System, device, and method for over-current relays protecting motors

Abstract

A system, method, and device for protecting an induction motor are disclosed. The exemplary system may have a module for determining the current drawn by the motor and a module for determining the state of the motor. The system may calculate a used thermal capacity based on a first formula when the motor is in an active state. The system also calculates the used thermal capacity based on a second formula when the motor is in an inactive state. When the used thermal capacity attains a threshold, the relay is tripped thus removing current to the motor and preventing motor from overheating. A method to derive thermal time constants from desired trip time limits (such as those defined by IEC standards or the thermal limit curves provided by motor manufacturers) is also presented. For example, the thermal time constants can then be used in tacking motor used thermal capacity throughout various motor states.

Description

TECHNICAL FIELD

The present invention relates to controls and protection for motors and more particularly, to a device, method, and system for determining the thermal properties of a motor.

BACKGROUND INFORMATION

In current practice, there are two types of control algorithms for solid-state over-current relays, namely, electromechanical relay algorithm [1][2] and 1²t algorithm. The electromechanical relay algorithm is derived from the model of electromechanical relay using the following: $\begin{array}{cc}\mathrm{For}0<M<1t\left(I\right)=\frac{t_r}{M^2-1}& \mathrm{A1}\\ \mathrm{For}M>1t\left(I\right)=\frac{A}{M^P-1}+B& \mathrm{A2}\end{array}$

Where

• • t(I) is the reset time in Eq. (A1) or trip time in Eq. (A2) in seconds,
  • M is the I_input/I_pickup (I_pickup is the relay current set point),
  • t_r is the rest time (for M=0) defined in IEEE Std C37.112-1996 Table 1,
  • A, B, constants defined in IEEE Std C37.112-1996
  • p Table 1 to provide selected curve characteristics.

The relay will trip if $\begin{array}{cc}{\int }_o^{T_P}\frac{1}{t\left(I\right)}dt>1& \mathrm{A3}\end{array}$

The discrete form of equation A3 is $\begin{array}{cc}\sum _{k=0}^{k=n}\frac{\Delta t}{t\left(I_k\right)}>1& \mathrm{A4}\end{array}$

Where

• • T_p trip time in seconds,
  • Δt Sample period in seconds,
  • t(I_k) t(I) calculated from Eq. (A1) or Eq. (A2) for k^th sample of current I.

For 0<M<1, t(I_k) is a negative number. If the summation in equation A4 keeps going, the sum value will go to—∞. Because this algorithm simulates the reset dynamics of electromechanical relays, the summation shall be stopped if $\begin{array}{cc}\sum _{k=0}^{k=n}\frac{\Delta t}{t\left(I_k\right)}<0& \mathrm{A5}\end{array}$

This is equivalent to saying that a motor will reach the same equilibrium temperature without regard to what state the motor is operating. The algorithm does not take into account whether the motor is operating at 50% I_FLA or at 80% I_FLA. In reality, a motor reaches different equilibrium temperatures when different currents are supplied to the motor. Therefore equation A1 does not simulate motor thermal dynamics for 0<M<1, which results in the algorithm not accurately tracking the used thermal capacity of AC motor under varying load.

The 1²t algorithm uses a locked rotor current I_LR and safe stall time t_LR as motor thermal limit. The cold trip time t_trip-C and the hot trip time t_trip-H for currents above pickup are defined by $\begin{array}{cc}{t}_{\mathrm{trip}-C}={\left(\frac{I_{\mathrm{LR}}}{I}\right)}^2{t}_{\mathrm{LRC}}& \mathrm{B1}\\ t_{\mathrm{trip}-H}={\left(\frac{I_{\mathrm{LR}}}{I}\right)}^2{t}_{\mathrm{LRH}}& \mathrm{B2}\end{array}$

The used thermal capacity θ_n is calculated by $\begin{array}{cc}{\theta }_n=\frac{\Delta t}{t_{\mathrm{trip}}}+{\theta }_{n-1}& \mathrm{B3}\end{array}$

There is a θ_hot or t_hot to determine the switch of t_trip from t_trip-c to t_trip-H. Note that equation B3 is usually implemented in DSP and is updated only when M>1. There is no update of θ_n for M<1 so this algorithm does not consider the cooling effect of 0<M<1.

