Motor Starting Study

What is Motor Starting Study and Analysis?

The definition of electric motor starting study and analysis is an activity to determine and analyze the behavior of a motor during its starting stage. This study is usually applied to large motors. Motor starting analysis includes evaluation of the starting current, voltage drop, voltage dip, and required start time. Motor starting studies are carried out in large commercial and industrial systems where a large capacity motor can have undesirable consequences on the performance of the motor, system and surrounding equipment — Ohm Engineering Works is a consultant who serves motor starting studies consulting services and analysis. If you are looking for motor starting study consultants company for your project or power systems facilities in India     and South East Asia Contact Ohm Engineering Works, with experiences certified power system engineers, by sending an email to : support@ohmengineeringworks.com.We conduct motor starting study and analysis mostly using ETAP software.

The capacity of the electric motor used in modern industrial systems is getting bigger and bigger. A motor is categorized as “large” by comparing the motor capacity (kW) to the total installed capacity of the power source in a system. Starting a large motor can cause severe disruption to the motor and locally connected loads, as well as to buses which are electrically away from the motor starting point. Incorrect motor-starting method can cause damage to the motor, power quality problems (such as operational breakdown) or even black-out. Ideally, the study and analysis of starting motors should be carried out before the purchase of large motor is made.

Basic Knowledge

An electric motor is an electro mechanical device that converts electrical energy into mechanical energy. Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo; some electric motors can also be used as generators; for example, a traction motor on a vehicle may perform both tasks. Electric motors and generators are commonly referred to as electric machines.

AC and DC Machines

AC Machines

• Induction motor

• Synchronous motor

Two classes of ac motors are recognized—induction (asynchronous) motor and synchronous motor. An asynchronous or induction motor requires slip—relative movement between the magnetic field generated by the stator and a winding set (the rotor) to induce current in the rotor by mutual inductance. The most ubiquitous example of asynchronous motors is the common ac induction motor which must slip to generate torque.

In the synchronous types, induction (or slip) is not a requisite for magnetic field or current production. In synchronous machines, rotor-winding currents are supplied directly from the stationary frame through a rotating contact or induced via a brushless excitation mechanism.

DC Machines

A direct-current machine is a machine consisting of a rotating armature winding connected to a commutator and stationary magnetic poles that are excited from a direct-current source or permanent magnets. Direct- current motors are of four general types: shunt-wound, permanent-magnet, series-wound, and compound-wound.

• DC shunt motor

• DC permanent magnet (PM) motor

• DC series motor

• DC compound motor

Motor Standards

The motor standards can be grouped into two major categories: NEMA and IEC (and its derivatives). In North America, the National Electric Manufacturers Association (NEMA) sets motor standards, including what should go on the nameplate (NEMA MG1). In other parts of the world, the International Electrotechnical Commission (IEC) sets the standards, or at least many countries base their standards very closely on the IEC standards. For example, Germany’s VDE 0530 standard and Great Britain’s BS 2613 standard are close to IEC with minor exceptions. Note that the major IEC standard for motor is IEC 60034 series.

NEMA MG1 [B37] specifies that every motor nameplate must show these specific items:

• Manufacturer’s type

• Rated volts and full-load amps

• Rated frequency and number of phases

• Rated full-load speed

• Rated temperature rise or the insulation system class

• Time rating

• Rated horsepower

• Locked-rotor indicating code letter

• Service factor

• Efficiency

• Frame size

Motor-Starting Methods

• Direct on-line (DOL)

• Series impedance

• Shunt capacitor

• Reactor/choke

• Reactor–capacitor

• Partial winding

• Wye/delta (Y-∆)

• Captive transformer

• Auto-transformer

• Electronic soft-starters

• Variable frequency drive/adjustable speed drive

• Voltage and frequency variation

Parameters Useful for Motor-Starting Evaluation

Some of the parameters useful for the motor-starting evaluation are:

• Bus voltage (Vbus)

• Motor terminal voltage (Vterminal)

• Motor input current

• Real power and reactive power

• Power factor

• Motor speed

• Motor torque

• Load torque

• Accelerating torque

Why We Need to Conduct Motor Starting Study?

At least there are 5 (fives) reasons why we need to conduct motor starting analysis. The reason are problems revealed existed, voltage dips, weak source generation, there is special torque requirements, and type of starting method.

