Servo drive of an injection molding machine

The servo drive of an injection molding machine is the power electronics unit in the electrical cabinet that powers and controls the servo motor. It takes the three-phase mains supply, converts it to DC and then to a variable three-phase output, and uses the motor's encoder to control speed and torque precisely. On a servo-hydraulic machine it runs the pump motor to give the flow and pressure that the controller asks for in each step of the cycle.

Where the servo drive sits

The drive is mounted in the electrical cabinet next to the controller boards, usually with its own heatsink and cooling fan. The drive interface board brings it the speed and pressure commands from the CPU board, and the motor cable and the encoder cable run from the drive to the servo motor.

How a servo drive works

1. Input and filter. The mains supply enters at the input terminals, where film capacitors filter out electrical noise.
2. Rectifier. Diodes turn the AC into DC.
3. DC bus. Electrolytic capacitors store energy and smooth the DC voltage.
4. Power module. Power transistors (IGBTs) switch the DC bus on and off at high frequency to produce a three-phase output of variable voltage and frequency.
5. Control board. It reads the encoder and the commands and adjusts the output, running current and speed control loops and, on many servo-hydraulic systems, the pressure loop as well.

The capacitors guide has a drawing of this power section. When the motor slows down, it works as a generator and pushes energy back into the DC bus; many drives send this energy to a braking resistor so that the bus voltage does not rise too far.

Servo drive or frequency inverter

A frequency inverter also varies the speed of a motor, but it usually drives an induction motor without the fast closed loop of a servo system. Some energy-saving hydraulic machines use an inverter on the pump motor. Servo-hydraulic machines use a servo drive with a permanent-magnet motor and an encoder, which responds faster and holds pressure more precisely. The two are not interchangeable without changing the motor and the settings.

Common servo drive alarms and their causes

Alarm codes differ between drive makers, so look each code up in the drive manual. The usual groups are:

• Overcurrent or ground fault: a damaged motor cable, failed winding insulation in the motor, or a failed power module.
• Overvoltage: energy returning from the motor that the braking circuit cannot absorb, a failed braking resistor, or a high supply voltage.
• Undervoltage or phase loss: a blown input fuse, a loose supply terminal, or a weak mains supply.
• Overheating: a failed cooling fan, a heatsink clogged with dust, or a hot cabinet.
• Encoder fault: a damaged or poorly shielded encoder cable, a loose connector, or a failed encoder.
• Overload: the motor has worked above its rating for too long, often because of a hydraulic problem such as an internal leak.

Aged DC bus capacitors can cause several of these alarms, especially under heavy load.

Inspecting and replacing a servo drive

Work on a servo drive is for qualified electricians. The DC bus capacitors hold a dangerous voltage after the supply is switched off.

1. Switch the machine off at the main isolator, then lock out and tag it.
2. Wait at least the discharge time shown on the drive's warning label, then measure the DC bus voltage to confirm that it is safe.
3. Clean the heatsink and filters, check the cooling fan, and check the tightness of the power terminals.
4. Before replacing a drive, save or write down its parameters if it can still communicate.
5. Fit a drive of the same model and rating, and reconnect the power, motor, encoder and control cables exactly as before, with the shields connected.
6. Load the parameters, enter the motor data, and run the encoder alignment or auto-tuning if the drive manual requires it. Check pressure and flow against the machine manual before production.

What to send for a replacement quote

• A clear photo of the drive nameplate: model, power, input voltage and output current.
• The model of the servo motor it drives.
• The alarm code and when it appears.
• The machine brand, model and serial number.

See the servo motors and drives category, the servo pump guide for the hydraulic side, and the electrical system guide for the overview.