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When choosing a motor for a two-stage industrial compressor, what matters as much as the power and speed on the block nameplate is how many seconds the motor needs, in star connection, to bring the heavy flywheel up to speed with the unloader open: on these 15–30 kW machines a 4-pole motor with a 400/690 V nameplate (for example the 22 kW DRG-180L-4, 1470 rpm, 39.7 A at 400 V) runs in delta on a 400 V supply and is suitable for star-delta starting. If the unloader valve is faulty, even the right motor cannot start in star; that is why on a two-stage compressor motor selection and starting arrangement are considered together.
A two-stage block first compresses air in the low-pressure cylinder and then, after passing it through the intercooler, in the high-pressure cylinder; this reaches pressures in the 12–15 bar class with a lower outlet temperature than a single stage. These machines mean higher flow, a heavier flywheel and a longer run-up. Panel and motor connection must be done by a qualified electrician. You can see suitable 4-pole models and current prices among the 120 models on the Buy Electric Motors Online page.
3 situations that strain the motor on a two-stage compressor
- Starting with a loaded cylinder: if the unloader valve does not vent the cylinder and intercooler at start, the motor tries to turn the heavy flywheel against pressure. In star the torque drops to one third, so the motor cannot accelerate and a large current surge occurs when it switches to delta.
- Star-delta timer set too short: if the switch to delta happens before the motor reaches speed, it draws a current close to direct-on-line starting and the fuse or overload may trip.
- Clogged intercooler: when the intercooler fins are covered with oil and dust, hot air enters the high-pressure stage, the block draws more power and motor current rises above the nameplate value.
What kW motor is needed, and why 4 poles?
On two-stage blocks the motor power is the value stated by the block manufacturer, and because of the higher pressure it is usually larger than for a single-stage block of the same flow. Belt drive with a 4-pole motor is common in this class: at the same power a 4-pole motor gives roughly twice the rated torque of a 2-pole one, and a smaller pulley ratio is needed for a slow block with a flywheel. Power, speed and connection data (400/690 V, Δ) on the old motor's nameplate must be the same on the new motor.
If the nameplate cannot be read, do not try to guess the power; ask the manufacturer for the flow and pressure on the block label, or measure motor current on a running machine. Compared with the rated current in the table below, the current measured at 400 V shows which power class the motor is loaded in.
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What happens to current and torque in star-delta starting?
In star-delta starting the motor is first connected in star; because the winding voltage drops by √3, both starting current and starting torque fall to about one third. In the DRG range motors of 4 kW and above have a 400/690 V nameplate, so they run in delta on 400 V and are suitable for star-delta. In the table below, star starting current is calculated as nameplate current × Is/In ÷ 3:
| Model | Power / speed | Current 400 V | Star starting current |
|---|---|---|---|
| DRG-160M-4 | 11 kW / 1465 rpm | 20.4 A | ≈ 52 A |
| DRG-160L-4 | 15 kW / 1465 rpm | 27.3 A | ≈ 71 A |
| DRG-180M-4 | 18.5 kW / 1470 rpm | 33.5 A | ≈ 87 A |
| DRG-180L-4 | 22 kW / 1470 rpm | 39.7 A | ≈ 103 A |
| DRG-200L-4 | 30 kW / 1475 rpm | 53.8 A | ≈ 131 A |
As current falls to one third, so does torque; that is where the real risk lies. Starting torques of the same models and their values in star:
| Model | Rated torque | Starting torque (Δ) | Torque in star |
|---|---|---|---|
| DRG-160M-4 | 71.7 Nm | 157.7 Nm | 52.6 Nm |
| DRG-160L-4 | 97.8 Nm | 215.2 Nm | 71.7 Nm |
| DRG-180M-4 | 120.2 Nm | 240.4 Nm | 80.1 Nm |
| DRG-180L-4 | 142.9 Nm | 285.8 Nm | 95.3 Nm |
| DRG-200L-4 | 194.2 Nm | 388.4 Nm | 129.5 Nm |
You can do the star-delta calculation for your own motor with the starting methods and star-delta tool. The general importance of starting torque on compressors is the subject of a separate article: why compressors need high motor starting torque.
How does flywheel inertia lengthen the run-up time?
