Maximum Voltage Drop Recommended at Motor Start Up

[Bob Deroeck]

Hi,

My workshop is located away from my house and the power supply line runs underground in conduit. The total distance from the main panel in the house to the subpanel in the workshop is about 170 ft.

I'm curious as to how many horsepower worth of machines I could operate through this power line and the possible negative effects on motor life due to high voltage drop during motor startup.

I've done voltage drop calculations and there is no problem running considerable amps at 240 v through this line during continuous motor operation. My question pertains to starting up a large motor. My understanding is that instantaneous amp draw when starting a motor ranges from 3-8 times the continuous load amps. Can any of you guys confirm if this is true? If so, what would be a "typical" number, or is the variation so great that there really is no "typical" number? It's quite likely that the largest motors I would run would be 5 HP, and likely to be Baldor or Lesson single-phase motors, but that a number of these motors could be operating simultaneously.

I've used the advanced voltage drop calculator at electrician.com, which includes the effect of conductor temperature on resistivity. Cases where starting up the motor at 3 or 5 or 8 times the continuous motor amps result in high amps and considerable voltage drop. While I recognize that this high amp draw is for a relatively short amount of time, I wonder what the effect would be on the lifetime of the motor. What would be the maximum voltage drop you guys would recommend for motor startup currents before you would be concerned about possibly damaging the motor?

Thanks for your input.

Bob

[Bob Deroeck]

I sent an email to Baldor asking for their imput on this subject. This is the response I got from Dave Redman, the Baldor district manager based in Boston.

"A single phase or 3 phase would have an in-rush current up to
6 times the Full Load Current (FLA). This is a momentary inrush of about 1
second. The motors are designed to start with 80% of the voltage. You do
lose 40 % of your starting torque (voltage loss squared). So as long as the
motor isn't under Full load you should be able to start. A fan motor may or
may not have some problems starting. The load on a fan is linear. So
as the motor is coming up to speed the load comes up to speed at the same
rate. Half speed half load, full speed, full load. If all the other motors
are already running you may not have a problem. If you are starting other
motors at the same time you may have problems.

If the single phase motor fails to start, the motor will hang up in the
start winding and draw 6 time the current until you trip a circuit breaker
or if it does that long enough (20 seconds) the start winding will burn out
and the motor is done.

You can add soft starts which will limit the in rush current to an
adjustable amount of 2-4 time the FLA. This works or you can buy Variable
speed drives that will accept 115 volt or in larger motors 230 volt single
phase and output three phase. These VFD's would need to be integrated into the
machine but that would give you the soft start and give you variable speed
on some of your machines.


If you don't have a major voltage drop during startup then your motor
should start fine. You should try to stagger the starting of each motor. Try
not to start more than one motor at a time. That will help greatly."

So, it appears that as long as the startup voltage drop is no more than 20%, there should not be a problem with potential damage to the motor, at least for a Baldor motor.

Bob