( TECHNICAL NEWS Issue 10

Please circulate to

Quarterly Technical Newsletter of Australia's leading supplier of AC VARIABLE low-voltage motor control and switchgear. FREQUENCY DRIVES AND BRAKING When an AC motor is This field induces current The will try to controlled by an AC in the rotor conductors variable frequency drive and the magnetic field match the speed of (V.F. drive) there is a associated with these significant difference in currents imparts a torque the rotating the way the installation which accelerates the responds to overhauling rotor in the direction of magnetic field, but loads compared with the the rotating field. The motor being connected rotor will try to match the due to bearing and directly to the mains. speed of the rotating other losses cannot The article will analyse magnetic field, out due to the difference in bearing and other losses run synchronously performance and the cannot run synchronously strategy used by V.F. with it. The difference with it. drives to absorb between the rotor speed regenerative energy - and the synchronous namely, dynamic braking. speed is called "slip". Also discussed will be DC injection braking and the difference between these methods of braking will become apparent. • AC variable The AC drives and braking 1 operates on the same principle as all three • Generation region phase AC motors - that is, overhauling interaction of two loads magnetic fields. A 2 rotating magnetic field is set up by the when • Dynamic brake it is connected to the resistor 3 supply. The speed of rotation of this field is • DC injection directly proportional to braking 4 the supply frequency. Adjustabl9 fr9qu9ncy AC motor driv9s (

JIC varia6Ce frequency arives ana 6raf(jng Generating {continuea from page 1) region­ Torque overhauling loads

For negative slips - that is, when the rotor is turning in the same direction but at a higher speed than the travelling field, 2 0 -1 Slip the "motor" torque is in fact negative. The machine Figure 1 develops a torque which A V.F. drive alters the speed of opposes the rotation and acts l the motor by changing the as an induction "generator", speed of the rotating magnetic converting mechanical power field. The AC supply is It can be seen that from the shaft into electrical rectified, the DC voltage is the decisive factor energy. filtered by a capacitor and When the motor is connected transistors are switched at high as far as the directly to the AC mains the speed using a Pulse Width direction of torque regenerated energy can be fed Modulation (PWM) technique back directly into the supply. to supply the motor with a is concerned is slip In contrast, when the motor is variable voltage and variable rather than speed. supplied by a V.F. drive the frequency supply. This regenerated energy cannot be provides control of the speed returned to the supply. The of the rotating magnetic field uncontrolled bridge on and hence motor speed. the input of the V.F. drive acts Field strength is maintained as a block to the regenerated constant providing minimal It can be seen that the decisive energy. This leaves all reduction in motor torque at factor as far as the direction of regenerated energy to be speeds up to base frequency. torque is concerned is slip dissipated by the motor and rather than speed. When slip is V.F. drive. A typical torque speed curve positive the torque is positive for an induction motor and vice versa. The torque Dynamic brake covering the full speed range therefore always acts so as to which is likely to be urge the rotor to run at zero resistor encountered in practice is slip - that is, synchronous shown in Figure 1. (Ns is speed. Inherent in each V.F. drive synchronous speed). installation is the ability to 15 BR~ RESISTOR dissipate to 20 per cent of regenerative energy as losses RECTFIER DC LINK INVERTER within the V.F. drive and motor. Where the load inertia is large and/or rapid accelera tionl deceleration occurs, regenerated energy increases substantially. This results in a rise in DC voltage and may cause damage or activate overvoltage protection Figure 2 to isolate the V.F. drive.

