# Allow only one motor to start, at any given time

Discussion in 'Sensors and Actuators' started by chrisv2, Oct 24, 2013.

1. ### chrisv2

15
0
Oct 24, 2013
I know enough about electronics to know that I don't know enough about electronics.

Here's what I'd like to understand: If I have three 120VAC electric motors, rated 2HP (which is likely the 'stall' HP at the limit of the design point of the motor) - and I want to run them off of a single 15A circuit, I realize there's a good chance that if two or three motors start at once the inrush current will cause the breaker to trip.

These motors may start or stop automatically based on an environmental sensor that each motor is connected to, so it's not simply a matter of the initial power-on of the motors.

Is there a simple circuit I could use that would be an in-series switch along with the motor's environmental sensor, which would allow the motor to start only if the overall current for the array of motors was below a set-point current threshold, and no other motor was in its start-up phase?

For example, if one motor is running steady-state (e.g. 2A) and another motor is in the process of starting up (e.g. 10A), and motor #3 wants to start at the same time, the current detector would disallow it until the two other motors are at steady-state and drawing only 4A.

It would also have to be "smart" enough to disallow all three motors from starting at once, at initial power-on -- or to disallow two motors from starting simultaneously if one motor is already running.

So, a black box that latches 'on' if the system is below a current threshold and also knows enough to disallow more than one motor at a time from starting up.

Does this make sense, and if it can be easily implemented do you think it's a reasonable way to allow multiple motors to be powered from a single 120V 15A household circuit?

2. ### duke37

5,334
751
Jan 9, 2011
According to my abacus, 2 HP = 1892W

1892W on 120V is 15.8A

So, you can only run one motor on this supply. Motors can take up to 8 times full load current to start so even one may be too much depending on the motor and trip mechanism.

3. ### chrisv2

15
0
Oct 24, 2013
ah - I didn't realize that a 2HP motor that is lightly-loaded would constantly draw 1892W.

Thanks for that clarification; I thought that the motor, once spinning and stable, would be less than that.

Set aside the HP of the motor for a minute - assume all three can fit within the supply circuit -- is the control circuitry I'm asking about a simple matter or will it require digital logic to pull off?