SE2150627A1 - Method of controlling electric motor braking with a soft starter - Google Patents

Abstract

A method of controlling braking of an electric motor by means of a soft starter having a first brake mode suitable for braking the motor in a high speed range and a medium speed range, and a second brake mode suitable for braking the motor in a low speed range, the method comprising: a) receiving a stop command when the motor is operating at a motor speed, b) obtaining a plurality of motor speed values, each motor speed value being an estimation of the motor speed using a different estimation method, c) determining the motor speed based on the estimated motor speed values obtained in step b), and d) switching from the first brake mode to the second brake mode in case the motor speed determined in step c) is below a threshold value.

Description

1O METHOD OF CONTROLLING ELECTRIC MOTOR BRAKING WITHA SOFT STARTER TECHNICAL FIELD The present disclosure generally relates to a method of providing braking of electrical motors by means of soft starters.

BACKGROUND Soft starters are devices for controlling an electrical three-phaseasynchronous induction motor. The “dynamic brake” feature in such devicesmay for example have two main states, which are dynamic brake, and DCbrake.

For dynamic brake, the braking torque is generated by continuouslyestimating the magnetic flux in the motor and firing the thyristors only whena braking torque is predicted, not when a driving torque is predicted. The fluxestimation relies on the electrical feedback that the rotor gives to the statorvoltages and currents, which can be measured by the soft starter. Thefeedback gets weaker at lower rotor speed (high slip). When the speed isgetting low enough, the firmware is not able to make accurate torquepredictions anymore. This leads to either no predictions or faulty predictions.This can cause absent breaking or pulsating torque with random polarity and magnitude.

For DC brake, the braking torque is generated by firing thyristors in onepolarity only, creating a pulsating DC current with 50 or 60 pulses per second(at 50/ 6oHz grid frequency). At low rotor speed the motor “locks to” the DCpart of the current and brakes into a full stop. At higher speed, the lockinggets weaker and the breaking torque is lower. The current variation of thepulses forms a magnetic field with 50 or 6oHz frequency. Closer tosynchronous speed the motor can lock to this instead of the DC field and start accelerating instead of braking. 1O SUMMARY The present inventors have found that: o Dynamic brake works at high and medium speed but may fail at low speed. o DC brake works at low speed, works with low efficiency at medium speed and may fail at high speed. o It is critical to switch braking methods (from dynamic brake to DC brake) only at medium speed. o For optimal breaking efficiency, switch should be done just before speed decreases to “low”. o If the switch is based on a sensor-less speed estimation algorithm, it is required to be:o Highest priority: Reliableo Second priority: Accurate In view of the above, a general object of the present disclosure is to provide amethod of controlling braking of an electrical motor with a soft starter which solves or at least mitigates the problems of the prior art.

To meet the requirement of high reliability, a redundancy approach was chosen.

There is hence provided a method of controlling braking of an electric motorby means of a soft starter having a first brake mode suitable for braking themotor in a high speed range and a medium speed range, and a second brakemode suitable for braking the motor in a low speed range, the methodcomprising: a) receiving a stop command when the motor is operating at amotor speed, b) obtaining a plurality of motor speed values, each motorspeed value being an estimation of the motor speed using a differentestimation method, c) determining the motor speed based on the estimated motor speed values obtained in step b), and d) switching from the first brake 1O mode to the second brake mode in case the motor speed determined in step c) is below a threshold value.

According to one embodiment the threshold value is 40 % or less of the nominal speed of the motor.

The threshold value may be set by the user, in the interval of o %-1oo % of the nominal speed of the motor.

According to one embodiment the first brake mode is a dynamic brake mode and the second brake mode is a DC brake mode.

According to one embodiment the estimation methods include at least someof the following methods: flux-based estimation, time-based estimation, trig- based estimation, and torque-based estimation.

The estimations of the motor speed value may be done independently ormore or less independently of each other. Each estimation may for exampleresult in a per unit value where 1 means synchronous speed and o means standstill.