The Amd2 [3] of IEC 60947-4-2 [4] imposes new thermal memory test requirements on solid-state relay protecting AC induction motors, which state: electronic overload relays shall fulfill the following requirements (note table and figures references are provided in Amendment 2 to standard IEC 60947-4-2):

• • apply a current equal to Ie until the device has reached the thermal equilibrium;
  • interrupt the current for a duration of 2×T_p (see Table 2 of [3]) with a relative tolerance of ±10% (where T_p is the time measured at the D current according to Table 3 of [3]);
  • apply a current equal to 7.2×Ie; and
  • the relay shall trip within 50% of the time T_p.

From the analysis above, it is seen that both electromechanical relay algorithm and 1²t algorithm may fail the thermal memory test and may not provide sufficient protection to AC induction motors. Accordingly, a need exists for a device, method, and system relay algorithm based on motor thermal model. The algorithm may accurately track a motor's used thermal capacity when the motor's current varies at any value satisfying M≧0.

SUMMARY

The present invention is a novel device, system, and method for an over-current relay protecting an induction motor comprising the following actions. The exemplary method may determine the current drawn by the motor. The method may also determine the state of the motor. The method may then calculate a used thermal capacity based on a first formula when the motor is in an active state. The method may also calculate the used thermal capacity based on a second formula when the motor is in an inactive state. Once the used thermal capacity attains a threshold the method may trip the relay, remove current from the motor, and prevent the motor from overheating.

The invention may include the following embodiments. In one exemplary embodiment, the first formula for used thermal capacity is ${\theta }_n=M_n^2\times \frac{\Delta t}{T_h}+{\theta }_{n-1}\times \left(1-\frac{\Delta t}{T_h}\right).$ In another exemplary embodiment, the second formula for used thermal capacity is ${\theta }_n={\theta }_{n-1}\times \left(1-\frac{\Delta t}{T_c}\right).$ In other exemplary embodiments, the method may calculate the used thermal capacity based on a third formula when the motor is in an acceleration state. In yet another exemplary embodiment, the method may calculate the used thermal capacity based on a fourth formula when the motor is in a deceleration state.

It is important to note that the present invention is not intended to be limited to a system or method which must satisfy one or more of any stated objects or features of the invention. It is also important to note that the present invention is not limited to the exemplary embodiments described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be better understood by reading the following detailed description, taken together with the drawings herein:

FIG. 1 is the operation status of a motor.

FIG. 2 is the difference in Eq. 19 and Eq. 22 versus time.

FIG. 3 are the results of a simulation for IEC thermal memory test.

FIG. 4 is the used capacity of different M values.

FIG. 5 is typical thermal limit curve provided by motor manufacturers for motors above 250 HP.

FIG. 6 is a flow chart of a standard trip algorithm 600 according to a first exemplary embodiment of the present invention.

FIG. 7 is a flow chart of a custom trip algorithm 700 according to a second exemplary embodiment of the present invention.

FIG. 8 is sample trip curves derived from IEC standard 60947-4-2 and its amendment 2 using algorithm 600.

DETAILED DESCRIPTION

The present invention provides a control algorithm for solid-state relay protecting for induction motors. The algorithm utilizes a thermal model of an AC induction motors. The algorithm calculates used thermal capacity of the motors based on motor currents. The relay trips once the used thermal capacity reaches limit. This invention also provides a method to derive thermal time constants from motor thermal limit curves and IEC standard 60947-4-2 [3][4]. The thermal time constants may then used in the control algorithm 600 and 700 for calculation of the used thermal capacity. The resultant algorithm may then satisfy the requirements of motor thermal limit curves and IEC standard 60947-4-2 [3][4].

The following is a list of nomenclature used throughout the application.

