Sizing Resistors for Reduced Voltage Primary Resistance

Hello Guest,

1. AC or DC?
2. Types of motors (stepper, servo, etc.)?
3. Sizes of motors?
4. Types of motor controls (inverters, H-bridge, etc.)?

Need more info!!!

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Mike

In his post he stated squirrel cage and wound rotor. I believe these to be ac motors.

It is simple. You first decide what percentage voltage you want to apply to the motor during starting. Then calculate the balance voltage to be dropped in the resistor taking this as the IR Drop, the I being the starting current.

To put it simply, let me take a 400V, 100A rated current motor, the starting current being 600A. Now, to reduce the starting current, I like to reduce the voltage applied to the motor to be only 50% of the rated voltage. That is 200V to be dropped in the external resistance.

I x R = 200V; I = 600A; then, R = 200/600 = 0.3333 Ohms. And the wattage rating would be: 200 x 600 = 120kW.

Is a 400v three phase motor draws 90amps what would the voltage across the motor have to be if i want it to start at 30amps??

is it as simple as 400/3?

how would i get my resistor size?

Bear in mind that, under some circumstances, a series resistance on a parallel-wound motor may be sufficient for to take a long time to come up to speed, or even to not start at all. In this case either of the circuit protective device(s) or the motor protective device must operate before the motor and its supply cables become a convection heater. This may be achieved by:

• selecting the circuit protective device(s) correctly so as to protect the cabling, and
• the motor protective device to the full load current rating of the motor.

This [resistance starting] method is used with slip ring motors where the rotor poles can be accessed byway of the slip rings. Using brushes, variable power resistors are connected in series with the poles. During start-up the resistance is large and then reduced to zero at full speed.

At start-up the resistance results in the stator's field strength being weakened less. As a result, the inrush current is reduced. Another important advantage is higher start-up torque. As well, the resistors generate a phase shift in the field resulting in the magnetic force acting on the rotor having a favorable angle^{[citation needed]}.

http://en.wikipedia.org/wiki/Induction_motor from Wikipedia (yes, I know) I had never heard of resistance starting, only having experience of much lower-power motors, but you state you are applying them to squirrel cage and not wound rotor. Is there some confusion here?

I don't believe he is referring to a Wound Rotor Induction Motor, but rather a Primary Resistor starting method for a standard Squirrel Cage Induction Motor. It used to be a common method before the advent of Solid State starters, hardly anyone uses them any longer.

About the only place I still see them used is in older Crane Controls and Fire Pump controllers. You can start here for resources on how to select and size them.

The use of "ballast resistors" is considered old technology although we still see them being used in overhead cranes for the trolley & bridge motions. Little by little, they are being replaced with solid-state soft-starts. Power Electronics (www.peinfo.com) & Benshaw (http://benshaw.cwfc.com/) make excellent products.

As an instructor,why not apply commom starting methods(DOL,STAR-DELTA) which still exist in most industries today,why u later take time to get your resistance starting formular,or even auto transformer starting methods?

There are no direct formular as far as i know.Just apply your usual ohms law and do some good calculations with the datas on the motors,just as electricalexpert is doing,then u may get your result.

Patrick Whowha

What u need is stator starters which comprise of (a)active resistors (b)star point starter (c) with chokes.

Taking active resistors for example,u will need to connect the resistors in series with the asynchronous motor in your lab.Note here that the starting voltage of the stator winding will reduce in direct proportion to starting current.

On the data plate of the motor,record the voltage(V) rating and current(I) rating.Take for example V=400V, I=5A. Motor could be say 2.2kw power rating, R=Resistance.

Apply V=IR, R=V/I =400/5 =80Ω. U will have to connect 80Ω x 3 Resistors in series with the motor if its a three phase motor with appropriate power rating of the motor so as not to burn the resistors.

Either use a contactor or manual 3pole switch for the line supply to motor and also for the three resistors to the motor.

Switch ON the resistors first in circuit,then switch ON supply through the Line contactor or manual switch,to reduce high starting current,then next,switch OFF the resistors through the contactor or manual switch connected across the resistors,leaving the motor to run with only the line contactor or manual switch and the line protection fuses or breakers.

U may connect ammeters on the lines to check the current or use clamp-on ammeter to check the current and your students will appreciate how motors could be started or controlled.Note,if u are connecting motor in star,use a star-point starter.The resistors in this case are connected to the W2,U2,V2 of the motor and having their ends bridged(STAR),while supply is fed to the U1,V1 and W1.

Note that using starters with active resistors have a drawback of causing energy losses.With chokes losses are lower,but power factor cosφ is poorer.With Stator starters,starting TORQUE is greatly reduced along with starting current.

Patrick Whowha

Good for you. We need to be teaching this. Many of these systems are still in use and we need our wireman to be able to service them. Still one of the smoothest starting methods available, hence there use in cranes. We probably don't need to know how to engineer them, just repair them and therefore understand how they work. This website has some good info: http://www.scribd.com/doc/17252612/Motor-Starter