I'd like to thank all founders of this forum because it's really very useful.

I have a motor drived by a soft starter and I want to make sure that the motor is suitable for the application. Kindly consider the following specifications for the motor, fan, soft starter.



1- Specifications of the fan are:



Induced fan,

Type: Y5- 48N016.5D

Volume: 85000 m3/ h

Full pressure: 2000 Pa

Fan rotational speed: 960 r.p.m

Motor power: 75 KW

Medium temperature: 270 ºC

Medium <A name=OLE_LINK1>titer: 0.648 Kg/ m3



2- Specifications of the motor are:



Type: 315S-6

Power: 75 KW

Voltage: 380 V

Current: 142.4 A

Frequency: 50 HZ

Efficiency: 92.8

Speed: 990 r.p.m

Insulation class: B

T_st / T_N : 1.6

I_st / I_n : 6.5

T_max / T_n : 2



3- Specifications of the soft starter are:



Power: 132 KW

Current: 250 A





We tried to start the motor by the soft starter many times by different parameters setting but each time the soft starter trips during the starting period and generate alarm (thermal motor model overload), some of the settings used were:





1- Rated current: 142 A,

Starting mode: voltage ramp+ current limit,

Starting voltage: 40%,

Starting time: 30 sec.

Current limit: 400 %

Tripping class: 10



2- Rated current: 142 A,

Starting mode: voltage ramp+ current limit,

Starting voltage: 40%,

Starting time: 25 sec.

Current limit: 450 %

Tripping class: 20



3- Rated current: 142 A,

Starting mode: voltage ramp+ current limit,

Starting voltage: 40%,

Starting time: 30 sec.

Current limit: 500 %

Tripping class: 20



The motor started only after changing the tripping class parameter to trip class 30 and the motor run successfully and drew a current of 80 A. the starting time has also to be parameterized to at least 30 sec. to start the motor.



Now we adjusted the soft starter for the following parameters:

Rated current: 142 A,

Starting mode: voltage ramp+ current limit,

Starting voltage: 40%,

Starting time: 30 sec.

Current limit: 450 %

Tripping class: 30





We are interested in the starting time and tripping class 30, Are these settings suitable for the motor or it is required to replace the motor by another one can withstand this heavy starting load.

Hello msayed

Welcome to the forum.

Fans tend to have quite a high inertia so take along time to start.
It is important to ensure that the motor is rated for the inertia of the driven load.
Motors are usually rated for either maximum load inertia or maximum locked rotor time.
Provided that the motor is rated higher than the inertia of the load, you should not have a problem.

When you start a high inertia load, you have high slip losses in the rotor causing a high rotor temperature.

The protection class of the starter must be set lower than the maximum withstand of the motor. If you have a motor that can withstand locked rotor current for around ten seconds, then you must not set the protection for a longer time as you will damage or compromise the motor.
In order to start a high inertia load, you need an extended start time, so you need an extended trip class. - ensure that the motor is protected at the higher trip class.

In order to develop sufficient torque to accelerate the motor to full speed, you will need a moderately high start current.
In would recommend a short ramp and a current limit so that the starter limits the current for most of the start time. This will maximise the acceleration rate and shorten the start time.
I would suggest a voltage ramp of around 5 seconds and a current limit of around 450%.
Reducing the current limit will extend the start time. If you reduce the start curent too much, the motor will not easily accelerate to full speed.

Best regards,
Mark.

Hello Mr. mark



Thanks a lot for your reply.



You suggest to set the voltage ramp at 5 sec. do you mean the starting time? We use the voltage ramp + current limit mode. I am sorry to ask u are u sure this time is enough to accelerate the motor and reach the full voltage.



Another question please, how I know the maximum locked rotor time of a motor?

Our motor is Chinese and I searched on the web and found only the data I posted before. How I know that a motor can be used for heavy starting loads and its protector can be adjusted at tripping class 30 with no fear for the rotor?



Best regards,

Msayed.

Hello Msayed

With most starters where there is a voltage ramp plus a current limit, when you set a fast ramp time, the voltage will rise quickly to the point where the current limit is reached. At that time, the voltage ramp is frozen until the current begins to drop, so if you set a voltage ramp time of 5 seconds, that does not mean that the voltage will reach full voltage in 5 seconds unless the current limit is not reached.
With an inertial load, you need to get as much energy transferred to the motor as possible without exceeding the current limits of the supply or the starter. I would suggest that the starter should be on the current limit for most of the start time in order to minimize the start time.

In regards to the maximum load inertia rating for your motor, the only way that you can determine this is from the manufacturer. It is not possible to calculate this from current rating.

Best regards,
Mark.

Hello Mr. mark



Thanks a billion; your reply was really a good advice.

I set the new parameters, the motor started in 32 sec. (it was taking 42 sec When setting old parameters).



I also set the new parameters to crusher motor; the starting time was 10 sec. shorter than the old setting. And the motor started to move from standstill 2 sec. after the starting signal although it was taking about 15 sec. at the old setting.







Best regards,

Msayed.