Variable Speed Drives

Speed drives improve the energy efficiency of AC or DC motors are installed upstream of a motor in its electrical circuit. Speed drives may be referred to as inverter drives or micro drives and can generally be one of two types: Variable Speed Drive (VSD) and Variable Frequency Drive (VFD).

A Variable Speed Drive is a type of industrial speed drive/motor controller and is used to control motor speeds and torque by adjusting the voltage and frequency supplied from the mains power supply to a motor device. Variable speed drives can be AC-AC or DC-AC types. A Variable Frequency Drive is alternative type of speed drive and varies the speed of an AC motor by varying the frequency of the motor. Variable frequency drives can only be used with AC drives.

With over 25% of all the electricity generated in the world consumed by electric motors, improving their energy efficiency can help to reduce carbon footprints and emissions. Many sites use motors to run fans, pumps and compressors and the installation of a suitably sized speed drive can help to improve energy efficiency and reduce energy usage. Heat and noise output may also be reduced as further advantages of installing a speed drive.

Typical clients that have benefited from the installation of speed drives by Synchronised Power range from industrial plants to distribution depots and retail outlets.

How Variable Speed Drives Work

A variable speed drive is installed between the mains power supply and the motor unit. The drive generates a source of DC supply from a built-in AC-DC rectifier. The DC supply is smoothed using DC capacitors and a choke before the supply is inverted back into AC using an output stage DC-AC inverter via a network of IGBTs (integrated gate bipolar transistors). The IGBT network is a smaller inverter drive or ‘Intelligent Power Module (IPM)’ with its own protection and basic control circuits.

The inverter section generates an AC output sine wave supply using Pulse Width Modulated (PWM) control. PWM refers to the chopping or switching (on/off) of the DC supply via the IGBT network. The sine wave is made by a series of DC pulses with the IGBTs controlled to switch at any time base. The pulse frequency is known as the switching frequency and is typically 3-4kHz.

The output voltage is supplied to the motor and the voltage and frequency of the supply are used to control the motor speed. The Voltage is controlled by the width of the fixed voltage pulses and the Frequency by spreading the pulse widths over the switching frequency base.

The IPM module controls the voltage and frequency supplied to the motor and will operate to a range of set-up parameters. The parameters are normally set at installation allowing an inverter drive to run a 230Vac Delta configured motor from a 240Vac single-phase mains power supply (50Hz frequency) or a 400Vac Star configured motor from a 400Vac three-phase configured mains power supply or any other combination required to flux the motor.

Some inverter drives can also apply electrical braking when a braking resistor (DBR) is available. This is because the output of an inverter drive is bi-directional allowing power to flow both ways. In such an instance the motor load will return stored mechanical energy to the inverter drive when it is actively slowed to a slower speed rather than allowed to decelerate or coast.

When braking, the bus voltage rises, and the smoothing DC capacitors are charged and apply braking in return to the motor shaft with the amount dependent on the size of the DC capacitor bank. For this type of application, a brake switch or chopper is required to diver the braking energy into the braking resistor.

If your site uses fans, compressors and pumps our energy consultants can provide a cost-effective quotation for the supply and installation of energy saving speed drives.