• • q heat input to a motor per second, its unit is Joules/sec, i.e., Watts,
  • dt an infinitely small time interval in seconds,
  • τ temperature rise in ° C., defined as the temperature difference between the motor and its surroundings,
  • τ_fin temperature rise at thermal equilibrium in ° C.,
  • R_th thermal resistance in ° C./Watt, defined as the temperature difference that will cause 1 Watt to flow between the motor and its surroundings,
  • C_th thermal capacitance in Joules/° C., i.e, in Watts*seconds/° C., defined as the energy required to change the motor's temperature by 1° C. if no heat is exchanged with its surroundings (adiabatic process),
  • T_th thermal time constant in seconds. If power input and ambient temperature remain constant, temperature will change 63% of the remaining excursion in 1 T_th. T_th=R_thC_th,
  • T_h thermal time constant used when motor is running with current inputs, i.e., I>0. In some applications, acceleration thermal time constant T_h-acc is different from running thermal time constants T_h-run for motors above 250 HP,
  • T_c thermal time constant used when currents are removed from the motor, i.e., I=0,
  • I_FLA rated full load amperage of the motor,
  • SF service factor of the motor,
  • I_pickup pick up current of a relay, I_pickup=SF×I_FLA,
  • M multiples of I_pickup, M=I/I_pickup,
  • τ_max maximum permissible temperature rise above ambient temperature of the motor
  • θ used thermal capacity=τ/τ_max,
  • Δt sampling period of current samples in seconds,
  • I_LR locked rotor current,
  • t_LRC cold locked rotor safe stall time,
  • t_LRH hot locked rotor safe stall time,
  • t_trip relay trip time.

FIG. 1 shows the different operational states or conditions of motors. Motor thermal characteristics during states A, B, C, D, and E may be significantly different. For example, when a totally enclosed fan cooled (TEFC) motor is stopped, the fan is stopped, so heat can not be dissipated as fast as when the motor is running. For motors above 250 horsepower (hp), motor thermal characteristics during state B may be significantly different from those during C and D. An accurate trip curve derived from the thermal limit curves [5] provided by motor manufacturers may need to be used. First a thermal model of the AC motor will be described, from which the used thermal capacity and trip time can be calculated based on thermal time constants. Second, implementation of International Electrotechnical Commission (IEC) standard trip classes using the thermal model algorithm are provided. Third, customized trip curves for motors above 250 HP are discussed. Finally, exemplary flowcharts are provided for applications using standard or custom trip curves. Embodiments and aspects of the invention may be implemented by a variety of Digital Signal Processing (DSP) devices.

As for any other objects, the first order thermal dynamics of a motor is described by: $\begin{array}{cc}\mathrm{qdt}-\frac{\tau }{R_{\mathrm{th}}}\mathrm{dt}=C_{\mathrm{th}}d\tau .& 1\end{array}$

For an infinitely small time interval dt, qdt is the heat energy input to the motor, $\frac{\tau }{R_{\mathrm{th}}}\mathrm{dt}$ is the heat energy dissipated from the motor to the surrounding, and C_thdτ is the resultant energy change of the motor. Multiplying both sides of Eq. (1) by R_th, yields: R_thqdt−τdt=R_thC_thdτ.   2

Substituting T_th=R_thC_th into Eq. (2) gives: R_thqdt−τdt=T_thdτ.   3

At thermal equilibrium, the heat input and heat loss cancels each other, temperature rise attains a final value τ_fin, and the motor temperature will cease to rise, i.e., dτ=0° C. Hence, Eq. (3) becomes: R_thqdt−τ_findt=0.   4

Whence: τ_fin=R_thq.   5

Substituting Eq. (5) into Eq. (3) gives: τ_findt−τdt=T_thdτ.   6

Rearranging Eq. (6) yields: $\begin{array}{cc}\frac{\mathrm{dt}}{T_{\mathrm{th}}}=\frac{d\tau }{{\tau }_{\mathrm{fin}}-\tau }.& 7\end{array}$

Integrating both sides of Eq. (7) gives: $\begin{array}{cc}\frac{t}{T_{\mathrm{th}}}=-\ln \left({\tau }_{\mathrm{fin}}-\tau \right)+k.& 8\end{array}$

If at the initial moment t=0, the motor has an initial temperature rise τ₀ above the ambient temperature, then Eq. (8): k=ln(τ_fin−τ₀).   9

Substituting for k in Eq. (8), and solving for τ, gives: $\begin{array}{cc}\tau ={\tau }_{\mathrm{fin}}\left(1-e^{-\frac{t}{T_{\mathrm{th}}}}\right)+{\tau }_0{e}^{-\frac{t}{T_{\mathrm{th}}}}.& 10\end{array}$

Since motor thermal resistance may be different at different operating conditions, different thermal constants may be used in Eq. (10). When the motor is running, the motor can properly dissipate heat and T_th=T_h. When currents are removed, the motor will slow down and eventually stop. When the motor is stopped, the motor heat dissipation capability may be reduced and T_th=T_c. T_c is usually three times of T_h for AC induction motors [7]. The different operating conditions of motors are shown in FIG. 1. Table 1 shows the variants of Eq. (10) used for different motor status.

