A servo pump system drives the hydraulic pump of an injection molding machine with a servo motor whose speed follows the cycle. Instead of a constant-speed motor and valves that throw away surplus oil, the servo drive turns the pump only as fast as each movement needs, and slows it down or stops it when the machine is waiting. A machine built this way is called servo-hydraulic: its movements are still hydraulic, but flow and pressure are controlled electrically through the motor. On the drawing, the servo motor is the electric motor that drives the pump through the bell housing.
The parts of a servo pump system
- Servo motor. A permanent magnet AC servo motor with a built-in encoder that reports its speed and position to the drive. See the servo motor guide.
- Servo drive. The electronic drive that powers the motor. It receives the pressure and flow commands from the machine controller and runs the motor to meet them. See the servo drive.
- Pump. A fixed-displacement pump, most often an internal gear pump; vane and piston pumps are also used.
- Coupling and bell housing. A flexible jaw coupling, often called a plum coupling, joins the motor and pump shafts. See the pump coupling.
- Pressure transducer. Fitted at the pump outlet, it feeds the actual pressure back to the drive. See the pressure transducer.
The relief valve stays in the circuit as protection, but on many servo machines the proportional pressure and flow valve of a fixed pump machine is no longer needed.
How a servo pump works
- For each step of the cycle, the machine controller sends the drive a flow command and a pressure command as electrical signals.
- Flow control. While the movement is free to run, the drive turns the motor at the speed that gives the commanded flow, because the flow of a fixed pump is its displacement multiplied by its speed.
- Pressure control. When the movement meets resistance, for example when the clamp is locked or the screw is holding pressure, the drive switches to pressure control. It compares the transducer signal with the command and adjusts the motor torque so the pressure stays at the set value, while the pump turns only fast enough to make up for leakage.
- Standby. When no movement needs oil, for example once screw recovery has finished and the part is still cooling, or while the machine waits, the motor slows down or stops.
Because the pump does not deliver flow that nobody uses, less energy is turned into heat in the oil, the oil cooler has less to do, and the machine is quieter between movements. The size of the saving depends on the machine and the cycle.
Our servo motor data
The AC servo motor in our range covers the Hysis U31 and U1 series, from 7.5 kW to 45 kW at 380 V three-phase, fan-cooled, with a 2500 ppr incremental encoder, and is listed for Haitian MA and MA II series machines. Plum couplings for joining the pump to the servo motor are listed with it as accessories.
Common problems and their symptoms
- Drive alarms. Overcurrent, overload, overvoltage, encoder faults and motor overtemperature are shown as codes on the drive. Look the code up in the drive manual before changing parts.
- Pressure oscillation or overshoot. Pressure loop settings that do not suit the machine, air in the oil, a faulty pressure transducer or cable, or a worn pump.
- Pressure does not reach the setpoint. A worn pump, a leaking relief valve, a transducer that reads wrong, or a torque limit in the drive.
- Motor runs faster than usual during hold pressure. Internal leakage in the pump, valves or cylinders, which the motor has to make up.
- Noise or vibration at the pump. A worn coupling insert, misalignment, cavitation in the pump, or worn motor bearings.
- Motor overheating. A blocked or failed cooling fan, high ambient temperature, or long periods of high pressure holding.
Replacing the servo motor or pump
- Switch off and lock out the machine, and wait for the drive's DC bus to discharge as the drive manual requires before touching any terminal: its capacitors stay charged after the power is switched off.
- Match the replacement motor's rated power, torque, speed, voltage, encoder type, flange and shaft to the original.
- After fitting a new motor or encoder, the drive usually needs the motor parameters entered and the encoder position identified, following the drive manual.
- After any change to the motor, pump or drive, check the pressure and flow calibration on the machine controller and compare the pressure with a gauge.
- For pump replacement, follow the steps on the hydraulic pump types page.
What to send for a replacement quote
The motor nameplate (model, rated power, torque, speed, voltage and encoder type), the drive model, the pump model code, the coupling size, the shaft and flange dimensions, and the machine model and serial number. Photos of the nameplates and of the complete pump unit help. Browse our servo motors and drives, then request a quote. The hydraulic system guide covers the rest of the circuit the servo pump feeds.
Updated by the IMSPARES technical team.