Problems Revealed

Motor starting study is required if:

• Motor rating exceeds 30% of the kVA rating of the transformer (if there is no generator).

• Starting a large motor will cause interference with locally connected motors, systems and loads as well as the buses connected to it.

• Rating or motor capacity exceeds 10-15% of the kVA rating of the generator (if the system is supplied only by the generator).

• Many motors start simultaneously.

Voltage Dips

• The motor load torque is directly proportional to the square of the motor terminal voltage, so any voltage variations will directly affect the torque characteristics of the motor load on the motor (T∞ V2).

• During starting, the voltage at the motor terminals shall be maintained at least 80% of the rated voltage or more for NEMA standard design B motors.

• If the voltage drop caused by the motor starting is interrupted, the load on the running engine can exceed the damaged torque and can slow down significantly or even experience a crash condition.

• Voltage dips also affect other types of loads such as electronic devices, sensitive control devices, lighting loads, etc

Weak Source Generation

• Motor starting studies are useful for analyzing the performance of small systems combined with generators.

• Smaller power systems are usually served by limited capacity, which generally exacerbates the voltage drop problem when starting a large capacity motor.

Special Torque Requirements

• Any special load must be accelerated by careful and precise control without exceeding the torque limits set on the equipment.

Determination of Starting Method

• The study can be used to select a motor or motor starting method or both.

• Detailed studies are used to determine the size of the starting resistor in motor rotor windings.

Objectives of Motor Starting Analysis?

A motor-starting study is performed to determine the voltages, currents, and starting times involved when starting large motors or a group of motors, either sequentially or simultaneously. Motor-starting studies are carried out to help ensure that:

• Motor(s) will start with appropriate/acceptable voltage drop

• Voltage drop at time of start will not disrupt other loads

• Motor feeder(s) are sized adequately

• Motor(s) will accelerate within acceptable start-up times

• An accurate evaluation of motor/load speed–torque characteristics and accelerating time is made

• An accurate evaluation of thermal damage characteristics of motors is made

• The motor will not experience nuisance tripping on the start

• In the event of direct on line (DOL) start is not possible, that the type and size of starter/drive required to start the motor is known

• Motor protective devices are sized/set properly

Electric motor starting studies can assist in the selection of the right motor design, to determine the best method to start the motor with minimum impact to the rest of your distribution system, and to reduce voltage flicker and voltage drop problems.

How To Conduct Motor Starting Analysis?

Standard Reference

Most common standard that use to conduct motor starting studies and analysis either by individual or consultant is IEEE 3002.7-2018: IEEE Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems.

General Methodology

• Data collection and verification

• Modelling

• Parameter and Model verification and validation

• Simulation

• Static

• Dynamic

• Analysis and Recommendation

• Reporting

Calculation Methods

There 2 (two) calculation method in motor starting analysis. They are:

• Mathematical relationships and hand calculations

• Software-based Calculations

Mathematical Relationships and Manual calculations

Short-circuit (impedance) method

This method involves reduction of the system to a simple voltage divider network (see Manning [B31]), as shown in figure below, where voltage at any point (bus) in a circuit is found by taking known voltage (source bus) multiplied by the ratio of impedance to the point in question over total circuit impedance.

where

• E is source voltage

• V is motor terminal voltage

• Z1 is system impedance

• Z2 is motor internal impedance

Current method

In general, in order to calculate any bus voltage in the system represented in Figure 12, the basic equations for the current method are as follows:

where

• Zpu is the total impedance between source bus and the load bus (p.u.)

• Vdrop is the voltage drop across the impedance (p.u.)

• Vbus is the voltage at a specific bus (p.u.)

Load flow method

The bus voltage and voltage drop can be determined with a conventional load flow program. This is true, by modeling the starting motor as a constant impedance load, and consequently, the load flow calculation produces the bus voltage during start.

Software-based Calculations and Simulation

There are two kinds of calculations and simulations: the static motor starting model and the dynamic motor starting model.

Static Motor Starting

It is assumed that starting the motor can always be performed and that the duration of the motor starting is given. During the starting period, the motor is represented by locked-rotor impedance, which draws the maximum current from the system and has the most severe impact on the system. After the starting period has been elapsed, the motor starting is changed to a constant kVA load.

