Single phase motor restart time restriction

Thread Starter

Jensenlogix001

Joined Sep 28, 2021
5
Good day,
I am trying to determine the reason for an 120VAC, 1/2 HP, 9A pump motor requiring a 3 minute minimum time delay to restart it.
Also, it has a 20 start per/day limit as well. These warnings are stated in the motor booklet only, not on the motor label.
Why can't I start it, stop it, and start it again as many times as I need to.
Here's what it reads under Pump Operation, "Max. 20 Starts / Hour: Time gap between Motor Stop & Restart must be min. 3 minutes.
Its a fairly large motor, it uses a capacitor for starting as most motors also use. I have not checked the Cap for ID information yet.
My theory for this limit is for the time needed for the recharging of the capacitor to reach full charge.
Does anyone out there have any other theories?
 

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crutschow

Joined Mar 14, 2008
28,204
Does anyone out there have any other theories?
The capacitor is an AC capacitor so it charges and discharges with the AC voltage during starting.
There is no delay for that charging.

The time delay between turning off the motor and restarting is likely to prevent overheating of the motor, as the large AC motor startup current (around 10 times the running current) can generate significant additional heating of the motor windings above their running temperature.
 

Thread Starter

Jensenlogix001

Joined Sep 28, 2021
5
Good day,
Thank you for the feedback.
Your theory for preventing overheating makes perfect sense.
As the motor is running under load there will be some normal heating developing.
I imagine if a stop is requested and not enough time is allowed before a restart, excessive additional heat will be introduced.
This additional increase in temperature during a restart, in theory, could damage or degrade the windings.
The manufacturer calls for 3 minutes min. between restarts with min. 20 starts/hour.
I wonder how they came to this solution.
Ambient Temp / Load current x Time, (the quadratic formula), Best guess perhaps....
Any ideas?
 

GetDeviceInfo

Joined Jun 7, 2009
1,920
frequent starts are probably the biggest reason for burning out motors. At least in my experience. Bearings #2.
An under rated O/L or Breaker can initiate the sequence of reset/start, with poor outcomes for the motors.
 

michael8

Joined Jan 11, 2015
233
120VAC, 1/2 HP, 9A pump motor

1/2 HP is 373 watts, 120 vac * 9A is 1080 watts, where does the excess 707 watts go? Or is the 9A just the
starting current?

Also what is this pumping? Could the wait be for the backpressure to go away before a new motor start?
 

LowQCab

Joined Nov 6, 2012
1,341
Another alternative is to Over-Rate the Motor.
Check the "Service-Factor" rating of the Motor,
Motors are commonly rated at up to 120% (1.2 Service-Factor),
yours may be much lower.
It's possible to have a Motor rated at ~85%, or a .85 Service-Factor.

Restricting the flow of Water, either with a Valve, or
by increasing the "Head-Pressure",
will reduce the average Current demanded by the Motor,
which will reduce the average Heat generated.
( the Load is determined by Volume, NOT PRESSURE ).

You can also simply double the Horsepower rating of the Motor,
( while keeping the work-load the same ),
and then do as You please with Starting and Stopping.

You can also use a quality VFD-Inverter to smoothly ramp-up the Power on each Start,
or simply run it continuously at a reduced Power,
or vary the Pump-Speed instead of switching it on and off repeatedly.

You can also use a Solenoid operated "Diverter-Valve" to recirculate the Water,
provided that You insure that the Water does not become over-heated
by using a large reservoir for cooling the Water before it returns to the Pump.
Then You can simply run the Pump continuously.
( the return plumbing should be designed with substantial restriction to Flow,
this should be done with a valve, smaller Pipe plumbing, or increased Head-Pressure,
retain at least ~10% of the maximum Flow-Volume
back to the Pump for Water-Cooling ).

Never "Dead-Head" the Pump, it will create high-pressure Steam and melt things.
.
.
.
 

Thread Starter

Jensenlogix001

Joined Sep 28, 2021
5
Good Day,
Thank you for all the excellent feedback on this subject.
I enjoy reading about everyone's ideas.
Allow me to add more details.
We know the pump runs on 120 Volts AC @ 9Amps with 1/2 Horse Power
I will be using this pump as an oil transfer pump for a specialized mineral oil type substance with a rather high viscosity level.
The pump is rated for a viscosity level of SAE 90 which is very viscus and hard to pump.
The material is a contributing factor to the heating of the pump.
Another contributing factor is run-time.
The longer we run the pump the higher the temperature will increase.
Fortunately my application experiments will consist mostly of low volume levels, around 8 liters per tank.
My run-times I expect will be around 2-3 minutes per tank with a 4 tank maximum per session.
We have decided to move forward with this pump and to deploy it for further experimental use and abuse.
I do not expect to have any issues with over heating but will closely monitor the pumps temperature with some thermocouples strategically placed upon the motor housing. Also, I will closely monitor Volts, Current, Watts, (E x I = W) throughout the process to check for any increased electrical anomalies.
I love my job...
Thanks to all.
J
 

Thread Starter

Jensenlogix001

Joined Sep 28, 2021
5
120VAC, 1/2 HP, 9A pump motor

1/2 HP is 373 watts, 120 vac * 9A is 1080 watts, where does the excess 707 watts go? Or is the 9A just the
starting current?

Also what is this pumping? Could the wait be for the backpressure to go away before a new motor start?
I am transferring mineral oil from one tank to another.
 

LowQCab

Joined Nov 6, 2012
1,341
Positive-Displacement-Pumps operate on totally different principles.
I didn't know You were pumping Oil.
They are fixed Volume, and the Pressure determines the Load,
the opposite of a common Centrifugal-Pump.
.
.
.
 

Thread Starter

Jensenlogix001

Joined Sep 28, 2021
5
Positive-Displacement-Pumps operate on totally different principles.
I didn't know You were pumping Oil.
They are fixed Volume, and the Pressure determines the Load,
the opposite of a common Centrifugal-Pump.
.
.
.
Quite true, in my case however there will be little to no pressures involved thankfully.
I was merely concerned with the start/stop/start limits set forth in the manual.
Thankfully, it is no longer a concern and will be moving forward with testing phase.
At that time we should have a better understanding of pump/motor performance.
I am 99% certain it will work fine.
Thank you for your excellent feedback.
J
 
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