Summary

Because of low voltage at the motor, a motor adequately size to drive a pump at a given pressure when operating at speed may not be able to start the pump against the same pressure.

Normal Operation

A typical AC induction motor used to drive a small displacement, high pressure hydraulic pump is a single-phase unit designed to operate efficiently at normal (near-synchronous) operating speed.  Ideally, a pump driven by such a motor is started unloaded.  This allows the motor to reach running speed before it sees a load, thus ensuring satisfactory operation (See article – Single-Phase Electric Motor Performance Characteristics).

Starting Against a Load

Starting voltage is less than running voltage

• AC induction motor starting current can be 600% of the full-load current.  This high current often causes significant voltage drop in the electrical power supply line from the main facility transformer to the motor (See article – Electrical Power Distribution and its Effect on Motor Operation).  Start-up voltages as low as 80 volts are not uncommon.

Low voltage reduces output torque

• AC motor ratings are given at specific voltage.  Operation at voltage other than rated will change the performance of characteristics of a motor.  Torque output of AC motors vary with the square of the voltage.  For example, a motor which has 120 in-lb torque at 120 VAC will only have 97 in-lb torque at 108 VAC.

Locked-rotor (starting) torque can be less than running torque

• At rated voltage, a given motor’s starting torque is typically its highest output torque.  However, since voltage at the motor is less at start than at any other operating speed, effective starting torque can be less than full-load torque.

Motor can be damaged

• If system pressure is too high, the motor will not turn.  Continued operation at this point will overheat and damage the motor.  Some other means (such as manual release valves) must be used to reduce system pressure to enable the motor to again start.

Solution

Ideally, the motor used to drive a pump can output just enough torque to accelerate the pump to full speed under all conditions.  In practice, the motor is always slightly oversized to provide some margin.  Excess margin is of course costly and can lead to a physically larger package size.  Other options available:

• Ensure electrical circuits leading to the power unit are adequately sized and do not impose significant voltage drop.  Drawback: Impractical when dealing with portable equipment
• If so designed, use the motor at 208/320 VAC instead of 120 VAC.  Higher voltage operation means less current and thus less voltage drop to motor.  Drawback: High voltage service outlets are not common in North America
• Use a motor which has special windings to increase starting torque.  Drawback: Such motors typically sacrifice full speed/breakdown torque to achieve higher start torque

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