S7 PLC and VFD 1

[krsh]

Dear sir,
I want to design a system whitch is using a S7 315-2DP CPU, MICROMASTER 440 and a Mobile Panel 170 to control the speed of a 22 KW motor which has a nominal speed of 1500rpm and is connected on a speed reducer with an output of 53rpm but i have some questions:

1.If i increase the freq. to 60hz how will the 4000 Nm will be affected?
2.What size should i use for the termomagnetical protection of the motor?
3.The length between the VFD and the motor is approx. 20 m should i use a normal 5 wire cable or a special one?
4.What software to use for parameterization of the VFD (which one is easier).
5.Before the use of the VFD i measure the load on the electric motor by using an clampmeter, will this still be effective?

 

[2b2]

1)If the system was designed for 60Hz you should not have problems with it (I had use VFDs without any issues) in case of torque you can tweak it based in the application.
2)The micromaster has a parameter to set the thermal protection, personally I just depend on this setting to input the value indicated by the motor manufacturer following NEC recommendations.
3)I normally use shielded cable. (if is a 3 phase system you will need four wires 3 for each phase and one for ground,I do not understand what is the fifth wire for)
4)I would identify the parameters to be set and program the drive from the keypad (is rather quicker for me)
5)Use a True RMS clampmeter, however, you can read the current from the keypad of the VFD.

 

[krsh]

In our contry nominal freq is not 60 Hz is 50 Hz and the electric motor is a 3 phase induction motor. The point is that the torque value of 4000 Nm is reached when the motor is runing at 50 hz but what about when i turn it up to 60 Hz what happens with the torque? ... how about the current?

 

[djs]

The torque the motor supplies is determined by the load. What torque will your load require at 53 rpm * (60/50) = 63.6 rpm? If it exceeds the torque rating of the motor, then you will need a larger motor, or limit the application to 50 hz.

 

[CanadianTaff]

Just to make a point about number 2.

You would not size any overload protection for the motor, the overload should be prior to the VFD and it will be to protect the VFD, therefore should be sized for the VFD.

As mentioned by 2B2, the VFD will protect the motor.

You would only need motor protection between the VFD and the motor of you are running multiple motors.

 

[krsh]

djs i do not know exactly what torque is needed for the load but i know that when i am checking with the clampmeter when the motor is on full load it shows approx 30 A per phase.

 

[jraef]

Torque in a motor is determined by the motor design voltage and the applied frequency. The motor output torque capability will remain the same when using a VFD as long as the original V/Hz ratio is maintained. So if your motor was originally designed for let's say 380V 50Hz, the V/Hz ratio is 7.6:1. If you apply 60Hz to it at the same 380V, then the V/Hz ratio will diminish to 6.33:1, or 83% of the design torque. So if you had 4000Nm at 50Hz, you would have 3333.33Nm at 60Hz.

One trick to consider is that you can boost your line voltage to compensate. For example in this case if you boost your line voltage to 460V and program your VFD to put out 460V at 60Hz, then the V/hz ratio is 7.6:1 again and you will be back at the original torque capability of the motor.

But be careful. As others alluded to, the torque REQUIREMENTS of the load may change as well. In centrifugal pumps and fans, the power requirements change as the cube of the speed. So at 60Hz you are at 117% of design speed and if your load is a centrifugal pump or fan, the load POWER requirement will go up to 160% of what it had been at 50Hz. Even with the voltage boost trick increasing the motor output power, you may still find yourself in a severe deficit unless your motor was over sized to begin with.

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[ozmosis]

krsh
1.If i increase the freq. to 60hz how will the 4000 Nm will be affected?
as jreaf points out, you need to advise more about the application as it is this that will determine the torque charateristics when overspeeding or dropping in speed. Maybe it is better to look at the torque at the motor than the gearbox output at this stage. The 4000Nm mentioned sounds like you have simply taken the 22kW motor and, based on 1500rpm calculated the torque produced and multiplied by the ratio of the gearbox and assumed 100% efficiency.
2.What size should i use for the termomagnetical protection of the motor?
The MM440 will have IsqT calculations to protect the motor for thermal protection assuming you know the method of cooling on the motor. The default is self cooled with the MM440.
3.The length between the VFD and the motor is approx. 20 m should i use a normal 5 wire cable or a special one?
Depending where you are in the world, you will have certain limitations on the motor cable to ensure compliance with EMC limits. Usually 4 wire with screen is advised.
4.What software to use for parameterization of the VFD (which one is easier).
if using the S7 to communicate direct with the MM440 then you could use the USS protocol direct from the PLC port to the MM440 and the TD170 will also talk on this too. Alternatively, use Profibus but you will need an option module for the MM440 to allow this.
5.Before the use of the VFD i measure the load on the electric motor by using an clampmeter, will this still be effective?
the MM440 measure current too, so you could always use this.

 

[krsh]

Ozmosis, the torque is not calculated is what the manufacturer says in the tehnical manual which is 3960 to be more exact. The application is about an loading elevator which loads bulk chemical fertilizer from a conveyor belt into a railcar. The idea is to increase the speed of the elevator to load faster if the fertelizer density is lower and to decrease the speed to gain a higher torque if the density of the fertilizer is high.(fertilizer is not standard it comes in different chemical formulas). Until now we used a flow restriction on the conveyor not to overload the elevator electric motor.
About the connection between the PLC-MP-VFD i am thinking of using Profibus-DP but i have some choices to make about the software to use on the VFD from : Starter, Drive ES Basic, SIZER etc. and i whant to choose the easyer one.