The static motor starting method is the recommended approach in one of the following conditions:

• New system conceptual design

• Motor and connected load dynamics are unavailable and cannot be estimated

• Acceleration times of the motors are not required to be calculated

• Objective is to determine the voltage impact on buses to size feeders and/or check protection settings

• Accelerating motors are primarily low-voltage motors

• Motors are connected to a system fed by utility grid(s) only

• Motors are connected to a system fed by generator(s) only, but the size of the starting motor is less 10% of the generator kVA rating

This method is suitable for checking the effect of motor starting on the system when the dynamic model is not available for starting motors. The static motor-starting calculation method involves:

• Time domain using a static model

• Switching motors modeled as ZLR during starting and constant kVA load after starting

• Running load flow when any change in system

Dynamic Motor Starting

In the dynamic motor starting method, using motor circuit models, the entire dynamic model for the motor and connected load is used to simulate the acceleration behaviour and voltage impact on the entire network. This method assumes the generator to be modelled as a constant voltage behind impedance.

Dynamic motor starting using a circuit model is a recommended approach under any of the following conditions:

• Existing system design change or expansion

• Motor and connected load dynamics are available and/or can be estimated

• Acceleration times of the motors are required to be calculated

• Accelerating motors are primarily medium-voltage motors

• Motors are connected to a system fed by generator(s), but the size of the starting motor is greater than 10% of the generator kVA rating

Software

• ETAP

• SKM

• EasyPower

Information & Data Requirement for Motor Starting Analysis

Basic Information

• Utility and generator impedance

• Transmission lines

• Cables

• Transformers

• Other components

• Load characteristics

• Machine and load data

Motor Starting Analysis Data

Bus Data

• Nominal kV

• angle

• load diversity factor etc.

Branch Data

• Branch includes three-winding transformer, two-winding transformer, transmission line, cable, reactor, and impedance.

• Branch data also includes Branch Z, R, X, or X/R values and units, tolerance, and temperatures, if applicable

• Cable and transmission line length and unit

• Transformer rated kV and kVA, tap and load tap-changing (LTC) settings

• Impedance base kV and base kVA

Power Grid Data

• Rated kV

• Minimum short-circuit megavolt ampere (MVA) (i.e., higher grid impedance and consequently more conservative to perform voltage drop study)

• Voltage at the point of interconnection (POI)

Static Load Data

• Rated kV, kVA, and power factor

• Operating load

Motor-Operated Valve (MOV) Data

• Rated kW/HP and kV

• Locked rotor (LR), no load (NL), normal, and rated torque (rated T)

• Current, PF, and time duration for each operation stage

• Operating load

• Voltage limits for start, seating/unseating, and travel time

Capacitor Data

• Rated kV

• Rated kvar/bank and number of banks

• Delta or wye connection

Lumped Load Data

• Rated kV, kVA, and power factor

• Operating load

Variable Frequency Drive (VFD) Data

• Bypass switch status

• Rated input/output kV, kVA, frequency, efficiency, and input power factor

• Operating input power factor, frequency, and V/Hz ratio

• Starting control type, control parameters, and current limit

Synchronous Generator Data

• Required synchronous generator data includes:

• Operating mode (swing, voltage control, kVAR control, or power factor control)

• Rated kV, kW, power factor, efficiency, poles

• X’di and X/R ratio

• Operating generation data (voltage, kW, and kVAR)

Synchronous Motor Data

• Rated HP or kW

• Rated kV

• Power factors & efficiencies at 110%, 100%, 75%, 50% & 0% shaft loading

• Operating load

Induction Motor Data

• Rated kW/HP and kV

• Power factors & efficiencies at 110%, 100%, 75%, 50% & 0% shaft loading

• Operating load

Study Results (Deliverables) and Reporting

After carrying out the motor starting analysis using ETAP, a report should be produced by the individual or motor starting analysis consultant in the format preferred by the client or users. The minimum required results or deliverables, and additional results to facilitate understanding of the simulation, should be included in the report.

Typical Report Format

•     General description

•     System input data

•     Switching motor and static load

•     Switching event data

•     Show load flow tab

•     Event load flow tabulation

•     Tabulated simulation results

•     Motor-starting alerts

•     Motor-starting plots and one-line diagram