According to one embodiment in case the soft starter is in full speed modewhen receiving the stop command, the determining in step c) includesremoving the highest and the lowest estimated motor speed values, anddetermining a mean value of the remaining estimated motor speed values as a candidate motor speed value.

According to one embodiment in case the soft starter is in a starting stagewhen receiving the stop command, the determining in step c) includesremoving the motor speed value obtained using torque-based estimation, anddetermining the lowest of the remaining motor speed values as a candidate motor speed value.

One embodiment comprises predicting the motor speed using extrapolationbased on settings and history to obtain a predicted motor speed value, wherein the determining in step c) includes selecting the lowest value of the 1O predicted motor speed value and the candidate motor speed value as the motor speed value.According to one embodiment the motor is an asynchronous motor.

Generally, all terms used in the claims are to be interpreted according to theirordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to "a/ an /the element, apparatus, component, means,etc. are to be interpreted openly as referring to at least one instance of theelement, apparatus, component, means, etc., unless explicitly stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:Fig. 1 schematically shows an example of an electrical motor assembly; and Fig. 2 shows a flowchart of a method of controlling braking of an electric motor by means of a soft starter.

DETAILED DESCRIPTION The inventive concept will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplifyingembodiments are shown. The inventive concept may, however, be embodiedin many different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided byway of example so that this disclosure will be thorough and complete, andwill fully convey the scope of the inventive concept to those skilled in the art.

Like numbers refer to like elements throughout the description.

Fig. 1 shows an example of an electric motor assembly 1. The electric motorassembly 1 comprises an electric motor 3. The electric motor 3 may be anasynchronous motor or a synchronous motor. Typically, the electric motor 3 is an asynchronous motor. 1O The electric motor assembly 1 comprises a soft starter 5. The soft starter 5 isconfigured to control soft starting and braking of the electric motor 3. Thesoft starter 5 may for example be the PSTX Softstarter by ABB® with updated software as described herein.

The soft starter 5 comprises semiconductor switches, for example anti-parallel connected thyristors. The soft starter 5 comprises a controller for controlling the semiconductor switches.

The soft starter 5 comprises processing circuitry 7 and a storage medium 9comprising software or a computer program, which form the controller.When the computer program is executed by the processing circuitry 7 itcauses the soft starter 5 to carry out the method as disclosed herein by suitably controlling the semiconductor switches.

The soft starter 5 is configured to control braking of an electric motor 3. Thesoft starter 5 has a first brake mode suitable for braking the electric motor 3in a high speed range and a medium speed range, and a second brake modesuitable for braking the electric motor 3 in a low speed range. The first mode is a dynamic brake mode. The second mode is a DC brake mode.

The processing circuitry 5 may for example use any combination of one ormore of a suitable central processing unit (CPU), multiprocessor,microcontroller, digital signal processor (DSP), application specificintegrated circuit (ASIC), field programmable gate arrays (FPGA) etc.,capable of executing any herein disclosed operations concerning motor braking.

The storage medium 7 may for example be embodied as a memory, such as arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM), a flash memory in amicrocontroller, or an electrically erasable programmable read-only memory(EEPROM) and more particularly as a non-volatile storage medium of adevice in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. 1O The operation of the soft starter 5 will now be described in more detail with reference to Fig. 2.

In a step a) the soft starter receives a stop command when the electric motor 3 is operating at a motor speed.

In a step b) a plurality of motor speed values is obtained by estimating themotor speed using a plurality of different estimation methods. Thus, each motor speed value is obtained using a different estimation method.

The estimation methods may be flux-based estimation, time-based estimation, trig-based estimation, and torque-based estimation.

Time-based estimation is a basic speed-estimation based on break time-outuser settings, which sets the total time for the brake process including both dynamic and DC braking.