TABLE 1
Motor Temperature Rise Formula
Motor
Status	Temperature Rise Formula	Description
B, C, D	$\tau ={\tau }_{\mathrm{fin}}\left(1-e^{\frac{t}{T_h}}\right)+{\tau }_o{e}^{\frac{t}{T_h}}11$	obtained by setting Tth = Th in Eq. (10).
E	$\tau ={\tau }_o{e}^{\frac{t}{T_h}}12$	obtained by setting τfin = 0 and Tth = Tc in Eq. (10).

The function of a digital overload relay is to translate the current drawn by an AC Induction motor into temperature rise τ, and detect whether this rise of temperature has reached the maximum permissible temperature rise or not. Therefore, the temperature rise may be correlated with the current. The temperature rise at thermal equilibrium of the motor is proportional to current square, i.e, τ_fin=k₂I²,   13

where k₂ is a constant. If I=SF×I_FLA=I_pickup, then τ_fin=τ_max=maximum permissible temperature rise, as indicated in: τ_max=k₂I_pickup².   14

Substituting Eq. (13) into Eq. (11) and dividing Eq. (11) and Eq. (12) by Eq. (14) yield: $\begin{array}{cc}\frac{\tau }{{\tau }_{\max }}=\frac{k_2{I}^2}{k_2{I}_{\mathrm{pickup}}^2}\left(1-e^{\frac{t}{T_h}}\right)+\frac{{\tau }_0}{{\tau }_{\max }}{e}^{\frac{t}{T_h}},& 15\\ \frac{\tau }{{\tau }_{\max }}=\frac{{\tau }_o}{{\tau }_{\max }}{e}^{-\frac{t}{T_c}}.& 16\end{array}$

Since $\frac{\tau }{{\tau }_{\max }}$ is the used thermal capacity θ of the motor at time t, substituting $\frac{\tau }{{\tau }_{\max }}=\theta \mathrm{and}M=\frac{I}{I_{\mathrm{pickupx}}}$ into Eq. (15) and Eq. (16) yields: $\begin{array}{cc}\theta =M^2\left(1-e^{-\frac{t}{T_h}}\right)+{\theta }_o{e}^{-\frac{t}{T_h}}& 17\\ \theta ={\theta }_o{e}^{-\frac{t}{T_h}}.& 18\end{array}$ Given Δt as the sampling period of current samples, the discrete forms of Eq. (17) and Eq. (18) are: $\begin{array}{cc}{\theta }_n=M_n^2\left(1-e^{-\frac{\Delta t}{T_h}}\right)+{\theta }_{n-1}{e}^{-\frac{\Delta t}{T_h}},& 19\\ {\theta }_n={\theta }_{n-1}{e}^{-\frac{\Delta t}{T_h}}.& 20\end{array}$

To implement Eq, (19) and Eq. (20) in DSP, $e^{-\frac{\Delta t}{T_{\mathrm{th}}}}$ can be pre-calculated and stored as a constant. A simplified form of these two equations can also be used by noting that when $\frac{\Delta t}{T_{\mathrm{th}}}$ is small enough, and $\begin{array}{cc}{e}^{-\frac{\Delta t}{T_h}}\approx 1-\frac{\Delta t}{T_{\mathrm{th}}}.& 21\end{array}$

