Correctly sizing your motor and drive saves your money and can give your machine the performance you require. Motors are available in a range of speeds and torques. Drives are available in a range of maximum voltages and continuous currents.

The motor speed determines the maximum machine speed. The motor torque, or turning force, needs to be sufficient to accelerate your machine up to the maximum speed. Maximum motor torque is required to accelerate the machine and to provide the maximum cutting force you need. The higher the acceleration rate you want, the higher the motor torque you need. Formulae are available to calculate the motor torque required using a number of machine mechanical parameters, these are beyond the scope of this brief article.

The maximum motor speed available from a specific motor is proportional to the voltage at the motor.

Consider the first AXOR SAX 165S motor. This has a stall torque of 165oz inches and a nominal speed of 2,100 rpm.

The motor winding is rated at 65VDC at 4.6 amps. If this motor is driven by 65VDC it will run at 2100rpm. If this motor is driven by 40VDC it will run at 2100 * 40/65 = 1292rpm.

You can run any motor below the rated rpm and torque limits. You should not run any motor above the rated rpm limits. However you can run a motor above it's torque limit for a short proportion and period of time, say for a few seconds, during machine acceleration.

The maximum motor torque is proportional to the current flowing through the motor coils. If the current flowing through the SAX 165S motor is 4.6 amps the torque available will be 165oz. in. If the current flowing through the motor is 2.5 amps the torque available will be 165 * 2.5/4.6 = 89.67 oz.in.

Motor drives are rated by maximum voltage and current. The drives can be operated below these voltage and current limits. AXOR drives operate throughout a wide range of voltages and can easily be set to provide lower continuous currents. AXOR drives can provide twice the rated torque for 2 seconds, this current can easily be reduced. The drive should be set so as not to exceed the rated motor stall current to protect the motor. Excessive currents can burn out the motor.

Initially it is best if you know what you want the maximum traverse speed of the machine to be, it is this speed which determines one of the motor's vital parameters. From this you can determine the maximum motor rpm. As an example assume that you want the maximum traverse speed to be 400ipm and that your machine is driven by a ball screw with a pitch of 0.25 inches, i.e. the machine will move 0.25 inches for each rotation of the screw. The maximum motor speed is given by: maximum traverse speed / machine movement distance per motor rev, i.e 400/0.25 = 1600rpm.

Assuming the SAX165S motor torque of 165 oz.in. is sufficient to operate the machine we need a motor drive capable of providing 4.6 amps continuously. The voltage required at the motor is given by: rated motor winding voltage * required maximum rpm/ rated motor rpm, 65 * 1600/2100 = 49.5 volts. From these figures we could use an AXOR MicroSpeed MCS60 6/12 drive rated at 60 VDC and 6 amps continuous current, or an AXOR Minispeed MS60 8/16 drive rated at 60 VDC and 8 amps continuous current. Both drives would need to be setup to limit continuous motor current to some 4.6 amps.

Some AXOR drives require a separate motor power supply. This should be generally rated to provide around 1.2 * the voltage required by the motor to achieve the required rpm, and at a current which is the sum of the motor continuous currents.

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