Set the drive and the compressor controller to restart automatically after a supply failure, with a delay long enough for the DG set to reach stable voltage and frequency, and for the compressor to finish its blow-down before it restarts.

Summer heat reduces what both machines can deliver. DG sets are derated by their makers for high ambient temperature, and the compressor delivers less air from hot intake air. In a hot generator room in Hyderabad, where air temperatures of 40 to 45 °C are common in summer, check both derating figures together, not separately.

Dust and heat also shorten the life of drive cooling fans and filters. A drive that overheats on a DG supply, where harmonic losses are higher, will trip first in the hottest week of the year. Keep the drive’s air path clean and the panel ventilated.

Light-load running and how to avoid it

A diesel engine that runs for long periods well below its rated load does not reach its proper operating temperature, which leads to unburnt fuel and carbon in the exhaust system, often called wet stacking. Many generator makers advise avoiding long running below about 30% of rated load. A VSD compressor makes this more likely, because at low demand it draws only a small fraction of its full power.

• Choose a low-harmonic drive so that the DG set does not have to be oversized.
• Share the DG set with other loads that run at the same time, so its total load stays higher.
• Avoid oversizing the compressor itself; check the demand with a data logger first.
• Where a set must run lightly loaded, follow the generator maker’s guidance on periodic running at higher load.

How to interpret this guidance, and its limits

The screening ratios on this page are a budgeting tool, not a design calculation. The actual voltage distortion depends on the drive’s harmonic spectrum, the alternator’s subtransient reactance, cable lengths, other non-linear loads such as UPS systems and LED lighting, and whether any power factor correction capacitors are connected. A harmonic study, or the generator maker’s own calculation with the drive data, is the reliable answer for a large or critical installation.

Power factor correction capacitor banks deserve a special note. On a DG supply they can create a resonance with the harmonic currents and can push the alternator into leading power factor, which some voltage regulators cannot control. Most installations switch the capacitor bank off when the plant runs on the DG set.

How to set up a VSD compressor on a DG set, step by step

1. Get the compressor’s package input power at full speed, the drive’s input power factor and its harmonic data from the data sheet.
2. Find out whether the drive is a standard six-pulse type with a choke, or a low-harmonic type.
3. List every other load the DG set will carry, with its running and starting kVA.
4. Apply the screening ratio and ask the generator maker to confirm the rating with the drive data.
5. Prefer an alternator with a permanent magnet generator (PMG) supply to its voltage regulator, which keeps the regulator’s power independent of the distorted output voltage.
6. Arrange for power factor correction capacitors to be switched off when the plant runs on the DG set.
7. Set automatic restart after a supply failure on both the drive and the compressor controller, with a suitable delay.
8. After commissioning, measure the voltage distortion at the compressor panel on DG supply at full and part load, and record it.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka in Secunderabad, has worked with industrial compressors since 2002 as an independent multi-brand specialist in new and used compressors, spare parts, service and rentals. Our service engineers cover all major brands across Telangana and Andhra Pradesh, and we supply equipment across India. We can help you choose between a fixed-speed and a VSD compressor for a site that runs on a DG set, read the drive data that the generator maker will need, and check the compressor’s settings for restart after a power cut.

What to send us

• The compressor’s make, model and data sheet, or its nameplate photographs.
• The DG set’s kVA rating, and the alternator’s nameplate if available.
• A list of the other loads on the same DG set.
• How many hours a day the plant runs on the DG set.
• Any trip or fault messages seen on the drive or the controller during DG running.
• Your typical air demand, or a pressure and power log if you have one.

Frequently asked questions

Can a VSD compressor run on a diesel generator?

Yes, and it is often easier on the set than a fixed-speed machine at starting. The point to manage is voltage distortion: a generator’s internal reactance is much higher than the grid’s, so the drive’s pulsed current distorts its voltage more. Give the generator maker the drive’s harmonic data, choose a low-harmonic drive where the set will carry the compressor for long hours, and switch off power factor capacitors on DG supply.

What size DG set do I need for a VSD compressor?

Start with the drive’s kVA, which is the package input power at full speed divided by the input power factor. For a 64 kW package at 0.95 power factor this is 67.4 kVA. Multiply by about 2 to 2.5 for a standard drive, giving about 135 to 170 kVA, or by about 1.25 to 1.5 for a low-harmonic drive, giving about 85 to 100 kVA. Then add every other load on the same set and its starting demand.

Is a VSD or a fixed-speed compressor better on a generator?

A VSD is usually better for starting and for load changes, because its soft start keeps the current to about the rated value and it ramps its load smoothly. A fixed-speed compressor on star-delta starting draws about 2 to 2.3 times its full-load current at each start and switches between unloaded and full load repeatedly. The fixed-speed machine’s advantage is that it draws almost no harmonic current, so the better choice depends on the drive type and how lightly the set will be loaded.

Why does my VSD compressor trip when the power changes over to the DG set?

Most trips at changeover are undervoltage faults: the supply disappears while the DG set starts and takes the load, and again for the brief break on return to mains, and the drive stops to protect itself. Enable automatic restart after a supply failure on both the drive and the compressor controller, with a delay that lets the DG set settle and the compressor finish blowing down. If the compressor trips while already running on the DG set, voltage distortion, a weak set or a leading power factor from capacitors are the usual causes.

Should power factor capacitors be switched off on DG supply?

In most installations, yes. A power factor correction capacitor bank can resonate with the harmonic currents from drives, which raises the voltage distortion, and it can push the alternator into a leading power factor that its voltage regulator may not control. VSD compressors already run at a displacement power factor of about 0.95, so they need no correction. The common practice is to interlock the capacitor bank so that it disconnects when the plant runs on the DG set.

Is it bad to run a diesel generator at low load with a VSD compressor?

Long periods well below rated load are bad for a diesel engine, because it runs too cool and builds up unburnt fuel and carbon. Many makers advise avoiding long running below about 30% load. A VSD compressor at minimum speed may draw only 19 kW, which is under 15% of a 160 kVA set’s kW rating. Choosing a low-harmonic drive, which allows a smaller set, and sharing the set with other loads both help.

Related pages

• Fixed-speed or VSD compressor
• Air compressor running cost calculator
• Integrating a VSD compressor with fixed-speed machines
• VSD reliability and the drive system
• Compressor motor tripping
• Altitude and temperature derating calculator
• Compressed air knowledge and calculation centre