Motors, Drives and Starting Large Blowers: 800 HP Considerations

At 800 HP, about 597 kW, the motor stops being a routine component and becomes a major electrical load. Starting it directly can dip the supply voltage across a plant, the inertia of a large wheel can take many seconds to accelerate, and the choice of voltage, starter, and drive affects cost, reliability, and energy for the life of the installation. This guide covers the points that matter when a blower reaches this size: current, voltage, starting methods, acceleration, and control.

What 800 HP Means Electrically

800 HP is about 597 kW of shaft power. The current the motor draws depends on the supply voltage, as the table shows for illustration, assuming a motor efficiency of 95% and a power factor of 0.88.

Supply voltage	Full-load current (approx.)	Starting current at 6 times full load
415 V	993 A	about 5,960 A
3.3 kV	125 A	about 750 A
6.6 kV	62 A	about 375 A

Direct-on-line starting commonly draws several times full-load current, with six times being a widely used figure. At low voltage, a 597 kW motor would draw nearly 1,000 A at full load and thousands of amps at start. That is why motors of this size are often supplied at medium voltage, where the currents, cable sizes, and switchgear are more manageable. The figures above depend on the motor’s efficiency and power factor, so use the motor data for real designs.

Why Starting Matters

Starting a large motor across the line has three effects.

1. Supply voltage dip. The inrush current drops the voltage at the motor and on the rest of the plant, which can trip other equipment or fail to start the motor.
2. Mechanical shock. The sudden torque stresses couplings, belts, shafts, and the wheel.
3. Heating. A long acceleration heats the motor, and repeated starts can overheat it.

Starting Methods

Method	How it works	Points to note
Direct-on-line	Full voltage at once	Highest inrush and shock; only where supply and load allow
Star-delta	Starts at reduced voltage, then switches	Reduced inrush, but a torque and current transient at changeover
Autotransformer	Reduced voltage from a tapped transformer	Smoother, more expensive
Soft starter	Ramps voltage electronically	Controlled inrush and torque, no speed control
Variable-speed drive	Ramps frequency and voltage	Lowest inrush, full speed control, higher cost
Fluid coupling	Slip coupling between motor and blower	Motor starts nearly unloaded, blower accelerates smoothly

For a large blower with a heavy wheel, a variable-speed drive or a soft starter is the usual answer, and a fluid coupling is also common where the load inertia is high.

Load Inertia and Acceleration

A large centrifugal wheel has significant rotational inertia, and the blower’s load torque rises with the square of speed. The motor must accelerate the wheel to speed before the motor’s thermal limit is reached. If the acceleration takes too long, the motor overheats. Check the acceleration time against the motor’s permitted start time, and check the number of starts per hour the motor can take. A blower that starts and stops frequently needs a motor and starter rated for it.

Belt Drive or Direct Drive

Direct drive couples the wheel to the motor shaft, so the blower runs at motor speed, and it is compact and efficient. Belt drive lets the blower speed differ from motor speed, which is useful for setting the duty after commissioning and for keeping the motor away from hot gas. At 800 HP, belt drive is limited by the power a belt set can carry, and many large blowers use direct or coupled drive. State the arrangement you need, and where the motor sits relative to the airstream.

Motor Efficiency and Regulation

Motor efficiency classes are defined in IEC 60034-30-1, and higher classes save energy over the life of a large machine. An 800 HP blower runs on a motor above 500 kW, so the EU fan ecodesign regulation, which covers fans between 125 W and 500 kW, does not apply to the complete unit, but the motor and the fan’s efficiency still deserve comparison on a lifecycle basis.

Variable-Speed Drives on Large Blowers

A drive on an 800 HP blower saves a great deal of energy on variable duty, and it starts the machine gently. It also introduces considerations: harmonics on the supply, motor insulation stress, bearing currents, cable length, cooling of the motor at low speed, and heat from the drive itself. Specify a motor suitable for inverter supply, and check the drive’s rating against the load. See flow control for blowers.

Altitude and Ambient Temperature

Motors are rated for a maximum ambient temperature and altitude. At high altitude, thinner air cools the motor less effectively, and at high ambient temperature the rating falls. Confirm the motor is suitable for the site, and remember that the blower’s own power also depends on the gas density. See gas temperature, altitude and density correction.

Vibration Standards at This Size

At 800 HP, the blower falls under ISO 20816-3 for vibration criteria, since that standard applies to fans above 300 kW. See bearings, balancing and vibration.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure, with backward-curved and forward-curved wheels and single-inlet and double-inlet designs. Motor voltage, starter or drive, and drive arrangement are confirmed for each selection, and depend on the supply, load inertia, and duty.

What to Specify

1. Available supply voltage and short-circuit capacity.
2. Permitted starting current and voltage dip.
3. Load inertia, or the blower wheel data.
4. Starts per hour and duty cycle.
5. Speed control requirement.
6. Site altitude, ambient temperature, and motor enclosure.
7. Drive arrangement and motor position.

Tell Us Your Supply and Duty

Send your supply voltage, starting limits, and duty, and we’ll recommend a motor and starting method. Contact us for a free quote.

Frequently Asked Questions