Eq. (21) is derived from a Taylor series. Substituting Eq. (21) into Eq. (19) and Eq. (20) gives: $\begin{array}{cc}{\theta }_n=M_n^2\times \frac{\Delta t}{T_h}+{\theta }_{n-1}\times \left(1-\frac{\Delta t}{T_h}\right).& 22\\ {\theta }_n={\theta }_{n-1}\times \left(1-\frac{\Delta t}{T_c}\right).& 23\end{array}$

where θ_n is the used thermal capacity at n^th sample, θ_n-1 is the used thermal capacity at (n-1)^th sample, and M_n is the M at n^th sample. Because motors may be running for days or months without stopping, it is very important that the error introduced in Eq. (22) and Eq. (23) by the approximation will not diverge as ΣΔt→∞. FIG. 2 shows the difference between Eq. (19) and Eq. (22) converges to 0 as ΣΔt→∞. Therefore Eq. (22) and Eq. (23) can also be implemented in the DSP to calculate the used thermal capacity dynamically. The relay shall trip if the used thermal capacity is greater than 1. Setting θ=1 in Eq. (17) and solving for trip time gives: $\begin{array}{cc}{t}_{\mathrm{trip}}=T_h\ln \frac{M^2-{\theta }_o}{M^2-1},& 24\\ T_h=t_{\mathrm{trip}}÷\ln \frac{M^2-{\theta }_o}{M^2-1},& 25\end{array}$

Eq. (24) can be used to calculate trip time for different M when T_h is know while Eq. (25) can be used to calculate T_h when t_trip is known. In the next section, Eq. (25) is used to derive the thermal time constants for the IEC standard trip classes. Note that the derivation from Eq. (1) to Eq. (18) is similar to the derivation in [6]. Further derivation is then different from [6] whose algorithm updates trip time instead of thermal capacity. Both standard and custom trip curves can be derived from Eq. (25). The derivations are demonstrated herein.

The derivations are first demonstrated on IEC standard trip classes. The thermal time constants T_h and T_c in Eq. (22) and Eq. (23) usually can be obtained from motor manufacturer. In the exemplary case, discussed herein, however, IEC standard 60947-4-2 is used to derive the thermal time constants of IEC standard trip classes. Details of the derivation follow. For demonstration, the derivation is only performed on Class 10A, 10, 20, and 30. The derivation for other standard trip classes can be obtained similarly.

TABLE 2
Requirements of IEC 60947-4-2 on performance of non-
compensated overload relays
	Class 10A	Class 10	Class 20	Class 30
A (1.0 × Ipickup)	ttrip > 2 h	ttrip > 2 h	ttrip > 2 h	ttrip > 2 h
B (1.2 × Ipickup)	ttrip < 2 h	ttrip < 2 h	ttrip < 2 h	ttrip < 2 h
C (1.5 × Ipickup)	ttrip < 2 min	ttrip < 4 min	ttrip < 8 min	ttrip < 12 min
7.2 × Ipickup	2 < ttrip ≦10**	4 < ttrip ≦ 10**	6 < ttrip ≦ 20**	9 < ttrip <= 30**
8 × Ipickup	1.6 ≦ ttrip ≦ 8.1*	3 ≦ ttrip ≦ 8.1*	5 ≦ ttrip ≦ 16.2*	7 ≦ ttrip ≦ 24.3*
7 × Ipickup	2 ≦ ttrip ≦ 10.6*	4 ≦ ttrip ≦ 10.6*	6 ≦ ttrip ≦ 21.2*	9 ≦ ttrip ≦ 31.7*
6 × Ipickup	3 ≦ ttrip ≦ 14.4*	6 ≦ ttrip ≦ 14.4*	9 ≦ ttrip ≦ 28.8*	13 ≦ ttrip ≦ 43.2*
5 × Ipickup	4 ≦ ttrip ≦ 20.7*	8 ≦ ttrip ≦ 20.7*	12 ≦ ttrip ≦ 41.5*	19 ≦ ttrip ≦ 62.2*
4 × Ipickup	6 ≦ ttrip ≦ 32.4*	13 ≦ ttrip ≦ 32.4*	19 ≦ ttrip ≦ 64.8*	29 ≦ ttrip ≦ 97.2*
3 × Ipickup	12 ≦ ttrip ≦ 57.6*	23 ≦ ttrip ≦ 57.6*	35 ≦ ttrip ≦ 115.2*	52 ≦ ttrip ≦ 172.8*
2 × Ipickup	26 ≦ ttrip ≦ 129.6*	52 ≦ ttrip ≦ 129.6*	78 ≦ ttrip ≦ 259.2*	112 ≦ ttrip ≦ 388.8*
Note:
1. All ttrip is counted from cold state at 40° C. except for B (1.2 × Ipickup) the ttrip is counted from 2 h at 1.0 × IFLA or from relay terminals reach thermal equilibrium at 1.0 × IFLA whichever is less, and for C (1.5 × Ipickup) the ttrip is counted from thermal equilibrium at 1.0 × IFLA.
2. All ttrip are in seconds except those stated otherwise.
3. For M >= 2, only one set of tests need to be done. Preferably at 7.2 × Ipickup, i.e., the requirements denoted by ** are preferred tests. Multiple or single set of requirements denoted by * can be used as alternatives.