The motor speed (name, per unit, determined by time-based estimation isgiven by:1 _ tsince stop signal wtime =tuseïgsetting Trig-based estimation of the motor speed (omg, per unit, makes use of thedynamic brake function while it is running. The occasions when thesemiconductor switches may fire (trig) to produce a breaking torque occuronly when the magnetic field in the rotor is in opposite phase of the magneticfield of the stator. This means that the frequency of these occurrences isproportional to the slip. The trig-based estimation of the motor speed is givenby: Tpowef grid wtrig = 1 _ Ttrig Ttrig = ttrig n _ ttrig n-l 1O Tpowei giid is the period time of the power grid, and Tiiig is the trig period which is the time difference between two consecutive firings of the thyristors.

If the soft starter 5 does not find any suitable trig occasions within areasonable time, it may enter a special mode where it tries to regain normaldynamic braking. If there is success the trig occasions again appear at asensible rate, and the soft starter 3 returns to normal mode. The trig-basedestimation does not output any new motor speed values during the special mode.

It is not trivial to register trig occasions. There can be a “group” of severalconsecutive, or almost consecutive trig occasions. There can also beadditional “false” occasions that are not synchronized with the motion of therotor. The approach is to accept only the first trig in a group and ignore trigs between the groups. A new trig period, Tiiig, is accepted only if it is both:o More than half of previous trig period0 More than half of a power grid periodOro More than 150 ms Torque-based estimation uses a model of the mechanical properties of themotor and its load, specifically the load torque and the angular momentum.When the stop command is received, the electric motor 3 is assumed to run atfull speed, very close to synchronous speed, as applications in need of brakinghas a low load torque. The present angular momentum Lii is selected as areference, and when the angular momentum is down to zero, the electricmotor 3 is at stand still. The decrease of angular momentum and thus motorspeed consist of load torque, 'tioad and torque I generated by the soft starter 3,via the electric motor 3. Braking torque is denoted negative sign. The motorspeed wioiqiie, per unit, is estimated as the remaining part of the initial angularmomentum Liiiii.Ln wtorque I Tinit 1O Ln 2 Ln_1 "l" Tdt "l" Tloaddt Before every motor start the motor speed, wtofque, and the angular momentumis assumed to be zero. During the start process, typically a soft start, thetorque I generated by the soft starter 3 is integrated. When the acceleration isdone and the electric motor 3 is at full speed, the value of the integratedtorque is latched. At this point the motor speed is constant and therefore motor torque I is in balance with load torque, 'tioadjfieffitaft = -t. This is alsolatched.

The load torque during start is assumed to be proportional to speed, and theacceleration is assumed to be constant. This means the average of load torqueduring start, that did not contribute to increase the angular momentum, canbe estimated as half full speed torque, and the integral of this torque should be considered in the angular momentum calculation. tacc 1Linit = I (Tdt) + šrloadflfteïgstart * tacc0 When the stop command is received the torque is latched again: 't10ad_bef0fe_st0p= -'t, and the present angular momentum is set to be the same as directlyafter the start: Ln=Ln-1=L1n1t. During dynamic brake the load torque isassumed to be proportional to the speed. In this design there may be a feedback from the motor speed wtofque calculated during the last execution.

Tload I wtorque (n-l) * Tload_before_stopThe motor speed values obtained by estimation in step b) may be filtered.

The motor speed value obtained using flux-based estimation may be filteredwith a first-order low pass filter with a time constant of for example 0.2 s.Both the estimation and the filtering are halted when any thyristor is conducting and are active only between thyristor firings.

For the trig-based estimation the motor speed value may be filtered with a rolling max filter of 6 values. A new value is produced at every trig occasion.

In a step c) the motor speed is determined based on the estimated motor speed values obtained in step b). 1O Step c) may involve obtaining a candidate motor speed value based on themotor speed values and obtaining a predicted motor speed value anddetermining the motor speed based on the candidate motor speed value and the predicted motor speed value.

