Slip ring V/s Squirrel cag

[maypot]

Hi,

Basically what are the differences between a 300 k W slip ring motor and a 300 k W squirrel cage motor ?
I mean that the slip ring motor is available and a new VSD is meant to be installed.
Wll the maximum torque be the same for the two cases ?
I understand that by shoring the rotor, the locked rotor current will be very high and a relatively low torque available ? Will the VSD provide this shortcoming ?

Bob

 

[laundry]

Hi maypot
A cage motor has a starting torque of 130% full load torque as the motor accelerates,the torque increses to its maximum pul out torque of 200/250%f.l.t.at 75% synchronouse speed.
A slip ring motor has a much larger starting torque and is much better on large loads.but you can get cage motors that will deliver high starting toques.slip ring motors have starting touques in the regen of +300%.
so a standard 300kw cage does nor have the same torque as a 300kw slip ring.

 

[Marke]

Hello maypot

If you are going to connect the motor on to a VSD, then the differences between the starting conditions of a slip ring and a standard cage motor are irrelevent as you never operate the motor under high slip conditions on a VSD.

When you start the motor with a VSD, you begin at oHz and increase the frequency of the VSD and the motor follows. It is always operating under low slip conditions. If it is not, you have major problems.

You can use a slip ring motor on the output of a vsd with the rotor shorted and it will behave normally. It may set up differently if you use a vector drive as the rotor characteristics will be a little different from a standard motor but the drive should be able to cope with that.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

Hello Maypot.

If you are connecting the motor trough a VFD you can use any of those motors Types.
The wound rotor with the secondary or rotor winding short-circuited will provide higher breakdown torque and better efficiency when operated at full load.
As Marke said, during the starting step, the motor operates after the break down torque zone developing high torque with low current as controlled by the variable frequency driver starting ramp. Up to breakdown torque, it will develop torque almost proportional to the stating current. This means that full load torque will be developed with a current close to the nominal or nameplate full load current, increase or decrease of such current will affect the torque in the same rate. Most probably you will need to get the equivalent circuit parameters to optimize the driver performance.

 

[maypot]

Thanks all.
I have got another query, in the case of an electric motor, does it matter for a VSD if the motor is connected in star or dlta ?

Bob

 

[laundry]

Hi maypot.
Yes depends on motor voltage,But in your case I would think that you should connect in delta.As I would gues your motor is 400v+ and not 220v.

Barry.

 

[fsmyth]

Just got a 20hp Baldor -
286TZ frame, 1425/1675 rpm, spec: 10P46W694,
stk no.: 39H314BAH, 208/240/60 and 220/380/50.
Talked to manufacturer - seems that this was
OEM'd to a mixer company, IIRC. Other than
that, no other specs.

I was hoping that someone had run across this
particular type, and could give me some pointers.
All of the documentation I can find refers to
3-phase rotor and stator windings - note that this
one has only 2 slip-rings, hence only one rotor
winding (more like a 3-phase alternator).
From what I have found, starting a wound-rotor
motor with the armature shorted has a 1000-1400%
starting current (as opposed to 500-800% on a
"standard" squirrel-cage). Is this also true
for this (single-winding) type?
What I need is practical application info, such as
how much resistance to use (near as I can tell so
far, about 3x the rotor resistance), when to switch
it out ( again, NAICT, at over 80% speed), and what
the effects of more slip have on operation.

It appears to my small mind that this could be the
equivalent of delta-Y start, without the massive
switching transient, and (because of more slip)
be more responsive to load variations when used as
an RPC idler.

I can probably interpolate values from a 3-phase
rotor type, if I could just find them. I realize
that this is a small motor for the type - most of
the literature I have found refers to 500HP+, but
any hints are welcome.
<als>

 

[electricpete]

I have seen a number of wound rotor motors in crane applications that have had the rotor fail for a variety of reasons, not entirely understood. Maybe if the cause of failure lies in resistor bank or the application (operating at high slip) these failures would not have occurred on vfd drive.

At any rate, all other things being equal, I would rather have a squirrel cage induction motor from reliability standpoint. Failure rate on the rotors of squirrel-cage motors is much much lower.

Again this is based on my own experience not knowing exactly what causes the failures.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[aolalde]

maypot:

As far as the wye or delta connection matches the VFD output base voltage and frequency, It does not matter.

For instance a dual voltage motor designed to be connected Delta at 220 Volts 50 HZ and Wye connected for operation at 380 Volts, 50 HZ, it has to be connected according with the VFD base operating parameters, say 220 Volts, 50 HZ.
The connection must be delta.
Now a dual voltage motor designed to be connected 2 circuits Wye for 220 Volts and 1 circuit Wye for operation at 440 volts, 50 HZ; it has to be connected to the very same VFD with base 220 Volts, 50 HZ.
Now the connection must be 2 Wye.

 

[Marke]

Maypot

A VSD can control star connected or delta connected motors, there is no problem. The only isue is the flux in the stator of the motor. To keep the flux within limits, you must copnnect the motor as you would if the VSD was not used. i.e. if on your supply, you would connect the motor in delta, then connect the motor in delta on the output of the VSD.
There should be an indication on the motor nameplate for the connection for your supply voltage.

Best regards,

Mark Empson

http://www.lmphotonics.com