The above table summarizes the requirements of IEC on non-compensated overload relays. T_h can be calculated based on the requirement in the above table using Eq. (25). For example, 7.2×I_pickup and Class 10A, 2<t_trip<10, the middle value of $t_{\mathrm{trip}}=\frac{2+10}{2}=6$

sec, substituting t_trip=6 sec., M=7.2, and θ₀=0 into Eq. (25) gives the thermal time constant T_h=308 sec. Because IEC prefers tests done at 7.2×I_pickup and tests done at other M×I_pickup are just alternatives, the table below only shows the T_h calculated from 7.2×I_pickup.

TABLE 3
Thermal time constants Th calculated for M = 7.2 and θo = 0
	Class	Class	Class	Class
	10A	10	20	30
Th	308	359	667	1001
(sec)

The following demonstrates that the thermal algorithm may meet the requirements of IEC standards. The trip time for different M (note again that $M=\frac{I}{I_{\mathrm{pickup}}})$

can then be calculated using Eq. (24). Table 4, below, shows the calculated t_trip for different M based on the T_h values in the above Table 3.

TABLE 4
Trip time ttrip calculated using the Th values in Table 3
	Class 10A	Class 10	Class 20	Class 30
M = 8	4.85	5.66	10.51	15.77
M = 7.2	6.00	7.00	13.00	19.50
M = 7	6.35	7.41	13.76	20.64
M = 6	8.68	10.12	18.80	28.20
M = 5	12.57	14.67	27.24	40.87
M = 4	19.88	23.19	43.07	64.61
M = 3	36.28	42.33	78.61	117.91
M = 2	88.61	103.38	192.00	288.00
M = 1.5	54.93*	64.08*	119.01*	178.51*
M = 1.2	135.90*	158.55*	294.45*	441.67*
M = 1.05	731.72	853.67	1585.39	2378.08
M = 1 + 1.0E−11	7588.41	8853.15	16441.56	24662.35
Note:
All ttrip is calculated from cold state at 40° C. except that ttrip of M = 1.2* and M = 1.5* is calculated from thermal equilibrium at 1.0 × IFLA. SF is set to 1.15.

Observe that t_trip in above table 4 meets all the requirement shown in table 2. For demonstration, FIG. 8 shows the cold trip curves of IEC class 10A, 10, 20, and 30. Note that the hot trip curves and trip curves of other IEC trip classes can be obtained similarly. The hot and cold trip times for M=7.2 are listed below in Table 5. The hot trip time and cold trip time for M=7.2 can also be simulated using Eq. (22).

TABLE 5
Hot and cold trip time for M = 7.2
	Class 10A	Class 10	Class 20	Class 30
Cold trip time (sec)	5.9994	6.9928	12.9922	19.4981
Hot trip time (sec)	1.4738	1.7178	3.1916	4.7899

Simulation of IEC thermal memory test may also be accomplished. The new thermal memory test verification in [3] requires:

• Applying a current equal to I_pickup±10% for two hours (equivalent to the thermal equilibrium);
• interrupt the current for a duration of 2×T_p±10% (T_p is defined in Table 2 of [3]);
• Apply a current equal to 7.2×I_pickup;
• The relay shall trip within 50% of the cold trip time listed in above Table 5.

FIGS. 3A and 3B show the simulation results for the IEC thermal memory test verification. FIG. 3B shows an enlarged view around the point of t=7233 sec. Table 6 compares the IEC thermal memory test trip time to the cold trip time. The IEC thermal memory test trip time is less than 50% of the cold trip time.