On two-stage blocks the large pulley is also a flywheel and smooths out the torque pulsation of the pistons during the cycle. At start, however, this inertia is a mass the motor has to accelerate. The run-up time is approximately:
t ≈ J × ω / (TY − TL)
J is the total inertia referred to the motor shaft (kg·m²), ω = 2π × n / 60 the motor's rated angular speed, TY the average motor torque in star and TL the load torque at start. Example: 22 kW DRG-180L-4; rated torque 142.9 Nm, starting torque in star about 95.3 Nm, ω ≈ 153.9 rad/s. Let us assume the total inertia of flywheel and motor referred to the shaft is J = 1.5 kg·m² (the real value depends on the flywheel and pulley ratio). Taking load torque as 0.1, 0.4 and 0.7 times rated torque:
| Load at start | Load torque | Accelerating torque | Run-up time |
|---|---|---|---|
| Unloader open | 14.3 Nm | 81 Nm | ≈ 2.9 s |
| Valve partly leaking | 57.2 Nm | 38.1 Nm | ≈ 6.1 s |
| Cylinder loaded (valve faulty) | 100 Nm | -4.8 Nm | cannot accelerate |
The calculation is simplified (torque in star is taken as constant) but the message is clear: with the unloader working the motor reaches speed in a few seconds; when the valve leaks the time more than doubles, and with the cylinder left loaded the motor cannot accelerate in star at all. Set the star-delta timer according to this time, and check the unloader valve first when replacing the motor.
Checklist before replacing the motor
- Note power, speed, voltage (400/690 V) and connection from the old motor's nameplate.
- After stopping the compressor, listen to check that the unloader vents cylinder and intercooler pressure.
- Check that the check valve does not let tank pressure leak back to the block side.
- Compare the pulley bore and keyway width with the new motor's shaft (on the 180L frame the shaft is 48 × 110 mm).
- Readjust belt tension and pulley alignment; excessive tension strains the motor bearing (6311-C3 on the 180L).
- Verify that the star-delta timer switches to delta only after the motor has reached speed in star.
- Set the overload relay to the phase current (nameplate current ÷ √3) and select contactor and fuses with the overload, fuse and contactor selection tool.
- At first start compare the direction of rotation with the arrow on the block; intercooler fan and lubrication depend on it.
For the causes of compressor motor burnout and the pulley-speed calculation, see our article on why a compressor motor burns out; we do not repeat those calculations here.
Which DRG motor for a two-stage compressor?
Example model data for a 22 kW two-stage workshop or factory compressor:
- Model: DRG-180L-4
- Power: 22 kW
- Rated speed: 1470 rpm
- Frame: 180L · cast iron
- Voltage: 400/690 V (Δ/Y)
- Current 400 V: 39.7 A
- cos φ: 0.86
- Rated torque: 142.9 Nm
- Starting torque: 2 × Tn
- Starting current: 7.8 × In
- Shaft D × E: 48 × 110 mm
- Bearing: 6311-C3
DRG Motor is a manufacturer, supplier and seller of electric motors; DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate). Periodic instalment campaigns may be available with Halköde; for the current option message us on WhatsApp or see the payment options page.
Motors in this power range
- DRG-160M-4 – 11 kW – 1465 rpm – 160M cast iron
- DRG-160L-4 – 15 kW – 1465 rpm – 160L cast iron
- DRG-180M-4 – 18.5 kW – 1470 rpm – 180M cast iron
- DRG-180L-4 – 22 kW – 1470 rpm – 180L cast iron
- DRG-200L-4 – 30 kW – 1475 rpm – 200L cast iron
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Frequently Asked Questions
What speed should a two-stage compressor motor have?
In this class belt drive with a 4-pole (1500 rpm synchronous) motor is common; in the DRG range rated speed is between 1465 and 1475 rpm. The exact speed is set by the old motor nameplate and the pulley ratio.
How many amps does a 22 kW compressor motor draw?
The DRG-180L-4 draws 39.7 A at 400 V and 41.8 A at 380 V. In star starting the current falls to about 103 A.
The compressor starts in star but the fuse trips when it switches to delta. Why?
Usually the motor switches to delta before reaching speed in star. Check that the unloader vents the cylinder and that the timer is set to a sufficient time.
Can a two-stage compressor be started direct-on-line?
It can if the supply and panel allow it, but on a 22 kW motor starting current rises to 7.8 times rated current. At these powers star-delta or a soft starter is the usual choice.
Send us the block and motor nameplates of your compressor and we will settle the right DRG model and starting arrangement together: Phone +90 (542) 666 11 11, e-mail info@drgmotor.com, contact page. For all models and current prices, see Buy Electric Motors Online.