Normally, the acceleration of the electric motor 3 has been completed and thesoft starter 3 has gone into full speed mode before the stop command is givenin step a). In this case a median filter which removes the highest and thelowest estimated motor speed value is used to determine a mean value of thetwo remaining signals. This mean value is a candidate motor speed value.This means that if one or possibly even two estimation of the motor speed is completely wrong, it does not affect the result at all.

If the stop command in step a) is received during the starting stage (usuallyduring soft start) the torque-based estimate is removed and the lowest of the three remaining motor speed values is the candidate motor speed value.

A parallel special case is that the dynamic brake mode fails to find suitabletrig occurrences. In this case both the flux-based, and the trig-basedestimations are removed, and the lowest of time-based and torque-basedestimations is the candidate motor speed value. If these trig problems arepresent after an interrupted start, a decreased version of the time-based estimate selected as the candidate motor speed value.

The predicted motor speed value is obtained based on settings and history.The motor speed determined in step c) is the lowest one of the candidate motor speed value and the predicted motor speed value.

In the beginning of the braking, when elapsed time is less than 15 % of thebreak time-out (user setting) or the last (limited) speed estimate is above 50%, the predicted motor speed value is an extrapolation that is done with astraight line from last (limited) speed estimate and forward with the same slope as the time-based estimation. 1O 1O After 50 % time elapsed the predicted motor speed value is an extrapolationthat is done from full speed and initial time through last (limited) speed estimate and its timestamp.

This creates a limited time before the switch to DC brake, even if the speed estimation fails.

The motor speed value obtained in step c) is compared to a user-settablethreshold value.

In a step d) the first brake mode is switched to the second brake mode in case the motor speed determined in step c) is below the threshold value.

There may also be a user settable switch-over delay time available. When themotor speed moves below the threshold value and the delay time has elapsed,a signal is generated alerting that it is time to switch from the first brake mode to the second brake mode.

Braking is then controlled using the second brake mode instead of the first brake mode.

The inventive concept has mainly been described above with reference to afew examples. However, as is readily appreciated by a person skilled in theart, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims (8)

1. A method of controlling braking of an electric motor (3) by means of asoft starter (5) having a first brake mode suitable for braking the motor (3) ina high speed range and a medium speed range, and a second brake modesuitable for braking the motor (3) in a low speed range, the method comprising: a) receiving a stop command when the motor (3) is operating at a motor speed, b) obtaining a plurality of motor speed values, each motor speed value being an estimation of the motor speed using a different estimation method, c) determining the motor speed based on the estimated motor speed values obtained in step b), and d) switching from the first brake mode to the second brake mode in case the motor speed determined in step c) is below a threshold value.

2. The method as claimed in claim 1, wherein the threshold value is 40 % or less of the nominal speed of the motor (3).

3. The method as claimed in any of the preceding claims, wherein the firstbrake mode is a dynamic brake mode and the second brake mode is a DC brake mode.

4. The method as claimed in any of the preceding claims, wherein theestimation methods include at least some of the following methods: flux-based estimation, time-based estimation, trig-based estimation, and torque- based estimation.

5. The method as claimed in any of the preceding claims, wherein in casethe soft starter is in full speed mode when receiving the stop command, the determining in step c) includes removing the highest and the lowest 1O 12 estimated motor speed values, and determining a mean value of the remaining estimated motor speed values as a candidate motor speed value.

6. The method as claimed in claim 4 or 5 dependent on claim 4, wherein incase the soft starter is in a starting stage when receiving the stop command,the determining in step c) includes removing the motor speed value obtainedusing torque-based estimation, and determining the lowest of the remaining motor speed values as a candidate motor speed value.

7. The method as claimed in claim 5 or 6, comprising predicting the motorspeed using extrapolation based on settings and history to obtain a predictedmotor speed value, wherein the determining in step c) includes selecting thelowest value of the predicted motor speed value and the candidate motor speed value as the motor speed value.

8. The method as claimed in any of the preceding claims, wherein the motor (3) is an asynchronous motor.