TABLE 6
Cold trip time and thermal memory test trip time
	Class 10A	Class 10	Class 20	Class 30
Cold trip time (sec)	5.9994	6.9928	12.9922	19.4981
Thermal memory test	1.3	1.5	2.6	3.9
trip time (sec)

Examining the used thermal capacity for M<1, when motors are running at current below pickup, the temperature rise at thermal equilibrium is proportional to the square of motor line current values. FIG. 4 shows the used thermal capacity for five different M values.

For motors above 250 HP, manufacturers may provide thermal limit curves that specify the safe time for running overload and acceleration overload. Curves 2 and 3 in FIG. 5 are the thermal limit curves. One standard trip class curve cannot provide required overload protection for both running overload and acceleration overload. For example, if a standard trip curve is selected based on acceleration safe time curve 3 in FIG. 5, the trip time of the standard trip curve for running overload (the dashed line) is longer than the running safe time specified by manufacturer (curve 2). Therefore a custom trip, for example, curve 1 can be used, which means thermal time constant during acceleration is different from that during running. Running thermal time constant T_h-run and acceleration thermal time constant T_h-acc can be calculated from the curves 2 and 3 using Eq. (25). Cooling thermal time constant T_c=3×T_h-run·M_acc-run is the multiple of pickup at the changing point of thermal time constants.

FIGS. 6 and 7 demonstrate exemplary methods for applications using IEC standard or custom trip curves. In FIG. 6 a standard trip algorithm 600 is provided. The algorithm 600 is initiated with the current at zero and the thermal capacity at zero (block 602). The method determines if the current is above zero (block 604). If the current is not above zero, the method proceeds to initiation. If the current is above zero the motor is started, the thermal capacity is calculated using Eq. (22) and T_h values in table 3 (block 606). The method determines if the thermal capacity is above one (block 608). If the thermal capacity is above one, the relay is tripped (block 610). If the thermal capacity remains below or equal to one and the current is not equal to zero (block 612), the motor remains active and the thermal capacity is calculated using Eq. (22). If the relay is tripped or the motor is deactivated, the thermal capacity is calculated using Eq. (23) and T_c equals 3×T_h (block 614). Once the thermal capacity is back to zero (block 616), the method is reinitiated (block 602).

In FIG. 7 a custom trip algorithm 700 is provided. The algorithm 700 is initiated with the current at zero and the thermal capacity at zero (block 702). The method determines if the current is above zero (block 704). If the current is not above zero, the method proceeds to initiation. If the current is above zero, the motor is started and the method determines if the status of the motor is active or in an acceleration process (block 706). If M_n≦M_acc-run, the method determines if the previous status of the motor was acceleration (block 708). If the motor was accelerating, set status to run (block 710) and calculate the thermal capacity using Eq. (22) with T_h=T_h-run (block 712). If the motor was not accelerating (block 708) and the motor was run or active status (block 714), calculate the thermal capacity using eq. 22 with T_h=T_h-run (block 712). If M_n>M_acc-run, the method determines if the previous status of the motor was run (block 718). If the status was run, the method calculates the thermal capacity using Eq. (22) with T_h=T_h-acc (block 716). If the status was not run, the method sets the status to acceleration (block 720) and calculates the thermal capacity using Eq. (22) with T_h=T_h-acc (block 716).

The method determines if the thermal capacity is above one (block 722). If the thermal capacity is above one, the relay is tripped (block 724). If the thermal capacity remains below or equal to one and the current is not equal to zero (block 726), the motor remains active and the thermal capacity is calculated as disclosed earlier in the algorithm 700. If the relay is tripped or the motor is deactivated, the thermal capacity is calculated using Eq. (23) and T_c equals 3×T_h-run (block 728). Once the thermal capacity is back to zero (block 730), the method is reinitiated (block 702).

The systems and methods may be implemented using analog components and/or digital components. The systems and methods may be implemented within software that utilizes various components to implement the embodiments described herein. Aspects disclosed in the exemplary embodiment may be utilized independently or in combination with other exemplary embodiments. Moreover, it will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. Persons skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration rather than of limitation, and the present invention is limited only by the claims that follow.