NHP Technical News, issue I 0 (

V.F. drives employ a device to dissipate excessive regenerative energy called a "Dynamic Braking Resistor". As shown in Figure 2, the resistor is connected across the DC bus via a transistor. When the DC voltage reaches the reference level in excess of rated capacity the transistor is turned on and the regenerative Figure 3 energy is dissipated in the resistor as heat. For this reason selection of a A typical torque speed curve V.F. drive must include for DC braking an induction When the speed setting of a consideration of how the V.F. motor is shown in Figure 4 motor connected to a V.F. drive will respond to below. Note the braking drive is reduced the regenerative energy. (negative) torque falls to zero synchronous speed of the Preferably, the V.F. drive as the rotor comes to rest, motor decreases. Figure 3 should have dynamic braking since there will only be below shows the effect of fitted as standard and the induced currents in the rotor changing the frequency from ability to readily accept (and hence torque) when the Fl to F2 and the braking braking resistors of higher rotor is cutting the flux. DC torque generated -T2. capacity to cater for varying injection braking is a When the synchronous speed load conditions. dissipative process and all the falls below the rotor speed, kinetic energy is turned into slip becomes negative, thus DC injection heat inside the motor. For this torque generated by the motor reason DC injection braking is switched to braking torque. braking should not be applied for At speeds above base excessive periods, as motor frequency braking torque is It was previously stated that over heating may occur. reduced - as is the positive the speed of rotation of the Where DC injection braking is motor torque. field in the air-gap is directly provided by V.F. drives several proportional to the supply Selection of the appropriate programmable parameters frequency. Dynamic Brake Resistor controlling the application of depends on the inertia of the Also, the rotor always tries to DC injection braking are motor and load, permissible run at the same speed as the provided. current limit of the transistor air-gap field, so if the field is These parameters set the and resistor and duty cycle of stationary and the rotor is not, frequency (or speed) when DC the load. Braking torque is a braking torque will be injection is activated, the usually expressed as a exerted. magnitude of DC voltage percentage of the motor torque available and can range from 30 per cent to 150 per cent of rated torque. The situation where the load overhauls the motor can occur quite often. When the motor is connected to the AC mains this is of no consequence. However, when the motor is connected to a V.F. drive the regenerative energy must be dissipated. Figure4

NHP Technical News, issue I 0 NHP Electrical Engineering Products Pty ltd A.C.N. 004 304 812 Internet http://www.nhp.com.au MELBOURNE 43-67 River Street, Richmond, Vic. 3121 Phone: (03) 9429 2999 Fax (03) 9429 1075 SYDNEY 'DC injection 6rafjng 30-34 Day Street North, Silverwater, N.S.W. 2128 (continuetf from page 3) Phone: (02) 9748 3444 Fax: (02) 9648 4353 BRISBANE FMAXI----. 25 Turbo Drive, >­ (.) Coorparoo, Old. 4151 c Q) Phone: (07) 3891 6008 :J Fax: (07) 3891 6139 0" ADELAIDE ~ *1 50 Croydon Road, Keswick, S.A. 5035 ~ 0'------~~-Time Phone: (08) 8297 9055 8 0 *2 __j Fax: (08) 8371 0962 PERTH DC voltage I 0 '*3 38 Belmont Ave., f Rivervale, W.A. 6103 Phone: (08) 9277 1777 Figure 5 Fax: (08) 9277 1700 NEWCASTLE 575 Maitland Road, applied (usually as a Dynamic braking The ability of V.F. drives Mayfield West, N.S.W. 2304 to provide these facilities Phone: (02) 4960 2220 percentage of supply Fax: (02) 4960 2203 voltage) and the time DC provides improved should be considered TOWNSVILLE injection is applied during its selection. 62 Leyland Street, dynamic response Garbutt, Old. 4814 Figure 5. to speed change Dynamic braking Phone: (07) 4779 0700 provides improved Fax: (07) 4775 1457 The above discussion has and overhauling ROCKHAMPTON outlined the operation of dynamic response to 208 Denison Street, speed change and Rockhampton, Old. 4700 a motor and V.F. drive loads. Phone: (07) 4927 2277 with overhauling loads, DC injection overhauling loads. Fax: (07) 4922 2947 the use of dynamic TOOWOOMBA braking provides DC injection braking Cnr Carroll St. & Struan Crt., braking to dissipate provides controlled Toowoomba, Old. 4350 regenerative energy and a controlled stopping stopping of motors for Phone: (07) 4634 4799 method of providing Fax: (07) 4633 1796 of motors for position control or CAIRNS positive braking torque to elimination of over-run. 14/128 Lyons Street, a motor - DC injection position control or Bungalow, Old. 4870 braking. Phone: (07) 4035 6888 elimination of Fax: (07) 4035 6999 DARWIN over-run. 3 Steele Street, Winnellie, N.T. 0820 Phone: (08) 8947 2666 Fax: (08) 8947. 2049 Agents: HOBART 199 Harrington Street, Hobart~Tas . 7000 Phone: (03) 6234 9299 Fax: (03) 6231 1693 LAUNCESTON 59 Garfield Street, Launceston, Tas. 7250 Phone: (03) 6344 8811 Fax: (03) 6344 4069 Contact NHP for all your BURNIE 6 Wellington Street, Burnie, Tas. 7320 Phone: (03) 6432 2588 switchgear requirements Fax: (03) 6432 2580 from the one source

Editorial content:- Please address all enquiries to 'The Editor- 'NHP Technical News ' PO Box 199, Richmond Victoria 3121. NHP Technical News, issue 10