In most cases the motor will be loaded more heavily. When the cut-out value of the pressure switch is raised, the piston compresses the air to a higher pressure on every revolution, the required shaft power increases and the motor draws high current for longer while filling the tank. If the motor's existing margin is small, overheating and protection trips begin; in addition, the maximum pressure permitted for the compressor block must not be exceeded.

DRG-132S-4 5.5 kW 1500 rpm DRG three-phase motor with cast iron frame – motor for a piston compressor pressure increase

The relationship between pressure and power

A piston block running at the same speed delivers slightly less air at higher pressure, because the air left in the cylinder re-expands; on the other hand, the compression work per unit of air increases. The net result is that motor power usually rises. The size of this increase depends on whether the block is single- or two-stage, the cylinder dimensions and the cooling; the correct value is read from the manufacturer's pressure-power table.

What to check first

  1. The maximum working pressure on the block's rating plate; the rating of the tank and safety valve
  2. The current of the existing motor measured close to the new cut-out pressure
  3. Time taken to reach pressure and the number of cut-ins per hour
  4. Ambient temperature; if the compressor room heats up, the motor's cooling margin shrinks

Options if the motor is not enough

  • A motor one power rating higher at the same speed; the suitability of the pulley and belt arrangement for the new power is also checked.
  • Reducing the block speed via the pulleys; the motor load falls, but so does air output.
  • If high pressure is needed by only one machine, consider a separate booster arrangement

The pulley and belt calculator can be used for the speed calculation.

Starting condition

If the unloader valve is faulty, the motor tries to start with pressure in the cylinder. A high pressure setting makes this worse; if starting becomes noisy and sluggish, the valve should be checked. Details are on the piston air compressor motors page; you can choose a suitable power rating in the online motor list.