Summary

Voltage drop in long electric supply circuits can have an adverse effect on motor performance.  Consider true voltage available at motor under load when specifying motors/power units

Article

Electrical codes call out minimum building feeder and branch circuit wires sizes based largely on current requirements and distance to equipment (or outlets).  All circuits must be some minimum size based on current; some circuits must be sized larger for a given current due to the distance from the circuit breaker panel to the outlet.  This ensures some minimum voltage at the outlet when the line is carrying its full load.  Electrical codes require wiring such that total voltage drop at the outlet is no more than 5% of the nominal line voltage.  Using 120 VAC nominal, voltage at the farthest outlet from the panel in a 20 Amp circuit should always exceed 114 VAC when serving 20 AMP load.

Most users of portable machines use extension cords to reach outlets.  By increasing the “electrical distance” from the outlet, the voltage at the motor will decrease at a given current.  The following shows how the voltage at the motor varies with motor current when starting against a given load:

Assume the motor draws 10 Amps when operated normal speed against the load.  The motor is connected to the outlet described above with a 100 foot, 12 gauge extension cord.  This cord will cause about a 3 volt drop when carrying 20 Amps.

• At rest, current is of course zero so voltage drop is zero – V_rest = 120 VAC
• Upon startup, current can momentarily jump to 600% of nominal operating current.  This means the 10 Amp nominal motor can draw 60 Amps upon startup.  At 20 Amps, the voltage drop would be 6 volts in the building wiring and 3 volts in the extension cord.  It follows that at 60 Amps the drop would be 27 – V_startup = 93 VAC
• Once the motor comes up to normal operating speed and load, it will operate at 10 Amps.  Voltage drop is about 5 volts, so V_running = 115 VAC

The above example is almost a “best-case” example.  Perhaps a worst case is a 115 VAC nominal line with a 15 Amp building circuit and a 100 foot, 16 gauge extension cord.  In this case, V_rest = 115 VAC, V_startup = 66 Volts, and V_running = 107 VAC

Except how it affects V_startup and V_running, V_rest is unimportant to motor operation and is not, but itself, an adequate indicator of a suitable power supply.  Using this value is much like using open-circuit voltage to assess the condition of a battery – it can have near nominal voltage but still not be able to maintain that voltage under a load.  The critical factors predicting how a motor will operate are V_startup and V_running, which can only be measure under load.

When selecting a motor-driven device, the minimum values of V_startup and V_running must be known to properly evaluate the device.  Test procedures, if used to prove capabilities of motors, must be done at these minimum values if the test is to be meaningful.

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