In rush current for 3-phase motor ???

Hej Everyone,

I don't have a lot of experience with this, but I believe
many in this forum do. Here's the question:

How many amperes "in-rush" current should I
expect from a 3-phase 5.5 kW motor ? What
amperage fuses should be good?

I have set in 20 amp fuses, and can run the motor
at half-speed ... but the fuses blow when I
switch the lathe to full-speed. It draws only
7 amps in steady-state at half-speed...but
the start-up transient is quite big.

I am hiring an electrician to beef up the wiring.
The question is, should 25 amps be good, or
should I pay the many extra bucks and go
straight to 32+ amps?

Thanks for any pointers received!

Left Hand Thread

Your motor is probably about 25A full load current. Fuses for such a motor here would be time-delay type, rated at something over 25A.

What do your electrical rules say? Those rules will be what you must follow, not what we say.

Fuses must not be larger than the wires can handle. And in the US, it would be required to use a "motor starter", which includes a time-sensitive overload device to open the circuit if the motor draws excess current.

JST,

Thanks for your reply. Yes, if I choose the 32 amp option, that
means bigger wire (and bigger expense). I am guessing your
estimate of 25 amps is about right ... wonder if I could find
any "even slower blowing fuses". It's not the steady-state
that's causes problems, it's the in-rush... I just need to buy
myself a few extra milliseconds!

mvh,

James (Left Hand Thread)

Motor in-rush current

Motor in rush is usually about 8 x Full load current. For a 5.5 Kw motor Full load current is about 12 amps at ( 415 volts in Australia) when started Direct on line.

I usually try to use a Motor protection circuit breaker and set for the full load current ,they have an adjustable setting pot on the face and an on/off switch. I have used lots of Telemecanique units have been running for over 25 years with very few replacements. These are units will handle the in-rush current. Buy one that puts your full load current in the middle of the setting, then you have a bit of room to move if problems occur.

You could of course buy a Variable speed Drive which are quite cheap these days and gives a lot of parameters such as ramp up, ramp down , torque settings, the manual is usually very comprehensive.

Hope this helps

Being that you are in Sweden, any suggestions made by most of the people here are going to be made based on US codes, which may not be the same as yours.

But in general, your fuse selection is based on the specifics of your installation, you have to look at the motor nameplate Full Load Amps at the voltage you are using, not the kW rating. You must also use fuses specifically designed for motor starting duty. Conductor sizes however are often based on kW (HP) ratings because of the possibility that a replacement motor may not have the exact same FLA as the one initially used, so the conductors are sized for the worst case scenario.

It also depends on the inertia you are spinning up. I had an air compressor that used a 20 hp motor, which actually ran at about 14 hp at maximum air pressure, but it had a huge flywheel and running on 240V would blow anything less than a 200 amp slow blow fuse. You might consider going to a circuit breaker if your electrical code allows it. They come in a wide variety of overload characteristics ranging from opening instantly at the rated current to tolerating multiples of the current for long intervals. A three phase motor can easily draw five times its rated running current at startup. The motor in question accelerated so fast by itself that I could not see a transition. I would hear the starter close and it would be running at full speed. With the belts on the flywheel, it took a long time, making the lights blink as it ran up.

Bill

"How many amperes 'in-rush' current should I expect from a 3-phase 5.5 kW motor ? What amperage fuses should be good?"

For NEMA-type motors, the "code" stamped on the motor's nameplate data relates to a table within the NEC which gives the factor.

Common motors are about 7 * FLA, for a short duration, thereafter settling down to FLA or less, depending, of course, on the load.

If you use the recommended time-delay fuses, you should have plenty of time for the motor to settle down to its FLA or less, after which the fuse will provide running protection.

However ...

Good machine design is to use a magnetic motor starter for motor running protection plus a fusible safety switch for operator protection (this assumes the safety switch accommodates a "lock-out/tag-out" function).

Then, there is the motor feeder and its protection which is usually a circuit breaker in the premises panelboard.

The NEC provides a worst-case example, where everything, everywhere has a protective device, and a specification to back it up.