Motors

When you start a motor the inrush current is much higher than the normal running current. This inrush current is only there for a brief period, but as you can imagine that builds up a lot of heat very quickly. Most larger motors do have motor protection equipment that will prevent a motor from being started too soon. It should show you how long until it can be restarted, as well as being able to say how many starts per hour are allowed. Searching the internet will get you a lot of info. For example here is a link to a primer on the subject:

http://baen.tamu.edu/users/stark/AGSM325_files/Lecture/6D%20Motor%20Protection%20Handout.325.pdf

Good Luck!

If you have a need to start and stop a motor quite frequently, may I suggest that, depending upon the HP, torque and the RPM you need, a servo from any of the major manufacturers might be a suitable choice. I'm a Pacific Scientific fan. PacSci is now owned by Danaher. Here is a URL for you to look at.

http://www.danahermotion.com/

Rick...

Is it OK to use a soft-start ckt.? These are availble thru Emerson...

Motors are rated for duty cycles. As per Indian Standards I.S:325 there are various duty cycles such as S1 to S4 which means Light duty, Medium Duty, Heavy duty.......... Contineous duty.Some motors used for Hoists and Cranes are suitable for reversing operations too. Frequent On and Off heats up motors so you have to select motor according to your application. For Pumps you have to normaly select contineous duty motor.There are also FEM European Standards for motors.

Your question is a bit surprising considering that you are an electronics engineer...

The answer to your question has nothing to do with electronics. Haven't you heard the word Inertia? what about acceleration? The electric part deals with in-rush currents and is not exclusive to motors. It will be the same for any inductor. or did you skip this class?

However, serious manufacturers will specify, or supply upon request the maximum recommended number of starts per time units, and this usually be from full stop to full RPM. You can take it from there. Obviously, the bigger the motor the less start-stops you are going to have. (remember?-bigger mass bigger inertia.) Stopping the motor will be easier than starting it again. You can assist stopping with mechanical breaks, or electrically, (DC motors), In both cases heat build up is your main enemy. You did not specify if you have to reverse the motors, (CW to CCW) But if you do there's a whole lot of different rules now. It is an interesting and not so complicated subject. you will enjoy studying it.

The issues are two:

*The high start current heats the motor more than the running current.

Frequent starts might result in overheating.

*The motor will experience high stress if not allowed to stop before restarting it.

if it is possible you can use a clutch for the mechanical transmission of power

AT 19.2 amps and ??? volts?

Lets assume 19.2 Amps and 230V.

Although there is a duty cycle for starting and stopping, the cost of the small motor (19.2 Amps at assumed voltage of 230V) is so small, you can probably beat it up and not worry if you have adequate overload protection (and spare motors) in place.

At one extreme, a high HP AC motor (1000 + HP) may be rated for 3 "across the line" starts per hour and then a 3 hour cool down period must follow. Inrush current for starting high inertia loads can be brutal.

Now, going to another extreme, motors and drives for press feeders and veneer lathes can have extreme starting and stopping requirements in the range of 10 to 100 start/ stop cycles per minute! However, the controls involved and the design of the system is not trivial. Literally weeks of design will go into a system with a 500Hp lathe motor doing 10 start stop cycles per minute running 24/7 production. The RMS of both drive and motor has to be carefully modelled and guarantees made on production rates.

Pump motors can be spinning backward, starting them in this state will draw even more amps and possibly trip overloads, so the control system needs a backspin delay timer to keep them from restarting too soon.

Motor application determines starting procedures, so this should be done with control systems.

Distinguished guest #1, exactly how is it you got to be an electronic engineer without understanding electrical engineering?

<Can I stop a motor and start it again immediately.Is there a rule which specify waiting period before you can start a motor again.>

Yes, though there are limits for larger motors.

A motor coming up to speed consumes more current than when it is running at full speed. That means more heat is dissipated in the motor at this time, and so it will run hotter. The higher current causes the starting equipment and the cables feeding it to run hotter too. Some manufacturers of starters and motors impose constraints so that one errs on the side of caution. "Maximum 5 starts per hour" might be a typical declaration, for the simple reasoning above. Ignore it, and one is in warranty-free, uncharted waters. As to whether ignoring it is worth the risk is a judgement only able to be taken locally.