The servo motor of an injection molding machine is a permanent-magnet AC motor controlled by a servo drive. On a servo-hydraulic machine it drives the hydraulic pump and changes speed in every step of the cycle, so the pump delivers only the oil flow and pressure that step needs, and slows down or stops while the machine waits. On an all-electric machine, separate servo motors drive the clamp, the injection, the screw rotation and the ejector directly.
The parts of a servo motor
The section drawing beside this page numbers the parts of an AC servo motor:
- Encoder: at the rear of the motor, it measures the shaft position and speed for the drive. See the encoder guide.
- Rear bearing: supports the rear end of the shaft.
- Power and encoder connectors: the power cable to the stator and the signal cable from the encoder both run to the servo drive.
- Stator windings: copper coils in a laminated steel stack. The drive feeds them current to make a rotating magnetic field.
- Rotor with magnets: permanent magnets on the rotor follow the stator's field and turn the shaft.
- Front bearing: carries the shaft and the load from the coupling.
- Mounting flange: bolts the motor to the pump bell housing; its spigot centers the shaft.
- Shaft: the output shaft with a keyway drives the pump through a flexible coupling.
How a servo motor works
The servo drive reads the encoder and feeds the stator windings with three-phase current whose frequency and phase follow the rotor position. Because the rotor carries permanent magnets, it turns in step with the field (it is a synchronous motor), and the drive controls its speed and torque directly.
On a servo-hydraulic machine, the motor speed sets the pump flow and the motor torque sets the pressure the pump can build. That is why such a machine uses less energy than one whose pump motor runs at a fixed speed all the time: when no oil is needed, the motor barely turns. The servo pump guide covers the hydraulic side, and the pump coupling guide covers the connection between motor and pump.
Specifications that identify a servo motor
A replacement must match the original on:
- rated power, torque and speed, and the maximum speed;
- supply voltage and rated current;
- encoder type and resolution, which must suit the drive;
- flange, spigot, shaft diameter and keyway;
- cooling (fan-cooled or natural) and protection rating.
Our AC servo motor is a permanent-magnet motor for servo-hydraulic drives, listed from 7.5 kW to 45 kW at 380 V AC three-phase, 0–2500 rpm, with a 2500 ppr incremental encoder, IP54 protection and fan cooling, in Hysis U31 and U1 series models.
Signs of a worn or failing servo motor
- Bearing noise: a rising whine, rumble or grinding, often with vibration. A worn bearing lets the shaft run off center and can damage the encoder.
- Vibration: misalignment between the motor and the pump, or a worn elastomer insert in the coupling.
- Overheating: a stopped cooling fan, a blocked fan cover, oil and dirt on the housing, or an overload caused by a hydraulic fault.
- Drive alarms: overcurrent or ground fault from damaged winding insulation or a damaged motor cable, and encoder alarms from the encoder or its cable.
- Loss of torque: the pump cannot reach its set pressure although the drive and the hydraulics are healthy. Severe overheating can weaken the rotor magnets.
Inspecting and replacing a servo motor
Electrical tests and motor replacement are for qualified electricians, with the machine switched off at the main isolator and locked out, and the DC bus of the servo drive discharged.
- Listen to the motor and feel for vibration while it runs, and check that the cooling fan turns.
- With the machine isolated, disconnect the motor cable at the drive and test the insulation of the windings to ground. Never test insulation with the drive connected: the test voltage can damage it.
- Check the encoder cable and connectors for damage and oil.
- To replace the motor, support its weight, remove it from the bell housing, and inspect both coupling halves and the elastomer insert.
- Fit the new motor and coupling, aligned to the pump as the machine manual describes.
- Enter the motor parameters in the drive and run the drive's encoder alignment or auto-tuning if its manual requires it, then check the direction of rotation at low speed before running the pump at pressure.
What to send for a replacement quote
- A clear photo of the motor nameplate: model, power, torque, speed, voltage and current.
- The encoder type, if it is marked, and the model of the servo drive.
- The flange and shaft dimensions, and the coupling size.
- The machine brand, model and serial number.
The electrical system guide shows where the motor and drive fit in the machine, and the servo motors and drives category lists what we supply.
Updated by the IMSPARES technical team.