Reduce Starting Load Current

Depends on how much mechanical load has to be brought up to speed.

If its mainly just the motor and gearbox getting up to speed then you probably wont see any change in the starting amp load.

Electric motors by nature take a high inrush of power to get moving and the only two easy options you have are using a VFD unit to control the starting power and ramp up time or a soft start unit to limit the inrush amp load.

Starting current doesn't vary much with load. It would be same even if the motor were bare shaft. But increasing load means the current falls more slowly. So your starting current might last a bit shorter time with 100:1 gearbox, but it wouldn't be easy to tell how much, if you're just using an ordinary ammeter.

Where can one find a graph showing variation of starting current with load?.

I suppose you mean showing variation of current with time at different connected loads. As I indicated in #2, the current at time zero is same irrespective of load. But sorry, I don't know, and to echo Wal, why do you want it?

Also it depends on the type of load. For a given power at full speed, the power at intermediate speeds during acceleration is different for a squared-torque load (eg centrifugal fan or pump) and a constant-torque load (eg PD pump, Roots blower or winch). This affects the evolution of the current drawn.

If you study the speed-torque curve of the motor, and the load characteristics - power, moment of inertia etc you might be able to arrive at an estimate.

It doesn't exist:

• The shape of the current/time curve varies according to the inertia of the load attached to the motor.
• The starting current is the same regardless of the load attached to the motor.

Let's start here instead. Why are you concerned with starting current on something like a hoist?

is that not normal if you dont want high starting current you may design one with a dc motor powered by a large battery bank, wheras your incoming ac would be used only for charging

Which planet are you on?

if it will be useful for reducing the starting load current

NO.

Consider a soft starter. eg star/delta or VFD.

Why is the start current an issue for you? How is it causing you grief?

What is the Slip of the motor. What comes to my mind is that You have to examine the possibility of increasing the Slip.

Pl. post the nature of duty to be performed, so that we can understand and CR4 MEMBERS can suggest some way out.

DHAYANANDHAN.S

Change your starter to vfd or soft starter

Never NEVER put a Soft Starter on a hoist motor!

NEVER...

And if it is a VFD, it takes VERY SPECIFIC knowledge on how to program what is called a Torque Proving circuit on a closed loop vector drive, meaning one that used an encoder feedback as opposed to an open loop sensorless vector. If you don't know what I just said there, DO NOT ATTEMP IT!!!

And back to the Soft Starter, did I say NEVER?

So I come back to this... why are you concerned?

I was mulling over the hoist application too...OP just said he was looking for a way to reduce start current....he has that now I guess, whether it's prudent or not is another matter.

It would be nice to know what OP's real objective is.

Elevator hoist motors start '"softly" don't they?

You say Never NEVER put a Soft Starter on a hoist motor! I assume you have a good reason for saying that, but can you please explain why not? As a non-electrical engineer I'm curious.

It seems that nobody bothered to tell these guys it shouldn't be done.

A possible solution?

Interesting. They only refer to fairly small powers, if that's significant.

That's just the first one I came across.

I reckon the concept is totally scalable.

"You say Never NEVER put a Soft Starter on a hoist motor! I assume you have a good reason for saying that, but can you please explain why not? As a non-electrical engineer I'm curious."

This is now a tangent of this thread, but the OP seems to have been scared off, so I guess it's OK.

2 main reasons:

1. The main issue is the failure mode of a soft starter. When you have a hoist, you always must have a brake motor. If you end up with shorted SCRs in a soft starter, you can have power flow through the motor circuit without control even if you think the motor controller is "Off". If your brake is being powered by the line, as most are, then you can have the situation where you have turned off the motor, but the brake coil is still getting power through the shorted SCRs, and the load free falls.
2. When you have a separately controlled brake to avoid having that happen then(such as an air powered or even a separate brake coil circuit), you must have full torque ALREADY ON the shaft BEFORE you release the brake. But with a soft starter, you are REDUCING the torque that the motor puts out. So if you release the brake too soon, you will not have enough torque to prevent the load from falling, yet if keep the shaft locked until the soft starter has completed ramping BEFORE you release the brake, what was the purpose of having the soft starter to begin with?

Mosy elevators that use soft starters use them on the hydraulic motors, and the hydraulic system has fail safes built into it. There are soft starters for traction elevators as well, but they have very specifically designed reundant safety systems that were intended to have soft starters from the very beginning, not retrofitted into another design.

Hoist drums can't free spool. The gear drive (worm) prevents this absolutely.

The brake stops inertial overrun to permit inching.

It doesnt have to be that High-tech :). VFD with the right dynamic braking resistors and setup, coupled with a mechanical pneumatic brake will do it. I have done several this way.

Dear visaseller,

I agree with Mr.Raef. - See Post No11.

The VFD should NOT BE USED - the starting torque, for VFD will be comparitively lesser than other system, because of its inherrent VFD's character. What is needed is more Torque, and Increasing SLIP of the Motor will help, as the Torque is proportional to s/R.

VFD application needs BEARING INSULATION or INSULATED BEARING, and conventional motor SHOULD NOT BE USED. Soft Starter, in my opinion, is not the option.

THANKS,

DHAYANANDHAN.S

With a 100:1 gearbox, the required power would decrease to about 10 KW; the starting current for a 10-KW motor would be about 100 A.

The starting current for a 10kW motor would be about 100A, if you had one wound for 10kW, but nearest standard output is 11kW.

I don't think he's considering changing the motor, just the gearbox. In that case the starting current is unchanged, but decays to the running current more quickly.

As often happens, nothing more has been heard from the OP, despite several requests for clarification.