I do electro hydraulic design and troubleshooting, also performing technical services on EH systems.

I'm working on one where I seriously question the design; both measurements and calculations show the motor cycling on to maintain pressure for under 0.1 seconds. This does not normally occur often, but occasional hydraulic problems have this cycling occurring about every 3 seconds. How do you spell warm?

(PLEASE DON'T SHOOT THE MESSENGER!)

I am trying to gain some general feel for the time to start a "typical" small squirrel cage motor across-the-line and to determine if the user without the motor manufacturer's inertia data can calculate that (I'd probably ASSUME starting power of 4* running as an approximation over the acceleration time, initially perhaps 7* dropping as speed increases.) But to add to the "less smart design", it starts for this "plug" at full shaft torque in addition to the inertial torque.

This motor, FWIW, is a 112M-B5 6.6kW 4P 480/3/60. My personal feeling is that it will probably reach 95% speed in under 0.5 sec ... supply is satisfactorily stiff.

There are a lot of things I would have done differently, but let's not go into those; I don't have redesign or replacement options, just explanation of what is happening so a manager can make an informed decision.

I've worked with medium frame motors where 10 seconds would not have been unreasonable ... limit of 3 starts then cool for an hour ... this isn't that type of application.

Any guesses? Any answers?

Thanks! George, South Carolina USA

Hello GWBynum

You do need to know what the total moment of inertia is, i.e. motor, coupling and driven load, and what the speed torque curve of the motor is before you can make any estimation.
The problem is that motors vary tremendously and motors that seem much the same can have starting characteristics that vary by a factor of greater than two to one.

Best regards,

Marke, I understand that there may be a factor of 2 (or more) between different motors of "similar" constructions ... I'm well familiar with NEMA design B and C which likely have that much difference ... but I'll be quite happy with a "typical" for a motor that size ... is it 10ms, 100ms, 1 sec ... or longer? I know I can take inertial optimized servo motors and go from 100% CW to 100% ACW with some motors in 10ms. I know that starting a particular 800 HP 900 rpm fan motor takes 12-15 seconds ... just looking at generalities.

If push comes to shove, I can connect a CT and monitor current ... that will give me a +/- 20% number ...

Thanks!