REFERENCES CITED

• [1] IEEE Std C37.112-1996, IEEE Standard Inverse-Time Characteristic Equations for Overcurrent Relays.
• [2] Benmouyal, G.; Meisinger, M.; Burnworth, J.; Elmore, W. A.; Freirich, K.; Kotos, P. A.; Leblanc, P. R.; Lerley, P. J.; McConnell, J. E.; Mizener, J.; Pinto de Sa, J.; Ramaswami, R.; Sachdev, M. S.; Strang, W. M.; Waldron, J. E.; Watansiriroch, S.; Zocholl, S. E.; “IEEE standard inverse-time characteristic equations for overcurrent relays,” IEEE Transactions on Power Delivery, Volume 14, Issue 3, July 1999 Page(s): 868-872.
• [3] Amendment 2 to IEC 60947-4-2, Ed.2, Document No. 17B/1406/CC.
• [4] IEC 60947-4-2 Edition 2.1 2002-03, Low-Voltage Switchgear and Controlgear—Part 4-2: Contactors and Motor-Starters—AC Semiconductor Motor Controllers and Starters.
• [5] IEEE Std 620-1996, IEEE Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction Machines.
• [6] Abou-El-Ela, M. S.; Megahed, A. I.; Malik, O. P.; “Thermal model based digital relaying algorithm for induction motor protection,” Electrical and Computer Engineering, 1996. Canadian Conference on, Volume 2, 26-29 May 1996 Page(s): 1016-1019 vol.2.
• [7] Samir F. Farag; T. Cronvich; “Motor Controller,” US patent number US005206572A, Apr. 27, 1993.

Claims

1. An over-current relay protecting an induction motor comprising: a module for determining the current drawn by the motor; a module for determining the state of the motor; a module for calculating a used thermal capacity based on a first formula when the motor is in an active state; a module for calculating the used thermal capacity based on a second formula when the motor is in an inactive state; and a module for tripping the relay when the used thermal capacity attains a threshold.

2. The relay of claim 1, wherein the first formula for used thermal capacity is selected from the group consisting of

3. The relay of claim 1, wherein the second formula for used thermal capacity is selected from the group consisting of

4. The relay of claim 1, further comprising: a module for calculating the used thermal capacity based on a third formula when the motor is in an acceleration state.

5. The relay of claim 4, wherein the third formula for used thermal capacity is selected from the group consisting of

6. The relay of claim 1, further comprising: a module for calculating the used thermal capacity based on a fourth formula when the motor is in a deceleration state.

7. The relay of claim 6, wherein the fourth formula for used thermal capacity is selected from the group consisting of

8. The relay of claim 1, wherein the relay complies with International Electrotechnical Commission standard 60947-4-2 and amendment 2 of International Electrotechnical Commission standard 60947-4-2.

9. A method for an over-current relay protecting an induction motor comprising the following actions: determining the current drawn by the motor; determining the state of the motor; calculating a used thermal capacity based on a first formula when the motor is in an active state; calculating the used thermal capacity based on a second formula when the motor is in an inactive state; and tripping the relay when the used thermal capacity attains a threshold.

10. The method of claim 9, wherein the first formula for used thermal capacity is selected from the group consisting of

11. The method of claim 9, wherein the second formula for used thermal capacity is selected from the group consisting of

12. The method of claim 9, further comprising the following actions: calculating the used thermal capacity based on a third formula when the motor is in an acceleration state.

13. The method of claim 12, wherein the third formula for used thermal capacity is selected from the group consisting of

14. The method of claim 9, further comprising the following actions: calculating the used thermal capacity based on a fourth formula when the motor is in a deceleration state.

15. The method of claim 9, wherein the fourth formula for used thermal capacity is selected from the group consisting of

16. The method of claim 14, wherein a standard IEC trip class curves is based on induction motor thermal model IEC standard 60947-4-2 and amendment 2 of IEC standard 60947-4-2.

17. An over-current relay protecting an induction motor comprising: a module for determining the current drawn by the motor; a module for determining the state of the motor; a module for calculating a used thermal capacity based on a first formula when the motor is in an active state; a module for calculating the used thermal capacity based on a second formula when the motor is in an inactive state; and a module for tripping the relay when the used thermal capacity attains a threshold.

18. The relay of claim 17, wherein the first formula for used thermal capacity is selected from the group consisting of

19. The relay of claim 17, wherein the second formula for used thermal capacity is selected from the group consisting of

20. The system of claim 17, wherein the used thermal capacity tracks the thermal capacity throughout various motor states.